Sometimes, before you protect something, you need to truly understand it first.

And the medium voltage network – although it forms the backbone of modern power systems – still remains a mystery to many engineers, who only know fragments of it. Yet it’s precisely the understanding of its structure, logic, and components that determines the effectiveness of any protection design.

We don’t teach from dry manuals. We share the knowledge we use daily in our own work – designing transformer stations, selecting switchgear, implementing protection systems, and configuring energy storage solutions. We share our experience with you because we know you want to design wisely and safely.

If you create or analyze power systems, configure protection, or work on modernizing existing lines – you need to know the anatomy of the MV network.

This article will help you: understand what voltage levels medium voltage networks cover and why the range is so broad, distinguish and intentionally apply network topologies (radial, looped, mesh), understand the function of the main MV components – stations, transformers, breakers, and switchgear, and prepare to create clear, logical functional diagrams that not only meet standards but also protect lives and assets.

What will you find in this article?

• In the first section, we’ll explain what exactly a medium voltage network is, what voltage levels it includes, and why different countries apply different classifications.

• Next, we’ll go through the most common MV network topologies, showing how the chosen structure affects reliability and protection design.

• Finally, we’ll thoroughly discuss the functions and placement of the main components of the medium voltage network – such as transformer stations, MV/LV transformers, switchgear, power breakers, and protection relays. We’ll show how each of these fits into the network structure, what role it plays, and where it is typically installed to ensure continuity of supply and safety for people and infrastructure.

With this knowledge, you will:

• be ready to independently create clear and logical functional diagrams,
  gain tools for communicating with designers, contractors, and distribution system operators,
  easily understand the relationships between MV devices and translate them into concrete technical decisions – such as choosing the type of protection or placement of current transformers.

This won’t be an academic manual. It will be a practical knowledge pill written from the perspective of those who work daily with voltages between 1 and 52 kV – not just read about them.

Reading time: 7 minutes

What is a medium voltage network and why is it worth understanding?

A medium voltage (MV) network is a segment of the power system that operates in the range from 1 kV to 52 kV – although it’s worth noting that this is a definition from the “upper shelf.” In practice, depending on the country, technical standards, and the grid operator’s adopted guidelines, these limits can vary significantly. In Poland, common voltage levels include 15 kV, 20 kV, and 30 kV, while in Germany it’s often 10 kV, in Spain and Italy – 24 kV and 36 kV, and in the UK and India, anything up to 33 kV is still considered medium voltage.

Why do these differences exist?

They stem from multiple factors – from historical investment decisions to equipment availability, to the geographic structure and population density of each country. Countries with more dispersed power systems (like Scandinavian countries or Australia) may use different voltage standards than those with densely packed consumer zones. These differences influence not just operating voltages but also transformer types, protection parameters, and even the layout and design of underground or overhead lines.

From the perspective of an engineer responsible for protection – knowing the local voltage classification is the foundation of every technical decision. It’s not enough to know IEC standards – you need to know what applies here and now, in your specific distribution region, and how those regulations influence the selection of relays, breakers, and transformers.

Functionally, the MV network is the transmission route between high-voltage substations and the final energy distribution points – that is, MV/LV transformers, switchgear, and power panels for end users. You could compare it to a highway system linking major power hubs to the local street grid. In this space, it’s not just megawatts that flow – but also immense responsibility. This is where the vast majority of disturbances, overloads, and faults occur.

That’s why understanding the medium voltage network is not just an academic issue – it’s a practical necessity. It enables engineers to act with awareness, safety, and efficiency. It helps determine where to place measurement points, how to select breaker parameters, when to use compensation, and when to change topology.

You might say the MV network is like the network of arteries between the heart and the capillaries – not as flashy as transmission at 220 or 400 kV, but absolutely essential for getting the “energetic oxygen” where it needs to go, on time, to every corner of the industrial organism. It’s at this stage that voltage is transformed from high to medium, and then – through MV/LV transformers – into the familiar utility voltage (230/400 V) we use every day.

Why is understanding the MV network so important?

Because this is where most failures, overloads, faults, and supply interruptions occur. This is where overcurrent protection, earth fault relays, and sometimes even reactive power compensation devices come into play. If you don’t understand this part of the network – you don’t know where responsibility begins and ends, can’t define measurement points correctly, and your protection design becomes guesswork.

On a practical level: an engineer who understands the logic of MV network operation can not only design better protection systems but also diagnose faults faster, assess risks more accurately, and collaborate more effectively with distribution system operators. And that translates into one thing – real impact on the reliability and safety of energy supply.

From the perspective of power system designers and integrators, the medium voltage network is not a dry definition. It’s a daily set of challenges that teach humility, precision, and respect for energy. Understanding how this network works is the first step toward designing smart and failure-resistant solutions. Without this knowledge – no technology will perform reliably.

MV network topologies: radial, looped, mesh – what to know before placing a relay

Before you start selecting protection devices in a medium voltage network, you need to ask yourself one key question: what topology are you dealing with? Whether the line is designed as radial, looped, or meshed determines not only the logic of power flow but also the protection strategy, selectivity, and overall system stability.

Each MV network topology has its own characteristics, strengths, and weaknesses – and understanding them is a foundation for every protection engineer, designer, or contractor working with energy systems. Below, we break them down in detail.

Radial network – the simplest, but the least resilient

The radial network is the most classic and intuitive form of power distribution – electricity flows in one fixed direction: from the supply station to the end consumer. Imagine a large tree: the trunk is the main station, and each branch is an individual distribution line, ending in MV/LV transformers. Sounds simple? That’s exactly what it is – and that’s both the strength and the weakness of this topology.

In a typical radial network, each point has only one source of power. There’s no backup supply from another direction – so if a fault occurs on any segment (for example, a cable failure, short circuit, or surge), the entire section beyond that point immediately loses power. In Poland, radial networks dominate in rural and suburban areas, where the number of consumers per kilometer of MV line is relatively low – often below 20 customers per kilometer.

The advantage of this approach is its low design complexity and ease of fault location. Construction costs for such networks are the lowest among all topologies – both in terms of infrastructure and protection equipment. For many DSOs, it’s the first choice when planning supply to distributed consumers with moderate demand.

But not everything that’s inexpensive is golden. The downside of the radial network is its low fault tolerance and lack of redundancy – even a single failure can cut power across a wide area. Statistically, the average restoration time in radial networks ranges from 30 minutes to 2 hours, depending on the network's reach and the speed of intervention. Furthermore, the protection must be extremely precise – any delay or improper selectivity can result in disconnecting too large a portion of the network.

In engineering practice, radial networks require the use of non-directional overcurrent relays, which react quickly to faults but cannot detect the direction of current flow. In more advanced systems, automatic transfer switches (ATS) and local fault notification systems are used – although implementing them increases costs, which may be unacceptable for smaller projects.

Where it's used: Most common in rural networks, low-density areas, small industrial facilities, and distributed backup power systems.

Advantages:
Simple structure = easy design and operation
Low implementation cost
Low protection complexity

Disadvantages:
No redundancy – one failure cuts the entire section
Long restoration time
Limited reconfiguration options

Protection takeaway:
Full selectivity is a must – relays must act precisely to avoid disconnecting upstream segments. This often includes directional or non-directional overcurrent protection, earth fault protection, and sometimes auto-reclosing functions.

Looped network – a balance between simplicity and reliability

The looped or closed-ring network is an elegant blend of radial simplicity and meshed resilience. Its foundation lies in the ability to supply a point from two different directions – under normal conditions, only one side is active, while the other remains on standby, ready to take over in the event of a fault.

In practice, imagine a ring where each consumer is connected to both sides of the circuit. If a fault occurs – say, a short circuit or cable damage – power can be restored quickly from the alternate direction. The whole switchover can take just a few seconds, provided the right automation and directional relays are in place.

Looped networks are typical in urban areas, industrial zones, critical infrastructure (hospitals, subway systems, data centers), and anywhere uninterrupted power must be guaranteed regardless of the circumstances. In Warsaw, Wrocław, and the Tricity area, many residential developments are supplied using looped systems – allowing operators to limit outage durations to just a few minutes per year.

However, this topology requires more advanced system management. The designer must consider not only the direction of current flow but also the possibility of simultaneous supply from two sources – which, if not properly secured, can lead to serious equipment damage. Time-coordination of protection devices is essential here, along with correctly configured interlocks and directional overcurrent relays (50/51 and 67).

It’s also worth noting that looped networks are often designed in an “open” form and only closed during faults – allowing a combination of control simplicity and reconfiguration flexibility. Increasingly, such systems are equipped with digital reclosers and IEC 61850-enabled devices, making fully automated switching possible without operator intervention.

Where it's used: Common in urban environments, large industrial facilities, hospitals, and data centers – wherever continuity of supply is critical.

Advantages:
Increased reliability
Network reconfiguration possible during faults
Minimized downtime

Disadvantages:
Higher system and protection complexity
Requires time-coordinated relays
Needs intelligent switching systems (e.g., reclosers)

Protection takeaway:
Protection must detect the direction of current flow, which is why directional relays and SCADA systems enabling dynamic source switching are commonly used. Interlock logic must be planned to prevent simultaneous supply from both ends of the loop.

Meshed network – maximum flexibility, maximum responsibility

The meshed network is the most advanced and complex distribution topology, where MV lines form a dense web of interconnections. Current can flow through multiple paths, and each consumer can receive power from several directions at once. It’s the electrical equivalent of a major city’s transportation system – with alternative routes, bypasses, and many access points.

In practice, meshed networks are used in large urban agglomerations, industrial zones, logistics hubs, ports, airports, and around large power plants or PV farms. Wherever power cannot be interrupted – even for a split second – and where redundancy and flexibility are absolute requirements.

The key strength of this topology is its high fault tolerance – damage to one or even several lines doesn’t cut power, as energy can “find” another path. In systems integrated with energy storage and distributed renewables (e.g., PV + ESS), the mesh enables dynamic power balancing and reduced transmission losses.

But that flexibility comes at a cost. Meshed networks require the most meticulous protection planning and relay coordination, as current flow is variable and unpredictable. Without appropriate solutions – such as multifunctional relays, digital control systems, substation automation (SAS), and full SCADA or DMS integration – a mesh network can quickly become... a trap.

Protection response times in mesh networks often need to be within 50–100 ms, and any selectivity error can trigger multiple protections simultaneously, leading to cascading disconnections. That’s why mesh systems are mainly designed where infrastructure operates at the highest reliability class – for example, large hospitals (N+1), subways, airports, or Tier IV data centers.

Where it's used: Primarily in central urban areas, industrial districts, ports, and airports – where response time must be counted in seconds and interruptions are unacceptable.

Advantages:
High flexibility and network resilience
Immediate power rerouting possible
Excellent redundancy and voltage stability

Disadvantages:
Complex protection logic
High implementation and maintenance costs
Risk of simultaneous relay operation (lack of selectivity)

Protection takeaway:
In such networks, selectivity and coordination are tasks for advanced systems. Sensitive relays with communication functions (e.g., IEC 61850), digital switchgear, and often station automation (SAS) and integrated DMS protection are required.

Choosing the right medium voltage network topology influences everything – from project cost and switchgear selection to protection architecture. A good designer understands not only the strengths and limitations of each topology but also how to match the technology to the terrain, consumer type, and operational conditions. So before you install your first relay – pause and ask yourself: what topology am I dealing with, and what does it demand of me?

Stations, transformers, switchgear and breakers – four MV pillars that determine reliability

A medium voltage network is not just about cables and diagrams. It’s a living system that – like any well-functioning body – needs organs: stations that coordinate, transformers that adjust voltage, switchgear that distributes power among users, and breakers that protect before things get serious.

Each of these components plays a specific role in the network structure, and their proper selection and placement are the foundation of effective and safe power distribution. Below, we break that foundation down – with insights from construction sites, container station projects, and grid modernization efforts.

Transformer and switching stations – the heart and command center of the network

An MV/HV or MV/LV station is where energy changes its form. It’s a hub of conversion and distribution, where voltage is transformed and redirected to different parts of the grid. In an MV system, you’ll encounter various types of stations – from classic pole-mounted units rated 160–400 kVA, to advanced container stations handling transformers of 1000–2500 kVA or more.

In practice, a station is:

a point of voltage transformation (e.g., 20/0.4 kV),
an energy distribution management center,
a communication node for automation systems (e.g., IEC 61850, SCADA),
and a physical location for protection devices that must respond within 50–150 ms to prevent fault propagation.

Modern MV stations are often equipped with energy storage systems (ESS), arc protection, optical sensors, and fault recorders – because today’s grid is not just about transmission, but also about monitoring, optimization, and resilience.

Check out our article:
Transformer stations and substations: Power for progress behind the scenes

MV/LV transformers - voltage translators, without which nothing works

MV/LV transformers – the voltage interpreters that keep everything running

A transformer is a device that, in practice, converts voltage from medium level (e.g., 15 or 20 kV) to the usable 400/230 V. It may appear silent, massive, and unimposing. But in reality, it’s the most heavily loaded device in the local system – carrying thousands of amps through its windings every day.

For example, a 1000 kVA transformer powered by a 15 kV network at full load will generate a primary current of around 38.5 A, and on the secondary side (400 V) – 1443 A. This level of power demands not only top-tier resin or oil insulation, but also effective thermal protection, overcurrent protection, and regular diagnostics.

From a protection standpoint, the transformer is a critical point – any fault on the MV side must be cut off instantly to avoid overheating or arc flash explosions. That’s why relays and power circuit breakers or high-voltage fuses are always located nearby, depending on the size of the installation.

Explore our range of medium voltage transformers:
Energeks Offer

MV switchgear – the controlled command center

Medium voltage switchgear is a compact unit that integrates switching, protection, and measurement equipment. This is where routing decisions are made – whether power flows left or right, to building A or zone B. In modern distribution systems, switchgear is packed with technology: from current transformers and arc protection, to digital relays with Modbus, IEC 61850, or Profibus communication.

Depending on the application, we design:

AIS (air-insulated switchgear) – economical, space-intensive (typically: 1.5–2 m² per bay),
GIS (gas-insulated switchgear) – compact, safe, hermetically sealed, ideal for urban stations,
Hybrid or automation-integrated switchgear – acting as a "digital gatekeeper" of energy flow.

A well-designed switchgear allows for segment replacement without cutting power, reducing service downtime and increasing reliability. In the field, relay response time in MV switchgear (e.g., SEPAM, REF615, MiCOM) must fall within 40–80 ms to effectively limit arc fault consequences.

Learn more:
The ultimate 2025 buying guide for switchgear: What your vendor won’t tell you

Power circuit breakers – the border guards of your network

A power circuit breaker is not a “regular fuse.” It’s a sophisticated device that interrupts short-circuit currents of several thousand amps in a fraction of a second – without generating arcs, smoke, or destruction. Together with a relay, it decides whether and when to disconnect the supply to prevent transformer damage, winding burnout, or threats to human safety.

In medium voltage networks, the most commonly used are:

• vacuum circuit breakers (up to 36 kV) – fast, durable, and resistant to environmental conditions,

• 
  SF6 circuit breakers (for GIS) – compact, sealed, ideal for urban networks,

• 
  air-insulated breakers (older types) – less common but still found in industrial networks.

The choice of breaker depends on the network’s short-circuit capacity, rated voltage, and the number of switching operations per hour. In field conditions, the typical breaker mechanism response time is 30–60 ms, and its lifespan is measured in tens of thousands of cycles.

Without these four elements – stations, transformers, switchgear, and breakers – the medium voltage network cannot operate or protect. Each has its own language, constraints, and design requirements. But only by working together – as a well-coordinated team – can they guarantee what matters most: uninterrupted energy delivery and the safety of people.

Relay selection and selectivity in MV networks – how to do it right

Let’s begin with a fact that can’t be ignored: the medium voltage network does not forgive mistakes in protection logic. High power levels, rapid short-circuit current changes, dynamic source switching – it’s an environment where a relay reacting too fast or too slow can cost far more than replacing a device. It can lead to a full facility shutdown, a critical asset being taken offline, or worse – danger to human life.

And it all starts with one question: does the relay know when it should act – and does it only act when it’s supposed to?

Protection selection isn’t just “pick a model, enter a trip current, click save.” It’s a conscious alignment of relay function with the network topology, energy flow direction, and the timing of other system components. It’s about knowing “who is responsible for what” and programming that responsibility so that everyone acts in turn – neither too early, nor too late.

In a radial network – the most intuitive type, where current flows in a single direction – classic overcurrent protection 50/51 is often sufficient. Its role? React quickly to faults and do it with finesse – so the last point in the line has time to respond before the upstream device cuts power to everyone. Time differences between relays must be thoughtfully designed – often with 100–150 ms spacing, which in the world of faults and dynamic loads feels like an eternity. Poorly chosen times? The network reacts like dominoes. More customers go dark than necessary. And users remember the blackout – not the selectivity.

But close the loop – even partially – and everything changes. In a looped network, power can flow from two directions. That means a regular overcurrent relay isn’t enough. It can’t “see” where the fault is coming from. That’s where directional relays 67 come in. They analyze current and voltage vectors and know whether the fault is “on their side” or further down the line. Without them, your seemingly elegant loop will behave like a misconfigured radial system in a fault, shutting down both power sources simultaneously.

And if you’re designing a mesh network – where everyone can power everyone else, and current directions change in real time – you’re now in territory where basic protections can’t keep up. You’ll need intelligent, multifunctional IEDs that not only detect faults but also communicate with each other in milliseconds. These relays don’t decide alone – they consult other devices. They use GOOSE messages, SCADA systems, and analyze the state of the entire grid. They act exactly where and when needed.

In practice, relay response time in a well-designed medium voltage protection system must fall within 40 to 150 milliseconds. But that’s not all. Selectivity thresholds also matter – the difference between the current that trips relay A versus relay B. If the gap is too small – both relays act at once. Too large – and the fault travels farther than it should.

And one more thing: don’t forget earth fault protection. In MV networks with resonant grounding, earth faults are the silent killers – often non-dramatic but dangerous because they remain undetected for too long. That’s why 50N/51N relays, directional 67N units, and devices that detect zero-sequence components should be present anywhere reliability matters.

It all comes down to this – relay selection isn’t about choosing a device, but about designing logic. It’s a conversation between the relay, the grid, and the engineer. And only when that conversation is clear and precise can we be confident that in the moment of truth, the network will react as designed – and not as it “thought it should.”

Before you raise the voltage – make sure you know the structure of the medium voltage network

Effective protection begins with understanding. And understanding the medium voltage network means more than just knowing catalog specs – it’s about being aware of how energy moves through topologies, how breakers respond, and why relays operate when they do.

If you’ve made it this far, you’re among those professionals who design with intention and build systems with responsibility. These are the engineers who shape the industry – project by project, schematic by schematic.

And if you feel ready to move from theory to real solutions – check how we can help.

Explore our ready-to-ship transformer offer
Join the community of power engineers and enthusiasts on LinkedIn

Thank you for being with us. And remember – reliable protection begins where chance ends!

Sources:

Electrical Engineering Portal – Relay coordination for MV systems
Siemens Energy – Protection and substation automation

Cover Photo: Michael Kucharski