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Mitsubishi SF6 Gas Circuit Breaker Type 100 SFMT-40E Breaker Failure Report

Prepared by: Jeffery M. Wright Manager – Transmission Field Services Vermont Electric Power Company, Inc.

April 6, 2004 – 13:08 • VELCO experienced a breaker failure in the Essex FACTS switchyard. The 115 kV SF6 gas circuit breaker is a Mitsubishi 100SMFT-40E. • Although there were no visible external damages, relay and protection engineers analyzed the relay information and all indications led to an internal failure of the K98 circuit breaker. EXAMPLE ONLY Mitsubishi 100SMFT- 40E Circuit Breaker Location: Georgia C80 Cap Bank Breaker

DFR analysis indicates the fault lasted nearly 4 cycles with phase 3 volts dropping to .02 p.u.

IBM, the largest manufacturer in Vermont is electrically connected to redundant VELCO 115 kV sources. They are located .7 miles from the Essex substation. IBM 115 kV phase 3 bus voltage dropped to approximately the same as what VELCO recorded at Essex. Approximately .02 p.u. for nearly 4 cycles. The power quality interruption cost IBM in excess of $1.5M in lost tooling hours and production.

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An internal inspection directed by Mitsubishi technical engineers verified that an internal fault had indeed occurred. The breaker was shipped back two days following the failure to Mitsubishi (Pittsburgh, PA) for a thorough failure analysis.

Formal Mitsubishi Response Friday, April 16, 2004 Conference Call Failure Analysis Report 1) Repair Efforts & Flashover Location The tanks from Breaker SG006800004 were dis-assembled and inspected. The flashover was noted to have occurred at Bushing Assembly #4, center phase, at the moving end. The evidence indicates that the arc struck from the Bushing #4 conductor to the bushing adapter plate. Fotos are attached. The quantity of arc byproduct powder in the center phase tank was moderate, indicating the flashover was not severe. Arc byproduct powder was collected and the SF6 which came inside the breaker was also collected, for possible future analysis. Breaker components which were damaged or suspected of being damaged were replaced; major replaced components included center phase interrupter, Bushing Assembly #4 (conductor & insulator), all 6 bushing adapter plates, the SF6 gas system, desiccant, and associated hardware, O-rings, etc. The other 2 phases were also disassembled; all major components from these 2 phases were re-used. All 3 tanks were thoroughly cleaned; all re-used components were thoroughly cleaned. Breaker SG006800004 was re-assembled and re-tested following written instructions. Breaker SG006800004 was shipped on 4/15/04. 2) Investigation into Cause of Flashover Our investigation indicated that randomly-occurring water was the likely cause of the flashover. Water was detected using our ultrasonic (UT) inspection technique, which can detect water in boltholes without dis-assembling the breaker. Boltholes in every nozzle were evaluated for presence of water by the ultrasonic method, and every bolt was removed for visual examination. This gave us an opportunity to visually confirm the ultrasonic indications. This investigation indicated that water was present randomly, at different levels, in 3 of the 6 bushing nozzles, in boltholes Bushing Nozzle # 2 (2 of 12 boltholes, very minor …