Tuesday, March 18, 2014

Unused Taps of CT and Differential Relay


This is a case study of the unusual false tripping of a 110/11 kV, 15 MVA transformer in a large chemical plant. As a part of renovation, the old air-blast circuit breaker and current transformer were replaced as a retrofit. Brand new SF6 breaker and CT (both Crompton Greaves make) were installed on the 110 kV side. After commissioning, the load on the transformer was very low, normally about 1 MW as the transformer was feeding power to a section of the plant which was mothballed for a long time. When it was paralleled with two other similar transformers and loaded, it tripped on differential protection, causing production loss. Tests of insulation resistance showed no problem and after two spurious trips, it was decided to conduct a thorough study of the issue.



The details of the current transformer (CT) was 150/75/1 A, with three cores and three windings – The Class 0.5 winding for metering, Class 5P20 for protection and Class PS for differential relay. It was a multi-tap CT, with the terminal connections as shown below.




Nobody knew how the shorting link between 75A and 150A tap came about. It was assumed that since the CT came from the manufacturer with the terminals short-circuited, the commissioning staff might have let the shorting in place in order not to keep any portion of the CT winding in open circuit. It was suspected that this shorting link might be the cause of false tripping. A search on the Internet showed some examples where similar tripping of differential relay had occurred due to identical causes. But, nobody came out with a theoretical explanation or a practical illustration of the phenomenon. Those people simply declared that when they removed the shorting link, the differential relay did not trip thereafter.



So we decided to demonstrate what would happen with the shorting link in place. We conducted a primary current injection test on the CT terminals and measured the secondary current to the differential relay and in the shorting link. The readings are tabulated below.



Case A: With Shorting Link



Primary Injected Current (A)
Current to Differential Relay (A)
Current in Shorting Link (A)
30
0
0.37
60
0.02
0.65
75
0.2
0.78

 Case B: Without Shorting Link



Primary Injected Current (A)
Current to Differential Relay (A)
30
0.35
60
0.77
75
0.98



It is easily seen that practically no current flows to the differential relay when shorting link is present. The relay is set to trip at 30% of the normal current (1A), that is, 0.3 A. The second CT connected to the relay is on the transformer secondary (1000/1A) where no shorting link was present. So, when the secondary current reaches 300A, corresponding to a load of approximately 5 MVA, the differential relay, which is seeing only the secondary current will pick up and cause the transformer to trip. Only after the shorting link was removed did the CT supply the required current to the relay. The transformer was then successfully loaded.



Result: Don’t short-circuit the unused taps of CT winding if a differential relay is connected across one of the taps.

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