2018 14 Paper: Maraetai CB Failure

During a routine operation when hydro control issued an OFF command for Maraetai Unit 2 one phase of the circuit breaker failed to open. This event led to a call out for operator assistance and finally the bus being manually cleared after 2 hours of motoring action on the generator. The effect was not only the immediate removal of around 410 MW of generation at the Maraetai bus but the consequential damage on the generator rotor poles as a result of the generator motoring.
The paper provides a detailed analysis on the failure of the protection systems to clear the bus. The paper discusses how the event triggered a fleet wide counter check on the circuit breaker dashpot oil levels along with the a review into the efficacy of the CB fail protection including discussion with the grid owner Transpower. The repairs performed on the generator rotor are also discussed and how a temporary innovative PLC based CB fail
scheme had to be put in place to address the circuit breaker failure shortcomings. The authors believe that sharing these details including the human learnings from this rare event will be hugely beneficial to the participating utilities.

• Author: Anirban Dey, Russell Hyde
• Exchange: 2018 - Albury AGL
• Company: Mercury
• Topic: Circuit Breaker, Protection, Switchgear, Testing, Transmission
• Key Words: Circuit Breaker, Motoring, Protection, Switchgear, Testing, generator