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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