Power Transformer are used for stepping-up the voltage for transmission at generating stations and for stepping-down for further distribution at substation. Usually naturally cooled, oil immersed, On type, two winding. three phase transformer are used up to the rating of 10 MVA. The transformers of rating higher than 10 MVA are usally Air-blast cooled.  For very high rating transformer the forced oil, water cooling or air blast cooling may used.

For regulating the voltage the transformers used are provided with on load tap changer. They are put in operation during load hours and disconnect during light load hour, that is they are usually operated at approximately full load condition. This is possible because they are arranged in bank and parallel with other  unit or disconnected at will.

Design: Power Transformer

So power transformer are designed to have maximum efficiency at or near full load condition. that is iron loss to full load copper loss ration is 1:1.

Power Transformer are designed to have consider leakage reactance than permissible in distribution transformers because in power transformers inherent voltage regulation is not as much important as current limiting effect of the higher leakage reactance.

Power Transformer usually make use of flux density of 1.5 to 1.77, have percentage impedance ranging from 6-18% and regulation 6-10%.


  1. KVA rating
  2. Rated voltage
  3. Number of phase (single phase or three phase)
  4. Type of Core ( shell type or core type )
  5. Tapping – if any
  6. Frequency (in INDIA 50hz, USA- 60hz)
  7. Construction ( Delta type or Star Type in case of three phase)
  8. Type of cooling ( air cooling, oil cooling)
  9. Ambient Temperature ( Average 40° C)
  10. Voltage Regulation
  11. No-load Current in ampere or percentage of rated current at at rated voltage and rated frequency.
  12.  Efficiency in percent at full load, half load, 3/4 load at unity or 0.8 pf (power factor).


Power Transformer are generally installed upon lengths of rail fixed on concrete slab having foundation 1 to 1½ meter deep.


Know about Transformer- click here

classification of transformer

Instrument Transformer


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