High frequency discharges and their effects on insulation in SF6 filled circuit breakers

This paper presents some experimental results taken during an investigation into the behaviour of EHV circuit breakers when switching low current reactive loads. This particular switching duty produces unusually large over voltages leading to repetitive breakdowns and high frequency arcing in the circuit breaker, starting approximately tens of microseconds after current zero and after the thermal recovery period is complete. These restrikes and associated high frequency arcs not only occur between the upstream and downstream electrodes and therefore are contained inside the nozzle but, under certain conditions, they can also occur outside of the nozzle containment principally from the downstream electrode to another metallic part of the interrupter which acts as a surrogate electrode. The latter is known as parasitic arcing and such arcs are difficult to control and extinguish. Even worse, if a power frequency arc follows the path taken by the high frequency arc, which is highly probable, then damage to the interrupter may ensue. It is, therefore, desirable to avoid the formation of these parasitic discharges. Unfortunately, there are many factors which lead to the onset of parasitic arcing further complicated by a dynamic interaction between these various factors. Broadly these factors are related to the design of the interrupter, the nozzle material and geometry, the SF6 gas and the interconnecting circuit. The results presented here highlight some of the more interesting and relevant aspects contributing to high frequency parasitic arcing relating to the PTFE nozzle