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Saturday, June 4, 2016

Non-Tripping Operation of a 69kV Power Crcuit Breaker

I posted a problem in the REE forum regarding the non-operation of a 69kV power circuit breaker. Here is the discussions made about the problem and what really happened on the breaker as found by maintenance group.

Just want ask your idea regarding our problem. Our 69kv power circuit breaker failed to trip when operated via SCADA(remote), switchboard and on the equipment local control. Breaker status was at close position. The SF6 pressure was normal, the spring mechanism was “charged”, and the DC power supply of the breaker was ok. We troubleshoot the local remote switch S12 by measuring the voltage at 111-to-grnd when the switch was at local and remote positions. 111-to-gnd reading was 0v when the switch is at local position; 111-to- gnd was -62.5v when the switch was at remote position. Next we checked the gas supervision relay K2 if it is working. Measured 34-to-gnd and the voltage was -62.5v while 31-to-gnd was also -62.5v. We concluded that the contact of SF6 supervision was closed and the trip coil Y2 was not an open circuit because of the presence of voltage reading. That was as far as we are allowed to troubleshoot. Anything that we might have missed in the tracing of trip circuit diagram? Thanks in advance.

When we first measured the voltage of the K2 gas supervision relay, there were voltages that appear on terminals 31 and 34. When the voltages were measured a day after, the voltages disappeared. In that case, it measn that the SF6 pressure was already near the lockout value so when voltage first appeared, the pressure was a little bit over the lockout value because of gas expansion due to hot weather so the tripping circuit was still complete; however, when it was measured again, that was the time that the pressure so low it could not expand to exceed the lockout value anymore.

There was a micro leak on the top cover of our breaker that is why the SF6 pressure was already low.

When the pressure switch was checked, it was found out that the contacts for SF6 low alarm is defective so there was no initial warning for shift engineers that the pressure was already low so schedule for recharging was not reqeusted prior to lockout actuation.

The SF6 gauge was defective. The value that appeared on the reading was actually lower by more than 1MPa compared to actual reading. Hence, eventhough the real SF6 pressure of the breaker was already low, the SF6 reading was still good due to inaccurate gauge.

Action Plan:
-The defective gauge was replaced
-The pressure switch was replaced to accommodate the alarm circuit
-comprehensive SF6 leak correction was conducted and sealed all the source of gas leaks


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