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Tuesday, February 22, 2011

Root Cause Analysis: Stuck-up Worm Gear for Spring Charge Mechanism of 69kV Power Circuit Breaker

I would like to share the substation equipment trouble that happened in our substation last February 15, 2011. We conducted maintenance on 69kV breaker and during energization of the line, the breaker failed to close after series of attempts. After series of troubleshooting, we found that the worm gear for spring charge mechanism was stuck on its bushing. Below is the full text of my report:



I. Background of the Problem:
February 15, 2011
1. Scheduled shutdown of 69kV Power Circuit Breaker to replace defective SF6 Gas fittings, base insulators for Lightning Arresters, and bolted connectors of breaker terminals with bolt-tap connectors. Power Circuit Breaker was tripped at 0801H.
2. The different groups proceeded with their respective activities after obtaining clearance from Area Control Center.
3. Maintenance group conducted testing on power circuit breaker (contact resistance, insulation and BMT) after the replacement of fittings and charging of SF6 gas. The Maintenance group returned the breaker position to open position after the completion of their activity. They noticed that upon tripping of the breaker, the miniature circuit breaker (MCB) of the motor for spring charging simultaneously tripped but they did not coordinate with the Engineer-on-duty.

4. Around 1500H, all maintenance group and the customer (electric cooperative) had given their work clearances. The Engineer-on-duty coordinated to Control Center for the energization of the line.
5. At 1506H, remotely (via SCADA) and locally (via Control Switchboard) closed power circuit breaker but failed; observed from the inspection that spring for closing operation was discharged.
6. The MCB supply was tripped as found out by the Engineer on duty. Maintenance group confirmed that the MCB tripped during the last operation of power circuit breaker.
7. When the MCB supply was turned on, the motor operated but the spring for closing failed to charge.
8. Tried to manually charge the spring for closing operation but was unable to move the hand crank.
9. Conducted troubleshooting on the spring mechanism charging and found out the worm gear connected to coupling shaft and main shaft was stuck-up to its bushing.

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