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Participant

Join Date: Jul 2012
Posts: 1

SF6 GCB Bushing CT Problem

07/12/2012 4:58 AM

Two cores of Bushing CTs on one 230 kV SF6 circuit breaker have very low secondary to ground insulation resistance (in the kilo ohm range). Now, the primary to secondary insulation resitance of the same BCTs had the same problem. Since the primary is the bushing of the breaker and is fully insulated from the ground and the BCT secondary winding, is there a possibility that the test was even correct? If the HV bushing was faulty in the first place, it would have exploded a long time ago due to the supposed low insulation resistance.

The option would be to replace the two BCTs and that would entail removing the SF6 gas, breaker bushing and replacing the gaskets which would involve considerable down time and resources. Moreover, the replacement BCTs have to be sourced out still.

The test engineer would not budge from his assumption that the equipment was defective hence it was shutdown immediately.

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Guru

Join Date: May 2010
Location: Liverpool, NY
Posts: 960
Good Answers: 131
#1

Re: SF6 GCB Bushing CT Problem

07/12/2012 11:17 AM

Do the insulation resistance (IR) test again with a different megohmeter. I could imagine the insulation on the secondary of a BCT being damaged or deteriorated to get low IR readings to ground, but to get a low IR reading from secondary to primary insulation of the BCT on a breaker that has good SF6 pressure and has been in service up to the time of the test is not likely.

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