Voltage Transformer Testing

2. Voltage Transformer Testing

2.1 Tests to be conducted

1. Insulation Resistance measurement Test 
2. Secondary Winding Resistance Test 
3. Polarity Test 
4. Tan Delta & Capacitance measurement Test 
5. Ratio Test 

2.1.1 Insulation Resistance Measurement Test

This test is conducted in order to find out the insulation resistance value of VT, for any abnormal conditions such as presence of moisture, dirt, dust & crack in insulator and degradation of insulation.
0-5/10kV Insulation tester is used for measuring the IR value of a VT.
It will be measured across primary to earth, primary to secondary, across secondary winding & secondary to earth.
Before conducting this test, cables connected to the secondary marshaling box shall be removed. Apply 5kV/10kV voltage across various combinations mentioned above for 1 minute & measure the corresponding insulation resistance value.

2.1.2 Secondary Winding Resistance Test

This test is conducted to cross check the secondary resistance values measured at field with respect to the same measured at manufacturing place.
Multimeter with resistance measurement is used for measuring the secondary winding resistance of a VT.
Secondary winding resistance will be measured across various secondary winding of a VT.
Note down the readings & compare the same with respect to the factory results.

2.1.3 Polarity Test

This test is conducted to confirm the polarity markings of the primary winding matches with that of the secondary winding.
A DC battery & a galvanometer shall be used to carry out this test.
Connect the '+ve' terminal of the battery to the P1 terminal of the primary winding & connect the '-ve' terminal of the battery to the P2 terminal of the primary winding. Similarly, connect the '+ve' of the Galvanometer to the S1 terminal of the secondary winding & connect the '-ve terminal of the Galvanometer to the S2 terminal of the secondary winding. Briefly apply DC source and note down the deflection in the Galvanometer. 
If it deflects in the '+ve' direction, then the polarity of the VT windings are proper.

2.1.4 Tan-delta & Capacitance Measurement Test

Purpose of this dissipation factor measurement of high voltage insulation is to detect initial weaknesses in HV insulation & the tan delta and capacitance values shall be compared with factory test results.
Tan-delta test kit shall be used to carry out this test.
HV lead of the Tan-delta test kit shall be connected to one of the primary terminals and other lead to tan-delta point. Apply recommended voltage & measure the tan delta and capacitance value. Cross check this with factory test results.
Similarly, repeat the test by connecting leads of the kit across tan-delta point and ground.

2.1.5 Ratio Test

Purpose of this test is to check whether the ratio error between the primary and secondary voltages are within the acceptable value.
Primary Injection test kit and a Multimeter shall be used to conduct this test.
Connect the leads of the Primary Injection test kit across the primary terminals of the VT. Connect the leads of the Multimeter across the terminals of the secondary windings. Now apply the primary voltage gradually & measure the corresponding secondary voltage values using Multimeter.

The ratio between the applied primary voltage & the respective secondary voltage shall be with in the acceptable limit.