How to Test Electrical Protection - Power Transformer Stability Test

Introduction

Differential protection is the most important protection applied to Power Transformers. Its is unit type protection which detects fault in the area covered between primary and secondary side current transformers.
It is very important to check / prove stability of differential protection before charging to ensure protection will operate incase of fault inside protected zone and also, it won't operate otherwise.

Setup

Material / Equipment's required for test is as listed below :

Variac with high current rating is required to get proper CT secondary current in controlled manner to conclude the stability / instability present. 

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

As shown in setup scheme above, Test voltage is applied in between 'Circuit breaker Cable Side' contact and 'Primary side' CT. Temporary short circuit with sufficiently thick wire required to be placed on secondary side of the transformer after CT to ensure current flow in both primary & secondary winding of the transformer under test.

Measurement points are marked in Capital Letters.

• A, B, C : Input current to transformer primary in B, Y, R phases respectively

• D, E, F : Primary side CT Secondary Currents of R, Y, B phases respectively

• G, H, I : Secondary side CT Secondary Currents of R, Y, B phases respectively

Procedure 

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.
5. Now, measure currents at all point from "A" to "I" and ensure that current are in proportion and balance in all phases.
6. Measure current inside relay and note down as per table given below. Make sure to note down current magnitude as well as angle on both sides.

If CT polarity and relay connections are done properly, relay current angles for power transformer with Dyn1 vector group will be as per table below:

Above is the case for stable condition of differential protection. You can observe that differential current is zero and small restraining current will be there always.

Now, to check instability ( i.e. in zone fault operation), we need to reverse polarity of CT's one by one and note down readings foreach combination. 

Following are some sample readings of stability test of 8MVA transformer carried out at field with variac of 15A rating.

Similar workout can be done on secondary side circuit also. But its not needed normally.

Remember to remove shorting on secondary side of the transformer!

 

That's all with proving stability of power transformers. 

Feel free to contact in case of any problem / suggestions.

See you in next one.