National Grid U.S. experienced a large amount of arcing when opening 138-kV gang-operated disconnect switches to isolate 138-kV SF6 breakers with capacitors. According to Timothy True, manager of substation maintenance, the capacitors are connected line-to-ground across the breaker’s busings to lower the transient recovery voltage. The switching sequence is to open the breakers and then to open the gang-operated disconnect switch, which isolates the breaker. The gang-operated disconnect switch (GODS) opens slowly by manually cranking a handle through a gearbox.

True will be presenting a session on “138-kV Gang-Operated Disconnect Switches: Arcing During Switching of SF6 Circuit Breaker Capacitors” at the Finepoint Circuit Breaker Conference in October. National Grid came to the conclusion, after the analysis of the situation, that the 138-kV gang-operated disconnect switches (GODS) should not be used to de-energize capacitors on breakers. The GODS operate slowly and the arcing that happens can pass high-frequency current through your potential transformer circuits and damage equipment.

“Either use air break switches, change the location of the breakers' capacitors, or use breakers that do not require capacitors,” True said.

True will offer detailed diagrams of the switching process, explaining the arc and what corrective actions National Grid took to deal with the situation.

According to True, the arc was established when the switch blade first opened and continued until the switch was almost fully open. Originally, it was thought it was due to Ferro resonance, and resistors were put on the low side of the potential transformers to attempt to dampen it. This did not work since it is not Ferro resonance.

“When switching, we were destroying electronic equipment such as oscillographs, remote terminal units, and digital meters in the substation. We also had the secondary fuses blow on the 138-kV potential transformers, and in one case, we found a weld mark in the PT fuse box where it flashed over. We stopped all switching, which involved using 138-kV GODS to drop breaker’s capacitors due to safety and equipment concerns. When the breakers had to be switched open for work, the clearances had to be enlarged so the switch could be opened dead. This presented system problems since a large amount of equipment had to be cleared,” True said.

True graduated from Rensselaer Polytechnic Institute in 1975 with an electrical engineering degree and has a New York state professional engineering license. He worked two years for S&C Electric Co. and for the past 30 years has worked for the electrical utility industry on Long Island, which is now part of National Grid. Since 1989, Tim served as manager of substation maintenance, which includes the engineers that are involved in equipment problems and failure analysis.