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Home / Technical Articles / Selection of distribution transformers for supplying power to LV networks

Distribution transformers (50-2500 kVA)

The characteristic data of distribution transformers are determined by the requirements of the network. The effective power determined must be multiplied by the power factor cosφ to give the rated power SrT. In distribution networks the value uk= 6% is preferred.

Selection of distribution transformers for supplying power to LV networks
Selection of distribution transformers for supplying power to LV networks (photo credit:

Transformer losses are made up of no-load losses and short circuit losses. The no-load losses are caused by the continuous reversal of magnetization of the iron core and are practically constant and independent of loading. The short circuit losses are made up of the ohmic losses in the windings and losses due to leakage fields. They are proportional to the square of the loading.

Oil-filled transformers and cast resin dry-type transformers are preferred. The use of askarel transformers is forbidden!

In this technical article, the most important criteria for the selection of distribution transformers in the power range from 50 to 2500 kVA for supplying power to low voltage networks will be discussed.

  1. Requirement of operational safety:
    1. Routine tests (losses, uk, voltage test)
    2. Type testing (heating, surge voltage)
    3. Special tests (short circuit strength, noise)

  2. Electrical conditions:
    1. Short circuit voltage
    2. Connection symbol / vector group (learn more)
    3. Transformation ratio

  3. Installation conditions:
    1. Interior and outside installation
    2. Special local conditions
    3. Environmental protection conditions
    4. Designs: oil-filled or cast resin dry-type transformer

  4. Operating conditions:
    1. Loading capacity (oil-filled transformers or cast resin dry-type transformers)
    2. Load fluctuations
    3. Number of hours in operation
    4. Efliciency (oil-filled transformers or cast resin dry-type transformers)
    5. Voltage regulation
    6. Parallel transformer operation (learn more)

  5. Characteristic data for transformers with examples:
    • Rated power SrT = 1000 kVA
    • Rated voltage UrOS = 20 kV
    • Lower side voltage UrUS = 0.4 kV
    • Rated lightning impulse withstand voltage UrB = 125 kV
    • Loss combination
    • No-load losses P0 = 1700 W
    • Short circuit losses Pk = 13000 W
    • Acoustical power LWA = 73 dB
    • Short circuit voltage ukr = 6%
    • Transformation ratio PV/SV = 20 kV/0.4 kV
    • Connection symbol Dyn5
    • Termination systems, e.g. lower voltage and upper voltage side flange systems
    • Interior or outside installation
Interior installation of liquid-filled transformers
Figure 1 – Interior installation of liquid-filled transformers


  1. Cable conduit
  2. Zinc-plated flat steel grate
  3. Exhaust opening with protective grate
  4. Unscrewed conduit with pump
  5. Ramp
  6. Air intake opening with protective grave
  7. Gravel or crushed rock layer
  8. Ledge

The installation of transformers should be free of underground water and flooding. The cooling must be protected against sunlight. Fire protection measures and environmental compatibility must also be ensured. Figure 1 illustrates a transformer with oil filling < 1000 liters. Here, an impermeable floor is suflicient.

For oil filling > 1000 liters, oil collecting troughs or oil sumps are mandatory.

The heat losses of transformers must be properly dissipated! This requires air intake and exhaust openings. The air intake must flow in underneath the transformer close to the floor and the exhaust must be led upwards.

The size of the exhaust opening is shown without grate in Figure 2 for a room heating of 15 K.

Ventilation for the interior installation of a cast resin transformer
Figure 2 – Ventilation for the interior installation of a cast resin transformer
Permissible overloading of Siemens GEAFOL cast resin transformers
Figure 3 – Permissible overloading of Siemens GEAFOL cast resin transformers

Pv = P0 + k × Pk75 [kW]

The meanings of the symbols are:

  • A – Air exaust opening and intake opening
  • Pv – Transformer power loss
  • k = 1.06 for oil-filled transformers
  • k = 1.2 for cast resin transformers
  • Po – No-load losses
  • Pk75 – Short circuit losses at 75° in kW
  • h – Difference in height in meters
Figure 3 gives the permissible overload capacity of cast resin transformers in the power range from 400 to 2500 kVA. The determination of the loading capacity of oil-filled transformers can be made on the basis of IEC 60076-1.

The heating losses which arise while operating a transformer (Figure 4) must be dissipated. If it is not possible owing to the conditions for installation to utilize natural ventilation, it is necessary to install a fan. The overall temperature of the transformer must not exceed 40°C.

Ventilation of the transformer room
Figure 4 – Ventilation of the transformer room

The overall losses in a transformer room are given by:

Qloss = ∑ Ploss

Ploss = P0 + 1.2 × Pk75 × (SAF / SAN)2

The total losses are dissipated through:

Qv = Qloss1 + Qloss2 + Qloss3

The individually dissipated amounts of heat can be calculated from the following:

Natural air current: Qloss1 = 0.098 × A1.2 × √HΔυL3

Losses dissipated by forced air currents (Figure 3):

Qloss3 = VL × CPL × ρ

Losses dissipated through walls and the ceiling (Figure 4):

Qloss2 = 0.7 × AW × KW × ΔυW + AD × KD × ΔυD

The meanings of the symbols are:

  • Pv – Transformer power loss in kW
  • Qv – Dissipated losses in kW
  • QW,D – Dissipated losses through walls and ceiling in kW
  • AW,D – Area ofwalls and ceiling in m2
  • KW,D – Heat transfer coeflicient in kW/m2K
  • SAF – Power for the cooling type AF in kVA
  • SAN – Power for the cooling type AN in kVA
  • VL – Air flow rate in m3/s, m3/h
  • Qv1 – Part dissipated in natural air current in kW
  • Qv2 – Part dissipated through walls and ceiling in kW
  • Qv3 – Part dissipated in forced air current in kW
The transformer noise is a combination of magnetic noise and the noise of additional ventilation.

Figure 5 shows the noise level of different transformers according to IEC Publication 551. The magnetic noise is the result of oscillations of the iron core (induction-dependent) and depends on the material properties of the core laminations.

Sound pressure level of transformers
Figure 5 – Sound pressure level of transformers

The acoustical power (Figure 6) is a measure of the noise level produced by an acoustical source.

Acoustical power level of transformers
Figure 6 – Acoustical power level of transformers

50KVA distribution transformer 20Kv-400V

Reference // Analysis and design of low voltage power systems by Ismail Kasikci (Purchase hardcover from Amazon)

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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.

One Comment

  1. Mackey Fernandes
    Sep 28, 2019

    Sharing valueable knowledge

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