Test on Power Transformer Before Commissioning


There are following test to be performed before commissioning of power transformers as per Indian Standard Code.


  • Junction box and marshalling kiosks
  • Control panel, relay panel etc.

Primary injection test

  • To check operation and stability of earth fault relay on high voltage side and low voltage side
  • line directional elements of high voltage relays
  • Over-current relays on low voltage side
  • Over-current relay on high voltage side ( when current transformers are not in transformer bushings )
  • Earth fault relay on low voltage side
  • Test on high speed neutral ammeter
  • Voltage compensation

Secondary injection

  • Performed on transformer protection relay

Ratio test

  • When 415 V applied on high voltage side, measure the voltage between all phase on low voltage side for every tap position of high voltage
  • Measure the voltage

        A to a , b and c

       B to a , b and c

       C to a, b and c

 Where A, B and C are three phase terminals of high voltage side whereas a, b and c are terminals of low voltage side

  • Magnetic balance test

Tripping tests

  • High voltage
  • Low voltage
  • Inter-tripping tests
  • Winding temperature trips

Calibrate earth resistance


Buchholz relays

  • Check that there is no air in protector before commissioning
  • When energizing transformer, close in on – trip – etc.
  • Test for angle, air injection etc
  • Check for stability when oil pumps are started – at ambient temperature and at winding temperature of 80 degree or above

Alarm circuit

  • Buchholz relay
  • Oil and winding temperature thermometer
  • Cooling gear failure

Fan & pumps

  • Check that oil valves are open in cooling circuit.
  • Check that rotation of pumps, automatic starting overloads devices etc.
  • Check stability of Buchholz relay

Tap changing test

  • Check mechanism, indication, buzzer , lamp etc.

Phasing tests

  • At 415 V
  • Between transformers in a bank
  • To prove internal and external connections for parallel operation
  • On auxiliary supply and voltage transformers


Insulation test

  • High voltage winding and low voltage winding
  • On current transformer and voltage transformers

Oil test

  • Check oil level
  • Dielectric test
  • Volume resistivity
  • Acidity
  • Tan delta
  • Water content

Load test

  • Voltmeter and ammeter on both high voltage and low voltage side
  • Over current
  • No spill in high voltage star point
  • No creeping of contacts on both high and low voltage earth fault relays
  • Directional elements – low voltage directional over current and earth fault relays
  • Voltage compensation

Voltage compensation test ( If compensating CTs are fitted )

  • Primary injection test
  • Load test

Check neutral earthing switches are closed before connect with live


Check transformer on equal taps before switching in

  • For transformer in bank
  • For transformer in parallel

If necessary, arrange temporary protection for soaking and switching in ( example – unrestricted earth fault, if soaking from low voltage side 113 V and 213 V open )


Tap changing test on load over full range of taps


When transformer rating above 1000 MVA, energizing of transformer for the first time, voltage should be build up gradually


Low voltage excitation current measurement


Single phase magnetic balance test



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