How to Test Electrical Protection - Stability Test of Motor High Impedance Differential Protection

Differential protection is generally applied to MV or HV motors with rating above 1000kW. This protection is normally of High Impedance Differential type which include simple Over current protection relay with 0.1A pick up and no time delay.

Material / Equipment's required for test is as listed in table.

Variac with high current rating is required to get proper CT secondary current in controlled manner to conclude the stability / instability present. If variac is not available, motor can be supplied from 3 phase 415V source.

Ensure actual CT Polarity is as marked on CT Terminals and scheme wiring is done correctly.

In case of Motor of 2500kW rating, motor will draw around 60 to 80Amp per phase i.e. 1/3rd of FLC appx. Make sure to use feeder of rating proper rating to ensure uninterrupted power throughout the test duration. Also, motor is going to rotate, so proper care should be taken to mark the area surrounding the motor for caution.

As shown in setup scheme above, Test voltage is applied in between 'Circuit breaker Cable Side' contact and 'Phase side' CT. AS previously mentioned, Test voltage can be injected through 3 phase variac or directly of source. Measurement points are marked in Capital Letters.

• A, B, C : Input current to Motors in B, Y, R phases respectively
• D, E, F : Phase side CT Secondary Currents of R, Y, B phases respectively
• G, H, I : Neutral side CT Secondary Currents of R, Y, B phases respectively
• J, K, L : Relay Current of R, Y, B phases respectively

Case 1 : Testing with Variac

1. Set variac output to 0V
2. Switch ON variac Input and start slowly increasing the output voltage
3. Keep monitoring current at points A, B, C. All currents should be balance.
4. Increase current slowly up to approximately 80% of the variac capacity i.e. 30Amp.
5. Now, measure and note down currents at all point from "A" to "L".
6. For CT of 300/1A ratio, CT secondary current will be 100mAmp in both sides of CT's and current through relay will be 0Amp.

Note Down Measurements

 If current through relay is not 0A, it implies one of the following issues exists

• CT specifications are not matching
• CT Polarity are not proper
• CT secondary wiring up to relay is not as required (i.e. CT secondary phases might be interchanged or CT is shorted through link or wrong wiring)

If current through relay is 0A in all phases, try shorting CT of any one phase on one side of the motor. This will make current flow through that particular phase. Swapping CT terminals will also flow current through relay. 

if relay is not tripping, then try reducing trip settings such that in come below relay current.

Case 2 : Testing without Variac

1. Switch ON the power supply
2. Motor will take current around 60 to 80Amps and will start rotating slowly
3. Ensure balance current is flowing through motor by measuring current at location A, B, C.
4. Measure and note down currents at all mentioned points.

Note Down Measurements

Repeat the procedure in case there is current or no current in relay for verifying stability.

This concludes stability verification test of MV / HV motors. 

Restore all shorting and setting changes to original before clearing for trial run.

Similar approach can be utilized for improving IR value of motor with blocked rotor condition. It evenly heats the winding showing results in short duration. Current will be a bit higher than during test.