Choice of MV/LV transformer

From Electrical Installation Guide

The transformers shall comply with IEC 60076. A transformer is characterized by its electrical parameters, but also by its technology and its conditions of use.

Characteristic parameters of a transformer

Rated power = the apparent-power in kVA on which the values of the design parameters and the construction of the transformer are based. Manufacturing tests and guarantee refer to this rated power
Frequency = for power distribution systems discussed in this guide, the frequency is either 50 Hz or 60 Hz
Rated primary voltage = the service voltage of the electrical network on which the transformer in connected
Rated secondary voltage = the voltage measured between the secondary terminals when the transformer is off load and energized at its rated primary voltage
Transformer ratio = RMS value of the rated primary voltage divided by the RMS value of the rated secondary voltage
Rated insulation levels = are defined by the values of the overvoltage power frequency withstand test, and high voltage lightning impulse tests. For the voltage levels considered in this guide, the encountered switching over voltages are generally lower than the expected lightning over voltages, so no over voltage switching tests are required for these voltages
Off-load tap-Changer switch = allows to adjust the rated primary voltage and consequently the transformer ratio within the range ± 2.5 % and ± 5 %. The transformer must be de-energized before the operation of the switch
Winding configurations = Star, Delta and Zigzag high and low voltage windings connections are defined by an alphanumeric code read from the left to the right. The first letter refers to the high voltage winding, the second letter to low voltage winding:

  • Capital letters are used for the high voltage windings
    • D = delta connection
    • Y = star connection
    • Z = zigzag connection
    • N = neutral point brought out to a dedicated terminal
  • Lower-case letters are used for the low voltage winding
    • d = delta
    • y = star
    • z = interconnected-star (or zigzag)
    • n = neutral point brought out to a dedicated terminal
  • A number between 0 and 11 indicates the phase shifting between the primary and the secondary voltages.
  • A common winding configuration used for distribution transformers is Dyn 11:
    • High voltage primary windings connected in Delta
    • Low voltage secondary windings connected in Star
    • Low voltage neutral point brought out to a dedicated terminal.
    • Phase shifting between the primary and the secondary voltage: 30°.

Technology and utilization of the transformers

There are two basic types of distribution transformer:

  • Dry type (cast resin encapsulated) transformer
  • Liquid filled (oil-immersed) transformer.

According IEC 60076, the standard conditions of utilization of the transformers for outdoor and indoor installation are the following:

  • Altitude ≤ 1000 m
  • Maximum ambient temperature: 40 °C
  • Monthly average temperature: 30 °C during the hottest month
  • Annual average temperature: 20 °C.

For other service conditions

  • For oil immersed transformer the IEC 60076-2 specifies the oil and winding temperature rise.
  • For dry type transformer the IEC 60076-11 specifies the thermal class.

The temperature surrounding the transformer is linked to the outdoor service condition, its cooling mode and efficiency when installed in a room, and its load. Two loading guides can help to verify if the transformer is correctly defined according to the expected lifespan, which are respectively the IEC 60076-7 and IEC 60076-12.

An annex within the HV/LV prefabricated substation standard IEC 62271-202 gives several examples of installation, based on these two guides.

Dry type transformers

(see Fig. B38)

The dry type transformers shall comply with IEC 60076-11:

Each individual winding of these transformers is casted in resin according to a vacuum dedicated process.

The high voltage winding, the low voltage winding and the frame are separate by air.

The encapsulation of a winding uses three components:

  • Epoxy-resin based on biphenol A with a viscosity that ensures complete impregnation of the windings
  • Anhydride hardener modified to introduce a degree of resilience in the moulding, essential to avoid the development of cracks during the temperature cycles occurring in normal operation
  • Pulverulent additive composed of trihydrated alumina Al (OH)3 and silica which enhances its mechanical and thermal properties, as well as giving exceptional intrinsic qualities to the insulation in the presence of heat.
  • This three-component system of encapsulation gives insulation system temperature 155°C (F) with average winding temperature rise limit at rated current Δθ = 100 K which provides excellent fire-resisting qualities and immediate self-extinction. The moulding of the windings contain no halogen compounds (chlorine, bromine, etc.) and no other compounds capable of producing corrosive or toxic pollutants, thereby guaranteeing a high degree of safety to personnel in emergency situations, notably in the event of a fire.

These transformers are classified as nonflammable. Transformers exposed to fire risk with low flammability and self extinguishing in a given time.

They are also exceptionally well adapted for hostile industrial atmospheres and comply with the following class of environment:

  • Class E3: up to 95 % of humidity and/or high level of pollution
  • Class C3: utilization, transport and storage down to -50 °C.
Fig. B38 – Dry type transformer

Liquid-filled transformers

The most common insulating liquid used in these transformers is mineral oil, which also acts as a cooling medium.

Mineral oils are specified in IEC 60296, they must not contain PCB (PolyChlorinated Biphenyl).

Mineral oil can be replaced by an alternative insulating liquid such as high density hydrocarbons, esters, silicones, halogen liquids.

The oil being flammable, dedicated safety measures against fire are mandatory in many countries, especially for indoor substations.

The dielectric liquids are classified in several categories according to their fire performance. This latter is assessed according to two criteria (see Fig. B39):

  • The flash-point temperature
  • The minimum calorific power.
Fig. B39 – Categories of dielectric fluids
Code Dielectric fluid Flash-point (°C) Minimum calorific power (MJ/kg)
O1 Mineral oil < 300 -
K1 High-density hydrocarbons > 300 48
K2 Esters > 300 34 - 37
K3 Silicones > 300 27 - 28
L3 Insulating halogen liquids - 12

There are two types of liquid filled transformers: Hermetically-sealed totally-filled transformers and Air-breathing transformer.

Hermetically-sealed totally-filled transformers up to 10 MVA

(see Fig. B40)

For this type of transformers the expansion of the insulating liquid is compensated by the elastic deformation of the oil-cooling radiators attached to the tank.

The protection against internal faults is ensured by means of a DGPT device: Detection of Gas, Internal Over Pressure and Oil Over Temperature.

The "total-fill" technique has many advantages:

  • Water cannot enter the tank
  • Oxidation of the dielectric liquid with atmospheric oxygen is entirely precluded
  • No need for an air-drying device, and so no consequent maintenance (inspection and changing of saturated desiccant)
  • No need for dielectric-strength test of the liquid for at least 10 years
Fig. B40 – Hermetically-sealed totally-filled oil transformer

Air-breathing transformer

(see Fig. B41)

This type of transformer is equipped with an expansion tank or conservator mounted above the main tank. The expansion of the insulating liquid is compensated inside the conservator by the raising of the oil level.

A conservator is required for transformers rated above10 MVA which is presently the upper limit for "totally filled type transformers".

In the conservator the top of the oil is in contact with the air which must remain dry to avoid any oxidation. This is achieved by admitting the outside air in the conservator through a desiccating device containing silica-gel crystals.

The protection of breathing transformers against internal faults is ensured by means of a buchholz mounted on the pipe linking the main tank to the conservator.

The buchholz ensures the detection of gas emission and internal over pressure.

The over temperature of the oil is commonly detected by an additional thermostat.

Fig. B41 – Air-breathing oil transformer

Choice of technology

As discussed above, the choice of transformer is between liquid-filled or dry type. For ratings up to 10 MVA, totally filled units are available as an alternative to conservator type transformers.

The choice depends on a number of considerations, including:

  • Local regulations and recommendations. In some countries dry-type transformers are mandatory for specific buildings such as hospitals, commercial premises etc.
  • Risk of fire
  • Prices and technical considerations, taking account the relative advantages of each technology.

Determination of the optimal power

The over sizing of a transformer results in:

  • Excessive investment
  • Un necessarily high no-load losses
  • Lower on-load losses.

Under sizing a transformer causes:

  • A reduced efficiency when fully loaded. The highest efficiency is attained in the range 50 % - 70 % of the full load,
  • On long-term overload, serious consequences for the transformer, owing to the premature ageing of the windings insulation, and in extreme cases, resulting in failure of insulation and loss of the transformer.

Definition of optimal power

In order to select an optimal power rating for a transformer, the following factors must be taken into account:

  • List the consumers and define the factor of utilization ku and the diversity factor ks for each load as described in chapter A
  • Determine the load cycle of the installation, noting the duration of loads and overloads
  • Take into account all possible future extensions of the installation.
  • Arrange for power-factor correction, if justified, in order to:
    • Reduce billing penalties in tariffs based, in part, on maximum kVA demand
    • Reduce the value of the required apparent power: P(kVA) = P (kW)/cos φ
  • Select the transformer, among the range of standard transformer ratings available.

To avoid over heating and consequently premature ageing of the transformer, it is important to ensure that cooling arrangements and temperature rise of the transformer are adequate.


  • A wrong choice of the winding temperature rise or thermal class can be at the origin of a reduced lifespan.
  • A wrong assessment of the service conditions linked to the load profile can be at the origin of a reduced lifespan. Ex: Photovoltaic production where the load is during the day and when a 70°C maximum ambient temperature gradient is met as in Russia between winter and summer.