Update: April 2026

In this article, we show how the quality control of an oil‑immersed transformer is carried out before it is put into service. We discuss voltage ratio measurement, winding resistance testing, insulation tests, loss assessment, a leak tightness test, and a check of mounting dimensions.

The text has been updated to clarify the permissible deviations for the voltage ratio according to IEC 60076-1. We thank the reader for their vigilance. We have also added an update for short‑circuit impedance values, described more literally in accordance with IEC 60076-1.

Imagine you are building a bridge. No matter how solid it looks, before anyone crosses it, it must pass a series of strength tests. With transformers it is similar – before the device goes into the grid, it must prove its resistance to voltage, load, temperature and extreme operating conditions.

In this article we will show you the testing process for our MarkoEco2 oil‑immersed transformer.

You will learn:

✅ How we check whether the transformer "understands" the grid and delivers the correct voltage,
✅ How we test the quality of the windings to avoid overheating and power losses,
✅ How we assess the strength of the insulation that protects against overvoltages,
✅ How we verify the transformer's energy efficiency and its pressure resistance,
✅ How we check whether the mounting dimensions allow trouble‑free installation.

This is a text that will show you what real quality control looks like – from strict standards to practical tests that decide the reliability of the device.

Reading time: 3 minutes – enough to gain knowledge that helps you avoid transformer operation problems.

1. Voltage ratio measurement and connection group test

This is the first and absolutely essential test, which checks whether the transformer "understands the language" of the power system it will be connected to. Imagine a band – if the guitarist plays in a different key than the rest, the whole song loses its meaning. With a transformer it is similar – if the windings are incorrectly configured, the voltages will be asymmetrical, and the whole installation will start to "play false", leading to losses, overheating and even more serious failures.

Why do we perform this test?

Every transformer has its own unique winding configuration and voltage ratio – this is the foundation of its correct operation. If the ratio deviates from the design assumptions, the system may not deliver the correct voltage to consumers, causing efficiency drops and the risk of overloads.

The test allows us to check two key things:

✅ Whether the voltages on the HV and LV sides are consistent with the designed ratio (e.g., 200 kV at the input and 5 kV at the output)?
✅ Whether the connection group is correct (e.g., Dyn5 – crucial for grid compatibility)?

How do we test it?

We connect the transformer to a test voltage source and make precise voltage measurements at the winding terminals.

Voltage ratio measurement – we check whether the voltage ratio between the high‑voltage and low‑voltage windings corresponds to the design assumptions. For the MarkoEco2 transformer, this should be 200 kV / 5 kV.

Connection group test – we verify whether the windings are connected according to the standard, e.g., Dyn5. The Dyn5 connection group means that the high‑voltage winding is connected in delta, and the low‑voltage winding in star with a phase shift of 150° (5 × 30°). In practice, this means that if phase L1 at the high‑voltage input reaches its maximum voltage, the corresponding phase at the low‑voltage output will have a phase shift of 150°.

What are the permissible deviations?

Update (April 2026): We thank an attentive reader, a student of electrical engineering, for their perceptiveness and good questions. Thanks to their vigilance, we can correct an inaccuracy in the previously given values. Previously, we gave reversed values for the permissible voltage ratio deviations. Correctly, in accordance with IEC 60076-1, they are ≤0.5% for the HV/LV winding and ≤1% for the LV (tertiary) winding. We have also clarified that these are deviations of the voltage ratio measurement, i.e., the ratio of voltages between windings, measured phase‑to‑phase between the corresponding transformer terminals. We thank you for your vigilance. Such comments are truly invaluable for a technical blog.

IEC 60076-1 specifies the maximum permissible voltage differences:

• ≤0.5% for the HV/LV winding

• ≤1% for the LV (tertiary) winding

If these values are exceeded, it may suggest:

❌ An error in winding,
❌ Problems with the quality of electrical connections,
❌ The transformer entering an asymmetric state, which increases the risk of overheating and power losses.

What if something is wrong?

Incorrect test results are an alarm signal that requires a thorough analysis. If the voltages deviate from the standard or the connection group does not match the design, we take several fundamental steps:

• Check internal connections – first, we verify whether the windings are correctly connected to the terminals and whether the connection group configuration is consistent with the documentation.

• Check the measuring equipment – measurement errors may result from incorrect connection of the test devices or their calibration.

• Winding analysis – in the case of larger deviations, it may be necessary to X‑ray the transformer using winding impedance measurement or to open the unit for mechanical inspection.

Why is this test so important?

Voltage ratio measurement and the connection group test are the absolute foundation of the quality control process. At this stage we can detect potential problems that could later lead to overloads, failures or even damage to the entire power system.

It is worth remembering that an error in the connection group configuration can:

❌ Affect grid compatibility, causing incorrect operation of other devices,
❌ Lead to dangerous operating states, such as winding overheating and overloads,
❌ Generate unexpected power losses, reducing the efficiency of the whole installation.

Thanks to testing according to the highest standards, we are sure that our transformers work perfectly with the grid, delivering stable and predictable voltage that meets both technical and economic requirements.

Because in power engineering, as in music – precise harmony is the path to reliable and efficient operation.

2. Winding resistance measurement – how do we check the internal strength of the transformer?

Every athlete knows that strong muscles are the foundation for enduring prolonged effort. In a transformer, the windings play this role – if they are weakened, the whole construction loses its efficiency. Resistance measurement allows us to ensure that the transformer will not only survive years of work but also not waste a single bit of energy. It is a test that checks whether the device is ready for intensive loads and will not overheat under long‑term operation.

Why do we measure winding resistance?

Winding resistance is one of the fundamental parameters determining the technical condition of a transformer. Even small differences can cause excessive heating, power losses and uneven phase loading.

Thanks to the measurement, we can assess:

✅ The uniformity of the windings – whether the wires are wound correctly and do not cause local overloads,
✅ The quality of electrical connections – whether there are loose contacts or soldering defects that could lead to overheating,
✅ Phase symmetry – whether each phase has a similar resistance, ensuring stability and even load distribution,
✅ Possible mechanical damage or inter‑turn short circuits – if the resistance of one phase deviates from the standard, it may indicate a short circuit or weakened insulation.

Measurement methodology

We measure winding resistance in accordance with IEC 60076-1, using the four‑wire Kelvin method. This eliminates the influence of the measuring leads, ensuring precise results.

Test stages:

1. Connection of measuring equipment – we use a DC current source of 1 to 10 A and precise voltage drop meters.

2. Stabilisation of the value – we wait until the current reaches a stable level, eliminating the influence of thermal effects on the measurement results.

3. Recording of results – we measure resistance separately for each phase of the HV and LV windings.

4. Temperature correction – because resistance changes with temperature, we convert the results to a reference value (usually 75°C).

What if the results deviate from the standard?

If the values are outside the permissible range, additional analysis is necessary:

❌ Too high resistance – may indicate damaged soldered connections or incorrect winding,
❌ Too low resistance – may suggest an inter‑turn short circuit, leading to local overheating and risking failure,
❌ Phase asymmetry – may indicate uneven winding, resulting in overload of one phase and unstable transformer operation.

Winding resistance measurement is one of the foundations of quality control. Even small deviations can result in increased energy losses, higher operating temperatures and a shorter transformer life.

Thanks to this test, we can guarantee that the transformer operates efficiently, does not generate unnecessary losses and is ready for many years of reliable operation.

3. Insulation strength and resistance – testing the protective armour

In power engineering, insulation plays a key role – it is a protective barrier that must withstand extreme operating conditions. If it is not strong enough, electrical breakdown can occur, resulting in damage to the transformer and even serious system failures.

This can be compared to a knight in armour – if its integrity is broken, he becomes vulnerable to attack. In transformers, the threats are not swords but high voltage and uncontrolled overvoltages.

Insulation tests allow us to determine whether the transformer is resistant to electrical overloads and will not lose its properties during many years of operation.

What are the insulation strength and resistance tests and why do we perform them?

Insulation resistance and electrical strength are the basic parameters determining the safety of transformer operation. Measuring these values allows us to assess:

✅ The quality of insulating materials – whether the dielectric meets the standards and is resistant to high voltages,
✅ The absence of mechanical damage – whether the insulation has micro‑cracks that arose during production, transport or assembly,
✅ Resistance to environmental conditions – whether the insulation effectively protects against moisture, dust and material ageing,
✅ The ability to withstand overvoltages and dynamic electrical loads that may occur in normal power grid operation.

These tests allow us to verify whether the transformer will operate safely and reliably for decades.

How do we perform insulation strength and resistance tests?

Transformer insulation testing consists of several basic stages:

1. Insulation resistance measurement (R60) – long‑distance test

Insulation resistance informs us about the condition of the dielectrics used in the transformer. It is performed using a megohmmeter (e.g., 2.5 kV or 5 kV), and the measurement lasts 60 seconds, hence the designation R60.

If the measured insulation resistance is too low, it may suggest moisture in the insulation, the presence of conductive contaminants, or mechanical damage to the dielectric.

2. Applied voltage test – simulation of extreme conditions

In this test, we apply a test voltage for 60 seconds to check whether electrical breakdown occurs. This is a simulation of extreme operating conditions.

Test voltage ranges for the MarkoEco2 transformer:

• HV → LV: 35 kV

• LV → HV: 5 kV

Test frequency: 150 Hz

During the test, the transformer insulation must withstand these values without any signs of breakdown, sparking or material degradation.

3. Induced voltage test – extreme load

We check the winding's ability to withstand long‑term overloads by increasing the voltage to 200% of the rated value for 40 seconds at an elevated frequency (150 Hz), which allows the test to be carried out in a safe time without saturating the core.

Test parameters:

• Test voltage: 200 kV

• Frequency: 150 Hz

• Duration: 40 seconds

If after this time no breakdown or other anomalies occur, it means that the windings and insulation meet IEC 60076-1 standards and can safely function in real operating conditions.

What if the results deviate from the standard?

If the tests show too low insulation resistance or signs of breakdown, we take the following diagnostic steps:

❌ Insulation moisture measurements – we check whether moisture has weakened the insulating properties,
❌ Microscopic insulation analysis – we assess the dielectric structure for micro‑cracks and mechanical damage,
❌ Impregnation quality check – poorly impregnated insulation can lead to reduced electrical strength.

If necessary, we carry out an insulation drying and additional impregnation process to restore its full properties.

What could be the consequences of poor results?

If the transformer does not meet the test requirements, this can lead to:

❌ An increased risk of electrical breakdown, which could result in failure of the entire system,
❌ Accelerated insulation ageing, reducing reliability and shortening the device's life,
❌ Reduced overvoltage resistance, increasing the risk of damage during sudden voltage rises in the grid.

How do we ensure maximum insulation resistance?

In our company we use:

✅ Highest quality insulating materials, resistant to extreme voltages and temperatures,
✅ Advanced impregnation techniques that increase resistance to moisture and ageing,
✅ Additional protective layers that reduce the risk of mechanical damage to the insulation,
✅ Precise quality control, including multi‑stage tests at every stage of production.

Measuring insulation strength and resistance is a key transformer safety test. Without solid and durable insulation, even the best‑made windings will not ensure long and safe operation.

Thanks to such meticulous tests, we are sure that our transformers are resistant to overloads, meet IEC standards and guarantee stability and safety for decades.

Because in power engineering, as in bridge building – a solid structure is the foundation of longevity.

4. No‑load loss measurement – is the transformer an "energy glutton"?

Even without load, a transformer consumes energy – the question is whether it does so reasonably.

The no‑load current measurement is an efficiency test – if losses are too high, it is a sign that something needs improvement. It is like assessing a car at idle – a good engine should not burn fuel unnecessarily.

What is the no‑load test and why do we perform it?

The no‑load test determines the transformer's energy efficiency. It allows us to check how much power the transformer consumes without load, i.e., when voltage is applied but no working current flows.

No‑load losses occur mainly due to:

✅ Losses in the magnetic core – resulting from eddy currents and magnetic hysteresis,
✅ Losses associated with leakage currents – although minimal, over a longer period they can affect transformer efficiency.

This test is crucial for assessing transformer efficiency – the lower the losses, the higher the efficiency, which translates into lower operating costs.

How do we perform the no‑load measurement?

We carry out the measurement in accordance with IEC 60076-1, applying rated voltage to the primary winding and measuring the current and losses in the transformer.

Permissible no‑load loss values – for a standard power transformer, no‑load losses should not exceed 2% of the rated power.

If they are higher, it may mean:

❌ Non‑optimal core material – e.g., using steel laminations of too great a thickness,
❌ Poor core lamination fit – which increases eddy currents and thermal losses,
❌ Problems with core impregnation – which can cause additional vibrations and uncontrolled magnetic losses.

Load losses and short‑circuit impedance

Short‑circuit impedance (Zk) is a parameter specified individually for each transformer on its rating plate – it is typically between 4% and 8% for distribution transformers.

IEC 60076-1 does not impose a specific Zk value, but specifies the permissible deviation of the measured value from the rated value: typically ±7.5% for two‑winding transformers. A deviation that is too large from the rated value can affect the transformer's ability to deliver power or its sensitivity to voltage fluctuations.

What could be the consequences of poor results?

If no‑load losses exceed the permissible standards, this can lead to:

❌ Increased operating temperature – which shortens transformer life,
❌ Non‑optimal energy efficiency – which increases operating costs for the user,
❌ Higher magnetic noise – resulting from incorrect core assembly,
❌ Faster insulation ageing – which in extreme cases leads to premature failure.

How do we minimise no‑load losses?

In our company we use the highest quality electrical steel with low magnetic losses, precise core cutting and advanced impregnation techniques that reduce uncontrolled core vibrations. Thanks to this, our transformers achieve an energy efficiency of over 99%, which means real savings for users.

The no‑load measurement is a key test of transformer energy efficiency. It assures us that the device is not an "energy glutton" and does not generate unnecessary losses.

By maintaining such procedures, we are confident that our transformers are optimised for efficiency, meet IEC standards and ensure reliability in long‑term operation.

5. Pressure strength and resistance test – transformer under pressure

Imagine that a transformer is like a submarine. It must withstand enormous pressure, both internal and external, to operate safely for decades. Pressure strength tests check whether the transformer construction is leak‑tight and whether there are any oil leaks that could lead to failure. It is like a structural test of a submarine's hull before its first dive – if something leaks, there is no question of a safe mission.

Why do we test transformer tightness?

Every oil‑immersed transformer is a hermetically sealed system in which the insulating oil plays a key role – it cools and insulates the windings. Any leak means not only oil loss but also the possibility of moisture and air entering the transformer interior, which leads to insulation degradation.

The pressure test allows us to check:

✅ Whether the transformer tank will withstand the expected mechanical loads,
✅ Whether there are micro‑leaks that could lead to oil loss,
✅ Whether the transformer construction meets IEC standards for mechanical strength.

We perform pressure tests in two main stages:

1. Overpressure test – simulation of extreme conditions

In this test, we introduce compressed air or nitrogen into the transformer at a specified pressure and observe how the construction reacts to overloads.

If after the test the internal pressure remains within the norm, it means the construction is completely tight. If the pressure drops below permissible values, an inspection of welds, seals and connection flanges is necessary.

2. Pressure change resistance test – simulation of real operating conditions

During operation, the transformer experiences pressure changes resulting from temperature fluctuations. The oil expands and contracts, and the tank must withstand this. Therefore, we perform a test that simulates cyclic pressure changes reflecting daily operation.

Test assumptions:

• Simulation of a daily cycle:

• Oil temperature change from -20°C to +90°C

• Internal pressure changes in the range of 20-35 kPa

• Transformer subjected to at least 100 cycles

Purpose of the test: To check whether structural elements do not deform and whether seals do not lose their properties.

Possible problems and their consequences

If the transformer fails the pressure test, we may be dealing with several problems:

❌ Leakage of welded joints or flanges – even micro‑leaks can lead to oil loss and moisture ingress.
❌ Tank deformation – if the transformer housing bends under pressure, it may indicate insufficient sheet thickness or design errors.
❌ Seal problems – leaks can lead to reduced insulation quality and shorter transformer life.

How do we ensure maximum transformer strength?

✅ We use high‑quality structural steel with increased pressure resistance,
✅ We precisely weld the tank and perform radiographic inspection (RT),
✅ We use advanced sealing techniques, eliminating the risk of leaks,
✅ We perform tests on every transformer to guarantee the highest quality.

Pressure strength and resistance measurement is one of the key tests guaranteeing long and trouble‑free transformer operation. Mechanical strength, absence of leaks and the ability to withstand pressure changes are the foundation of every unit's reliability.

Thanks to professional tests, we are sure that our transformers can operate even in extreme conditions – without leaks, deformations or risk of failure.

6. Insulation distance measurement – precision that saves from failure

In the world of high voltages, millimetres matter. If the insulation distance between conductors or between windings and the housing is too small, the consequences can be catastrophic – from overheating to complete insulation breakdown. Insulation distance measurement is one of those tests that at first glance may seem trivial, but in fact constitutes a key element of transformer operating safety.

It is like checking the clearance between high‑voltage lines – conductors placed too close can lead to sparkovers and failure.

Why do we measure insulation distances?

The insulation distance is the space between conducting elements of the transformer that prevents sparking, breakdown and excessive thermal losses.

This measurement allows us to verify:

✅ Whether the transformer meets IEC 60076 standards for insulation distances,
✅ Whether there are design errors that could lead to short circuits,
✅ Whether there is a risk of dielectric breakdown during operation, especially during overvoltages,
✅ Whether the transformer can operate safely in conditions of high humidity and contamination, which reduce insulation effectiveness.

How do we measure insulation distances?

We measure insulation distances in several areas of the transformer:

• Phase‑to‑earth distance – we measure the clearance between windings and the housing,

• Phase‑to‑phase distance – we check the clearances between phase conductors,

• Distances between conducting elements in the low‑voltage and high‑voltage areas,

• Clearances at connection points and terminals where the risk of voltage flashover is highest.

If the measured values deviate from these standards, it means that:

❌ The winding design does not meet the design requirements,
❌ Local electrical overloads and insulation breakdown may occur,
❌ The transformer will not pass further high‑voltage tests.

Problems resulting from incorrect insulation distances

If the insulation distances are insufficient, the transformer may encounter the following problems:

• Electrical breakdowns – the breakdown voltage of the insulation decreases if the conductors are too close together,

• Excessive heating – when clearances are too small, local temperature increases can occur, accelerating insulation ageing,

• Operational problems – under conditions of high humidity and contamination, the breakdown voltage can drop significantly, leading to partial discharges and gradual insulation damage.

How does our company ensure compliance with insulation standards?

• Precise design control – we design every transformer in accordance with IEC standards, taking into account appropriate clearances,

• Advanced insulation technologies – we use high‑quality dielectric materials that allow maintaining proper distances even during long‑term operation,

• Detailed tests at every production stage – from the design phase to final high‑voltage tests.

Insulation distance measurement is a key stage of transformer testing. The safety, reliability and ability to operate under high voltages depend on maintaining appropriate distances.

Thanks to tests, we are sure that our transformers are resistant to overvoltages, meet IEC standards and can operate without failure for decades – without the risk of breakdown or insulation failure.

7. Mounting dimensions – fitting reality

A transformer is not only a machine – it is an element of a larger system that must fit perfectly into the power infrastructure. Even the best transformer in the world will not fulfil its role if it does not fit into the intended mounting location or its connections do not meet the customer's requirements.

The mounting dimensions test is nothing other than a precise check of the device's geometry – we ensure that every bolt, every hole and every terminal is exactly where it should be. It can be compared to checking whether a key fits a lock – even a minimal difference can mean that the transformer cannot be installed correctly.

Why do we test mounting dimensions?

During transformer design, every dimension is precisely calculated, but production is a process in which unexpected deviations can occur. Measuring mounting dimensions allows us to check whether:

✅ The transformer fits the foundation or structure to which it is to be attached,
✅ The spacing of mounting holes is consistent with the customer's requirements,
✅ The connection terminals are correctly positioned, enabling safe connection,
✅ Transport and installation will proceed without complications – no need for modifications on site.

How do we perform dimension checks?

We measure the mounting dimensions on several key elements of the transformer:

• Length, width and height of the housing – we check whether the device will fit in the installation space,

• Spacing of mounting holes – we measure their position to ensure they match the foundation,

• Position of electrical terminals – they must be consistent with the design to enable quick and safe connection of conductors,

• Height and position of radiators and other external components,

• Clearances between structural elements – we check whether the planned service spaces allow proper access to components.

Standards and dimensional tolerances

Each transformer design specifies the maximum permissible dimensional deviations that cannot affect the correctness of installation.

Deviations outside these values may mean that the transformer will not fit the infrastructure, which will require additional installation work and may delay system start‑up.

Potential problems and consequences of incorrect dimensions

❌ Mismatch with the foundation – if the mounting holes are shifted by a few millimetres, it may be necessary to re‑drill them on site, lengthening the installation time,
❌ Problems with connecting conductors – poorly positioned terminals may make proper cable connection impossible,
❌ Difficult service and operation – if components are too close together, maintenance may be hindered, increasing upkeep costs,
❌ Transport problems – a transformer that exceeds the planned dimensions may not fit the transport vehicle or may require additional transport permits.

How do we ensure precision?

• Advanced measurement technologies – we use laser measurement systems and 3D scanners to precisely verify transformer dimensions,

• Strict quality control at every production stage – we check dimensions not only on the finished transformer but also during the assembly of individual components,

• Documentation for the customer – we deliver every transformer with a complete dimension report, allowing the avoidance of surprises at the installation site.

The mounting dimensions test is a key stage of quality control that prevents problems during transformer installation. Thanks to precise measurements, we are sure that our devices fit perfectly into the power infrastructure and are ready for immediate installation.

Every transformer that leaves our gates undergoes rigorous tests with one goal – absolute reliability. This is not just a set of procedures, but a multi‑stage control process that allows us to anticipate and eliminate potential failures before the device even goes into operation. We are aware that a transformer is the heart of a power system. If it fails, the consequences can be serious – from power supply interruptions, through costly downtime, to infrastructure failures.

Why is it worth working with Energeks?

✅ Safety confirmed by numbers – Our transformers meet IEC 60076 standards and have standard measurement deviations below 2%, guaranteeing high precision and longevity of the devices.

✅ Excellent efficiency – less loss, more savings – Thanks to optimised construction and the use of high‑class materials, our transformers achieve over 99% efficiency, which realistically reduces operating costs.

✅ Products tailored to customer needs – We adapt each transformer design to the specifics of the customer's infrastructure – in terms of dimensions, electrical parameters, and special requirements for mechanical and environmental resistance.

✅ Trust backed by experience – Our solutions work in hundreds of industrial plants, power stations and power grids across Europe. Every transformer delivered is the result of cooperation between a team of engineers who have been raising quality standards in the power industry for years.

✅ Comprehensive service – from design to service – Cooperation with Energeks is not only the purchase of a transformer, but also professional advice, precise tests and technical support at every stage of operation.

If your company is looking for proven and safe transformers that can meet even the most demanding conditions – Energeks is the right choice. Our products will provide energy stability, long life and trouble‑free operation for decades.

📩 Contact us today to discuss the technical specification and find a solution tailored to your needs.

Sources: Energeks Technical Documents