OVERVIEW:

Power transformer is a crucial asset in power system. Regular testing is done to check its performance and whether it fulfils the specifications. Manufacturer performs tests on transformers before delivering it and once it is delivered to desired location, again tests are performed to check for any loose connections or the insulation is not bridged. Once, the results are confirmed, transformer is ready to be energized and put into operation. This blog post provides with the basic procedures and tests that may be required to perform on transformers.

TYPES OF TRANSFORMER TESTS:
Transformer tests are classified based on the test performed at site or at the factory. Tests done at site includes pre-commissioning test, emergency tests and condition monitoring tests. Tests done at the factory are routine tests, special tests and type tests.

TYPE TESTS:
Type tests for a transformer are done at the factory to check if the transformer meets the customer’s specifications and design expectations. These tests are carried out on a prototype and not performed on every transformer produced. These tests ensures that the design criteria of the entire production are fulfilled.

TYPE TESTS OF TRANSFORMER INCLUDE:
1.
Winding resistance test
2.
Ratio test
3.
Vector group test
4.
Impedance voltage/short circuit impedance and load loss test
5.
Measurement of no-load loss and current
6.
Insulation resistance test
7.
Dielectric test
8.
Temperature rise test
9.
Tests on on-load tap-changer
10.
Vacuum tests on tank and radiators

ROUTINE TESTS:
Routine tests are performed on transformer to ensure that it works correctly and meets the necessary standards.
ROUTINE TESTS OF TRANSFORMER INCLUDE:
1.
Winding resistance test
2.
Transformer ratio test
3.
Vector group test
4.
Impedance voltage/short circuit impedance and load loss measurement
5.
Measurement of no-load loss and current
6.
Insulation resistance test
7.
Dielectric test
8.
Tests on on-load tap-changer.
9.
Oil pressure test on transformer

SPECIAL TESTS:
Special tests are carried out on transformers based on customer requests. These tests provide important information to the customer for operating and maintaining the transformer.

SPECIAL TESTS OF TRANSFORMER INCLUDE:
1.
Dielectric tests.
2.
Measurement of zero-sequence impedance of three-phase transformers
3.
Short-circuit test
4.
Measurement of acoustic noise level
5.
Measurement of the harmonics of the no-load current.
6.
Tests on Buchholz relay, temperature indicators etc.

Insulation Resistance Test:
The insulation resistance test is carried out to ensure the safety and reliability of the transformer. This test is carried out using methods like megger test, polarization index test and step voltage test. This test checks the insulation of the transformer to ensure it is in good condition. The insulation resistance should be at least 100 times the operating voltage of the transformer. Common procedure to perform this test is as follows:
1.
Isolate the transformer means disconnect the line and neutral terminal.
2.
Test insulation resistance using megger between windings and winding and the earth (transformer’s body).
3.
Perform the test and record the result.

The results are obtained at intervals of 15 seconds, 1 minute and 10 minutes. Constant DC voltage is applied to check if insulation can withstand the pressure and its value increases with the duration of applied voltage. There are other tests to check the insulation of transformers, like the lightning impulse test, switching impulse test, and partial discharge test.

Transformer Ratio Test:
The Transformer Turns Ratio test measure the turns ratio between the windings according to IEC 60076-1 and IEEE C57.12.90. standards. The results shall match to the nameplate ratings to see if there’s any insulation damage, shorted turns etc. During this test, a 415 V supply is applied to the high-voltage winding while the low-voltage winding is left open. The voltage is measured at both the high-voltage and low-voltage terminals to find out the actual transformation ratio of the transformer. Measured voltage ratio should be within 0.5% of the nameplate specification. General procedure to perform this test is as follows:

Set the tap changer to the lowest position and LV terminals open.
2.
Connect a 3-phase supply to the HV terminals. Measure the voltage between phases on the HV side and also the induced voltages at the LV terminals.
3.
Now increment the tap changer position and repeat the above step.
4.
Repeat this process for each tap position.

BUSHING POWER FACTORING (AC Hi-Potential Insulation Testing):
Test the bushings before inserting into the transformer. Using a Power Factor set or a capacitance bridge to measure the capacitance values of the bushing taps so that any internal problem is identified. Measure both C1 and C2 values, that are specified on the bushing nameplate. Megger can be used to test the bushing and its tap at 2500 volts. Check the bushing tap insulation rating before applying 2500 volts. Some small bushings may only be able to withstand 500 or 1000 volts at the tap.


TEMPERATURE RISE TEST OF TRANSFORMER:

The temperature rise test ensures the temperature of the windings and the oil inside it stays within the specified limits. This test is carried out using winding temperature indicators (WTI) and oil temperature indicators (OTI) to measure the temperatures. In this test, the rated voltage and frequency are applied to the primary side of the transformer. A short circuit is created in the secondary winding to allow the rated current to flow through it. This current causes the winding temperature to increase over time. The test continues until both the windings and the oil inside the transformer reach a stable temperature, and must be within the acceptable limits set by the manufacturer. The time taken to reach the stable temperature will determine the thermal time constant of the transformer, and the final temperature indicates the temperature rise of the transformer when operating at full load.