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Home / Technical Articles / MV/LV transformers – The most common power supply in low voltage networks

MV/LV Transformers, where everything starts…

The general term power supply in LV networks refers to the supply of electrical energy. The power supply, and more generally the different supplies, are provided by sources (mains supply, batteries, generator sets, etc.) which can be MV/LV transformers, diesel generator sets, and UPSs.

MV/LV transformers - The most common power supply in low voltage networks
MV/LV transformers - The most common power supply in low voltage networks

This technical article will explain the most common power supply for LV networks provided by an MV/LV transformer. Don’t be confused, the same transformer with or without some modifications can be used also as a backup power supply, special power supply for safety services, or auxiliary power supply.

Let’s see the most common power supply source – transformers that are used in MV/LV networks.

We won’t deal with the basics of a transformer. Well, only one sentence… The transformer is an electric electromagnetic induction machine whose function is to transfer electrical power between two different voltage systems at the same frequency.

MV/LV transformers are generally divided into three types depending on their construction: Oil, Air-insulated and Resin insulated dry-type transformers.


  1. Oil transformers
  2. Air-insulated transformers
  3. Resin insulated dry-type transformers
    1. Applications
  4. Medium-voltage winding
  5. Characteristics of MV/LV transformers
  6. Primary and Secondary Connection Configurations
    1. Time index
    2. MV/LV transformer common couplings
    3. Coupling group

1. Oil transformers

The magnetic circuit and the windings are immersed in a liquid dielectric that provides insulation and evacuates the heat losses of the transformer.

This liquid expands according to the load and the ambient temperature. PCBs and TCBs are now prohibited and mineral oil is generally used. It is flammable and requires protective measures against the risks of fire, explosion, and pollution.

The most commonly used protective measures are the DGPT or the DGPT2: Gas, Pressure, and Temperature sensor with 1 or 2 sensing levels on the temperature. This system cuts off the LV load (1st level) then the MV supply (2nd level) when there is a fault inside the transformer. A holding tank is used to recover all the liquid dielectric.

Of the four types of immersed transformer:

  1. Free breathing transformers,
  2. Gas cushion transformers,
  3. Transformers with expansion tank and
  4. Transformers with integral filling, only the latter are currently installed.
An example of oil transformer with integral filling
Figure 1 – An example of oil transformer with integral filling

Structural standards for immersed transformers

  • Power from 50 to 2500 kVA (25 kVA possible):
    • Primary voltage up to 36 kV
    • Secondary voltage up to 1.1 kV
  • Power > 2500 kVA:
    • HV voltage greater than 36 kV
  • IEC 60076-1, IEC 60076-2, IEC 60076-3, IEC 60076-4, IEC 60076-5

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1.1 Free breathing transformers

A quantity of air enters the surface of the oil and the cover allows the liquid to expand with no risk of overflowing. The transformer “breathes”, but the humidity of the air mixes with the oil and the dielectric strength deteriorates.

Free breathing transformer
Figure 2 – Free breathing transformer

1.2 Gas cushion transformers

The tank is sealed and a cushion of neutral gas compensates for the variation in volume of the dielectric (risk of leak).

Gas cushion transformer
Figure 3 – Gas cushion transformer

1.3 Transformers with expansion tank

To limit the previous disadvantages, an expansion tank limits the air/oil contact and absorbs the overpressure.

However the dielectric continues to oxidise and take in water. The addition of a desiccant breather limits this phenomenon but requires regular maintenance.

Transformer with expansion tank
Figure 4 – Transformer with expansion tank

1.4 Transformers with integral filling

The tank is completely filled with liquid dielectric and hermetically sealed. There is no risk of oxidation of the oil.

Transformer with integral filling
Figure 5 – Transformer with integral filling

The overpressure due to the expansion of the liquid is absorbed by the folds of the tank.

The overpressure due to the expansion of the liquid is absorbed by the folds of the tank
Figure 6 – The overpressure due to the expansion of the liquid is absorbed by the folds of the tank

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2. Air Insulated Transformers

The windings of air transformers are insulated by means of the wrapping of the windings themselves, the mounting of plastic partitions and compliance with adequate insulation distances.

These types are of limited use, because their specific construction characteristics make them very sensitive to humidity, to even limited pollution and to chemically aggressive substances. In fact the absorption of humidity and the deposit of dusts can lower the dielectric coefficient of the insulating materials used.

A careful commissioning procedure must thus be followed, so as not to affect operation, such as the drying of the windings by means of heating elements installed on the transformer.

Air Insulated Transformer
Figure 7 – Air Insulated Transformer

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3. Cast Resin Transformers

Dry-type transformers, with one or more enclosed windings, are usually called cast resin transformers. These types, due to developments in construction techniques, are more and more widely used because of their reliability, their lower environmental impact compared to oil transformers, and because they reduce the risks of fire and spreading polluting substances in the environment.

Medium-voltage windings, made with wire coils or, even better, insulated aluminium strips, are placed in a mould into which the epoxy resin is poured under vacuum, to avoid inclusions of gas in the insulation. The windings are then enclosed in a cylindrical enclosure, which is totally sealed, mechanically strong and has a smooth surface which impedes both the deposit of dust and the action of polluting agents.

Low-voltage windings are generally made of a single aluminium sheet, the same height as the coil, insulated by suitable material and heat treatment.

Cast resin transformers use class F 155°C insulating material, allowing for a maximum temperature rise of 100°K.

Dry-type cast resin transformer
Figure 8 – Dry-type cast resin transformer

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3.1 Applications

Cast resin transformers are used in a wide range of applications and represent the most reliable answer for distribution systems, power production, rectification, traction and for special requirements.

Distribution of electrical power
  • Service sector: hospitals, banks, schools, shopping and cultural centres
  • Infrastructures: airports, military installations, ports and off-shore installations
  • Industry in general

Conversion and rectification
  • Air conditioning systems
  • Continuity units
  • Railways, underground railways, tramways and cable cars
  • Lifting and pumping systems
  • Welding lines
  • Induction furnaces
  • Naval propulsion

Step-up transformers for power production
  • Wind parks
  • Photovoltaic systems
  • Cogeneration systems
  • Industrial applications

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4. Medium-voltage winding

The technology used to make the MV windings of strips, rather than of wire, puts less stress on the insulation between the turns.

In traditional windings, made with a circular-section conductor, each layer of the winding is made up of a number n of turns side by side.

In windings made with strip conductors, each layer is made up of just one turn. If the voltage of a single turn of a winding is denoted by us, in strip windings the voltage between turns belonging to two adjacent layers is always us, while in traditional windings this voltage assumes the maximum value of (2n – 1) × us, as shown in the diagram below.

Left: Winding made with wire conductors: The voltage increases with the number of turns; Right: Winding made with strip conductors: The voltage is divided uniformly.
Figure 9 – Left: Winding made with wire conductors: The voltage increases with the number of turns; Right: Winding made with strip conductors: The voltage is divided uniformly.

Transformers with strip windings thus have a greater resistance capacity to impulse voltages and at industrial frequencies, as well as a lower probability of occurrence of localised partial discharges. Strip winding also has the advantage of drastically reducing the axial forces due to short-circuit currents.

Division of the voltage between the turns of the medium-voltage winding.

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5. Characteristics of MV/LV transformers

Table 1 – Standard characteristics

Standard characteristics of MV/LV transformers
Table 1 – Standard characteristics of MV/LV transformers

Table 2 – Characteristics connected with the construction method

Characteristics connected with the construction method
Table 2 – Characteristics connected with the construction method

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6. Primary and Secondary Connection Configurations

Symbols used to designate the connections. Internal windings may be connected in star, delta or zigzag configuration. Depending on the connection method the system of induced voltages on the low-voltage side is out of phase with respect to the average voltage by angles which are multiples of 30°.

The winding connection method is identified by 3 letters (upper case for the primary and lower case for the secondary):

  • Y – star connection
  • D – delta connection
  • Z – zigzag connection
MV/LV transformer connection designation
Figure 10 – MV/LV transformer connection designation

Associated with these letters are numbers which represent the phase shift, dividing it into 4 groups:

  1. Group 0 – no phase shift
  2. Group 11 – 330°
  3. Group 6 – 180°
  4. Group 5 – 150°
The choice of the transformer switch-ON unit is one of the important factors for determining the operating regime as a function of the load. The ideal condition is when the load is balanced on all the phases, but this condition is often impossible to obtain.

For this reason it is necessary to know the phase shift between primary and secondary phases. The table below shows the typical insertion diagrams.

Table 3 – Typical MV/LV transformer connection configurations

MV/LV transformer connection configurations
Table 3 – MV/LV transformer connection configurations

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6.1 Time index

The designation of the connections (by letters) has an additional number that indicates the angular phase shift, for example Yy6, Yd11, Ynyn0 (external neutral).

Rather than expressing the phase shift angle between the primary/secondary voltage vectors (pole by pole or phase by phase) in degrees, a more descriptive method is used: the time index.

The voltage vector on the primary side is assumed to be located at midday. The time index indicates the position of the time at which the corresponding vector is located on the secondary side.

Example: Time index 5 (phase shift 150°)

Time index 5 (phase shift 150°)
Figure 11 – Time index 5 (phase shift 150°)

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6.2 MV/LV transformer common couplings

Transformer vector groups connections
Figure 12 – Transformer vector groups connections

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6.3 Coupling group

For two three-phase transformers to be able to operate in parallel, they must have:

  1. A ratio of their power < 2
  2. The same technical characteristics (transformation ratio)
  3. The same short-circuit characteristics (% of voltage)
  4. Compatible star or delta connections
  5. Identical time indices (terminal to terminal links) or belonging to the same coupling group if the operating state is balanced.

Parallel operation of transformers from different groups is possible by modifying connections, but they must be submitted for the approval of the manufacturer.

MV/LV transformer coupling group
Figure 13 – MV/LV transformer coupling group

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Source: Power balance and the choice of power supply solutions – Legrand

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Mcgillelectric
    Feb 17, 2021

    Do you have any article about Buck-Boost Transformers?

  2. José leiva
    Oct 28, 2019

    Excelent information for electrotecnic !!

  3. Kyaw Aung Naing
    Oct 28, 2019

    can I get PDF files for this.

  4. Ralph Ogerly
    Aug 05, 2019

    Thank you for all the material.I love it.It helps me understand and learn more.I love the field.

  5. Industrial Automation Specialist
    Jun 25, 2019

    Excellent reading.very beneficial to our young generation engineers and technicians.

    Jun 25, 2019

    The information are basic & comprehensive for Electrical Engineers to be kept in mind before entering in the field. This enable to approach the appropriate selection of MV/LV Transformers to it’s use. The application is equally imperative for senior network electrical engineers working with Utilities.

  7. Baharak Daneshnia
    Jun 25, 2019

    Document you share , are very useful to me thank u so much

  8. Haitham
    Jun 24, 2019

    Thanks for your efforts and helpful information.

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