Interrupting Characteristics of Small Current Using N2/SF6 Gas Mixtures

Abstract

1.INTRODUCTION Recently, the study for replacing SF 6 gas that is one of the potent greenhouse gases by N 2 /SF 6 gas mixture has been advanced in the gas insulated switchgear (GIS). 1-6 The sets of mixture ratio and pressure which dielectric performance is equivalent to that of pure SF 6 are being clear. However some component devices of GIS such as the gas circuit breaker (GCB), the disconnecting switch (DS) and the earthing switch (ES) are required of the capability of the current interruption besides the dielectric performance. While several reports on the high current interruption using N 2 /SF 6 mixture for the GCB 7 are published and describe it is very difficult because of the considerably inferior interrupting capability, the report on the small current interruption for the DS and the ES is very little. 8 However it will be more possible to apply the gas mixture to the GIS if the interrupting characteristics of the small current become clear and the interrupting capability becomes equivalent to pure SF 6. Then, we investigated the interrupting characteristics for the small current on the basis of the sets of the mixture ratio and pressure, which dielectric performance is identical to the pure SF 6. As a result even if the dielectric performance was enough, the interrupting capability was not. Lastly, we show to improve the interrupting capability up to that in the pure SF 6 by means of the easy method with a magnet, taking advantage of the high arc voltage. 2.EXPERIMENTAL SETUP Fig.1 shows the experimental equipment using the 72kV disconnecting switch (DS). The distance between the contacts was 55mm in full open. The speed of contact separation was 1.5m/s. The window was set to observe the arc between contacts by high-speed camera which speed was 4500frame /sec. Fig.2 shows the experimental setup. The interrupting current was 800Arms, which was produced by the 3.3kV/300V transformer and the reactor. That frequency was 60Hz. The transient recovery voltage (TRV) was 400V on peak value. The average of the rate of rising recovery voltage(rrrV) was 14.5V/μs, which was controlled by the parallel 60Ω resistor. Those current and voltage were based on the condition of the loop current interruption. If DS could not interrupt the current, the back up CB interrupted it after 4.5 cycles from the beginning of the current flow. * Mitsubishi Electric Corp, Amagasaki,Hyogo, Japan 661-8661.