Transformers, transformer stations, MV/LV switchgear and infrastructure for renewables are all part of the same puzzle. An energy storage system is not a lonely island. It is an element of a system that must communicate with the grid, the inverter, protection devices, automation, and the operator's requirements.

This article is for renewable energy investors, designers, general contractors, industrial plants, PV+BESS farm developers, and everyone who wants to understand why a transformer for an energy storage system should not be selected like a standard transformer from a catalogue.

We often imagine an energy storage system as one large battery: containers, lithium‑ion modules, a cooling system, inverters, monitoring, algorithms. But in practice, all this technology is of little use if the energy cannot safely enter the grid and return from it.

And this is where the transformer enters the stage.

It is the gateway between the world of batteries, inverters and the PCS system, and the medium‑voltage grid. A bit like a lock in a canal: on one side we have the dynamic, fast, electronic world of the energy storage system; on the other – the stable, demanding and unforgiving power grid.

If the lock is poorly chosen, the whole project begins to lose efficiency, reliability and predictability.

The energy storage market is growing very rapidly.

According to the IEA, energy storage was the fastest‑growing commercially available energy technology in the power sector in 2023, with global battery storage additions reaching 42 GW.

This means one thing: more and more investors will have to ask themselves not only "which energy storage system to choose?" but also: "which transformer to select so that this storage system really works well?"

In the text, we will cover:

• the role of the transformer in a BESS system,

• the differences between a transformer for PV, industry and energy storage,

• power rating selection in kVA/MVA,

• the choice between oil‑immersed and dry‑type transformers,

• the impact of harmonics, cyclicity and bidirectional operation,

• the mistakes that most often cost the most,

• a practical checklist for RFQ specifications.

Reading time: about 10 minutes

Energy storage is not a "large UPS"

The biggest mistake at the start?

Treating energy storage as a larger version of a backup installation.

Yes, a storage system can serve an emergency function. It can supply a facility when the grid is not working.

It can stabilise voltage, reduce peak power demand, or cooperate with photovoltaics. But a modern BESS, or Battery Energy Storage System, is not just a battery.

It is a power system with its own dynamics.

IEC TS 62786-3:2023 describes the requirements for stationary battery energy storage systems connected to distribution networks, including connection schemes, switchgear, operating range, active and reactive power response, power quality, protection, monitoring, control and grid‑connection tests.

The very list of these areas shows that an energy storage system is an active participant in grid operation, not just a consumer or a simple source.

A transformer in such a system must therefore do more than the classic "step up the voltage".

It must work with inverters, withstand variable load profiles, respond to bidirectional operation, and function in an environment where power electronics generate different phenomena than a traditional industrial consumer.

What exactly does a transformer do in an energy storage system?

In simple terms: the transformer matches the voltage of the PCS/inverter system to the grid voltage or the facility's internal installation.

On the battery side, we have direct current. The PCS system converts it to alternating current.

Then the transformer steps up the voltage to the level required by the LV or MV grid, most often in industrial and renewable projects to medium voltage.

But that is only the simplest description.

In reality, a transformer in a BESS system performs several functions simultaneously:

• it galvanically isolates the storage system from the grid,

• it matches voltage levels,

• it affects the earthing method and protection operation,

• it limits the transfer of some disturbances,

• it must withstand loads resulting from inverter operation,

• it operates both during charging and discharging of the storage system.

In a classic PV farm, the energy flow is essentially unidirectional: from the panels through the inverters to the grid. In an energy storage system, energy flows in both directions.

In the morning, the storage system may charge from the grid or from PV; in the afternoon, it may discharge; in the evening, it may operate in price arbitrage; and at night, it may provide a system service.

The transformer therefore does not have one "calm" operating profile.

It has a daily rhythm resembling breathing: inhale, exhale, pause, fast response, repeat.

The infographic shows a simplified diagram of a transformer's operation in an energy storage system. On the left is a battery container, from which energy goes to a PCS inverter that converts DC to AC. The central element is the transformer, which matches voltage levels, isolates the storage system from the grid, supports earthing and protection, and limits some disturbances from the power electronics. On the right, the power grid and industrial infrastructure are shown. Bidirectional arrows symbolise charging and discharging of the storage system, i.e., the flow of energy both from the battery to the grid and from the grid to the battery.

CC: ENERGEKS 2026

First question: what is this storage system for?

We do not select a transformer "just because".

We select it for a function.

A different transformer will make sense for an industrial plant that wants to reduce its contracted capacity. Another for a PV farm with a 10 MW/20 MWh storage system.

Another for a large grid‑scale storage system that is to provide balancing and regulation services or support a local grid node.

Before selecting a transformer, several questions must be answered:

• Will the storage system operate mainly behind the meter, i.e., on the consumer's side?

• Will it be connected as an independent unit to the grid?

• Is it to cooperate with a photovoltaic farm?

• Will it charge from the grid, from PV, or from both sources?

• Will it frequently switch from charging to discharging?

• Is it to supply reactive power?

• Does the operator require a specific voltage regulation range, communication and observability?

These are not formal questions. They are questions that determine winding temperature, losses, power margin, connection group, cooling type and insulation durability.

Transformer power: kVA, MW and MWh are not the same thing

This is the point where it is easy to fall into a trap.

An energy storage system is usually described by two parameters: power and capacity.

Example: 5 MW / 10 MWh. The first value tells you how much power the system can charge or discharge. The second tells you how long it can maintain that power.

We do not select a transformer directly based on MWh.

The transformer "sees" primarily apparent power, i.e., kVA or MVA, and the load profile over time.

If the storage system has a PCS power of 5 MW and is to operate at a power factor of 1, the minimum apparent power is theoretically about 5 MVA.

But if reactive power supply or absorption is required, operation at cosφ = 0.9, or a wider regulation range, the apparent power increases.

For example:

5 MW / 0.9 = 5.56 MVA

This means that a 5 MVA transformer may be too tight if the system is to operate dynamically and provide additional grid services. In practice, the designer may consider a 6.3 MVA unit, but the final choice depends on the PCS requirements, the operator, the operating profile, environmental conditions and the overall system architecture.

Similarly for a 2 MW storage system:

2 MW / 0.9 = 2.22 MVA

Here, a natural point of analysis may be a 2.5 MVA transformer, but not as an "automatic answer", only as a result of calculations and coordination with the rest of the system.

⚡ The most important rule: MWh capacity tells you how large the "energy tank" is.

MW power and operating requirements tell you how wide the "pipe" through which this energy flows must be. We select the transformer for the pipe, not for the tank itself.

Oil‑immersed or dry‑type transformer?

This is one of the most common questions from investors.

The answer is: it depends on the location, power, fire risk, environmental conditions and the facility's requirements.

Oil‑immersed transformer for energy storage

An oil‑immersed transformer is very often a natural choice for larger energy storage systems, PV+BESS farms and outdoor transformer stations.

It has high heat dissipation capability, good overload capacity, a wide range of available powers, and performs well in outdoor applications.

In containerised projects and MV stations, an oil‑immersed transformer can operate as part of a compact infrastructure: energy storage, PCS, switchgear, transformer, metering and protection system. For larger powers, oil gives the designer greater thermal flexibility.

It is worth remembering, however, the requirements for oil containment basins, environmental protection, clearances, fire protection and site approvals.

In special environmental conditions, the use of esters can be considered, but this too should result from the design, not from fashion.

Dry‑type transformer for energy storage

A dry‑type transformer, especially cast‑resin, is a good solution where fire safety, indoor operation, limiting the risk of insulating liquid leakage, or location near utility infrastructure are important.

In energy storage systems installed at industrial plants, logistics centres, commercial facilities or technical buildings, a dry‑type transformer may be more acceptable from the point of view of health and safety, the insurer and the building designer.

However, it is more sensitive to cooling conditions.

It does not like being "closed in a cupboard" without airflow and expected to work like a transformer in an ideal test hall.

Ventilation, ambient temperature, dust, humidity and service space are of great importance here.

BESS is bidirectional operation. The transformer must be ready for it

In a typical industrial consumer, energy flows from the grid to the plant. In a classic PV farm, it flows from the source to the grid. In an energy storage system, it flows in both directions.

During charging, the transformer works as an element supplying the storage system. During discharging, it becomes part of the energy export path. This affects:

• protection selection,

• directionality of measurements,

• automation,

• MV/LV protection settings,

• operator requirements,

• active and reactive power flow analysis.

For this reason, the transformer for an energy storage system should be designed together with the PCS system, switchgear, protection and connection scheme. It should not be added at the end like a missing piece.

CIGRE, in its guidelines for BESS connection stations, indicates that the design of such infrastructure covers the entire life cycle: from design and development, through commissioning, to asset management, including output power assessment and parameters at the PCC. This is important because the transformer is not a separate product in a vacuum. It is part of the entire storage system's capability to operate at the point of connection.

Harmonics: the silent enemy of the transformer

Inverters and power electronic converters are the heart of an energy storage system. Without them, the battery could not cooperate with the AC grid. But power electronics generate phenomena that cannot be ignored.

One of them is harmonics.

Harmonics can increase additional losses in the windings and structural components of the transformer. They can cause additional heating, affect insulation durability and require appropriate thermal margin. It is a bit like driving a car on a smooth motorway and on cobblestones. The average speed may look similar, but the suspension load is completely different.

Therefore, when selecting a transformer for BESS, it is worth requiring data on:

• the harmonic spectrum generated by the PCS,

• current and voltage THD,

• switching frequency,

• filtering requirements,

• permissible power quality levels,

• operation under partial load.

IEC TS 62786-3:2023 covers, among other things, power quality, EMC, interface protection, active and reactive power response, and grid‑connection tests. This shows that the transformer for an energy storage system must be selected in the context of the entire electrical environment, not only voltage and rated power.

Transformer losses: small watts, big money

In energy storage systems, much attention is paid to battery, inverter and cooling system efficiency. The transformer is often treated as an obvious element. That is a mistake.

A transformer has no‑load and load losses. No‑load losses occur when the transformer is energised, even if the storage system is not operating at full power. Load losses increase with current flow.

A simple example like a power bill:

If a transformer has 3 kW of no‑load losses and is energised all year round, that gives:

3 kW × 8760 h = 26,280 kWh per year

That is over 26 MWh of energy lost annually on no‑load alone. For larger units, several transformers or a long project life, the difference between an average and an optimised solution can mean tens or hundreds of megawatt‑hours of losses over the entire operating life.

In an energy storage system that earns money from price differences, flexibility services or peak demand reduction, every unnecessary loss is like a leak in a tank. Small, seemingly. But it works every day.

Voltage, connection group and earthing – technical details that determine system operation

The selection of a transformer for BESS starts with voltages.

On one side we have the PCS voltage, often at LV level. On the other side we have the MV grid, e.g., 15 kV, 20 kV or another level specified in the connection conditions. The transformer must match these worlds not only in voltage but also functionally.

Important parameters are:

• primary and secondary voltage,

• rated power,

• connection group,

• short‑circuit voltage,

• tap‑changing range,

• insulation level,

• neutral earthing method,

• parallel operation requirements,

• compatibility with protections.

The connection group is not cosmetic. It affects phase shift, behaviour during earth faults, zero‑sequence current flow and protection coordination. In projects with multiple PCS units, an architecture with several block transformers instead of one large unit can be considered. This can improve modularity, serviceability and availability.

Operator requirements: the transformer must fit the grid, not just the storage system

In Poland, an energy storage system does not end with the container and the detailed design. It ends when it can be safely connected, commissioned and operated in accordance with the operator's requirements.

TAURON Dystrybucja indicates that, in agreement with the DSOs associated in PTPiREE, procedures for obtaining a permit for use for type D electricity storage systems were developed, effective from 31 March 2026. The same page gives the maximum power thresholds for types B, C and D: 0.2 MW, 10 MW and 75 MW respectively.

This is important because, as the project power increases, not only the transformer power increases. The number of requirements regarding documentation, observability, controllability, tests, protections and connection conditions also increases.

URE also informed about new "General Application Requirements" resulting from NC RfG, which came into force on 1 December 2025 for type B, C and D units for which connection conditions are issued from that date. In practice, this means that BESS projects, especially hybrid PV+BESS or those connected as active grid resources, should be analysed not only from the equipment side but also from the formal‑technical requirements side.

Transformer for PV+BESS: one system, two operating profiles

A photovoltaic farm and an energy storage system look like a natural pair. PV produces energy when the sun shines. The storage system allows its use to be shifted in time, reduces curtailment, smooths the generation profile or increases self‑consumption.

But for a transformer, such a system can be more demanding than the PV farm alone.

Why?

Because PV generates energy in a specific daily profile, dependent on insolation. BESS can charge and discharge according to market, grid or industrial strategy. Sometimes the energy from PV feeds the grid, sometimes it charges the battery, sometimes the battery discharges to the grid, and sometimes the whole system operates with export power limitation.

The transformer must be selected for the real power flow scenario, not just the sum of nameplate powers.

Example:

A 20 MWp PV farm and a 10 MW / 20 MWh storage system do not automatically mean that the transformer must be 30 MVA. If the connection conditions limit export to 20 MW and the EMS strategy monitors the operating profile, the selection may look different. However, if the system is to have the ability for fully independent operation of the source and the storage system, the transformation power requirement may be greater.

This is the moment when simulations, connection conditions and a clear EMS strategy are needed. Without this, transformer selection is like choosing a bridge without knowing how many trucks will cross it and in which direction.

Transformer for industrial energy storage

In industry, energy storage often has a very practical function: to reduce peak demand, improve PV self‑consumption, provide reserve, stabilise the operation of loads or reduce contracted capacity costs.

Here, the transformer may be part of an existing plant station or new infrastructure dedicated to the storage system. The choice between a dry‑type and oil‑immersed transformer depends on the location.

If the storage system is located next to a production hall, in a technical building or close to people, a dry‑type transformer may be the natural choice. If the system is larger, installed outdoors and operates in a containerised station, an oil‑immersed transformer may be more advantageous in terms of thermal performance, power and cost per MVA.

For industry, the following are particularly important:

• noise,

• installation location,

• fire safety,

• service access,

• compatibility with existing switchgear,

• operation with sensitive loads,

• expandability.

An energy storage system in a plant should not be designed as a gadget for invoice optimisation.

It is a new active element of the internal power network.

Transformer for a large grid‑scale storage system

In large grid‑scale projects, the transformer becomes part of a block architecture.

Instead of one huge unit, several blocks are often used: PCS + block transformer + MV switchgear, followed by a power take‑off to the main station.

Such architecture gives greater flexibility, facilitates servicing and limits the consequences of failure of a single component.

For a 50 MW / 100 MWh storage system, several blocks of 5 MW, 10 MW or more can be considered, depending on the PCS used and the connection concept. Then the question is not only "which transformer?" but "which transformation architecture gives the best compromise between efficiency, availability, cost and risk?".

This is especially important when the storage system is to earn money from system services. Failure of one transformer in a modular system can limit the power of part of the system. Failure of one central element can stop a much larger part of the project.

How to prepare a RFQ for a transformer for a BESS energy storage system?

Selecting a transformer for an energy storage system does not start with the question: "how much does a 2.5 MVA transformer cost?"

It starts with a much more interesting question: how will this storage system really operate?

Because BESS is not a fridge that simply draws power from a socket. It is an active power system that sometimes draws energy, sometimes gives it back, sometimes supports the grid, sometimes charges from PV, and sometimes does everything so dynamically that the classic approach of "let's choose a transformer with some margin and it will be fine" starts to resemble driving a sports car on wheelbarrow tyres.

We advise on the selection of transformers for energy storage systems based on actual operating parameters, not just a single power value from a table.

Below, we show what data is worth preparing and why each item matters.

PCS power in MW – how wide the energy flows

PCS power determines how much power the storage system can charge and discharge. This is one of the most important parameters for transformer selection, because the transformer must handle the actual power flow between the inverter and the grid.

If the PCS has a power of 2 MW, 5 MW or 10 MW, the transformer must be selected not only for that value but also for the system's operating mode. Will the storage system operate continuously? Will it respond to demand peaks? Will it provide grid services? Will it frequently change the direction of energy flow?

PCS power is not just a number. It is the rate at which energy "breathes" through the transformer.

Storage capacity in MWh – how large the energy tank is

Storage capacity, expressed in MWh, tells us how much energy the system can store. We do not select the transformer directly based on MWh, but this value helps to understand how long the storage system can operate at a given power.

A 5 MW / 10 MWh storage system can operate at full power for about 2 hours. A 5 MW / 20 MWh storage system can do so for about 4 hours. For the transformer, this means a completely different thermal profile.

A short power pulse is one thing. Several hours of regular operation under high load is another conversation. The transformer does not get offended immediately, but the winding temperature remembers everything.

Maximum charging and discharging power – because BESS works in both directions

In a classic consumer, energy flows from the grid to the consumer. In a PV farm, most often from the source to the grid. In an energy storage system, we have bidirectional movement.

Therefore, we need to know the maximum charging power and the maximum discharging power. Sometimes they are the same, sometimes different. This affects the selection of transformer power, protection, measurement and the whole system logic.

A transformer in BESS does not have the quiet life of a retiree. It is more like a goalkeeper at an airport: sometimes it lets energy through in one direction, sometimes in the other, and all the time it has to keep order.

Required cosφ or reactive power range – not only active power keeps the grid alive

Active power, expressed in MW, does useful work. But the power system also needs control of reactive power. That is why we ask about the required power factor cosφ or the range of operation with reactive power.

Why is this important?

Because the transformer is selected for apparent power, i.e., kVA or MVA. If the storage system is to operate at cosφ = 1, the situation is simpler. If it is to operate at cosφ = 0.9 or supply/absorb reactive power according to the operator's requirements, the required apparent power increases.

Example:

5 MW at cosφ = 1 means about 5 MVA.5 MW at cosφ = 0.9 already means about 5.56 MVA.

The difference is not academic. It can decide whether the transformer will operate with a comfortable margin or whether it will ask the designer every day: "did you really think that about me?"

PCS side voltage – the starting point for transformation

The PCS, or Power Conversion System, operates on a specific voltage side. The transformer must be matched to the inverter's output voltage and safely step it up to the grid or facility installation level.

This is a basic parameter, but it should not be treated routinely. A different PCS voltage means a different winding configuration, different currents, different losses and different protection requirements.

Simply put: before the transformer steps up the voltage, it must know which step it is starting from.

Grid voltage on the MV/LV side – where the energy is to go

On the other side of the system, we have the low‑voltage or medium‑voltage grid. In industrial and renewable projects, we most often talk about connection to medium voltage, e.g., 15 kV, 20 kV or another level specified in the connection conditions.

This parameter determines the transformer ratio, insulation level, MV switchgear, surge protection and compliance with the operator's requirements.

The transformer is the interpreter between the language of the PCS and the language of the grid. And in power engineering, the interpreter must know both languages perfectly.

Frequency – a simple but mandatory detail

In Poland and most of Europe, we operate at 50 Hz, but for international projects or unusual applications, this parameter must be clearly specified.

Frequency affects the core design, magnetic losses and transformer operation. For standard projects, this is an obvious point. For a good technical enquiry – still mandatory.

Number of PCS units and their connection method – one large system or several blocks?

An energy storage system can have one central PCS or several smaller units operating in parallel. It can also be built modularly: PCS + block transformer + switchgear.

This has a huge impact on the architecture of the entire installation. Several smaller transformers can improve serviceability and limit the consequences of failure of a single block. One larger unit may be more cost‑effective and simpler in layout, but it increases the importance of a single element for the availability of the entire system.

There is no one answer for all. But there is good engineering.

Required transformer short‑circuit voltage – the parameter that keeps short‑circuit currents in check

Short‑circuit voltage affects short‑circuit currents, voltage drops, parallel operation of transformers and protection coordination.

Too low a short‑circuit voltage can mean higher short‑circuit currents. Too high a value can cause greater voltage drops and affect system operation. Therefore, this parameter is not "fine print" in the specification. It is one of those values that decides whether protections work elegantly or start improvising.

And protections in power engineering should not have a talent for improvisation.

Connection group – phase geometry matters

The transformer connection group determines the winding connection method and the phase shift between the primary and secondary sides. It affects the system's behaviour during faults, zero‑sequence current flow, cooperation with protections and the possibility of parallel operation.

In BESS systems, where we have power electronics, measurements, directional protections and operator requirements, the connection group must be selected consciously.

It is a bit like setting the choreography in a three‑phase dance. If one side takes a step sideways and the other a step forward, the system may look spectacular only for the first few seconds.

Tap range – a small correction with a big impact on voltage

Transformer taps allow the ratio and voltage level to be adjusted to the grid operating conditions. In energy storage systems, this is particularly important when the installation operates under variable conditions, at different load levels, and with the possibility of exporting energy to the grid.

The tap range should correspond to the connection conditions and voltage requirements. A well‑chosen transformer gives the designer a regulatory margin. A poorly chosen one leaves them with a problem that later returns in measurements, complaints and nervous phone calls.

Insulation level – resistance to reality

The insulation level must correspond to the grid voltage, overvoltage conditions and operational requirements. This applies to both oil‑immersed and dry‑type transformers.

In practice, it is about the device's ability to operate safely in an environment where overvoltages, disturbances, switching operations, faults and all that energy weather that is not visible but which the transformer feels very well occur.

Insulation is no place for creative savings. It is the foundation of durability.

DSO/TSO requirements – because the grid has its rules

The distribution or transmission system operator specifies requirements regarding connection, protection, measurement, control, power quality parameters and system operation.

Therefore, when enquiring about a transformer, it is worth attaching the connection conditions or at least information on what stage the project is at. The operator's requirements can affect the voltage, connection system, protection, automation, measurement and station architecture.

An energy storage system can be modern, intelligent and beautifully described in a presentation. But if it does not fit the grid requirements, it is still only a very expensive container with ambitions.

Expected operating profile – the transformer also has a daily rhythm

Will the storage system operate every day? Will it charge at night and discharge during the peak? Will it cooperate with PV? Will it provide system services? Will it operate rarely but intensively?

The operating profile tells us how the transformer will be loaded over time. This is key for assessing temperature, losses, insulation durability and possible overload capacity.

Two storage systems of the same power may require a different approach if one operates calmly for several hours a day, while the other responds dynamically many times a day. On paper, they look similar. In the windings – not necessarily.

Harmonic spectrum from the PCS – because the inverter does not sing a pure sine wave

The PCS converts energy between DC and AC. It is the heart of the BESS system, but like all power electronics, it can introduce harmonics.

Harmonics cause additional losses, heating and loads for the transformer. Therefore, it is worth knowing the THD, harmonic spectrum, switching frequency and filtering requirements.

This is one of the most important reasons why a transformer for BESS should not be selected like a standard transformer for a calm consumer.

A sine wave from an inverter can be like a conversation after three coffees: essentially understandable, but full of nervous tremors.

Loss requirements – efficiency works all year round

Transformer losses have a real impact on the economics of an energy storage system. No‑load losses occur when the transformer is energised. Load losses increase with current flow.

In a BESS that is to earn money from price arbitrage, flexibility services, peak reduction or PV self‑consumption, every unnecessary loss reduces the financial effect.

That is why we ask about loss requirements and recommend analysing not only the purchase price but also operating costs over the entire life cycle. The cheapest transformer on the invoice is not always the cheapest in operation.

Location: indoor or outdoor?

The installation location influences the choice of transformer type, cooling, enclosure, protection, fire protection, noise, service access and building requirements.

Oil‑immersed transformers are often a good fit for outdoor installations, especially at higher powers. For indoor, industrial installations and facilities with increased fire safety requirements, cast‑resin dry‑type transformers are often worth analysing.

This is not about a fashion for "dry" or "oil". It is about the working environment, risk, power and technical common sense.

Environmental conditions – the transformer does not work in a catalogue

Ambient temperature, altitude above sea level, humidity, dust, salinity, ventilation, solar radiation, risk of flooding, aggressive industrial atmosphere – all of this matters.

A transformer from a catalogue lives in a beautiful world of even temperatures and ideal assumptions. A transformer in the field lives next to dust, heat, frost, rain, containers, cables and people who sometimes block ventilation grilles because "it's just for a moment".

That is why environmental conditions must be stated at the beginning. Then a device can be selected that will operate not in theory but in a real location.

Noise requirements – silence is also a technical parameter

A transformer generates noise. For industrial installations, this may not be a problem. For commercial, residential, office buildings or close to property boundaries – it can be very important.

Acoustic requirements should be specified at the enquiry stage. This allows the construction, location, enclosure or solutions to limit noise emissions to be selected.

Because a transformer should operate stably. It does not also need to give a nightly concert for the neighbours.

Oil‑immersed transformer, dry‑type transformer or an analysis of both variants?

At Energeks, we can advise on an oil‑immersed transformer, a dry‑type transformer, or compare both variants.

An oil‑immersed transformer usually works well for higher powers, outdoor installations, containerised stations and renewable projects. It has very good cooling properties and a wide range of applications.

A dry‑type transformer is often chosen for indoor installations, facilities with increased fire safety requirements, industry, logistics centres and places where limiting insulating liquid is an important argument.

The best choice does not come from an advertising slogan. It comes from the place of work, the power, safety requirements, ventilation, costs and the maintenance plan.

Temperature monitoring – because it is better to know earlier than by smell

Temperature monitoring of the windings and core allows the transformer's condition to be controlled, overloads to be responded to and operation to be better managed.

In BESS, where the operating profile can be dynamic, temperature monitoring is not a luxury. It is a practical tool for maintaining reliability.

Temperature sensors, protection relays, alarm signals and integration with the supervision system help to avoid situations where the first diagnostic message is "something is heating up".

Dimensional and transport limitations – because the transformer also has to get there

Power and electrical parameters are one thing. But the transformer must be delivered, unloaded, positioned and connected.

Therefore, we need information about dimensional limitations, weight, access road, foundation, room height, gate width, crane capabilities and service space.

This is very down‑to‑earth data. Literally. But without it, even the best transformer can become the hero of a logistical comedy that no one wanted to produce.

Planned system expansion – think about the second stage before the first stage sets in concrete

Energy storage systems are often designed in stages. Today 2 MW, in two years 5 MW. Today cooperation with PV, tomorrow additional grid services. Today one PCS, tomorrow additional blocks.

If the investor plans expansion, it is worth saying so immediately. This allows for a power margin, space in the station, switchgear configuration, cable cross‑sections, parallel operation capability and future connection strategy to be considered.

Power engineering likes planning. Improvisation is great in jazz, but in a transformer station we prefer notes, diagrams and protection selectivity.

What we advise at Energeks

We advise that the transformer for an energy storage system should be selected not as a separate device but as part of the entire BESS system: batteries, PCS, switchgear, protection, automation, transformer station and operator requirements.

We analyse:

• whether an oil‑immersed or dry‑type transformer is better,

• what rated power gives a safe margin,

• what voltages and connection group suit the project,

• how the operating profile will affect temperature and losses,

• whether harmonics from the PCS require special attention,

• what DSO/TSO requirements must be met,

• how to prepare the transformer for future expansion.

⚡ The better the input data, the less guesswork.

And in power engineering, guesswork can be expensive, heavy and very awkward to transport.

Therefore, if you are planning a BESS energy storage system, a PV+BESS installation or an industrial energy optimisation system, it is worth starting with a well‑prepared technical enquiry.

We will help translate it into a specific transformer, station and MV/LV infrastructure selection.

Because an energy storage system starts with the battery only on a slide.

In reality, it starts where the energy must safely meet the grid.

A good transformer means a more relaxed energy storage system

A BESS energy storage system can do really beautiful things: charge when energy is available, discharge when it is needed, support photovoltaics, stabilise plant operation and help better manage energy costs.

But all this magic needs a solid gateway to the grid.

That gateway is the transformer.

We like to look at transformer selection not as a table of power, voltage and price, but as a conversation about the future operation of the whole system. About whether the storage system will charge calmly like a phone at night, or operate dynamically like an espresso machine on a Monday morning. About whether energy will flow in one direction, in two directions, often, rarely, stably or impulsively. About whether the transformer is simply "there" or truly supports the reliability of the investment.

We help select oil‑immersed and dry‑type transformers for energy storage systems, PV+BESS installations, industry, renewables, and MV/LV transformer stations. We advise, analyse operating parameters, operator requirements, PCS power, losses, harmonics, environmental conditions and future system expansion.

Because the transformer for an energy storage system should not be selected "by eye".

The eye is great for admiring the sunset over a PV farm.For BESS, it is better to use calculations, experience and a proper technical specification.

If you are planning an energy storage system or modernisation of power infrastructure, see our range of Energeks transformers.

If time is of the essence, also check the transformers available off‑the‑shelf in our warehouse.

Thank you for making it to the end of this technical walk through the world of BESS. If after reading you have more questions than at the beginning – that is a very good sign. In power engineering, good questions are often worth more than quick answers from a catalogue.

And if you want to stay up to date with technical analyses, market examples and a practical dose of power engineering knowledge, follow our profile on LinkedIn.

A well‑chosen transformer does not make noise around itself.

It simply works.

Stably, safely and exactly as it should.

Sources:

International Energy Agency, „Batteries and Secure Energy Transitions” via https://iea.blob.core.windows.net

IEC TS 62786-3:2023, „Distributed energy resources connection with the grid — Additional requirements for stationary battery energy storage system”

DNV-RP-0043, „Safety, operation and performance of grid-connected energy storage systems”

Cover Photo: DC Studio/magnific.com

Heatwaves are returning. Transformers remember every one of them.

When the air temperature exceeds 35°C, most electrical devices simply work harder. Air conditioners draw more current. Refrigerators don't rest. Industry runs at full capacity because deadlines chase deadlines.

And the transformer stands in the corner of the switchgear station, in a container or in a building basement, and does its job. Without a word of complaint. Up to a point.

That point has a name: critical winding temperature.

When you exceed it, the transformer does not always shut down elegantly with an error message. Sometimes it just accelerates its ageing. Quietly. Invisibly. Day after day, throughout the hot summer.

This text is about what really happens to a transformer during a heatwave, why standard parameters can be misleading, and what exactly is worth checking before high temperature does it for you.

Reading time: about 9 minutes.

First, some physics without which the rest makes no sense

When a spectacular explosion of a power station appears in the media — like recent well‑known footage from the Enstedværket site near Aabenraa, a facility linked to Energinet infrastructure — it is easy to focus on the effect itself: the flash, the bang, the smoke, the scale of destruction.

Photo via LinkedIn on profile of Emil Mahler Larsen

But in power engineering, such images are only the finale.

The end of a process.

Before a failure, before protection trips, before an electric arc, fire or mechanical damage to equipment, physics always happens first. Sometimes over seconds. Sometimes over months. Sometimes over years.

And that is precisely why the conversation about transformer overheating in heatwaves is not seasonal scaremongering. It is a conversation about what happens inside the device long before the problem becomes spectacular.

A transformer produces heat. Not by accident, but by the very nature of its work.

Core losses. Copper or aluminium winding losses. Eddy currents. Leakage fluxes. All of this turns into heat that must go somewhere.

In oil‑immersed transformers, this heat is taken up by the oil, which circulates naturally or with pumps and releases energy to the surroundings through radiators or tank fins.

In dry‑type transformers — and we will focus on these here, because they stand in buildings, indoor substations, data centres and industrial halls — this heat is released to the air.

And here the problem begins.

Air in summer is warm.

Warm air absorbs heat less effectively than cold air.

A transformer that calmly operates with a winding temperature of 80°C in winter can reach 95–100°C in summer at the same load. Or more.

Every additional 10°C above the rated value translates into roughly twice as fast insulation ageing. This is not an opinion — it is the well‑researched Arrhenius law, used in electrical engineering for decades.

Insulation that should last 30 years, with regular exceedance of the permissible temperature, can end its life in half that time.

Reason 1: Ambient temperature higher than the design assumed

Most dry‑type transformers are designed for a maximum ambient temperature of 40°C. This is a standard value, often visible in the technical documentation.

What does this mean in practice?

That the manufacturer designed the cooling so that at 40°C in the room, the transformer does not exceed the permissible winding temperature.

When the ambient temperature exceeds 40°C — which in Poland's climate during the summers of 2024 and 2025 happened increasingly often — the transformer begins to operate outside its rated thermal capabilities.

There is no fault in that. It is simply physics.

If you have a dry‑type transformer and you do not know what the actual temperature is in its room on a hot day, you are in the situation of a driver who does not know the engine oil temperature on a climb up to the Tatra Mountains. The engine may get you there. But it may not come back intact.

What is worth doing: check what the actual ambient temperature is at the transformer on the hottest days. Not in the office. Not outside the building. At the transformer, at 3:00 PM, when the sun has been heating the walls and roof for several hours.

Reason 2: Room ventilation that cannot keep up

A dry‑type transformer cannot dissipate heat into a vacuum. It dissipates it into the air in the room. That air must go somewhere — and must be replaced by cooler air.

If the room ventilation is too weak, the air temperature at the transformer rises. The transformer dissipates heat into increasingly warmer air, which means it itself becomes increasingly warmer.

This is a vicious circle and can lead to overheating even at a load well below rated power.

Summer heat does one specific thing here: it heats the air outside the building, which is the source of cooling. If the ventilation intakes draw air from the south, and the sun heats the south wall of the building — the intake air temperature can be 5–10°C higher than the air temperature in the shade.

And that is before the air even reaches the transformer.

What is worth doing: assess whether the ventilation openings are well placed and whether the grilles are not blocked by dust, dirt‑clogged protective meshes or accidental storage of items. Check whether the airflow actually passes through the room, rather than circulating in a closed loop.

Reason 3: Load increases together with temperature

This is the paradox of summer that hurts double.

When it gets hot, the demand for electricity rises. Air conditioning. Ventilation. Cooling systems. Food industry at full speed. Refrigerators, freezers, server room cooling systems — everything wants its share of power.

This means that transformers are more loaded in summer than in winter.

And at the same time they have a worse ability to dissipate heat.

Higher load → more losses in windings → more heat to dissipate. Higher ambient temperature → worse cooling conditions → slower heat dissipation.

These two effects act at the same time and in the same direction.

A transformer in July therefore works under a double unfavourable condition. It produces more heat and dissipates it more slowly.

What is worth doing: check historical load data for the transformer from previous years during the summer months. If the load has increased in recent years (because more air conditioning, new equipment or facility expansion has been added), the thermal margin may have shrunk.

Reason 4: Temperature sensors that do not alarm in time

A modern dry‑type transformer should be equipped with winding temperature sensors. These can be PT100 or PTC sensors, which work with a thermal relay. The relay sends an alarm signal at the first threshold and disconnects the transformer at the second — critical — threshold.

Sounds good.

But in practice, three problems appear.

First: the sensor measures temperature at a specific point on the winding. If it is poorly placed or outdated, it may not reflect the actual temperature at the hottest point of the coil.

Second: some transformers, especially older or cheaper installations, have alarm settings set too high. The alarm appears when the insulation has already been operating in excessive heat conditions for weeks.

Third: the alarm signal goes to the building's BMS system or to a local panel. If no one is actively monitoring this in summer, the alarm may burn unnoticed for several hours.

What is worth doing: check when the sensor system was last calibrated and tested. Check the alarm settings in the documentation and compare them with the temperature for class F insulation (155°C) or class B (130°C). Ensure that the alarm reaches someone who responds — not just logged in a file.

Reason 5: Overload during the summer peak

Transformers have the ability for short‑term overload — even up to 120–150% of rated power for a specified time. This is a design option, useful for temporary demand peaks.

Only this capability is calculated on the assumption that the transformer operated for some time at low load before the overload and is relatively cold.

In summer, the transformer can be warm around the clock. Cool nights, which in winter gave it time to rebuild its thermal margin, may not provide sufficient cooling in summer.

When such a transformer enters a peak load state — say, during a hot afternoon when air conditioning is pulling full power — it starts from a higher base temperature. The margin to critical temperature is smaller. An overload that would be fine in winter can be risky in summer.

What is worth doing: if the transformer is often used close to its rated power limits, review the overload curves in the technical documentation and check how they change with ambient temperature. Manufacturers provide this data. It is worth reading before August, not after.

Reason 6: Dust and dirt in summer block cooling ducts

Dry‑type transformers dissipate heat through air ducts in the windings and between structural elements. These ducts must be clear.

In summer, when windows and doors are open more often, fans work more intensively, and nearby construction and renovation work generates more dust — the amount of dust in the air increases.

Dust settles on the windings and in the ducts. If the winding is of the open‑wound or VPI type, dust can gradually restrict airflow. The thicker the dust layer, the worse the cooling.

In cast‑resin transformers, the problem is smaller — the epoxy resin creates a closed shield around the windings. But here too, dust on the external surfaces blocks heat exchange.

Additionally, dust saturated with moisture can pose a risk to insulation — especially if it contains metallic particles or chemical substances from production.

What is worth doing: check the cleanliness of the transformer and its surroundings. If previous inspections showed excessive contamination, it may be worth planning cleaning before the summer peak, not after.

Reason 7: The transformer simply has years on it

Transformer insulation ages. This is not an opinion — it is a technical and economic fact worth taking into account.

With each year of operation, with each heatwave, with each overload and each heating and cooling cycle, the insulation loses its properties. It becomes brittle. Loses flexibility. Its dielectric strength decreases.

A transformer that is 15–20 years old and for half its life has operated in difficult thermal conditions may have insulation in a condition corresponding to a much older device.

No one sees this with the naked eye. The insulation may look good and at the same time be on the verge of breakdown.

Heat is not a problem in itself. Heat is a test.

If the insulation is in good condition, the test will pass without complications. If it is tired — heat may be the moment that finalises what began several years earlier.

What is worth doing: if the transformer is over 15 years old and has not undergone diagnostics in the last few seasons, it is worth considering insulation measurements. Insulation resistance measurement, absorption coefficient, and in the case of oil‑immersed transformers — oil analysis — are tools that give a real picture of the device's technical condition.

What to specifically do when a heatwave arrives

Below is a practical list of actions that make sense before and during high temperatures. There is nothing exotic here — only things that really affect the safety of transformer operation.

Before the summer season:

• Check the temperature in the transformer room from the previous summer, if you have such data. Compare it with the ambient temperature assumed in the documentation.

• Assess ventilation: are the grilles clear, is the airflow correct, is the intake air not coming from places excessively heated by the sun?

• Check the temperature sensor settings and test whether the alarm signal actually reaches the responsible person.

• Assess historical load — is the transformer being increasingly loaded year on year?

• If the transformer is over 15 years old, consider a diagnostic inspection before the season.

During a heatwave:

• Monitor the temperature in the transformer room regularly — not just once a week.

• If the transformer has forced cooling (AF fans), check whether the fans are working correctly and whether their operation is triggered at the right time.

• If you have the ability to temporarily distribute the load or reduce non‑productive loads during the temperature peak, consider it.

• Do not ignore a temperature alarm, even if it has always turned out to be false before. In summer, every alarm needs checking.

Dry‑type vs oil‑immersed transformer — does this change the situation?

Yes, but not as many think.

An oil‑immersed transformer has oil that carries heat away from the windings to the tank walls and radiators. This allows for higher thermal capacity and slower winding temperature rise during short‑term peaks.

But the oil also has its own critical temperature. At too high ambient temperature and too weak cooling — for example with blocked tank fins or a damaged cooling pump — an oil‑immersed transformer will also overheat.

A dry‑type transformer reacts faster to ambient temperature changes because it does not have the thermal buffer of oil. On the other hand, there is no risk of oil leakage, no risk of oil fire, and it is generally less complex to operate.

In both cases, the principle is the same: high ambient temperature reduces the transformer's ability to dissipate heat. And in both cases, ignoring this for several seasons leaves a mark on the insulation.

A few words about what is not worth doing

It is not worth relying solely on the fact that "it has always worked."

Transformers are unfailingly patient devices. They can operate in poor conditions for months and years without giving clear signals. And then the failure happens suddenly — often during the first major overload or an especially long heatwave.

It is also not worth assuming that since the transformer has a temperature sensor, the problem will solve itself. A sensor can warn — but it cannot improve ventilation, reduce load or repair insulation that is already tired.

And it is not worth comparing a transformer to a device where "if it breaks, we will buy a new one." MV transformers are investments for decades. Premature failure is not only the cost of buying a new device — it is downtime, installation cost, risk of burnouts in the installation, potential production loss and a lot of nerves at the station.

Heat is not an enemy of the transformer if you help it

A heatwave is a test of the power infrastructure.

Most transformers that are in good technical condition and operate in well‑designed conditions will get through the summer without problems. These are devices built for long‑term operation.

But those that are a dozen or so years old, operate in poorly ventilated rooms, are increasingly loaded and have not been regularly checked — have much less margin in summer.

The good news is simple: most of these problems can be checked, assessed and improved. Ventilation. Load. Cleanliness. Sensors. Insulation condition.

This is not complicated diagnostics. It is taking care of a device that serves the entire installation and has no replacement during a failure.

If you do not know what condition your transformer is in and how it will react to the next heatwave — this is a good time to find out.

Want to check your transformer before the season?

At Energeks, we help select transformers for real operating conditions and assess whether an existing device has the right margin for summer peaks.

If you are planning a transformer replacement, station modernisation or want to check whether the current unit has the right power reserve, see:

—> our range of dry‑type and oil‑immersed transformers

—> transformers available off‑the‑shelf in our warehouse

—> further technical analyses on the Energeks LinkedIn profile

In a well‑designed transformer station, summer should not be a lottery.

It should be just another working season.

sources:

Energinet — Ensted–Kiskelund / Enstedværket przy Aabenraa

IEC 60076-7:2018 — Loading guide for mineral-oil-immersed power transformers

From this article you will learn how the transport of a power transformer is planned, what in practice dimensions of about 9.2 m length, 3.4 m width, 4.0 m height and a weight of about 165 tonnes without oil mean, how unloading looks, and why post‑delivery inspection protects the entire investment.

Power transformer transport is not a delivery. It is a technical operation.

There are loads that arrive on a pallet.

There are those that a courier leaves at the gate.

And there are those at which the gate itself starts asking existential questions.

A power transformer with a length of about 9.2 m, width of about 3.4 m, height of about 4.0 m and a transport weight without oil of about 165 tonnes does not travel like ordinary equipment. It does not "drive". It is led along the route like a technical operation on a living organism of infrastructure.

A powerful tractor and good intentions are not enough.

You need a route that really has a margin.

You need a trailer matched to the pressures.

You need an unloading plan that is not made over coffee five minutes before the manoeuvre.

You need people who know that 165 tonnes do not like sudden ideas.

At Energeks, we take this topic seriously, because a power transformer is not an ordinary delivery item.

It is the future heart of a power supply system.

Before it starts working, it must safely pass through transport, unloading, post‑delivery inspection, positioning and preparation for commissioning.

And that is what this text is about.

About logistics that looks spectacular but in practice is precision.

About unloading where patience weighs more than steel.

About post‑delivery inspection that tells you more than the best assurances.

And about why good power transformer transport is not an add‑on to the investment, but one of its first quality tests.

Reading time: about 9 minutes.

Dimensions, weight and centre of gravity: the transformer has its own physics

In power transformer transport, the numbers are not decoration in the documentation.

They are the instruction manual for the survival of the whole operation.

9.2 m length is not just information that the load is long. It is a forecast of how the whole combination will behave on curves, roundabouts, intersections, plant gates and the last metres to the foundation.

3.4 m width means that the comfort of a normal traffic lane ends. Logistics begins where the driver, pilot, traffic management and travel plan must act as one organism. At this width, even an ordinary road sign can suddenly become a technical problem.

4.0 m height forces you to look not only ahead but also upward. Bridges, overhead wires, gates, canopies, process pipelines and structures on the plant site cease to be scenery. They become a checklist.

And 165 tonnes of transport weight without oil?

That is no longer a number.

That is a character.

The weight of a transformer determines axle loads, trailer selection, lashing method, suspension behaviour, ground preparation, unloading options, support points and whether the foundation is really ready to meet a device of this class.

That is precisely why a transformer is not a load that simply needs to be transported.

It is an object that needs to be understood.

The most important thing is the centre of gravity. It does not look spectacular. It does not have its own nameplate with fanfares. Yet it decides how the transformer will behave during lifting, braking, turning, tilting, moving and positioning.

If the centre of gravity is treated superficially, you can have excellent equipment and still create a situation that no sensible person wants to see up close.

It is a bit like carrying a huge wardrobe with a hidden safe on one side. From the outside you see a rectangle. In your hands you feel the truth.

A transformer also has its truth. And you need to know it before the first move.

That is why transport drawings, marking of the centre of gravity, lifting points, lashing points, support points and manufacturer's instructions are so important. This is not paperwork. It is the nerve map of the whole operation.

The route: the shortest road is rarely the best

In power transformer transport, the shortest route does not win.

The route that does not pretend to be ready wins.

On paper, everything may look innocent. The road exists. The bridge exists. The gate exists. The curve exists. But a combination with a 165 tonne transformer does not ask whether something exists. It asks whether it can safely pass that way.

You need to check turning radii, bridge load‑bearing capacities, culverts, shoulders, height restrictions, overhanging wires, roundabouts, intersections, road surface, entrance to the site, and the last metres to the unloading point.

And those last metres can be the most malicious.

The transformer may travel hundreds of kilometres, passing bridges, gates and roundabouts, and then stop a few dozen metres from its destination because the ground is not ready, the turning radius is too tight, or the unloading equipment has no room to work.

That is why not only the public road is analysed. The plant site, site roads, manoeuvring areas, access to the foundation and the working area of the crane or sliding system are also analysed.

In heavy logistics, the map is the beginning. Reality always needs to be examined more closely.

Loading and unloading: here it is not strength that wins, but sequence

From the outside, transformer unloading may look simple.

A crane arrives. We attach. We lift. We move. Done.

A beautiful fairy tale.

In the real world, 165 tonnes do not respond to optimism. They respond to geometry, centre of gravity, working radius, ground load‑bearing capacity, lifting points, sling angles, sequence of movements and quality of communication between people.

It is not about having a big crane.

The crane must be selected for the specific operation. For the specific weight. For the specific radius. For the specific height. For the specific position of the transformer and the specific working space.

Rated capacity looks good in a table, but during unloading the real configuration counts. The larger the working radius, the more the available capacity drops. The more difficult the ground, the more important the outriggers become. The less space around, the more every decision begins to resemble a game of chess with gravity.

And gravity does not lose through inattention.

That is why before unloading you need to know where the transport combination will stand, where the lifting equipment will stand, where the supports will be, how the load will pass, who gives the commands, what the safety zones are, and what we do if conditions cease to be ideal.

The moment the transformer is in the air is the worst moment for creativity.

Good unloading looks almost boring. And that is a compliment.

No shouting. No guessing. No sudden corrections. No people running in different directions. Everyone knows where they stand. Everyone knows when to speak. Everyone knows who makes the decision. Everyone knows when to stop the operation.

With large masses, speed is not evidence of professionalism.

Control is.

Post‑delivery inspection: the transformer also leaves traces of its journey

When the transformer is finally in place, many people think that the hardest part is behind them.

That is exactly when one of the most important stages begins.

Post‑delivery inspection.

It sounds modest. Almost official. In practice, it is a moment of truth.

A transformer may look good and still require thorough verification. It may also arrive with minor transport traces that are harmless, but they need to be assessed correctly. The mere presence of the device on site does not yet mean that everything is ready.

It means that you can start checking.

Post‑delivery inspection includes visual inspection of the tank, paint coatings, nozzles, valves, seals, lashing points, transport securing, separately delivered accessories and completeness of documentation. If the transformer was transported without oil, you also need to check the protection conditions of the active part, for example dry air or nitrogen pressure and tightness.

One of the key elements is the shock recorder.

A small device with great significance. Its task is to record events that could have stressed the transformer during transport, transhipment, parking or unloading. We are talking about shocks, vibrations, accelerations and sometimes also tilts. In more modern systems, there is also time stamping, GPS location and temperature.

The shock recorder acts like the transformer's black box.

It is not interested in who said that everything went gently. It records what really happened.

This does not mean that every recorded event is a disaster. But every significant event requires assessment. You need to combine the recorder data with the external condition of the device, transport documentation, information from the crew and the manufacturer's recommendations.

It is not about looking for sensation.

It is about a technical decision.

Because a power transformer has inside it a core, windings, connections, insulation, a tap changer and a whole precise internal architecture. Not everything that matters is visible to the naked eye.

That is why post‑delivery acceptance should not be based on faith.

It should be based on data.

Foundation and positioning: 165 tonnes must have a place to sit calmly

The transformer has arrived.

It has been unloaded.

Now it needs to be positioned.

It sounds like the finale of the whole story.

In practice, this is the moment when many earlier decisions come to light. Not in theory. In the real contact of 165 tonnes with a prepared place.

The foundation for a power transformer is not a piece of concrete. It is part of the system.

It must carry static and dynamic loads. It must maintain geometry. It must work with support points, rails, rollers or a sliding system. It must enable operation, service, possible replacement and safe management of transformer oil if we are talking about an oil‑filled unit.

A transformer does not load the world generally.

It loads it specifically.

Through specific points, in a specific place, at a specific time. Therefore, the general load‑bearing capacity of the foundation is not enough. Local load under specific support points also matters.

Then there is levelling.

It is not cosmetic. It is not about the device looking nice in a photo. Incorrect positioning can affect load distribution, accessory assembly, access to valves, cooling performance, cable routing, earthing and future maintenance.

A power transformer should not fight with its own stand.

It is supposed to work on it.

That is why the positioning site must be ready earlier. Not almost ready. Ready. With an accepted foundation, checked level, prepared working space, provided service access, solved oil drainage and completed earthing.

With 165 tonnes, the word "almost" starts to become very expensive.

How much does power transformer transport cost?

This is one of those questions to which the honest answer is: it depends on how much reality likes to complicate logistics.

The cost of power transformer transport is not calculated like ordinary freight per kilometre. Here you do not pay only for the journey from point A to point B. The pricing includes the weight of the device, its transport dimensions, the route, the number of axles in the combination, escort, permits, analysis of bridges and viaducts, possible removal of road infrastructure, trailer selection, preparation for loading, unloading, crane or sliding system, and sometimes also additional securing, technical stops and the work of several teams simultaneously.

Therefore, transport of a power transformer weighing several dozen tonnes will be a completely different operation than moving a unit weighing about 165 tonnes without oil. In the first case, we are talking about demanding logistics. In the second, about an operation where every metre of the route, every turning radius and every support point begins to have financial significance.

The greatest impact on cost usually comes from:

• transformer weight

• transport length, width and height

• distance of transport

• number of countries and formalities along the way

• need for escort

• difficulty of the last metres of access

• selection of crane or unloading system

• ground and foundation preparation

• working time of technical teams

• post‑delivery inspection requirements

In practice, the cheapest transformer transport is rarely the best news. If the quote looks too light for the weight of the device, it is worth asking what is missing. Does it include unloading? Does it cover route analysis? Does it include escort? Is a shock recorder foreseen? Has anyone checked the last section to the foundation? Is the lifting equipment selected for real conditions, not for an optimistic scenario?

Because with a power transformer, transport cost is not just a line item in the budget. It is part of the protection of the whole investment.

A well‑planned transport may seem more expensive at the beginning, but it often saves money where it really hurts: in delays, damage, additional cranes, foundation corrections, assembly downtime and nervous decisions made already on site.

In short: power transformer transport costs as much as the calm delivery of a very expensive, very heavy and very necessary device without improvisation along the way.

And that is one of the things that sensible people do not try to do accounting magic on.

A large transformer does not need luck. It needs a process

Power transformer transport looks spectacular, but its real value lies not in photos from the road.

It lies in preparation.

In a route that has been checked. In equipment selected for the real weight and dimensions. In people who understand centre of gravity, lifting points, pressures, tilt and movement sequence. In post‑delivery inspection that relies not on impression but on data. In a foundation that is ready to receive the device without nervously writing the plan at the last minute.

A power transformer is not ordinary cargo. It is the future heart of a power supply system. Before it starts working, it must safely pass through transport, unloading, acceptance, positioning and preparation for commissioning.

And that is why good logistics is not an add‑on to the investment.

It is its first quality test.

If you are planning a project involving transformers, transformer stations, switchgear or solutions for power infrastructure, it is worth checking the full Energeks offer and seeing how broadly you can approach power supply from a technical, design and delivery perspective.

Depending on the working conditions, installation location and system requirements, a good starting point may be the choice between an oil‑immersed transformer and a dry‑type transformer. Each of these solutions has its specific applications, its advantages and its requirements, so the decision should result from real working conditions, not a random choice from a catalogue.

And if the project does not like to wait, it is also worth checking which transformers are available off the shelf.

Because sometimes the best news in power engineering is not a big promise, but concrete equipment availability when the schedule really needs it.

More technical inspiration, projects and power engineering specifics can also be found on the Energeks LinkedIn page.

references:
CIGRE Technical Brochure 673, Guide on transformer transportation
Mammoet, Transformer transport services
Hitachi Energy, Install and Commission

What does "dry‑type transformer" mean, and why is it not always cast‑resin?

A dry‑type transformer is not one type of device, but a group of transformers without insulating liquid. It can have air insulation, open‑wound windings, VPI impregnation, composite insulation, or full cast‑resin encapsulation with epoxy resin. The choice depends on the working environment, humidity, dust levels, fire safety requirements, cooling method, and ease of servicing.

A dry‑type transformer sounds simple. So simple that it is almost suspicious.

In the industry, a mental shortcut very often works: dry means cast‑resin.

Someone says "dry‑type transformer", and the other person immediately pictures windings cast up to the brim with epoxy resin. Solid, shiny, compact coils. No oil. No tank. No risk of leakage. Case closed.

Only technically, the case is not closed at all.

A dry‑type transformer is not one technology. It is a whole family of constructions where insulation and cooling are not based on an insulating liquid. There is no mineral oil or ester to remove heat while also providing insulation. Heat is dissipated mainly by air, and the winding insulation can be achieved in several ways.

And this is where the most interesting part begins.

A dry transformer can have air insulation.

A dry transformer can be VPI‑impregnated.

A dry transformer can be of the open‑wound type.

A dry transformer can have composite insulation.

A dry transformer can finally be cast‑resin – the most well‑known type – cast with epoxy resin.

Each of these belongs to the world of dry‑type transformers, but not all behave the same. They differ in resistance to moisture, dust, temperature, vibration, contamination, overloads, cooling method, and ease of servicing.

Therefore, the question "which dry‑type transformer to choose" should not start with the price.

It should start with the place of work.

Will the transformer stand in a clean technical room?

In an industrial hall with dust?

In a public building?

In an indoor substation? Near people?

In a humid environment?

In a place where low noise level matters?

Where any failure means costly downtime?

Only then does it make sense to discuss whether epoxy, VPI, air insulation or a special construction will be best.

This text organises the subject without unnecessary hype.

It will cover what a dry‑type transformer really means, what its types are, how air insulation differs from VPI and cast‑resin, where open‑wound works, and why epoxy resin is not always the only sensible answer.

Reading time: ~ 8 minutes

A dry‑type transformer is not one box, but several different design philosophies

Simply put, a dry‑type transformer is a transformer that is not immersed in an insulating liquid. In an oil‑filled transformer, the windings and core operate in oil or another insulating fluid. In a dry‑type transformer, this fluid is absent.

But the absence of oil does not mean the absence of insulation. This is very important.

The insulation must still withstand operating voltages, overvoltages, heating, ageing, vibrations, and mechanical stresses during short circuits. The difference is that this function is taken over by solid materials, air, varnishes, resins, insulating tapes, spacers, distancing elements, impregnation systems, and the design of cooling ducts.

That is why two dry‑type transformers of the same power can look similar in a table but behave completely differently in operation.

One will dissipate heat better but tolerate contamination less well.

Another will be more resistant to moisture but heavier and more expensive.

A third will be easier to service but will require a clean, well‑ventilated room.

A fourth will perform where a standard design would age too quickly due to chemicals, vibrations or elevated temperatures.

It is a bit like technical clothing. A sports shirt, a softshell, a rain jacket and a work coverall can all serve to protect the body, but no sensible person treats them as interchangeable. Each solution makes sense in a different environment.

The same is true for dry‑type transformers.

Which dry‑type transformer to choose – is every dry transformer an epoxy one?

Every epoxy cast‑resin transformer is a dry‑type transformer, but not every dry‑type transformer is epoxy.

This sentence is worth remembering, because it resolves half of the industry misunderstandings.

Epoxy, i.e. cast‑resin, is only one type of dry‑type transformer.

Very popular, often very good, but not the only one.

If a request for quotation only says "dry‑type transformer" without specifying the winding technology, offers for different constructions may appear.

One company will propose cast‑resin – a dry transformer with windings cast in resin.

Another will propose VPI – a dry transformer with vacuum‑pressure impregnated windings.

A third will propose an open‑wound construction – a dry transformer with open, ventilated windings.

A fourth, most interestingly, might propose dip and bake – a dry transformer with windings impregnated by dipping and baked in an oven.

Formally, all will be dry‑type transformers, but technically they will not be the same product.

This is where the risk of comparing apples to oranges begins.

The price may differ not because someone exaggerated their margin, but because different insulation systems, different environmental resistances, different cooling methods and different capabilities for working in harsh conditions are being compared.

Therefore, in a well‑prepared specification, it is not enough to write "dry‑type transformer 1000 kVA. It is worth specifying the winding technology, environmental, climatic and fire classes, cooling method, noise level, degree of protection of the enclosure, ambient conditions, room ventilation, temperature sensors and operational requirements.

A dry‑type transformer does not work in a vacuum.

It works in a specific building, a specific substation, a specific hall and specific air. And the air can be clean, dry and calm. Or it can carry moisture, dust, salt, chemical vapours and everything that electrical insulation very much dislikes.

Dry‑type transformer with air insulation

The simplest variant is a dry‑type transformer with air insulation.

In such a construction, air remains one of the basic elements of the insulation and cooling system. The windings are not fully encapsulated in a solid resin block. They are usually protected with an insulating varnish or resin in an impregnation process, for example by VPI or the simpler dip and bake method.

In practice, this means the winding is protected but not enclosed in a thick, solid mass of resin.

This gives several important advantages. Such a transformer can be lighter. It can dissipate heat well because air has easier access to the winding surfaces. It can also be easier to inspect and service because the construction is more open.

But there is another side.

If the working insulation largely remains air, the quality of that air becomes enormously important. Dust, moisture, conductive contaminants, aggressive chemical compounds and condensation can become real problems. This type of transformer needs a clean, dry and controlled environment.

This is not a flaw in itself.

It is simply a condition for correct application.

In a clean technical room, such a construction can work very well.

In a harsh industrial hall where dust is airborne and temperature and humidity change dynamically, much more caution is needed.

A transformer with air insulation is like a device that breathes well.

But since it breathes, it should not breathe dirt.

The illustration shows a dry transformer construction where the spaces between windings, insulators and supporting elements are clearly visible. This explains the principle of a dry transformer with air insulation: air participates in cooling and electrical separation, and the windings are not completely enclosed in a solid resin mass. Such a transformer likes clean, dry technical rooms.

Dry‑type VPI transformer

VPI stands for Vacuum Pressure Impregnation.

In this technology, the windings are saturated with resin or insulating varnish in a controlled process. First, air is removed from the spaces between turns, then the impregnating material is introduced under pressure. After curing, a structure is formed that is stronger, more stable and better protected than with simple varnishing.

The key point, however, is that VPI does not create the same effect as cast‑resin encapsulation.

In a VPI transformer, the windings are impregnated with insulation but are not completely sealed in a solid resin block. There is no uniform, massive epoxy block. Rather, we have an impregnated, reinforced and protected structure that still retains a more open character.

This gives an interesting compromise.

VPI can be more cost‑effective than cast‑resin.

It can dissipate heat well because the winding is not covered by a thick layer of resin. It can be lighter and more flexible in certain applications. It works well in many technical buildings, industrial facilities, switchgear rooms and applications where conditions are relatively controlled.

However, it is not a technology for every environment.

If the transformer is to operate in a place with high humidity, in air with conductive dust, in a chemical atmosphere or in an area exposed to salt, you need to check very carefully whether VPI is sufficient. Sometimes it will be. Sometimes VPE, cast‑resin or a completely different solution will be better.

Simply put: VPI is a reasonable compromise between price, cooling and resistance. But like any compromise, it works best when you know the operating conditions well.

he illustration shows a dry transformer with visible windings, supporting frame and insulators, which fits the VPI technology well. In a VPI transformer, the windings are saturated with varnish or resin in a vacuum‑pressure impregnation process, but are not completely cast in resin like cast‑resin. This gives a good compromise between insulation protection, cooling and cost.

Dry‑type open‑wound transformer

Open‑wound is a construction with open, air‑cooled windings.

Sometimes such transformers are described as AN (air natural) when cooling is by natural air movement without fans. In other cases, forced cooling (AF) with fans may appear.

In an open‑wound transformer, the windings are visible, ventilated and protected by electrical insulating materials. They are not enclosed in a resin mass.

Airflow is very important here, because it is responsible for heat dissipation.

The greatest advantage is effective cooling.

The open construction allows air to flow through the ducts and around the windings. This allows the transformer to dissipate heat efficiently to the surroundings. An additional advantage can be lower weight and simpler inspection.

The biggest limitation is sensitivity to the environment.

Open‑wound does not like moisture, dust, contamination or aggressive air.

In a clean indoor environment it can work very well. In a place where dust settles on the insulation, moisture creates conductive paths, and ventilation draws contaminants from the hall, problems can begin.

This solution is rather for interiors with controlled conditions.

Not for a random corner of a hall where "it will be fine".

In power engineering, "it will be fine" often later means "why is the protection tripping" or "why is the temperature rising faster than in the documentation".

The illustration shows a dry transformer with strongly exposed winding elements, insulators and air ducts. This captures the idea of an open‑wound construction – a transformer with open, ventilated windings. Such a design dissipates heat very well because air can flow more freely around the active parts. The price for this openness is simple: the transformer does not like moisture, dust and aggressive environments. It is a more precise technical piece of equipment than a "shovel‑for‑everything".

Dry‑type dip and bake transformer

Dip and bake is a simpler method of winding impregnation.

The windings are dipped in varnish or insulating resin, then dried and cured in an oven. Hence the name: dip and bake.

This is a well‑known, relatively simple method used in various electrical devices.

Compared to VPI, however, it usually has a lower ability to penetrate deeply into the winding structure. There is no such intensive air removal and pressure‑driven material injection.

Does this mean dip and bake is bad? No. It means it has its place.

It can be used in less demanding applications, at lower powers, in auxiliary devices, or where operating conditions are stable and do not require a higher level of protection. However, if the transformer is to operate in a more difficult environment, VPI or cast‑resin may provide a greater safety margin.

In practice, the difference between dip and bake and VPI is like the difference between painting wood on the surface and deep impregnation. Both protect. But not to the same degree.

Here we see a simplified representation of a dry transformer in a technical view that well illustrates the dip and bake method: the windings are protected by insulating material, but do not form a solid, full resin block like cast‑resin. In this technology, the windings are dipped in varnish or resin and then baked in an oven. The result is simpler, lighter and more economical, provided the transformer works in a clean and predictable environment.

Dry‑type cast‑resin transformer: resin rules

Cast‑resin, an epoxy‑cast transformer, is the most recognisable type of dry transformer. In this construction, the windings are cast in a resin medium which, after curing, forms a compact, mechanical and dielectric shield.

This is the type that many people have in mind when they say "dry‑type resin transformer".

Its greatest advantage is resistance. Epoxy resin protects the windings against moisture, contamination and mechanical damage. The construction is stable, compact and performs well in facilities where fire safety, absence of insulating liquid and operation inside a building are very important.

Such a transformer is often chosen for commercial buildings, hospitals, data centres, production halls, urban infrastructure, indoor substations, public buildings and installations where the risk of oil leakage would be hard to accept.

But cast‑resin is not magic ;-)

It is usually heavier and more expensive than simpler dry constructions.

The thick resin layer increases resistance but can also affect heat dissipation. Servicing the windings is more difficult because the coil is not open.

If serious damage occurs, repair can be less flexible than in more accessible constructions.

Therefore, a cast‑resin transformer is often a very good choice, but not always the optimal choice.

If the environment is clean, dry and controlled, and fire safety requirements are not particularly strict, VPI may be technically sufficient and economically sensible.

If the environment is very harsh, just the word "cast‑resin" does not exempt you from analysing environmental and climatic classes, enclosure, ventilation and the manufacturer's documentation.

The illustration shows a cast‑resin dry transformer – a construction with massive resin‑encapsulated windings. The red, compact winding blocks show what is most important in this technology: high protection against moisture, contamination and mechanical damage. This solution makes sense where the transformer cannot be a delicate princess of the infrastructure but must work calmly in a building, indoor substation or facility with higher safety requirements.

Dry‑type transformer with composite insulation

There are also dry transformers with composite insulation other than classical epoxy resin.

These can be solutions based on polyurethane resins, silicone resins or other special materials. They are used where standard solutions do not fully match the operating conditions.

This is a niche, but technically very interesting.

Such constructions can make sense in environments with elevated temperatures, strong vibrations, special chemical requirements, or where a certain flexibility of the insulating material is needed. It is not always about the insulation being as hard as possible. Sometimes it is more important that it withstands stresses, thermal cycles, vibrations or contact with a specific environment well.

In practice, such solutions require detailed agreement with the manufacturer. They are not chosen on the basis of "let's take something unusual because it sounds modern". They are chosen when the application truly requires it.

It is a bit like specialised tools. Most screws do not need to be turned with surgical instruments. But when you encounter an unusual problem, an ordinary wrench may not be enough.

The diagram presents a dry transformer as a modular construction where the windings, insulators and metal frame form a coherent system resistant to operation in demanding conditions. Such an image fits a dry transformer with composite insulation, where insulating materials are selected not only for voltage but also for temperature, vibration and chemical environment. It is a technology for situations where standard insulation says "I'm only here for a while", but the project needs something more robust.

Dry‑type transformer for a building. Air, VPI or epoxy?

In buildings, the topic of dry‑type transformers appears particularly often. The reason is simple. The absence of insulating liquid makes design easier in places where the transformer operates close to people, utility rooms, technical installations and high‑value infrastructure.

But not all buildings are the same.

In a clean, well‑ventilated technical room where humidity is controlled and dust is minimal, an air‑insulated or VPI transformer can be a sensible solution. It can dissipate heat well, be easy to inspect and cost‑effective.

In a building with high safety requirements – for example a hospital, data centre, shopping mall or infrastructure facility – a cast‑resin epoxy transformer can provide greater operational peace of mind, especially when resistance to moisture, contamination and limitation of fire risks are important.

In an industrial building, you need to look even more broadly. Is there dust in the air? Is it conductive? Is the transformer room separated from the production process? Does the ventilation draw clean air or air from the hall? Are there vibrations? Are there temperature spikes? Can condensation occur in winter?

Sometimes the difference between a good and a bad choice lies not in the transformer itself, but in the room where it is to operate.

A dry‑type transformer needs air. But not just any air.

Cooling of dry‑type transformers

In dry‑type transformers, heat must be dissipated to the surroundings. Most often by air. And this is a topic that is often underestimated at the purchase stage.

A transformer can have natural cooling, designated AN. This means that air flows by natural convection. Warm air rises, cooler air flows in from below, and the transformer dissipates heat to the room.

It can also have forced cooling, designated AF. Then fans support the airflow, increasing the cooling capacity and allowing temporary increases in load or improved thermal conditions.

Only a fan does not solve everything.

If the room is too small, poorly ventilated or hot, the fan will just mix warm air with even warmer air. If the air is dusty, the fan can deposit contaminants on the windings more quickly. If the ventilation grilles are poorly sized, the transformer may operate at a higher temperature than assumed.

And higher temperature means faster insulation ageing.

Insulation does not usually fail spectacularly on the first day. It ages quietly. Day after day. Cycle after cycle. Overload after overload. And then comes the moment when the system no longer has a margin.

Therefore, with dry‑type transformers you need to ask not only about the rated power, but also about losses, ventilation, ambient temperature, permissible overloads and how the winding temperature is monitored.

Insulation and the working environment

The biggest mistake when choosing a dry‑type transformer is thinking that because there is no oil, the environmental problem is smaller.

Sometimes it is smaller. But it does not disappear.

A dry‑type transformer can be very sensitive to the air that surrounds it. If the air is clean and dry, the situation is comfortable. If it contains dust, moisture, salt, metal particles, chemical vapours or conductive contaminants, the insulation has a much more difficult task.

In a cast‑resin transformer, the windings are better protected by the resin. In VPI, the protection is good but less massive. In open‑wound, the protection is more dependent on the cleanliness and stability of the environment. In composite solutions, everything depends on the specific material and purpose.

Therefore, environmental conditions are one of the most important selection criteria.

It is worth checking whether condensation can occur. Whether the room will be heated. Whether the station doors open directly to the outside. Whether the transformer will be periodically switched off, which can promote moisture absorption during temperature changes. Whether there are production processes nearby that generate dust or fumes. Whether the enclosure has the appropriate degree of protection but at the same time does not excessively restrict cooling.

There is no sense in buying a transformer resistant to everything if it works in ideal conditions. But there is even less sense in buying a more delicate construction if the environment is harsh.

Serviceability and access to windings

In dry‑type transformers, construction differences also affect service.

Open, air‑insulated and VPI constructions can be easier to inspect. More elements are visible. It is easier to assess contamination, hot spots, signs of partial discharges, the condition of the insulation surface and mechanical damage. In some cases, cleaning may also be easier.

Cast‑resin is more enclosed. This gives protection but limits access. If the winding is embedded in resin, it cannot be treated the same way as an open construction. In the event of serious damage, repair may be difficult or economically unviable.

This does not mean that cast‑resin is worse. It means it is different.

In many applications, higher resistance and lower environmental risk are more important than easier access to the winding. In other cases, service accessibility may be very important, especially when the transformer operates in a less critical application but requires regular maintenance.

Selecting a transformer is always a trade‑off. More protection may mean less access. More openness may mean better cooling but greater sensitivity to dirt. A lower purchase price may mean higher requirements for the room.

There is no free lunch. There is only a well‑calculated lunch.

When does which type make sense?

If the transformer is to operate in a clean, dry, well‑ventilated room and the application does not require high environmental resistance, an air‑insulated, open‑wound or VPI construction can be considered. Such solutions can be lighter, cost‑effective and thermally efficient.

If the environment is still controlled but the investor expects better winding protection and greater insulation stability, VPI is often a very sensible compromise. It gives better impregnation than simple varnishing and can perform well in industry and technical buildings.

If there is higher humidity, risk of contamination, higher safety requirements, or the transformer is to operate in a facility where reliability and resistance are particularly important, cast‑resin should be considered. This solution is more expensive and heavier, but often gives a greater safety margin.

If the application is unusual, for example involving high temperature, vibrations or a specific chemical environment, composite insulation or special designs agreed with the manufacturer may make sense.

The most important thing is not to select a transformer by its name alone.

"Dry" only tells you that there is no insulating liquid. It does not tell you how the windings are protected. It does not tell you how the transformer will withstand dust. It does not tell you how it will cope with moisture. It does not tell you whether it will be easy to service. It does not tell you whether it will be cost‑optimal.

That is just the beginning of the conversation.

The 6 most common mistakes when choosing a dry‑type transformer

The first mistake is assuming that dry means cast‑resin.

This leads to misunderstandings in offers, tenders and technical discussions.

The second mistake is comparing only power and price.

A 1000 kVA VPI transformer and a 1000 kVA cast‑resin transformer can have completely different properties. Power alone is not enough.

The third mistake is ignoring ventilation.

A dry‑type transformer dissipates heat to the air. If the room does not remove that heat, the problem will return as temperature, alarms and faster insulation ageing.

The fourth mistake is underestimating dust.

Dust in a house is annoying. Dust on electrical insulation can be much more serious, especially if it contains conductive particles or binds moisture.

The fifth mistake is choosing on the basis of "take the cheapest dry".

The cheapest variant may be good if it fits the conditions. If it does not, it becomes an expensive compromise.

The sixth mistake is not talking about service.

A transformer is supposed to work for years. Access, cleaning, temperature measurement, sensors, inspections and documentation are part of the real cost of ownership.

A simple decision map for the investor and designer

First, define the working environment.

Is it clean, dry and stable, or does moisture, dust, aggressive air or condensation risk occur?

Then, define the safety requirements.

Does the transformer operate in a building, near people, in critical infrastructure, in a public facility, in a production plant or in a separate station?

Next, check the thermal conditions. What is the ambient temperature? How does the ventilation work? What are the transformer losses? Is airflow provided for? Will the enclosure restrict cooling?

Only then comes the technology choice.

If conditions are mild, open‑wound, air‑insulated or VPI can be considered.

If conditions are moderately demanding, VPI often makes very good sense. If the environment is harsher or safety requirements are high, cast‑resin may be more appropriate.

If the application is special, composite insulation or a custom design must be considered.

Finally, price – but not as the only criterion.

Price should be compared only when comparing solutions with similar purpose and similar levels of resistance.

Otherwise, the tender table looks elegant, but the decision may be technically random.

FAQ in a nutshell

Is every dry‑type transformer cast‑resin?
No. Every cast‑resin transformer is a dry‑type transformer, but not every dry‑type transformer is cast‑resin. Dry means no oil or other insulating liquid. The windings can be protected by air, varnish, VPI impregnation, composite insulation or full epoxy encapsulation.

What is the difference between a VPI and a cast‑resin transformer?
A VPI transformer has windings impregnated with varnish or resin in a vacuum‑pressure process. A cast‑resin transformer has windings fully encapsulated in epoxy resin. VPI usually dissipates heat better and can be more cost‑effective. Cast‑resin gives higher protection against moisture and contamination but is heavier, more expensive and harder to repair.

When is a VPI dry‑type transformer a good choice?
A VPI transformer is a good choice when it operates in a clean, dry, well‑ventilated technical room. It is a reasonable compromise between price, cooling and resistance. It works well in many buildings, industrial plants and installations with controlled operating conditions.

When is a cast‑resin epoxy transformer better?
A cast‑resin transformer is a better choice where higher resistance to moisture, contamination and fire safety requirements matter. It fits indoor substations, public buildings, data centres, hospitals, shopping malls, production halls and facilities where operational stability is highly valued.

What is an open‑wound dry‑type transformer?
An open‑wound transformer is a dry transformer with open, ventilated windings. It dissipates heat very well but is more sensitive to moisture, dust and contamination. It works best in clean, dry, controlled technical rooms.

Which dry‑type transformer to choose for a building?
For a building, the transformer should be selected after analysing the working conditions. In a clean technical room, VPI or an air‑insulated construction may suffice. In a facility with higher humidity, risk of contamination or high safety requirements, a cast‑resin transformer more often makes sense.

Summary

A dry‑type transformer is not only the one cast to the brim with epoxy resin.

That is a convenient mental shortcut, but technically too narrow for this whole family of devices.

Dry primarily means the absence of insulating liquid.

It does not mean one single winding technology.

The simplest constructions use air insulation and varnish or resin impregnation.

VPI strengthens the windings through vacuum‑pressure impregnation.

Open‑wound gives very good cooling but requires a clean environment.

Composite insulations make sense in special conditions.

Cast‑resin provides high resistance thanks to full epoxy encapsulation, but usually means higher price, greater weight and more difficult service.

Therefore, the selection of a dry‑type transformer starts with one practical question:

Where will this transformer work?

Only the answer to this question leads to a sensible decision.

Will air insulation be enough?

Would VPI be better?

Is it worth choosing resin?

Is a special construction needed?

Or perhaps, for this application, an oil‑immersed transformer would be a better solution because the operating conditions, cooling, power or operational economics point to that technology.

In power engineering, a good decision rarely consists of choosing the most well‑known name.

More often, it consists of calmly matching the technology to the real life of the device.

And a transformer, like any device in infrastructure, has its own life. It breathes the air of the room. Well‑chosen, it works quietly and predictably. Poorly chosen, it quickly reminds you that mental shortcuts are convenient only until the first problem appears.

If you are at the stage of designing, modernising a substation or comparing offers, it is worth looking more broadly than just at power and price. At Energeks, we are happy to help select a solution for real operating conditions, without automatisms and without forcing one technology into every case.

You can check our offer for cast resin dry‑type transformers and oil‑immersed transformers, and if you want to follow more technical explanations about transformers, substations and power infrastructure, we also invite you to our Energeks LinkedIn profile.

Thank you for reading our technical articles.

Such topics are important because good power engineering begins not with flashy slogans, but with well‑asked questions.

SOURCES:

IEC 60076 11, Power transformers, Part 11, Dry type transformers.

GEAFOL® – Gießharztransformatoren in Schutzgehäusen mit Luft-Wasser-Kühlsystem by SIEMENS

Vacuum Pressure Impregnated (VPI) Transformers: All You Need to Know

Copper and aluminium in transformer windings have different properties, but the quality of the device is determined by the entire design. Check 4 facts about losses, durability, dimensions and operation.

On the table lie two offers.

In one, someone declares: copper, so premium.

In the other: aluminium, so economical.

Both sound confident.

Both try to win your attention with a single word.

And this is where the problem begins.

Because in transformers, a single word very rarely tells the truth about the whole device.

We are writing about this because these doubts often return in conversations with investors, designers and contractors. Each time we see the same mechanism:

The winding material is often sold as an ideology, although in practice what counts is the whole transformer design: its losses, cooling, short‑circuit strength, termination method and manufacturing quality.

The applicable efficiency requirements in the EU and the USA do not force a choice by definition of copper or aluminium. They demand a technical result. That is a fundamental difference.

The question "copper or aluminium" is often poorly framed.

A better question is: which transformer design gives me real technical, operational and economic benefit for my application?

This text is for people who do not want to buy a slogan.

After reading, you will be able to distinguish material properties from manufacturer marketing, understand when copper really makes sense, when aluminium is a sensible choice, and what questions to ask so that silence falls on the other side of the table, followed by the most valuable sentence: yes, that is exactly the point.

Concrete content awaits you inside.

First, we will dismantle the most common myths.

Then we will go through the physics of materials, the impact on efficiency and dimensions, behaviour during short circuits, connection and service issues, and how to read an offer.

Finally, we will give you a practical decision filter.

Reading time: ~ 8 minutes

What really determines the quality of transformer windings?

Do not jump too quickly.

First, name the game.

In the dispute over windings, the game usually looks like this: copper is presented as the choice of sensible, durable and professional people, while aluminium is presented as a cheaper substitute.

Or the opposite: aluminium is shown as modern, light and good enough, while copper is shown as a costly relic.

Both narratives are convenient for sales because they simplify reality to a single symbol.

Technology does not work that way.

Manufacturers and standards evaluate a transformer by the result of the whole design.

What counts are no‑load and load losses, temperature rise, insulation, impedance, mechanical strength, heat dissipation method, behaviour under overloads, and connection quality.

European ecodesign regulations for transformers focus on minimum efficiency levels.

Similarly in the United States, the DOE (Department of Energy) tightens energy efficiency requirements for transformers but does not impose one single correct winding material.

This is an important moment. Because when a salesperson starts with the material rather than with loss parameters, temperature, insulation class and operating conditions, it is very possible they are trying to close the conversation before you ask uncomfortable questions. In negotiations, this is a classic shortcut. In transformers as well.

Truth number one: copper conducts better, but that does not end the matter

There is no point pretending the differences do not exist.

Copper has very high conductivity and serves as the reference point for the IACS scale – the international standard of conductivity.

For annealed copper, the standard is 100% IACS.

At the same time, its density is high, about 8.89 g/cm³ according to the definition used for the IACS standard.

Aluminium conducts less well on a volumetric basis, but has a much lower density.

The Aluminium Association also points out something that copper marketing does not like to repeat: aluminium gives about twice the conductivity per unit mass compared to copper.

That is why for decades it has been the preferred material in many transmission and distribution applications.

And this is where the simple story ends and real design begins.

If aluminium has higher resistivity, the designer compensates with a larger conductor cross‑section.

In other words, you do not compare a naked piece of metal to a naked piece of metal, but two complete winding designs.

Therefore, the sentence "copper has lower losses" is too crude to decide anything sensibly.

Copper may have lower losses at the same cross‑section, but a transformer is not a competition for the same cross‑section.

It is a competition for the result of the whole construction.

Eaton explicitly emphasises that the common belief that a transformer with copper windings is by definition more efficient, more reliable or stronger under short‑circuit conditions is a simplification and a false assumption.

It is a bit like comparing two cars only by the material of their pistons, without asking about the engine, cooling, gearbox and aerodynamics.

Sounds impressive. Says little.

Truth number two: efficiency does not sit in the name of the metal, but in the design

In recent years, efficiency requirements have become tougher.

The EU has Regulation 2019/1783 amending the earlier ecodesign requirements for transformers, and the European Commission explicitly indicates that these regulations have pushed the market towards models with higher efficiency and lower life‑cycle costs.

In the USA, the DOE adopted new standards for distribution transformers, published in 2024, with compliance mandatory from April 23, 2029.

What does this mean in practice?

That the market is increasingly less tolerant of transformers based on slogans alone. The manufacturer must deliver the parameters.

If an aluminium design meets the loss and temperature requirements, it meets them really, not pretend. If a copper design does not close them or does so at the cost of unjustified price increases, the mere presence of copper does not save the offer.

And here we come to the point where the marketing narrative often breaks.

Copper is not an automatic guarantee of better overall transformer efficiency.

Aluminium is not an automatic guarantee of worse efficiency.

Efficiency is the result of the electromagnetic and thermal design, core selection, winding geometry, cooling method and loss control.

When someone tries to close the conversation with a single word, it is worth calmly replying:

"I understand – are you saying that the material itself is more important than the declared load losses, no‑load losses and temperature rise?"

Very often after such a question the conversation suddenly becomes more substantive.

Truth number three: copper more often wins where compactness and mechanical margin matter

Not to fall into the opposite extreme, one must honestly say: copper has real advantages.

Thanks to its higher volumetric conductivity, it allows the required resistance to be achieved with a smaller cross‑section than aluminium. In many designs, this translates into more compact windings and easier fitting of the design into a limited space.

Copper also has high mechanical strength and good thermal conductivity, which in practice is an advantage in constructions where compactness, high power density, high mechanical rigidity or specific short‑circuit conditions matter.

Industry sources highlight these features, and even a report comparing busbar systems indicates that the obvious advantages of aluminium are lower initial cost and weight, while copper offers more compact solutions and greater mechanical robustness.

This does not mean that every copper design beats every aluminium design.

It only means that under certain design conditions, copper gives the designer greater comfort. If the transformer has to fit into a tight enclosure, operate in more difficult thermal conditions, or the investor prioritises minimising dimensions, copper often becomes a strong candidate.

Here the truth is inconvenient for both sides of the marketing dispute.

The copper advocate cannot say: always better.

The aluminium advocate cannot say: there is never any difference.

A difference can exist. You just have to know how to locate it.

Truth number four: aluminium is not a poorer cousin; it is a material that requires honest design

The most damaging myth is: aluminium is just cost cutting.

No.

Aluminium is a full‑fledged engineering material, widely used in power engineering.

If aluminium is given the right cross‑section, well‑designed connections, proper winding geometry and is backed by a sensible manufacturing regime, it can create a transformer with very good performance.

And here it is worth pausing for a moment.

The problem is not aluminium itself.

The problem can be a poor design based on aluminium, or the way such a design is later sold. Because if someone wants to buy aluminium at the price of aluminium, yet expects the compactness of copper, its design margins and the psychological comfort of the word "premium", they stop talking to physics and start talking to their own imagination.

And physics remains calm. It is not interested in labels. It is interested in cross‑sections, losses, operating temperature, impedance, termination method and test results.

That is where marketing ends and the truth about transformer quality begins.

When does aluminium in a transformer make sense?

This is where the conversation becomes really practical.

Because even the best material can be spoiled by a poor connection.

For years, aluminium has had a reputation as a tricky material at terminals.

Part of this reputation grows from the history of old, poorly executed applications, but part comes from the real need for proper approach to connections and terminations.

ANSI C119 standards cover tests for aluminium‑aluminium, aluminium‑copper and copper‑copper connectors. NEMA also reminds that for conductors of different metals, appropriate certified connectors and proper installation procedures should be used, and the quality of terminations should comply with the requirements of the hardware and equipment manufacturer.

In other words, the problem is not "aluminium is bad".

The problem is: is the entire connection system designed and executed as it should be?

This is precisely the point where the buyer should stop hunting for a quick "yes" and start looking for a real "no".

Instead of asking "do you have copper?",

it is better to ask: how have you solved the material transitions, what connectors do you use, what are the tightening torque procedures, how do you validate connections, and what operational experience do you have? Then the other party will either engage in the technical details or stay with the slogan.

And you will know who you are talking to.

The infographic organises the key issues that return when asking: copper or aluminium in transformer windings? It shows the differences in conductivity, cross‑section, weight, losses and design requirements, making it easier to understand what really determines efficiency, durability and the selection of a transformer for a specific application. This is a synthetic summary for those looking for a practical answer to questions about copper and aluminium windings, no‑load and load losses, operational safety and total life‑cycle cost.

Copper or aluminium in a transformer – which is better?

The truth does not lie on one side of the barricade.

Copper indeed has higher volumetric conductivity, usually allows more compact designs to be built, and often gives greater comfort where dimensions, mechanical margin or demanding operating conditions matter.

Aluminium, on the other hand, has for years been a full‑fledged material used in power engineering. With properly designed windings, appropriate connections and a well‑calculated overall design, it does not have to mean either lower efficiency or lower reliability.

Marketing begins when someone tries to turn this technical difference into a worldview war. In one version we hear that only copper is professional. In another, that aluminium is always just as good and there is no point in paying extra. Both narratives are convenient.

Both look good in a catalogue. And both simplify the topic to a level that ceases to be useful for the investor.

A mature decision looks different.

If compact dimensions, a specific mechanical margin, a specific winding architecture or limited installation space are key, copper may be the right choice.

If the priority is a well‑calculated total cost, reasonable weight, adequate efficiency and a proven design with correctly solved connections, aluminium may be a fully rational choice. The problem, therefore, is not which material sounds better.

The problem is whether someone evaluates the transformer through parameters and design, or only through a label.

What is more important in a transformer than copper or aluminium itself?

The best questions usually do not sound spectacular.

They sound calm and precise.

Therefore, instead of starting the conversation with the winding material alone, it is better to ask about the declared no‑load and load losses, insulation class, temperature rise, short‑circuit impedance, dimensions, weight, termination method, connection type and warranty conditions.

It is also worth asking which standards and tests confirm the given solution, and what exactly copper or aluminium gives in this specific unit, not in a general sales presentation.

It is here that the difference between technique and storytelling very quickly becomes apparent. If the other side provides numbers, relationships, documentation and specific answers, the conversation stands on solid ground. If instead prestige, emotion and a mental shortcut appear, you are most likely entering not a world of parameters but a world of marketing.

And perhaps this is where the most honest answer to the whole question of copper and aluminium lies. The truth does not reside in the metal itself. It resides in the design, documentation, manufacturing quality and honesty of the conversation. Copper and aluminium are not heroes of a moral tale. They are tools. Both materials can work very well. Both can also be used badly.

The most expensive mistake appears when someone stops thinking and buys a narrative instead of parameters.

What you can expect from us

In transformers, as in life, the most confusion is usually caused by answers that are too simple.

At Energeks, we look at the subject of windings more broadly than just through the lens of the slogan "copper" or "aluminium". What matters much more to us is whether the whole transformer has been designed responsibly, coherently and with long‑term stable operation in mind.

Therefore, for MarkoEco2 oil‑immersed transformers, what counts for us is the whole: hermetic construction, oil compliant with IEC 60296, compliance with EN 50588‑1 and EN 60076‑1, monitoring capabilities and solutions supporting long life and loss reduction.

It is from such decisions that a device is created that is supposed to work calmly, stably and without surprises.

We look similarly at TeoEco2 dry‑type transformers. Here, quality is determined by disciplined engineering: compliance with EcoDesign Tier 2, reduction of no‑load and load losses, F1 fire safety, and readiness for real operating conditions and cooperation with protection systems. This is equipment that is supposed not only to look good in an offer but above all to find its place where certainty matters.

We offer both transformer types in variants with aluminium and copper windings.

Sometimes the best answer is copper, sometimes aluminium, and sometimes simply a well‑chosen design. And that, actually, is quite good news.

In power engineering, the most expensive thing is not the material itself.

The most expensive thing is oversimplification.

That is why it is worth negotiating with an offer the same way you negotiate good cooperation terms.

With an attitude towards real mutual benefit.

You get a transformer that does its job for years.

The manufacturer gains a customer who understands what they are paying for.

And only then does the conversation truly make sense.

Sources:

1. European Commission, Power Transformers, Ecodesign Requirements

2. U.S. Department of Energy, Distribution Transformers, 2024 final rule and compliance timeline

3. Eaton, Copper vs. Aluminum Conductor Information for Distribution Transformers

The power industry loves paradoxes.

The largest devices in the power system very often depend on the smallest details. A transformer can weigh several tons, have a power rating of several megavolt-amperes, and operate continuously for 30 years. Yet the part that often decides its reliability is only a few centimetres in size.

It is the transformer terminal.

More precisely, the component that connects the medium voltage cable to the transformer bushing.

To someone outside the industry, it looks like an ordinary piece of metal with a few bolts. A detail that few people pay attention to, as long as everything works.

For a power engineer, it is a completely different story. It is one of the most critical points in the entire installation. Right here, high currents meet, mechanical forces from heavy cables act, temperature changes occur, and the very practical question arises: will this connection safely withstand years of operation in real conditions?

Transformer terminals are connection components mounted on the bushings of a medium voltage transformer. They enable safe connection of MV cables, increase the contact surface area of the conductors, and improve the mechanical stability of the connection.

This brings very concrete benefits.

• Lower contact resistance.

• Lower risk of connection overheating.

• Greater predictability of transformer operation over a long service life.

That is why TOGA-type transformer clamps are often used in medium voltage transformers. They are not an aesthetic detail or a marketing add-on. They are a solution born from a very practical need. The need to better manage current, temperature, and connection mechanics in a place that looks unremarkable but in practice is of enormous importance.

And this article is about those issues.

We will show what TOGA-type transformer clamps are and how they are built.

We will look at why conventional cable connections at transformer bushings can be problematic.

We will explain how the clamp construction affects current, temperature, and contact resistance.

We will also examine why grid operators increasingly require stable connection solutions.

We will show, through examples, in which installations transformer clamps become fundamental to the reliability of the entire station.

Reading time: ~11 minutes

TOGA-type transformer clamps – the small component that keeps hundreds of amperes in check

Anyone who has ever stood next an open medium voltage transformer knows that moment.

You look at the massive machine. Several tonnes of steel, a magnetic core, oil, windings. Everything looks calm, heavy, almost majestic.

Then your eyes stop on something the size of a hand.

The clamp.

And this is where real engineering begins.

Because this is not an ordinary piece of metal.

It is a component that must flawlessly carry hundreds of amperes, withstand temperature changes, vibrations, and mechanical forces from cables, while maintaining very low contact resistance for years.

A TOGA-type transformer clamp acts as an adapter between two worlds.

On one side we have the transformer and its bushing – the point where energy exits the tank.

On the other side we have the medium voltage cable, often thick, heavy, and not very flexible.

The clamp introduces an additional conducting element between them, most often made of copper or its alloys. This element increases the contact surface, stabilises the conductor, and distributes mechanical forces over a larger area.

From the point of view of physics, three important things happen:

• The current has a larger surface area through which to flow.

• The metal-to-metal contact pressure is more even.

• The connection is less susceptible to movement and stress.

The effect is simple: less heat, fewer problems, more operational peace.

The photo shows a set of medium voltage transformer clamps mounted on the porcelain bushings of an oil‑immersed transformer. Each clamp serves as the connection point for the MV cables, enabling safe and stable connection of the conductors to the transformer winding. The massive construction of the metal connection blocks increases the contact surface area and allows even current flow, which limits local heating and reduces the risk of energy losses. At the same time, the clamps take up the mechanical loads from the heavy cables, protecting the bushings from stress.

It is in this unremarkable place that all the physics of the transformer’s operation comes together – current, temperature and connection durability – which must remain stable for decades of service.

Photo CC: ENERGEKS 2026

Why conventional cable connections at transformer bushings can be problematic

Cable lug, bolt, tighten – done.

On paper, it works perfectly.

In reality, three very concrete problems appear.

The first is the weight and stiffness of the cable.

Medium voltage cables with large cross-sections are not delicate. They are heavy, springy constructions that very often do not want to go exactly where the design intended. If the cable comes in at an angle or is under tension, it starts acting like a lever and loads the bushing terminal.

The second problem is the contact surface area.

Metal does not make ideal contact with metal. Current flows through microscopic contact points. If there are few such points, current density increases, and along with it, temperature.

And suddenly, a small resistance starts turning into a local heat source.

The third problem is time.

A transformer does not operate in a perfect vacuum. There are vibrations, temperature changes, material expansion and contraction, short-term overloads. If the connection relies on only a single pressure point, micro‑movements can occur over time.

And micro‑movements in power engineering have a bad reputation.

Because they always end with degraded contact.

And this is precisely where the need for better solutions begins.

But even then, the story is not over.

Because once we have improved the mechanics and the electrical connection, another level of challenges appears. One that does not arise solely from current, bolts and cable geometry, but from the fact that the transformer works in the real world, not in a sterile laboratory. In an open station, in an environment full of moisture, dust, temperature variations and all that unwanted biological activity that power engineering knows all too well.

MV bushing covers – what they are and what they really protect against

At first glance, they look a bit like little black hoods.

And that is why they are easy to dismiss. Someone looks at the transformer, sees the bushings, clamps, porcelain, metal, and treats these covers as an extra. A technical trifle that just happens to be there.

Yet in power engineering, such trifles very often do the dirty work that allows everything else to operate calmly.

MV bushing covers are installed to protect the most sensitive area of the transformer connection point. This is where we have live parts, metal components, and relatively small insulation clearances. Exactly the kind of combination we do not want to expose to chance, weather and the creativity of nature.

Most often they are referred to as bird guards. And this is no exaggeration or industry legend. Birds really can cause trouble in a transformer station. All it takes is for one to perch in an unfortunate spot, brush a wing, come close to two points at different potentials, and physics immediately takes over. An arc appears, protection trips, and suddenly we have an outage that nobody planned.

It sounds unremarkable, but this is exactly what some of the most irritating operational problems look like. Not a major failure from a movie. Just a small incident that stops the equipment.

And this is where bushing covers come in.

All black, without any unnecessary fanfare. 😎

Their role is very simple. They make accidental contact with live parts more difficult and reduce the risk that something or someone creates a bridge between potentials.

A bird, a small animal, a branch, a metal object, and sometimes even a tool during service work – all of this can become a problem if it gets too close to where theory ends and medium voltage begins.

A cover does not, of course, make the transformer armoured and indifferent to the whole world. But it very effectively reduces the risk of the simplest, most absurd and, unfortunately, entirely real events. The kind after which one looks at the report and thinks: really? because of that?

Well, yes.

That is why MV bushing covers are no gimmick. They are a practical safeguard that supports the reliability of the transformer from its most mundane side. They do not improve the catalogue glamour of the device. They improve its chances of calm, long-term operation in the real world.

And the real world, as we know, does not always cooperate.

The photo shows medium voltage bushing covers installed on an oil‑immersed transformer. These unassuming black covers protect the critical connection points against accidental contact with live parts and reduce the risk of flashovers caused by birds, small animals and other external factors. They are a simple but very important protective element that supports the safety and operational reliability of the transformer in daily service.

Photo CC: ENERGEKS 2026

From a project perspective, the most sensible approach is when the entire connection system can be selected as a coherent solution, rather than assembled later from random components. Depending on the needs of the investment, these can be transformers equipped with terminal clamps, clamps for a specific type of connection, or MV bushing covers that increase operational safety. Such solutions are available in the Energeks offer; therefore, for a specific project, it is best to simply discuss the configuration and match it to the real operating conditions of the station – and the easiest way to do this is to contact us directly.

How the clamp construction affects current, temperature and contact resistance

Here begins that part of power engineering that looks unremarkable from the outside but is pure physics on the inside.

And as is the case with physics, you can disagree with it, but it will do its job anyway.

At first glance, a transformer clamp is simply a metal component that connects the cable to the transformer. Except that current does not behave as politely as we would like to imagine. It does not flow ideally through the entire contact surface like a beautifully spread sheet of water.

In reality, it flows through those places where metal truly touches metal. And there are far fewer of those contact points than intuition suggests.

That is exactly why the construction of the clamp matters so much.

If the contact surface is larger and the pressure is more evenly distributed, more actual contact points appear. This in turn lowers contact resistance. And lower contact resistance means one thing: less heat where we least want to see it.

Because resistance and temperature are a pair that very quickly show their claws. Joule’s law clearly states: the power dissipated in the connection increases with the square of the current. This means that even a small resistance, under a high operating current, can turn into a local source of heating. First, a few extra degrees appear. Then the material starts to operate hotter, ages faster, and the connection gradually loses its original parameters.

A transformer clamp does three very important things at once.

First, it increases the contact surface area, so the current has more space to flow calmly.

Second, it distributes the contact pressure better, so the connection does not rely on only one small fragment of metal.

Third, it stabilises the whole assembly over time, reducing the risk of micro‑movements that, over the years, can degrade the quality of the contact.

The effect is simple, though extremely valuable from an operational point of view. The current does not concentrate in one tight spot but spreads over a larger area. The temperature of the connection remains lower. And a lower temperature means calmer, more predictable transformer operation.

It can be compared to traffic. The same number of cars squeezed onto a single narrow street quickly creates chaos. When they are given a wide road, everything flows much more calmly. Current behaves similarly. It also likes to have space.

That is why a well‑designed clamp is not a technical detail for the sake of principle. It is a component that helps keep three things in check at once: current, temperature and connection durability. And for a transformer operating for decades, that is truly no small matter.

Why grid operators increasingly require stable connection solutions

Grid operators have one big advantage over the rest of the market.

They do not see a single transformer; they see a whole repeated picture of operation.

For the designer, a transformer is a device selected to meet technical parameters. For the investor, it is an element of a larger puzzle. For the grid operator, it is part of a system that must operate calmly not for one or two years, but for 30, sometimes 40 years.

And it is this perspective that changes everything.

Because when you look at thousands of devices operating in different locations, under different weather conditions and different loads, you very quickly see which solutions age well and which only look good on the day of acceptance.

Every failure, every thermal imaging report, every overheated connection and every case of degraded contact goes into the analysis. At first, it is a single event. Then a second. A third. A tenth. And suddenly it becomes clear that this is no longer a coincidence, but a recurring pattern.

And power engineering does not like recurring problems.

That is why operators are increasingly looking not only at the transformer’s power, loss levels or insulation parameters, but also at how the cable connections are designed. Whether the connection is mechanically stable. Whether the contact surface is sufficient. Whether the arrangement can withstand the stresses from heavy cables, vibrations, temperature changes and years of operation.

Because practice shows something very interesting.

In many cases, the transformer itself, as a machine, works flawlessly. The windings are in good condition, the oil maintains its parameters, the core operates stably. The problem does not begin in the heart of the device.

The problem begins at its interface with the outside world.

Exactly where the cable connects to the transformer.

And that is the moment when a detail ceases to be a detail.

It becomes an element of the entire station’s reliability.

It is from this logic that the operators’ technical requirements arise. The more operational experience, the more attention is directed to the construction of bushings, the method of making cable connections, the stability of clamps and the resistance of the whole connection system to real operating conditions.

Because ultimately, the operator does not buy just the transformer.

The operator buys operational peace.

The photo shows a set of medium voltage transformer connection components: a transformer clamp, a porcelain bushing and a bushing cover that protects the critical point from environmental influences. It is here that current, mechanics and operating conditions meet, which is why each of these components must be consciously selected and work as a coherent system. In practice, this means one thing: reliability begins with a detail, and a well‑designed connection is not an accident but the result of properly selecting all the components that together create a safe and durable connection.

Photo CC: ENERGEKS 2026

Where transformer clamps show whether the project was truly well thought out

There are installations where the transformer has a rather comfortable life. It runs steadily, the cable arrives without too much acrobatics, the load does not do a rollercoaster every day, and everything looks as neat as in the nice drawing from the project.

But there are also places where reality quickly verifies whether the connection at the transformer was designed with intelligence or simply so that it could be bolted together and the matter closed.

And there, transformer clamps cease to be a technical curiosity.

They become a very practical test of the quality of the whole solution.

Take photovoltaic farms.

Everything seems simple.

There is energy production, there is a transformer, there is a power output to the grid. End of story. Except that the transformer in a PV farm operates under conditions that like to test the patience of materials. In the morning the system wakes up, then power rises, then full sun comes, a cloud passes, sun again, ambient temperature does its thing, and along with it the operating conditions of the connections change. This is not the calm, uniform life of an old distribution transformer that does roughly the same thing for half a day. Here current and temperature can change dynamically, and each such cycle means work for the material, the contact pressure and the contact interface.

Add to this the cables. Thick, heavy, serious, with character. Cables that have no intention of lying down gently just because someone drew a nice route on the plan. If the connection at the bushing is weak or too sensitive to stress, the PV farm will show it quickly. And it will do so without sentiment.

Very similar is the case in industrial installations.

Here the emotional stakes rise even higher, because on the other side of the cable there is often a process that really does not like downtime.

Steelworks, foundries, chemical plants, large logistics centres, data centres, plants with production lines operating in continuous mode. In such places, the transformer does not supply an abstract power from a table. It supplies concrete work, concrete machines, concrete money that either flows or stops flowing. If the connection at the transformer starts to heat up, age or lose stability, it is no longer a minor technical defect. It is the beginning of a problem that can affect the entire facility.

That is why, in industry, no sensible person wants the critical point of the system to behave like a moody paving stone after the first winter. The connection has to be stable, predictable and boring in the best possible sense. It simply has to work.

There are also container stations.

The place where theory very quickly meets tight reality.

Here every centimetre matters. Cables enter from below, the switchgear stands close, the transformer has its dimensions, and the person responsible for installation suddenly discovers that the planned geometry was beautiful until the real cable appeared. Not the one from the brochure, but the real one – stiff, heavy and moderately interested in cooperating.

Under such conditions, even a good connection can get out of breath if it does not have adequate stabilisation. The cable rarely comes in perfectly straight, the manoeuvring space is limited, and every unnecessary stress‑inducing twist later affects the terminal and the quality of the contact. This is where a well‑designed clamp shows its true value. Not in a folder, but when you have to manage physics, space and cable weight all at once.

There are also installations that are more environmentally demanding.

For example facilities with large temperature variations, outdoor infrastructure, or locations where the transformer has to operate in an environment of dust, moisture and constant changes of conditions. There, every detail of the connection matters even more, because the connection does not work in a comfortable laboratory but in a world that regularly checks whether everything was done properly.

That is precisely why solutions that increase the contact surface and mechanical stability are not a luxury for hardware aesthetes. They are simply a sensible response to operating conditions.

Because the truth is rather amusing, though for operation it is less amusing.

The transformer can be excellent.

The core solid, the windings well‑made, the oil within spec, everything looks as it should.

And then all that majesty of several tonnes of equipment can be put to the test by a few centimetres of metal at the connection point.

A related topic worth knowing:

Why an MV transformer bushing terminal has one or two holes?

f you want to better understand why even such a small detail as the cable attachment method matters, take a look at our article about the construction of MV bushing terminals. We show there where the difference between one and two mounting holes comes from and how it affects the stability of the connection and its durability over time.

Where to get such a transformer, clamps and those hoods?

And here we come to a very practical question.

Because theory is theory, physics is physics, and temperature curves look beautiful in an article, but in the end someone has to close the topic.

You need to select the transformer.

You need to select the clamps.

You need to plan the bushing covers. You need to make sure that everything fits together not only in the catalogue but also later on the real station, with the real cable, real installation and real operator requirements.

And this is where the difference begins between assembling a system from random components and designing a solution that makes sense as a whole.

You can look at the transformer as a separate product, the clamps as separate hardware, and the covers as yet another add‑on to order. But in power engineering practice, these things do not work separately. They meet at the same place, on the same connection, under the same current, temperature and the same pressure of reality.

That is why the most sensible approach is to think about them together.

In the Energeks offer you can find both low‑loss medium voltage oil‑immersed transformers and cast‑resin dry‑type transformers. You can contact us about selecting transformer clamps and medium voltage bushing covers.

In this way, the entire system can be selected coherently, for a specific project, for the cable routing method, for the installation conditions and for the requirements of a given installation. Without guessing, without improvisation at the end of the investment and without nervously wondering whether all the components will really work together as they should.

And that really matters in power engineering.

Because sometimes the reliability of a transformer is not only decided by what is inside the tank.

What happens on the outside can be just as important. On the bushings, on the clamps, at the interface between the cable and the device. In all those places that do not make a great impression in a long‑distance photo, but which can make a great difference after several years of operation.

If you like technical stories from the power industry told without pomposity but with respect for detail, we also invite you to our LinkedIn.

Referencje:

IEEE Power Transformer Handbook

Pfisterer – Technical documentation (MV connection technology)

Sometimes the most interesting things in the power industry are surprisingly small.

You're standing by a medium voltage transformer, looking at a porcelain bushing, and you see a metal terminal.

On one phase, one hole.

On another, two. Someone asks: is this a mistake? Is something missing?

No. It's a conscious design decision.

In the world of MV transformers, such small details aren't just cosmetic.

They are elements that affect the installation's durability for the next 30 years of operation.

In the place where the cable meets the transformer, enormous currents, electromagnetic forces, and temperature also meet.

And right there, one additional hole can make a huge difference.

Today, we'll take a look at one of the most underestimated elements of an MV transformer.

The bushing terminal and why it sometimes has one hole and sometimes two.

If you design transformer stations, work with MV transformer installation, set up PV farms, or simply want to understand the power industry more deeply, this article will show you something important.

You'll understand why the construction of the bushing terminal isn't an accident.

You'll learn how the number of holes affects currents, temperature, and connection durability.

And why, in power engineering practice, one extra hole can save a transformer from overheating.

In this text, we'll discuss:

• how an MV transformer bushing works and is constructed

• why terminals have one or two mounting holes

• how the number of bolts affects current, temperature, and contact resistance

• what distribution grid operators require

• which installation errors most often lead to connection overheating

It's worth reading, because the only thing truly worth accumulating in life is knowledge!

Reading time: ~12 minutes

How an MV transformer bushing works and is constructed

Before we move on to the mounting holes themselves, it's worth understanding the role of the bushing.

A medium voltage transformer typically operates in the range from about 6 kV to 36 kV. The windings are inside a tank filled with transformer oil. This oil serves two functions. It cools the windings and provides electrical insulation.

The problem appears where the conductor has to exit the tank.

The current must pass from inside the transformer to the outside, to the cable or busbar. At the same time, electrical breakdown through the housing cannot be allowed. The potential difference is enormous.

That's why bushings are used.

A transformer bushing is an insulated element, usually made of porcelain or composite, that conducts the conductor through the transformer tank wall. Inside it, there is a conductive pin connected to the transformer winding.

On the outside of the bushing, there is a terminal.

The metal fitting to which the cable or busbar is connected.

And it's in this fitting that the topic of one or two holes appears.

The bushing terminal, a small element with great responsibility

The bushing terminal is the meeting point of two worlds.

On one side, we have the transformer. A device that can have a power rating from several hundred kilovolt-amperes to several megavolt-amperes.

On the other side, the medium voltage cable or busbar leading the energy further into the grid.

At this single point, currents in the order of hundreds of amperes, and sometimes over a thousand amperes, flow. At the same time, the metallic contacts must maintain very low resistance.

If the contact resistance increases even minimally, the Joule effect appears.

Electrical energy starts turning into heat.

And heat in the power industry is enemy number one.

Why an MV transformer bushing terminal has one mounting hole

The simplest and at the same time very common construction of a medium voltage transformer bushing terminal has one mounting hole.

At first glance, this may seem like a minimalist solution, but in reality, it is a conscious compromise between electrical requirements, mechanical needs, and installation practice.

In such an arrangement, the cable lug is bolted to the terminal with one bolt.

The bolt presses the lug eye against the flat metal surface of the bushing terminal. This creates an electrical connection through which energy from the transformer can flow further to the medium voltage cable.

For many installations, this solution is fully sufficient and has been used in distribution power engineering for decades.

To understand why, it's worth looking at the scale of currents on the medium voltage side.

In distribution transformers with a power of several hundred kilovolt-amperes, the currents on the MV side are relatively small. This follows directly from the relationship between power, voltage, and current.

For example, a 1000 kVA transformer operating in a 15 kV network generates a current of about 38 amperes on the medium voltage side. Even with a 2500 kVA transformer, this value increases to about 96 amperes.

These are values that, from the perspective of electrical connection construction, are relatively small.

A properly made bolted connection with one bolt and an adequate contact surface carries such currents without any problem for many years of operation.

That's precisely why, in transformers with lower power ratings, using a terminal with one mounting hole is a completely rational solution.

One bolt ensures adequate pressure on the contact surfaces.

If the surfaces are clean and the bolt tightening torque is correct, the contact resistance remains very low. This means that no significant energy losses or excessive heating appear at the connection point.

The connection is also simple to install. The installer needs to fit one cable lug and tighten one bolt with the appropriate torque. In the conditions of constructing or modernizing a transformer station, this has practical significance because it shortens installation time and reduces the risk of errors.

A terminal with one hole also has construction advantages.

First of all, it is more compact. In container stations, where space between transformers, switchgear, and cables can be very limited, every centimeter of space matters. A smaller terminal makes it easier to route cables and maintain the required insulation clearances.

The second advantage is the lower weight of the entire bushing assembly.

In distribution transformers, which are often installed in large quantities in the grid, every structural element is optimized for cost and simplicity of production. A simpler terminal means less material and fewer technological operations during manufacturing.

There is also the aspect of compatibility with typical cable lugs used in medium voltage networks. In many cable systems, standard lug eyes are designed specifically for single-bolt connections.

Thanks to this, installation is quick and requires no special intermediate elements.

In power engineering practice, a terminal with one hole is therefore a good solution in several typical situations.

The first is a transformer with relatively low power, where the currents on the medium voltage side are not large. Under such conditions, a single bolted connection provides sufficient contact surface and mechanical stability.

The second situation is cable installations where the transformer is connected directly to an MV cable terminated with a standard cable lug. The cable is flexible and does not generate large mechanical loads on the terminal, so one attachment point is sufficient.

The third situation is transformer stations with limited installation space. A compact terminal makes it easier to route cables and maintain safe distances between phases.

However, physics and operational practice remind us that every solution has its limits.

One bolt means one pressure point.

It also means that the entire contact surface is pressed in one place. If the connection is made imprecisely, the contact surface may be smaller than assumed.

As transformer power increases, currents increase, and with them, the requirements for the quality of the electrical connection increase.

MV transformer bushing terminal with one mounting hole used in standard cable connections in MV transformer stations. The single-bolt construction enables quick and compact connection of the cable lug to the transformer bushing, ensuring adequate contact surface for typical operating currents in distribution transformers. This solution is often used in transformers with lower and medium power ratings, in cable installations, and in container stations where simplicity of assembly and limited connection space are important.

© ENERGEKS 2026

At a certain point, one bolt ceases to be the optimal solution.

That's when the construction with two mounting holes appears, which allows for increased mechanical stability and improved pressure distribution on the contact surface.

And it is this solution we will look at in the next step.

Why an MV transformer bushing has two mounting holes and when it is necessary

A terminal with two holes is a construction used where the electrical and mechanical requirements of the entire system increase. In transformers with higher power ratings and in industrial installations, a simple single-bolt connection ceases to be the optimal solution.

In such an arrangement, the cable lug or copper busbar is bolted to the bushing terminal with two bolts. At first glance, the difference seems small. In reality, it changes a great deal in the behavior of the entire connection during the transformer's many years of operation.

The first benefit concerns mechanical stability.

With one hole, the cable lug is pressed at a single point and can rotate minimally around the bolt axis. This movement isn't large, often fractions of a millimeter, but in power engineering, even such small changes matter. A transformer during operation is not a completely static element. There are magnetic core vibrations, temperature changes causing material expansion, and electromagnetic forces generated by fault currents.

If the connection has only one attachment point, the lug may shift slightly over time. Two mounting holes eliminate this problem. The cable lug becomes locked at two points, which practically prevents rotation and stabilizes the entire connection.

The second benefit is related to contact surface area.

Power connections work best when the contact surface area between metals is as large as possible. In practice, this means the conducting elements must be pressed together with adequate force over as large an area as possible.

Two bolts result in a more even distribution of pressure over the surface of the cable lug or copper busbar. Thanks to this, a larger part of the metal surface participates in conducting current. As a result, local current density decreases and energy losses at the connection point are limited.

The third benefit concerns one of the most important parameters of any electrical connection:

CONTACT RESISTANCE

Contact resistance always arises where two conductors are mechanically joined. Even very smooth metal surfaces actually only touch each other at many microscopic points. The better the pressure and the larger the contact surface, the lower the connection resistance.

If contact resistance increases, the phenomenon of heat generation appears according to Joule's law. Electrical energy starts being converted into heat at the connection point.

To illustrate the scale, it's worth looking at a simple example:

If the connection resistance increases by just 100 microohms, and a current of 600 amperes flows through the joint, the power loss will be about 36 watts at a single point.

On paper, this seems like a small value. However, in reality, this energy is released on a very small metal surface.

This means local heating of the joint to temperatures significantly higher than the ambient temperature. Over time, this can lead to surface oxidation, a further increase in resistance, and accelerated degradation of the connection.

Two bolts help keep contact resistance at a minimum level because they provide stable pressure and a larger effective contact area between metals.

In practice, terminals with two holes appear most often in several situations.

The first is a transformer with higher power.

As power increases, operating currents and requirements for the quality of electrical connections also increase.

The second situation is connections made using copper busbars instead of cables.

Busbars are rigid and heavy, therefore requiring more stable attachment.

The third situation is industrial installations or transformer stations operating in difficult operating conditions.

Vibrations, temperature changes, and high fault currents mean that the mechanical stability of the connection becomes critical.

In such cases, using two mounting holes in the bushing terminal is not a construction luxury.

It is a design element that significantly increases the reliability of the entire transformer over a long operating period.

MV transformer bushing terminal with two mounting holes intended for connections with higher current loads. The double-bolt construction enables stable connection of the cable lug or copper busbar, increases the contact surface area, and limits contact resistance. This solution is most often used in transformers with higher power ratings, in transformer stations with busbar connections, and in installations meeting distribution system operator requirements, where long-term connection stability and minimization of joint heating are crucial.

© ENERGEKS 2026

At Energeks, we take such details seriously. Our MV transformers can be equipped with various bushing termination configurations, tailored to the station design, cable connection method, and grid operator requirements. This applies to both single-hole and double-hole terminals, as well as various types of connection clamps used in power engineering, such as TOGA-type solutions, selected depending on the connection configuration and design standards. If you want to see more examples of such solutions, check out our Energeks transformer offer,

or contact our advisors directly to match the solution precisely to your needs.

How the number of bolts in an MV transformer terminal affects current, temperature, and contact resistance

In power engineering, there is something beautiful in the details.

From the outside, a transformer seems like a massive, calm machine. Several tons of steel, a magnetic core, an oil tank. Meanwhile, its longevity is often determined by elements you can hold in your hand. One of them is the bolted connection at the end of the bushing.

At first glance, the difference between one and two bolts seems like a trivial detail.

In reality, it is a decision that affects three very important physical phenomena.

The flow of current, the temperature of the connection, and contact resistance.

And it is these three parameters that decide whether the connection will work calmly for 30 years or start showing signs of fatigue after a few seasons.

#1 Let's start with current.

The greater the transformer's power, the larger the currents appearing in the system. In distribution transformers with a power of several megavolt-amperes, currents on the medium voltage side can reach hundreds of amperes. Under such conditions, even a small imperfection at the contact point begins to matter.

Current does not flow uniformly through the entire metal surface. In reality, it flows through many microscopic contact points where the metal surfaces actually touch. Each of these points carries part of the total current.

If the contact surface is small, the current density at these points increases.

And when current density increases, temperature also increases.

#2 This leads us to the second phenomenon: Temperature.

In every electrical connection, contact resistance appears. Even in the best-made connections, there is a slight electrical resistance resulting from the microstructure of the metal surface.

Joule's law states that the power dissipated as heat equals the product of resistance and the square of the current. The formula is simple, but its consequences are enormous.

If the current is 500 amperes and the contact resistance is only 50 microohms, about 12.5 watts of heat is dissipated at the connection point. That's not much, as long as the heat is distributed over a large metal surface.

The problem begins when the electrical contact is limited to only a small fragment of the surface. Then this energy concentrates in one place and the temperature starts to rise.

Two bolts act here as a very simple but extremely effective engineering tool. They increase pressure and distribute it over a larger surface. Thanks to this, the number of microscopic contact points between metals increases, and contact resistance decreases.

#3 The third phenomenon is equally interesting: Electrical stability over time.

A bolted connection is not a perfectly rigid structure. During transformer operation, temperature changes occur. Metal expands and contracts. The transformer core generates slight magnetostrictive vibrations. During grid faults, powerful electromagnetic forces appear.

If the connection is held by only one bolt, the cable lug may move minimally. These are very small movements, often on the order of tenths of a millimeter. However, over many years of operation, such micro-movements can gradually degrade contact quality.

Two attachment points stabilize the connection in a completely different way. The cable lug becomes immobilized in two places, and pressure is distributed more evenly. The connection is less susceptible to geometry changes during device operation.

That's why, in transformers with higher power ratings, manufacturers very often use double-bolt terminals as standard. This applies especially to units above several megavolt-amperes, where operating currents are already large enough that every construction detail matters.

A similar situation appears in the case of connections with busbars.

Copper busbars are much heavier and stiffer than power cables. They introduce additional mechanical loads into the system resulting from their own weight and from electromagnetic forces during faults. Two attachment points allow these forces to be distributed and protect the transformer bushing from excessive stress.

Do grid operators require terminals with two bolts in MV transformers?

In many projects, yes. Distribution system operators manage thousands of transformers working in very diverse environmental conditions. Every failure is analyzed, and conclusions later find their way into technical guidelines for new installations. Over the years, in many countries, this has led to the introduction of requirements for double-bolt bushing terminals in specific classes of MV transformers.

Power engineering is a field that learns from experience. Every overheated connection, every thermal imaging inspection report, and every grid event analysis becomes part of the knowledge that later influences design standards.

Therefore, when you look at a transformer bushing terminal and see two bolts instead of one, often behind it is not only the manufacturer's decision but also grid operator requirements and years of practical observation of equipment operation in real power systems.

Transformers such as MarkoEco2 are designed with real distribution grid operation in mind.

This means one thing: they must fit the operator's standards even before they reach the station.

That's why, already at the design stage, we consider the technical requirements of distribution system operators and investor specifications. This also applies to seemingly minor elements such as the configuration of MV bushings or the method of terminating cable connections.

In practice, this means the transformer arrives at the station prepared exactly for the conditions of a given project.

This approach is simple.

The transformer should not force the grid to adapt.

The transformer should be adapted to the grid.

That's why the bushing configurations, the arrangement of single-bolt or double-bolt terminals, and connection solutions in Energeks transformers are designed to seamlessly fit into operator requirements and the practice of working in real power stations.

Top 5 problems causing cable connections at MV transformers to overheat

In the operational practice of medium voltage transformers, very many problems do not start with the transformer itself. They start with the connection. The place where the cable or busbar meets the bushing terminal.

This is one of the most stressed points in the entire system. Large currents flow there, temperature changes occur, and at the same time, it is a mechanical connection dependent on installation quality. That's why minor installation errors can, after a few years, lead to overheating, metal oxidation, and in extreme cases, even failure.

Problem 1: Imprecise preparation of the contact surface.

Metal surfaces, in theory, should fit together perfectly. In practice, on their surface there are oxide layers, dirt, and sometimes even a thin layer of paint or residues from cable lug production. If such surfaces are bolted together without cleaning, electrical contact occurs only at a few microscopic points.

As a result, contact resistance increases, and the connection starts to heat up. That's why, in professional installation, contact surfaces are cleaned, and often also protected with a special contact paste that limits oxidation.

Problem 2: Incorrect bolt tightening torque.

Too little tightening causes insufficient pressure of the cable lug against the terminal. The metal surfaces then do not adhere properly, and contact resistance increases. After some time, connection heating appears.

On the other hand, too much tightening torque can deform the cable lug or damage the terminal thread. In extreme cases, it can also cause cracking of insulating elements in the bushing.

That's why transformer and cable lug manufacturers always specify the recommended bolt tightening torque. In professional installation, torque wrenches are used to achieve the proper pressure.

Problem 3: Using the wrong cable lug.

The lug must be matched both to the cable cross-section and to the construction of the bushing terminal. Too small an eye causes improper lug positioning, while too large an eye limits the contact surface. In both cases, connection resistance increases.

Sometimes a encountered problem is also a situation where the terminal has two mounting holes, but only one bolt is used during installation.

Superficially, the installation works correctly. Current flows, the transformer operates, and the installation passes technical acceptance. However, the connection lacks full mechanical stability. The lug may move minimally during temperature changes or transformer vibrations.

After a few years of operation, oxidation of the contact surface appears and connection temperature rises.

Problem 4: Improper cable routing.

A medium voltage cable has significant mass and specific stiffness. If it is routed at the wrong angle or is under tension, it can exert a constant force on the bushing terminal. Over a long period, this causes micro-movements of the connection and gradual deterioration of electrical contact.

That's why, in professional installations, cable supports and appropriate cable bending radii are used to eliminate stresses acting on the transformer terminal.

Problem 5: Lack of periodic connection inspection.

A transformer is designed for decades of operation. However, bolted connections can change over time under the influence of temperature, vibrations, and material aging. That's why, in many industrial installations, periodic inspections are performed using thermal imaging cameras.

Thermal imaging allows very quick detection of a point where the temperature is higher than in the other phases. Often this is the first sign that contact resistance is starting to increase and the connection requires inspection.

In power engineering, very often it is the small details that determine installation reliability. The cable connection at the transformer bushing is one of those places where installation quality has a direct impact on the operational safety of the entire station.

Small detail, big physics

The story of one or two holes in a bushing terminal says more about power engineering than might seem.

Because this is not an industry of spectacular gestures. It's an industry of decisions that at first glance look like trivial details, but in practice work for decades.

An MV transformer doesn't get a second chance every few years. It stands and works. Day after day. In winter, in summer, under load, after faults, in silence and without attention. For 30, sometimes 40 years.

And that's precisely why details like the method of attaching a cable lug matter. Because they decide whether everything will work as it should, without unnecessary losses, without overheating, without surprises.

So when you look at a bushing terminal with one or two holes, you are looking at the result of an entire industry's experience. Physics, tests, errors, and conclusions that someone once had to draw.

At Energeks, we like this level of thinking.

Because we know that a well-designed transformer is not just parameters on paper, but a fit to the reality of operation.

That's why our MV transformers can be equipped with various bushing termination configurations, tailored to the station design, cable connection method, and grid operator requirements.

If you want to see how different solutions look in practice, check out our offer.

And if you appreciate a technical perspective on power engineering without unnecessary noise, we also invite you to our LinkedIn, where we regularly share knowledge from projects and work with transformers.

REFRENCES:

IEEE Power Transformer Handbook, IEEE Press
Electric Power Transformer Engineering, James H. Harlow, CRC Press

It can pretend for years that everything is under control.

And then, in a very short time, it reminds you that the hard sciences also have a hard memory 🫣

A medium voltage transformer is a master of patience.

It can endure more than the table suggests. Work longer than someone planned.

Survive decisions that were borderline but were supposed to work out.

And that's precisely why it can be treacherous.

It doesn't break when things are really bad.

It breaks when, for a long time, things were almost good.

When the power margin was slowly dwindling, and no one noticed the moment when physics started charging interest.

This text isn't about failures.

It's about how to maintain control before the last 20% of margin disappears faster than you expect.

We see it more and more often.

Grids are working more intensively.

Load profiles are sharper.

Renewable sources, energy storage, chargers, inverters introduce dynamics into the system that older design assumptions simply didn't foresee.

The trusty old transformer copes and keeps working.

Only it's operating in a different world than the one it was selected for.

And that's not an unsolvable problem; it's a phenomenon to be understood.

This article is for those who prefer to know sooner rather than replace later.

For people who treat a transformer not like a grey box, but as an element of an energy strategy.

If you read on, you'll see how to recognize the moment when overload stops being flexible, why short episodes have long consequences, and how to make decisions that genuinely extend a transformer's life instead of heroically shortening it.

We'll look at why transformer aging accelerates non-linearly.

We'll explain how much operating outside rated parameters really costs.

We'll debunk the myth of momentary overload and show why many failures are the logical consequence of earlier choices, not equipment malice.

It'll be interesting, so stay until the end, where a small bonus also awaits you🥰.

Reading time: about 9 minutes

When overload stops being flexible

Every medium voltage transformer has a certain tolerance.

The designer isn't naive.

They know life won't be a catalog table.

They know load will spike temporarily, that summer will be hotter than the standard average, that someone will add another charger or inverter.

And for a long time, everything indeed works.

The problem begins when overload stops being flexible and starts being structural. The difference is subtle.

Flexible overload is an episode.

A dozen or so minutes of higher current, after which the transformer returns to thermal equilibrium. Structural overload is a situation where the operating point permanently shifts closer to the thermal limit.

The key indicator isn't the power percentage itself, but the hot-spot temperature of the winding.

IEC 60076 and IEEE guidelines clearly show that the aging rate of cellulose insulation increases exponentially with temperature.

An increase of 6 to 8 °C can double the aging rate.

This isn't a linear relationship. It's a chemical reaction accelerated by temperature.

In practice, the critical moment is recognized by several signals: shortened cooling time after a load peak, more frequent fan activation, an increase in no-load and load losses measured indirectly through active and reactive power analysis.

Add to this the analysis of gases dissolved in the oil, which shows whether the insulation is starting to react.

A transformer doesn't shout. It whispers in the data.

If we don't look at load profiles on an hourly and seasonal basis, it's easy to miss the moment when 80% of rated power stops being safe because the operational context has changed.

And today, context changes faster than ever.

Why short episodes have long consequences

Many investors think like this:

It was only 30 minutes.

Nothing happened.

From an operational point of view, they're right.

From the point of view of insulation chemistry, not necessarily.

Paper insulation in a transformer ages due to cellulose depolymerization.

Every temperature increase accelerates this process. A short episode of high load raises the hot-spot temperature. The cellulose chain molecules shorten.

We cannot reverse this process.

If there are a few such episodes a year, the impact may be negligible.

If they repeat daily during peak hours, we start building a permanent loss of dielectric strength. The transformer still works, but its safety margin decreases.

It's a bit like metabolic debt in the body. One sleepless night doesn't cause a revolution. Hundreds of such nights change biological parameters.

In systems with a high share of RES, high-load episodes often combine with higher-order harmonics generated by inverters.

Harmonics cause additional losses in the core and windings.

Losses mean heat. Heat means accelerated aging.

A short episode can mean a few percent of annual insulation life loss.

No one will see this at the moment of the event. We'll see it a few years later in the form of a failure that seems sudden.

Physics doesn't forget. It accumulates.

And at a certain point, a very specific question arises: since the transformer is still working, is it better to modernize it, regenerate it, or plan for replacement?

This isn't a zero-one decision.

Factors include oil analysis results, the degree of insulation polymerization, energy efficiency, compliance with Ecodesign Tier 2 requirements, and the real costs of losses.

Sometimes renovation makes sense and allows regaining several years of stable operation.

Sometimes economics and safety clearly indicate that it's better to replace the unit before a failure does it for us.

If you're facing such a dilemma, we discuss this topic more broadly in the article:

Is it worth investing in a new transformer when the old one still works?

It's a good complement to this conversation, especially when the decision concerns the next 20 years of installation operation, not just the upcoming season.

How to make decisions that genuinely extend a transformer's life

The most important decision is moving away from catalog thinking.

Rated power isn't an absolute.

It's a reference point for specific conditions.

If a transformer operates in an environment with higher ambient temperature, variable load profiles, and an increased harmonic level, this must be accounted for in the life model.

In practice, this means temperature monitoring, power quality analysis, and periodic oil diagnostics.

Decision number two is planning reserve with the future in mind, not just based on construction loads.

If we know that within three years, energy storage and high-power DC chargers will be added, it's worth planning for a transformer with a higher thermal class or greater power.

Decision number three is peak management.

EMS systems and energy storage control can realistically flatten the load profile.

Sometimes investing in intelligent control is cheaper than premature transformer replacement.

Extending a transformer's life isn't heroism.

It's consistent data management.

An MV transformer can work for 30 or even 40 years.

Provided we don't treat it like an unlimited resource.

Why aging accelerates non-linearly

Here we get to the heart of the matter.

The aging of paper-oil insulation is described by the Arrhenius law.

Simply put, it states that the rate of a chemical reaction increases exponentially with temperature.

If at 98 °C a transformer uses one unit of life per year, then at 110 °C it may use two or three. At 120 °C, the rate of increase is even greater.

The last 20% of the power margin often means operating in a temperature range where aging acceleration is dramatic compared to the nominal range.

That's why we talk about non-linearity.

In the first 60% of load, changes are gentle.

Near the limit, they become abrupt.

That's precisely why a transformer can work without problems for years, and then, in a short time, enter a phase of rapid degradation.

This isn't a whim of the device. It's a consequence of materials physics.

And it's at this moment that the real dilemma appears.

Should we still invest in renovation, drying, oil replacement, or is this already the stage where insulation parameters directly state that the construction is approaching the end of its technical life?

If the topic concerns units with 30, 40 years of operation, it's worth looking more broadly at the technical and economic aspects of such a decision.

We discuss them in detail in the article:

Refurbish or replace? Your transformer's last chance!

It's a natural complement to this part of the conversation, especially when you want to understand where cost-effective regeneration ends and responsible replacement planning begins.

How much does operating outside rated parameters really cost

The cost isn't limited to the energy bill.

First, we shorten the device's technical life.

If the designed service life is 30 years, and we realistically achieve 22, then the missing 8 years have their own capital value.

On the scale of a PV farm or industrial plant, this means millions of PLN shifted in time.

Second, the risk of unplanned downtime increases.

And the cost of downtime often exceeds the cost of the transformer itself.

Third, power quality parameters deteriorate.

Higher temperatures mean higher losses, higher losses mean lower efficiency.

Differences of one or two percent in large installations translate into significant annual amounts.

Operating outside rated parameters doesn't have to be a mistake.

It can be a conscious decision. There's one condition. We must know its price.

The myth of momentary overload

We hear this often. The transformer is oversized; momentary 110% won't hurt it.

It will hurt it or not, depending on the context.

If momentary overload occurs at low ambient temperature and the transformer has cooling reserve, the impact may be minimal. However, if it's 110% on a hot day, with an already elevated harmonic level, the effects are completely different.

The myth lies in looking at the power percentage, not at the thermal and electrical conditions.

A transformer doesn't feel %%. It feels temperature and electric field.

Momentariness isn't a time category. It's an energy category.

Why failures are the logical consequence of earlier choices

A failure is rarely a single event.

It's the result of a sequence of decisions.

Power selection on the edge. Failure to update load analysis after installation expansion.

Abandoning monitoring because nothing happened for years.

Each of these decisions is rational at the time it's made.

The problem arises when the system changes, but the assumptions remain old.

A transformer doesn't know the budget. It only knows the laws of physics.

That's why we say many failures are the logical consequence of earlier choices.

That's good news. Since they're logical, they can be prevented.

The transformer as part of a strategy, not a cost

In many projects, an MV transformer appears in the budget as a purchase item.

Power, voltage, delivery date, price.

Ordered, installed, connected.

It's supposed to work.

But the moment we start looking at it as a strategic asset, the conversation changes tone.

A transformer isn't just a device for changing voltage levels.

It's the energy node of the entire installation.

Every decision about power expansion, every new DC charger, every additional inverter, every energy storage unit passes through it.

If it's minimally selected, the company's entire energy strategy starts being constrained by one grey box in the station.

Life cycle planning means more than just writing "30 years" into the documentation.

It means analyzing how the load profile will change, what the power growth scenarios are, how the structure of loads will change. Today, a production plant has a specific consumption.

In 3 years, it might have a line that's 40% more energy-intensive.

If the transformer has no room for such a change, investment in development starts with infrastructure replacement.

TCO analysis, or total cost of ownership, often brings surprising conclusions.

A cheaper transformer with higher losses generates greater energy costs over 20 years than the difference in purchase price. A unit non-optimally selected for harmonics may operate with reduced efficiency and age faster. In the long-term balance, savings at the start turn out to be an illusion.

When energy storage enters the system, the transformer ceases to be a passive element.

It becomes part of the power control system.

You can smooth peaks, limit overloads, consciously manage reactive power.

That's specific kilowatts less during critical hours and specific degrees Celsius less in the winding.

In this perspective, the last 20% of power ceases to be a free reserve.

It's a zone we treat as an area of high responsibility.

We enter it when we know why, for how long, and with what consequences.

Not because it "still fits somehow."

This isn't a conservative approach. It's a mature approach.

BONUS: Answers to the most frequently asked questions on the topic

Does a transformer always have to operate below 80% power?

No. The key factors are temperature, load profile, and cooling conditions.

In many cases, 90% is safe if it's well calculated and monitored.

Does oil change extend a transformer's life?

It can help if the oil has degraded, but it won't reverse paper aging.

That's why diagnostics must be comprehensive.

Is it worth installing online sensors in older units?

In many cases, yes.

The cost of monitoring is small compared to the value of information about temperature and gases in the oil.

Does oversizing always pay off?

Not always.

Sometimes a better solution is intelligent load management or support from an energy storage system.

Summary and invitation

Transformer aging isn't linear.

The last 20% of power often tempts, because it looks like a safe reserve.

In practice, that's precisely where the technical cost grows fastest.

Fortunately, we aren't helpless. Data from monitoring, temperature and power quality analysis, sensible power planning, and updating design assumptions allow us to keep the situation under control. Without drama. Without fighting fires at the last minute.

An MV transformer can be just another device in the station. It can also be a consciously managed asset that works stably for decades. The difference lies in decisions made earlier, not in the failure itself.

As Energeks, we support investors, designers, and operators in the selection and modernization of MV units based on real work profiles.

Our offer includes oil transformers and resin-insulated transformers, all in Ecodesign Tier 2 standard, designed for high efficiency and a long life cycle. We also deliver complete transformer stations and solutions integrated with energy storage.

If the topic concerns your installation, it's worth talking sooner rather than later.

On our website and LinkedIn, we share knowledge from projects and implementations, showing how to approach a transformer not emotionally, but strategically.

References:

IEEE Std C57.91 Guide for Loading Mineral Oil Immersed Transformers
A classic document that details the relationship between temperature, load, and accelerated insulation aging. You'll find thermal models, life loss calculations, and a practical approach to short-term and long-term overloads.

CIGRE Technical Brochure 761 – Condition Assessment of Power Transformers via https://www.scribd.com/
A very concrete study on assessing the technical condition of transformers, interpreting oil tests, diagnostics, and making decisions about modernization or replacement based on data, not intuition.

This article is about what really happens at the interface between a PV inverter and a transformer, when DC from the modules turns into AC, and then still has to get along with the grid. A practical look.

You see a PV farm.

Rows of modules like a well-ordered army.

Inverters working quietly, smokelessly, without any theatrics.

And somewhere nearby stands a transformer.

The same type of device that in other projects can be a boring backdrop.

But in photovoltaic installations, a transformer can have its most intense life precisely when everything looks calm.

Because an inverter isn't an ordinary energy source.

It's fast power electronics that can perform wonders with current, but at the same time can introduce phenomena into the system that aren't visible at first glance: harmonics, rapid changes, reactive power control, sometimes minor unwanted components.

And all of this lands at the interface with the transformer.

In PV, one thing is particularly clear: most problems don't arise because the equipment is bad. They arise because the interfaces between equipment are often poorly coordinated.

This article is for designers, contractors, investors, and maintenance people who want the inverter-plus-transformer system to operate stably for years, without nervous adjustments after commissioning.

After reading, you will be able to recognize typical friction points and select solutions that genuinely improve power quality, operating temperatures, and reliability.

First, we'll establish a common language: what actually happens at the interface between the inverter and the transformer.

Then, we'll go through typical problems: harmonics, overheating, reactive power control, overvoltages, and resonances.

We'll discuss the most important tools, breaking them down into their basic elements.

At the end, you'll get five solutions to the most critical problems in transformer-inverter cooperation—including simple 'rule of thumb' methods that improve stability—and you'll receive answers to frequently asked questions on the topic, in a ready-reference cheat sheet.

Worth reading.

Reading time: about 15 minutes

What really happens at the interface between a PV inverter and a transformer

In a textbook, it looks simple: modules produce DC, the inverter turns it into AC, the transformer steps up the voltage, and the grid accepts the energy.

In practice, this interface is where two worlds meet.

The first world is power electronics.

An inverter doesn't generate a sine wave the way a generator does. It synthesizes it by switching transistors at high frequency and controlling modulation. This gives excellent control over active and reactive power, but leaves behind side effects: harmonics, high-frequency disturbances, steep voltage and current rise times.

The second world is the transformer, an electromagnetic device that likes predictability.

It is designed for a specific voltage shape, specific losses, specific temperatures, and specific load dynamics. When it receives a waveform with more content than a pure sine wave, things start to get interesting.

The most important thing to remember is this: a transformer in a PV system isn't just a voltage pass-through. It's the component where the side effects of inverter control and grid parameters materialize.

What language to use to understand each other

Remember the story of the Tower of Babel?

Everyone was supposedly building the same thing, yet each spoke a different language. In a project, it works the same way: if designers, contractors, automation engineers, and service personnel use different words for the same phenomena, diagnosis takes longer than the repair itself.

Harmonics are current or voltage components with frequencies that are multiples of the fundamental. In a 50 Hz grid, the 5th harmonic is 250 Hz, the 7th is 350 Hz, and so on.

For a transformer, this means additional losses and additional heating.

THD (Total Harmonic Distortion) is a measure of the total waveform distortion.

In practice, it's worth separating voltage THD from current THD.

An inverter most often introduces current distortion, while voltage distortion worsens depending on grid impedance and the transformer setup.

Reactive power is the control of voltage and the flow of reactive energy.

An inverter can supply or absorb it according to grid operator requirements, but this control changes the currents in the system and can increase the transformer's load.

Resonance is a situation where inductive and capacitive elements in the system begin to amplify certain frequencies.

In PV systems, there's plenty of capacitance: cables, filters, compensation capacitors, grid properties. Inductance too: chokes, transformers, lines.

It doesn't have to explode, but it can generate overvoltages, vibrations, and... strange protection errors.

Why harmonics make the transformer do extra work

A transformer has no-load losses in the core and load losses in the windings. When harmonics appear, three things happen simultaneously.

The RMS current increases, even if the active power doesn't. This means greater I²R losses in the windings. And that's the first reason for heating.

Added to this are additional losses, such as eddy currents in the windings and structural components. These increase faster with frequency, so higher harmonics can cause disproportionately large thermal damage.

The third thing is noise and mechanical vibrations. The transformer may start operating louder, and the winding mechanics experience greater fatigue over the long term.

The most insidious part is that on SCADA, everything might look decent because the power is stable, and only thermal imaging shows that something is wrong.

—>

If you want to go deeper and understand how to calculate this and translate harmonics into real requirements for the transformer, we recommend our article:

Transformer K-Factor: The Key to Protection Against Harmonics.

In it, we explain what the K-Factor is, what it tells us about non-linear loads, how it helps select a transformer for actual operating conditions, and how to limit the risk of overheating and insulation life reduction before the problem shows up in temperatures and alarms.

Where overheating comes from when parameters seem normal

There are three typical scenarios.

The first is apparent load.

Someone looks at the MW and is calm, but the transformer is loaded by currents resulting from reactive power and distortion. It doesn't heat up from MW. It heats up from current and losses.

The second is inverter operation in regulation modes.

For example, voltage control via reactive power, active power curtailment, operation under variable grid conditions. This changes the transformer's load profile over time, often faster than in conventional power systems.

The third is a design mismatch.

A transformer selected for a linear load may have too small a margin for additional harmonic losses. The power rating seems to match, but thermally, there's no breathing room.

This leads to a practical conclusion: in PV, checking kVA isn't enough.

You have to think about power quality, the share of reactive power, and the expected operating profile.

Reactive power control: a tool that helps the grid but loads the system

Grid operators increasingly require voltage support.

The inverter then has to implement curves: cos φ as a function of P, Q as a function of U, or a specific set Q.

First, let's break this down in plain language, without magical shortcuts.

Imagine the inverter has two knobs: one for active power P (the one you sell in kWh), and one for reactive power Q (which doesn't give kWh but affects voltage and currents in the grid).

The grid operator tells the inverter how to turn the second knob.

What does 'cos φ as a function of P' mean?

Cos φ is, simply put, information about the share of reactive power relative to active power.

When cos φ is close to 1, there's almost no Q. When it drops, Q increases.

Cos φ as a function of P means:

the power factor should depend on the current active power. The more P you produce, the more the inverter should change cos φ according to a set curve.

How it looks in practice:

When the farm produces little power, the inverter can operate near cos φ = 1.
When the farm enters high production, the inverter starts generating or absorbing reactive power to help keep voltage within the permissible range.
It's like an automatic transmission for voltage: it depends on the load.

Why do this?

Because during high generation, the voltage at the connection point tends to rise.

Reactive power can pull it down or push it up, depending on the direction.

What does 'Q as a function of U' mean?

Q as a function of U means: reactive power should depend on voltage.

This is pure regulation automation.

If voltage rises above a set threshold, the inverter starts acting to lower it.
If voltage drops, the inverter does the opposite to raise it.

It works like a thermostat, only instead of temperature, you have voltage, and instead of a heater, you have Q.

Now, an important detail: This isn't just an on/off state. It can be a smooth curve. For example, the higher the voltage, the more Q the inverter should absorb to reduce it. The lower it is, the more it should supply Q to boost it.

What does 'a specific set Q' mean?

This is the simplest version:

Someone tells the inverter upfront how much reactive power to produce, regardless of P and U.

For example:
We set the inverter to constantly absorb 1 MVAr.
Or constantly supply 0.5 MVAr.
Or maintain Q at a level resulting from the operator's dispatch.

Why do this?
Because sometimes the grid needs a specific amount of voltage support at a given moment, not automation dependent on local measurements.

From the grid's perspective, this is good.

From the perspective of the transformer and cables, it means higher currents for the same active power.

If the installation operates with a significant share of reactive power, the transformer may hit its current limit before reaching its nominal active power rating.

This is a classic source of situations like: theoretically I have reserve, but in practice, the temperature is rising.

What's treacherous for the transformer and cables in all of this

Here's the core of why we're mentioning this.

Reactive power increases the current in the system. Even if the active power P doesn't change.

If you have P (active power) and you add Q, the apparent power S increases, and along with it, the current.

Simply put:
More Q = higher current = greater thermal losses in cables and the transformer.

And that's why sometimes this happens:

On the screen, everything looks fine because the MW are stable.

But the transformer has a higher temperature because the current is larger.

Or the current limit appears earlier, before you reach full active power.

Control via cos φ from P, Q from U, or a set Q are ways the grid operator tells the inverter to support voltage, but this support is carried out by current, so it can increase the load on the transformer and cables even when active power doesn't change.

Additionally, if there's separate compensation in the system, you have to be very careful about who is controlling what. An inverter with its own regulation and a capacitor bank without coordination can enter into unpleasant interactions.

This rarely looks like a major failure.

More often, it looks like instability, fluctuations, protection errors, strange background harmonics.

Overvoltages and resonances: a problem that often reveals itself after commissioning

In PV, you have plenty of elements that create capacitances and inductances.

Long cables on the AC side, filtration, sometimes compensation, plus the transformer and grid parameters. Resonance doesn't have to be constant.

It can appear only in specific operating states, at a specific power, or with a specific grid configuration.

Symptoms can be misleading:

overvoltages, an increase in voltage THD, reactive power fluctuations, random protection trips, sometimes damage to filter components or overheating that doesn't match the load.

The most important design practice is this:

resonance must be treated as a systemic risk, not as bad luck. If there are capacitors, filters, and long lines in the project, frequency analysis of the system ceases to be a luxury.

What tools really solve these problems

When do you need compensating reactors and filters, and when are proper settings enough?

A line compensating reactors on the inverter output limits the steepness of current changes and suppresses some higher harmonics. An LCL filter does this more effectively but is more sensitive to grid parameters and requires proper tuning and damping.

If the problem is mainly current distortion and local harmonic amplification, passive or active filters might be the right solution.

A passive filter is simpler but requires good matching because it can interact with the grid.

An active filter is flexible but more expensive and requires sensible power sizing.

In many projects, the first step should be inverter settings:

THD limits, control strategy, filter parameters, Q regulation modes.

Sometimes the problem isn't that you need new hardware, but that the control is set up in a way that provokes the system.

If you want to understand when a compensating reactor is a real stabilization tool and when it's just a patch for a poorly selected system, check out our article:

Why low-loss transformers don't need compensating reactors?

We break down there where the need for these in compensation systems even comes from,

what low-loss transformers change in the reactive power and current balance,

and how to avoid situations where adding compensation elements starts creating new problems instead of solving them.

It's a text for those who prefer to calculate and select correctly once, rather than tune the installation later in the field ;-D (been there, done that…)

How to select a transformer for non-linear load

A transformer for PV should be selected not only based on apparent power, but also on the expected harmonic level, reactive power share, and cooling conditions.

In practice, what matters is thermal performance and additional losses, because these determine whether the unit will operate stably for years or live on the edge of its insulation.

If you anticipate significant current distortion, you have to account for the fact that harmonic current increases losses.

Some losses simply increase with current, while others increase faster because higher frequencies drive additional losses in windings and structural components.

The classic approach then calls for transformers adapted to non-linear loads, a power margin, and conscious cooling design.

This isn't oversizing for sport. It's a thermal reserve meant to allow the system to breathe in a real operating profile, without constantly pushing temperatures to the limit.

In PV, there's another layer rarely discussed openly until the hunt for the cause of strange currents and events begins.

That's earthing and winding configuration, i.e., the connection group.

The choice of group affects how third-order harmonics and zero-sequence components behave, where they can close their circuit, and whether they get the conditions to do so at all.

If the connection has a delta on one side, some components have a place to circulate locally.

If it doesn't, these same phenomena can flow into the grid or appear as currents in places no one suspected. This isn't a detail. It's the difference between an installation that is quiet and predictable and one that generates additional loads and diagnostic complications.

In the same basket is the tap changer—voltage regulation on the transformer side.

In PV projects, it's tempting to treat it as a one-time setting during commissioning. But it often becomes a tool for matching voltages in a real grid, with real drops and rises, with real reactive power control.

If you have the wrong tap range or the wrong regulation method, you can end up with a system where the inverter overcompensates with Q regulation because the transformer is set too high or too low relative to the connection conditions.

And again, this doesn't have to look like one spectacular failure. More often, it looks like long-term, unnecessary current loading and temperatures that are a few degrees higher than they should be.

That's why selecting a transformer in PV is worth treating as matching the interface between the inverter and the grid, not as buying a device with the right nameplate power.

Preparation for this involves analyzing the operating profile, power quality requirements, reactive power control, and thermal conditions, and then selecting transformer parameters and winding configuration so that the system is predictable.

With emphasis on what's hardest to fix after commissioning: thermal performance, harmonic interactions, and zero-sequence behavior.

—>

If you have doubts, we're happy to advise, and we also explore this topic in this article:

Which transformer should you choose for a 50, 100 or 150 kW PV system? Here’s what you need to know

5 solutions to the most critical problems in transformer-inverter cooperation

A transformer is a fan of a clean sine wave and predictable work.

An inverter is a waveform editor: it takes DC, assembles AC, regulates P and Q, plays according to grid requirements.

Usually, this works beautifully. Trouble begins when this digital finesse leaves traces in the world of iron: harmonics, high-frequency components, rapid current changes, reactive power operation.

That's why in PV, two things are crucial: grid conditions and control.

Below, we suggest solutions to the five most common problems related to this topic.

1. Harmonics and current distortion, or the bill for 'nice' electronics

Inverters are non-linear by nature. Even if they have a filter at the output and look well-behaved, in practice they can introduce current harmonics, especially at certain operating points and grid configurations.

What this does to the transformer:
Harmonics increase losses in copper and the core, as well as so-called additional losses, which in transformers grow faster than linearly with frequency and distortion.

The end result is boring and brutal: higher temperature. And temperature is the currency of insulation life.

What to do?

The simplest move is to check whether the problem lies in the emission itself or in grid resonance. Because sometimes the inverter is 'OK', and the grid turns its harmonics into a megaphone.

In practice, the following help: well-chosen line chokes, passive filters, active filters in larger installations, and conscious management of the impedance seen by the inverter. For MV PV farms, how the cable distribution and section lengths are designed is also crucial, because cable capacitances can shift resonant frequencies.

2. Reactive power and voltage control, or when the inverter helps a little too much

Modern inverters have volt-var and volt-watt functions, i.e., voltage-dependent regulation. Grid connection requirements in Europe strongly promote the ability to control reactive power and provide voltage support from distributed generation.

What this does to the transformer:
Reactive power itself isn't bad. The problem arises when its flow is unpredictable or too intense relative to the assumptions.

The result can be: currents increase, losses increase, the voltage drop across the transformer impedance rises, sometimes control oscillations appear if several devices 'fight' over the same voltage.

Solutions in three steps:
The first level is inverter settings consistent with requirements and the operator's philosophy.
Manufacturer documentation and guidelines for specific connection rules, such as VDE AR N 4105 in the German context, show how important reactive power control parameters are.

The second level is coordination: if you have compensation, an OLTC in the transformer, inverter regulation, and automation at the HV/MV substation, it's worth asking one basic question: who is the voltage leader here, and who is just supporting.

The third level is measurement and monitoring: without recording the Q profile, cos φ, and voltage over time, it's impossible to distinguish normal operation from automation chasing its own tail.

3. Transformer overheating despite correct rated power

This is a classic: everything 'fits in kW', yet the transformer still struggles more than it should.

Most common causes:
First, harmonics and additional losses, as discussed.
Second, high ambient temperature and cooling conditions, because PV stations often stand in places where summer air is like a warm compress.
Third, dynamic loads: fast power ramps, daily weather cycles, frequent changes in operating point.

Solutions:
A dual-track approach works here: selecting a transformer with the load profile in mind and ensuring power quality. Sometimes this means conscious oversizing, sometimes it means design parameters for distorted loads and choosing a winding connection group that helps close certain harmonics in the delta instead of pushing them into the grid.

If you want to approach this engineering-wise, the path looks like this:

current measurement, spectrum analysis, additional loss calculation, winding and hotspot temperature verification, and only then decisions about filters or setting changes.

4. Overvoltages, steep edges, and voltage surprises in cables

The inverter works in a pulsed manner. Cables have capacitance. The transformer has inductance. The system likes to create oscillations, and oscillations like to appear when no one invited them.

What happens in practice:
With long cable runs between inverters and the transformer, or between the transformer and the connection point, phenomena related to wave reflections and local overvoltages can appear. Add to this classic surges from the grid and switching operations, which in PV can be more frequent because automation works intensively.

Solutions:
Surge protection selected for the actual installation location, sensible earthing, control of cable lengths and their parameters, sometimes damping elements. In larger systems, designers also use solutions that limit the steepness of current changes seen by the transformer—so again we return to chokes and filters, only this time the motivation isn't THD, but insulation protection and spike limitation.

5. The common coupling point and the magic of weak short-circuit power

There's another unassuming hero: the short-circuit power of the grid at the connection point.

The weaker the grid, the more visible the impact of inverters on voltage and distortion.

This isn't an inverter flaw. It's a fact about the system's impedance.

Solutions:
Power quality analyses are performed, taking into account grid impedance and emission allocation, precisely in the spirit of the approach from IEC TR 61000-3-6.

Practically, this means that sometimes it's better to invest in a filtration system and setting coordination than to hope the transformer will SOMEHOW bear it—because a transformer is not a harmonic filter.

Simple ways to improve stability

First, start with a diagnosis: is the problem current-related, voltage-related, or resonance-related?

If current harmonics dominate, target filtration and control parameters.

If voltage sags or fluctuates, look at grid impedance, Q control, and regulation coordination.

If there are random events and overvoltages, suspicion falls on resonances, filter tuning, interactions with compensation, and cable lengths.

Then, get control in order: inverter settings, consistent regulation curves, no conflict between compensation and the inverter, control of power ramps and limits.

Next, selection and verification of the transformer for the real operating profile.

If data shows that currents and additional losses are high, the solution might be a transformer with better thermal performance, a different range of permissible distortion, or simply a properly chosen margin.

Finally, only then add filtration equipment where it makes quantifiable sense: chokes, LCL filters, passive or active filters, sometimes correction of compensation and protection settings.

Answers to the most frequently asked questions - FAQ

Can a photovoltaic inverter accelerate transformer aging?

Yes, if current harmonics, a DC component, or poorly set reactive power enter the grid, the transformer can heat up more than would result from the active power alone.

What is the most common PV problem affecting transformers?

Power quality surprises: harmonics, voltage fluctuations, and reactive power operation controlled by inverters.

Does a filter or choke really make a difference?

Yes, because it limits distorted currents and steep current edges, which increase losses and temperature in the windings.

What's more important: transformer power or its resistance to distortion?

In practice, both. A kVA reserve helps, but design for non-linear loads and grid conditions also matters.

What standards help set harmonic limits and connection requirements?

In Europe, the reference point is often grid connection requirements based on EN 50549, as well as compatibility and harmonic emission assessment rules from IEC 61000-3-6.

The interface between a PV inverter and a transformer is a bit like a big city intersection

On paper, the rules are simple, but in reality, what counts is traffic intensity, road surface quality, and whether the signaling is set up for the actual rush hours.

In photovoltaics, these rush hours repeat daily, and power quality, grid stiffness, and protection settings can turn an ordinary installation into a system requiring smart coordination.

The good news is that most tricky topics can be handled without stress if you approach them systemically.

First, understanding what's really happening in the currents and voltages.

Then, measurement and PQ monitoring to speak the language of data, not impressions.

Finally, design decisions that make a difference.

Sensible filtration, reasonable reactive power control, adaptation to grid conditions, and a transformer selected for the real operating profile, not just the nameplate.

If you are at the stage of selecting a transformer for PV or want to stabilize the operation of an existing installation, we invite you to explore our offer.

For low-loss oil transformers MarkoEco2, compliant with EcoDesign 2 ——> click here,

for TeoEco2, cast resin transformers Tier 2 ——> click here

In both cases, we're happy to help select a solution for your grid conditions, connection requirements, and inverter operating mode.

We also develop these topics on LinkedIn, more behind the scenes and more operationally. If you like specifics, follow us on LinkedIn and join the conversation.

Thanks for this shared journey through a topic that at first glance looks like a detail, but in practice determines the stability of an entire farm.

We are people for people, and we work best in partnership when both sides bring curiosity, precision, and a desire to do things properly.

REFERENCES:

IEC TR 61000-3-6. Electromagnetic compatibility (EMC) - Part 3-6: Limits - Assessment of emission limits for the connection of distorting installations to MV, HV and EHV power systems

Technical Requirements of Photovoltaic Inverters for Low Voltage Distribution Networks, K. Chmielowiec, Ł. Topolski, M. Dutka, A. Piszczek, Z. Hanzelka, T. Rodziewicz via MDPI

IEEE Standard for Harmonic Control in Electric Power Systems