How to choose a power transformer

How to choose the transformer power?

If a plant is directly supplied by a MV / LV transformer is to be supplied and the maximum apparent power load of the system has been determined, a suitable dimensioning for the transformer can be determined taking into account the following considerations:

  • The possibility of the Power factor to improve the system (see Chapter L)
  • Expected extensions to the installation
  • Installation restrictions (e.g. temperature)
  • Standard transformer ratings
Apparent power (kVA) In one)
- 237V -
- 410V -
100244141
160390225
250609352
315767444
400974563
5001218704
6301535887
80019491127
100024361408
125030451760
160038982253
200048722816
250060903520
315076734436

Number. 1 : Standard apparent powers for MV / LV transformers and associated nominal output currents

3-phase transformer

The rated full load current In on the low voltage side of a Three-phase transformer is given by:

where from:

  • P. a = rated kVA of the transformer
  • U = phase-to-phase voltage when idling in volts (237 V or 410 V)
  • I am in amps

Single phase transformer

For a single phase transformer:

where from

  • V = voltage between the low-voltage terminals when idling (in volts)

Simplified equation for 400 V (3-phase load)

The IEC standard for power transformers is IEC 60076.

Source: Schneider Electric

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