Turns ratio or Voltage ratio test of Transformer

Note: Kindly follow necessary safety rules for testing of transformer.

Transformer testing are mainly two types, one is type test and second is routine test. Transformer voltage ratio or turns ratio test is one type of routine test for transformers. Reference standard for this test is IS-2026.

Purpose:

Transformers have specified voltage rating on primary side and secondary side. We are conducting this test to verify voltage ratio of the transformer. If value of voltage transformer ratio does not come within limits, we can know about high resistance in leads of winding and tap changer connection. Shorting between turns of winding can also be detected by this test.

Voltage ratio equation of transformer:

Transformer voltage ratio= (Primary side voltage / Secondary side voltage)

Voltage ratio error:

The voltage ratio error should be within 0.5 %. Equation for error in voltage ratio is as below.

Error in voltage ratio= ((Calculated voltage ratio – Measured voltage ratio)/Measured voltage ratio) X  100 %

                                               

Test:

In this test we apply low AC voltage to primary side and measure the secondary side output voltage. AC low voltage is injected at high voltage side of transformer so that LV voltage will not be high for measuring and safety purpose. This test is done at no load position of transformer.

Suppose the transformer voltage ratio is 6.6 KV/ 433 V. The voltage ratio for this transformer as per division will come 15.24 for phase to phase value. Now suppose we are applying 415 V to primary side and we are getting 27.23 V from secondary output side then voltage ratio test is ok as 415/27.23 = 15.24.

For testing we are considering Delta to Star connected transformer with 5 taps, whose details are as below.

Tap number	HV voltage (Phase- Phase)	LV voltage(Phase- Phase)	Calculated Ratio (Phase- Phase)
1	6930	433	16.00
2	6765		15.62
3	6600 (Neutral tap)		15.24
4	6435		14.86
5	6270		14.48

Test steps:

1. This test is carried out on no load so first isolate the transformer from HV and LV side.
2. Put the transformer at first tap or last tap so that you will get voltage ratio values in ascending or descending order.
3. Connect the cables to HV and LV side of transformer for applying and measuring voltages. Make the cable connection without looseness.
4. Apply low AC voltage to HV side of transformer and measure the LV side voltage for phase to phase and phase to neutral. Here primary is delta connected and secondary is star connected so phase to neutral voltage will be 1/√3 of the phase to phase voltage for secondary side. 
5. Repeat the test for next tap up to last tap and record the values.

The values of voltage ratio of above transformer will nearly come as below. The voltage ratio error should be within 0.5 %.

Tap number	Applied HV voltage (Phase- Phase)	LV voltage(Phase- Phase)	LV voltage (Phase- Neutral)	Measured Ratio (Phase- Phase)
1	433	27.06	15.64	16.00
2	433	27.72	16.02	15.62
3	433 (Neutral tap)	28.41	16.42	15.24
4	433	29.14	16.84	14.86
5	433	29.90	17.28	14.48

Now a days, one electronic kit called as TTR (Transformer Turns Ratio) kit is available. By carrying out the test with this kit, the kit will automatically show the turns ratio or voltage ratio. So you don’t need to measure voltage either on primary or secondary voltage and calculation of it.