1. Testing of Transformers

2. Preliminary Tests
Following tests are carried out in the works at different stages, before the core and the coil assembly is placed in its tank
(1) Core insulation: After the core is assembled, 2kV test is done to ensure that the insulation between clamp plates, core bolts and core is adequate
(2) Core loss test:
Some turns are wound over the core and it is energised at normal flux density. Core loss and magnetising current are noted and compared with design values.

3. In order to get accurate ratio, a ratiometer is employed
(c)Check of Ratio,Polarity, Vector relationship and winding resistance of Transformer Assembly
Ratio test is conducted to ensure the correctness of voltage ratio between different windings on each tapping
The tolerance for ratio is ±0.5% of the declared ratio or ±10% of the percentage impedance voltage, whichever is smaller
In order to get accurate ratio, a ratiometer is employed
It also indicates the polarity of transformer windings

4. For a three phase transformer, it is more usual to carry out a vector relationship test
In this test, one of the high-voltage and low-voltage line terminals are joined together as shown in fig
Three-phase 400V supply is connected across high voltage line terminals
Voltage between terminals 2U-1N, 2V-1N,2W-1N,2V-1V,2W-1W and 2V-1W are measured

5. For YN, d11 vector relationship
2U-1N > 2V-1N > 1U-1N
And 2V-1W > 2V-1V or 2W-1W
The vector relationship for any other group can be checked in a similar manner
The dc resistance of each winding is measured by Kelvin’s double bridge to check that there is no faulty joint

6. 2U-1N > 2V-1N > 1U-1N
And 2V-1W > 2V-1V or 2W-1W

7. Final Tests
The completely assembled transformer is tested in accordance with the International standards.
Routine tests:
Measurement of winding resistance
Measurement of voltage ratio and check of voltage vector relationship
Measurement of impedance voltage (principal tapping), short circuit impedance and load loss
Measurement of no load loss and current
Measurement of insulation resistance
Dielectric tests
Tests on-load tap changers

8. Type tests:
All the tests listed as above and the temperature rise test and Dielectric type tests

9. Special tests Dielectric special tests
Measurement of zero-sequence impedance of three phase transformers
Short-circuit test
Measurement of acoustic noise level
Measurement of harmonics of the no load current

10. Special tests Measurement of power taken by the fans and oil pumps
Measurement of capacitances between
windings to earth and between windings
Measurement of transferred surge voltage on
low voltage windings
Measurement of insulation resistance to earth
of the windings, or measurement of dissipation factor of the insulation system capacitances

11. 1. Measurement of winding resistance:
For calculation of I2R losses in the winding, it is necessary to measure dc resistance of each winding
The resistance measurement should be done after the direct current circulating in the winding has reached a steady state
In some cases, it may take some minutes depending upon the winding inductance

12. Temperature of the winding must be stable and for this reason, this test is usually carried out before load loss measurement
The average oil temperature is determined as the mean of top and bottom oil temperatures and is taken as average winding temperature

13. 2. Measurement of voltage ratio and check of voltage group
These tests are conducted to check that all connections to the bushings, tap changers etc. have been made correctly during final assembly.

14. 3. Measurement of impedance voltage and load loss:
Load loss = I2R losses in the winding + stray losses due to eddy currents in conductors, clamps and tank
Stray losses vary with frequency
Hence in this test, supply is given to the transformer at rated frequency
The load loss and impedance voltage are guaranteed at 75oC but are measured at the ambient temperature of the test room
Measured load loss is corrected to reference temperature (75oC)

15. I2R losses are directly proportional to resistance, which varies with temperature
Stray losses vary inversely with temperature
The test is carried out by short circuiting, usually the LV winding and by supplying the impedance voltage to HV winding
The measured power will also include small core-loss.
Since the supply voltage during the test is a small fraction of normal voltage, this loss can be ignored

16. For a high impedance transformer ( Z > 15%) the core loss may become appreciable and can be deducted after separately measuring it at impedance voltage
As per IS: 2026(Part I) -1977, the measurements can be made at any current between 25% to 100%, but preferably not less than 50% of the rated current
Load loss and impedance voltage can be corrected for rated MVA as below

17. %Z = (test voltage/rated voltage) x (rated current/test current)x100
Computed loss at rated current =
measured loss at rated current x ( rated current/test current)2
%Z = (test voltage/rated voltage) x (rated current/test current)x100

18. While measuring load loss and impedance at different tap positions, readings should be taken quickly, and the interval between measurements at different taps should be adequate to avoid significant errors due to momentary temperature rise of windings
The difference in temperature between the top oil and bottom oil should preferably be small to enable the average temperature to be determined accurately

19. Three wattmeter method should be used instead of two wattmeter method to avoid large value of wattmeter multiplier constant
The power factor during load loss test can be less than 0.1 and wattmeters suitable for such low power factors should be used
Tolerances

20. 4. Measurement of no load loss and current
To assess the efficiency of transformer
Also as a check that high voltage tests have not caused any damage to the winding insulation
For large transformers, no-load loss measurement is carried out before and after completion of dielectric tests
Test is carried out rated frequency feeding usually LV winding
I2R will be negligible
p.f for medium power transformers - around 0.3
p.f for large power transformers - around 0.1

21. Core losses = hysteresis losses + eddy current losses
Hysteresis losses dependent on average value of voltage
Eddy current losses dependent on rms value of voltage
In this test, two voltmeters are used
(1) bridge rectifier type to indicate average voltage
(2) dynamometer type to indicate rms voltage

22. Supply is set such that specified value is indicated on the average voltmeter
Hysteresis component of no load loss is measured correctly
Eddy current component will be either lower or higher than the true value, depending upon the form factor of the supply voltage
Ratio between two components is known for any particular quality of core steel

23. Where, P = no-load loss for sinusoidal voltage
Pm = the measured no-load loss
P1 = ratio of hysteresis losses to total iron losses
P2 = ratio of eddy current losses to total iron losses
K = ( rms voltage / ( 1.11*average voltage) )2
For normal flux densities and frequencies of 50Hz and 60Hz, P1 and P2 are each 0.5 for grain oriented steel
P1 and P2 are 0.7 and 0.3 , respectively, for no-grain oriented steel

24. 5. Measurement of Insulation Resistance
IR is measured between all windings and tank with a megger
IR varies inversely with temperature
Thus, oil temperature is also recorded
IR at 15th second and 60th second are noted for determining Polarization Index