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:
- This test is carried out on no load so first isolate the transformer from HV and LV side.
- Put the transformer at first tap or last tap so that you will get voltage ratio values in ascending or descending order.
- Connect the cables to HV and LV side of transformer for applying and measuring voltages. Make the cable connection without looseness.
- 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.
- 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.
