How to do the transformer testing and what are the different types of transformer testing?

By testing a transformer we can confirm the performance and specification of a transformer. Some of the transformer testings will be done by the manufacturer and they are type tests and routine tests of the transformer. Certain types of tests will be carried out by the consumer to determine the ability and quality of the transformer. By testing the transformer we could obtain the equivalent circuit parameters. In case if the transformer is rewound with different primary and secondary windings then the equivalent circuit would change. So to determine the equivalent circuit parameters different types of tests should be conducted in a transformer.

What is the major objective of transformer testing?

  • Transformer quality could be determined
  • To make sure that the transformer has met the required customer standard
  • Testing can prevent accidents, if there is any component fault then we can determine that during the test
  • We can make sure that the transformer is capable to do its intended work

What are the preparations that must be done before doing the transformer test?

  • Check if the earthing connection in the transformer is rigid
  • Check the oil level
  • The air from the Buchholz relay and the bushing must be released
  • Clean the bushings
  • Check if the connections are properly done or not
  • The cables and shorting links must be of sufficient cross-section
  • Make sure that all the selected instruments have the required range
  • Make sure that the connections are done according to the diagram
  • Check if all the instruments are properly earthed

How to check a transformer when it is received from a factory?

If we receive a transformer from a factory or if the transformer is relocated to another location then we must verify the transformer properly. Make sure that the transformer is dry and also check for any damage during the shipping. Check the internal connection are proper and also make sure that the impedance, ratio, and polarity of the transformer is the same as in the nameplate.

What are the different types of transformer testing?

Factory tests

Routine test & Special test

Consumer site testing

Pre-commissioning test, emergency test, periodic condition monitoring test


  • Winding resistance measurement
  • Phase displacement measurement and voltage ratio measurement
  • Load loss measurement and short circuit impedance measurement
  • No-load loss and current measurement
  • Dielectric routine test
  • Load tap changer test


  • Special dielectric test
  • Transient voltage transfer characteristics should be determined
  • The zero sequence impedance measurement on a three-phase transformer
  • Determine the ability of the transformer to handle short circuit
  • Harmonic measurement of no-load current
  • Check the insulation resistance to the earth
  • Dissipation factor of the insulation system capacitance measurement


  • Insulation resistance test
  • Winding resistance measurement  
  • Voltage/ turn ratio measurement
  • Vector group/polarity test
  • Measurement of no-load loss & no-load current
  • Temperature variation test
  • Induced overvoltage withstand test


  • Resistance test of insulation
  • Dielectric absorption test
  • High potential test
  • Ratio test
  • Phase displacement test
  • Polarity test
  • Short circuit test
  • Resistance test of windings


The insulation resistance test of the transformer is done to determine that the all the insulation system of the transformer is proper or not. By conducting this test we could know if there is a low resistance path to earth or between windings to windings. Megger is used to do the insulation resistance test, in order to do the measurement of the LT to earth and HT we need to use 1000V, 2000mega ohms megger.

Procedure for the insulation resistance test of the transformer

  • Disconnect the line and neutral of the transformer
  • Connect the megger to the LV and HV sides to determine the insulation resistance between LV and HV windings
  • After that connect the megger leads to the HV bushing studs and also to the transformer tank earth point so that we can measure the IR value between the earth and the HV windings
  • Connect the megger leads to LV bushing studs and also to the transformer tank earth to determine the IR


This test is done to make sure that the rise of temperature of the windings won’t go beyond that of the specified limit during normal conditions. A thermometer would be used to do the temperature measurement of the transformer and hourly readings of the oil temperature will be taken. The inlet and outlet of cooler bank temperature will be taken hourly too and thus we can determine the mean oil temperature. While doing the temperature measurement we must consider that the top oil temperature doesn’t rise more than one degree Celsius during four consecutive hour readings. The last reading is done to know the final oil temperature rise.


This test would be helpful to determine the performance of the transformer, the transformer performance is mostly dependent on the voltage ratio.  The turn ratio of the transformer can be determined by the ratio test, this type of test will be conducted in the winding, before it is assembled in the tank.


  • The transformer tap changer should be in the lowest position and the LV terminals should be open
  • Apply the three-phase 415v supply to the HV terminals, then measure the voltage which is applied on each phase on the HV and LV
  • So after the measurement of the voltages in the LV and HV terminals, the tap changer of the transformer must be raised and should repeat the test too
  • This test should be separately done for all the tap positions


This test is done mostly for the three-phase transformer and this test is done to determine the imbalance in the magnetic circuit. In order to do this test first we should keep the tap changer of the transformer in a normal position and then the neutral of the transformer must be disconnected from the ground. After that apply the 230V Ac supply across one of the HV winding terminals and neutral terminals. HV terminal voltage must be measured regarding the neutral and then all three phases must be subjected to this test.


These tests should be done to determine if the losses and impedance are within the tolerance level. During this test, the LV windings will be shorted by shunt after that the current will be applied to the HV windings. The measurement must be done properly by using a voltmeter.


This test is done to determine the resistance of the winding and this test will be done by applying a small DC voltage to the windings and measures the current. By doing this test we can determine the proper ohmic resistance from each phase winding and the test will be conducted both in the high and the low voltages.


This test is conducted to determine the polarity of the transformer and to do this, two transformers are used. The low voltage windings and the high voltage winding is connected in parallel to each other. The transformer secondary winding has high voltage winding and this part will be connected to a fuse. The fuse which is used for this purpose would only have a very low rating. So when the autotransformer polarity is the same then the fuse won’t blow and when the polarity is different the fuse will blow off.


This test will be done to the high voltage part of the transformer and LV of the secondary will be short-circuited in this part. The HV/LV jumps must be removed and the neutral must be disconnected from the earth. The HV side must be energized by the LV supply. The supply voltage which is used to circulate the current through the transformer will be applied to the primary part of the transformer. The applied voltage will be really small when compared to the nominal voltage. So the reading of the wattmeter shows the full load copper loss. Measure the current in neutral, LV & HV line voltage, and also the HV line current.

What are the major devices that can be used to do the transformer protection?

  • HT fuse and DO fuse
  • LT circuit breaker
  • HT circuit breaker with earth fault relay tripping
  • Oil surge relay for OLTC
  • PRV for OLTC
  • Lightning arrester and HORN GAPS
  • Breather

What are the major causes of transformer failure?

  • Improper oil level
  • Seepage of water in oil
  • Prolonged overloading
  • Unbalanced loading
  • Termination fault
  • Prolonged short circuit
  • Installation check is not properly done
  • Operation fault of tap changer switch
  • Low quality of raw material
  • Design faults

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