Time Lags and Optical Investigations of Pre - Discharge in SF₆/N₂ - Mixtures at Very Fast Transient Voltages

Abstract

This paper deals with the statistical and formative time lags in SF₆ and SF₆/N₂ - mixtures under strongly non - uniform field distribution, stressed by very fast transient voltages (VFT).

In most of our experiments, the dielectric strength of the insulating gas, stressed with the highest investigated frequency of 20 MHz, was equal or less compared to a 8 MHz VFT. Nevertheless it was found that with certain parameters a 20 MHz VFT may provide a higher dielectric strength than a 8 MHz VFT.

In pure N₂ the time lags are distributed over the whole investigated range between 10 ns and 100 ms. By increasing the portion of SF₆ the events in the time range between 100 nanoseconds and some microseconds become rarer and have a minimum for a mixture ratio of 50 to 75% SF₆/N₂, depending on the other parameters.

Breakdown often occurs at a time when the damped VFT surges are already vanished and the DC voltage remains at the test gap. Obviously the preceeding charge injection has greatly reduced the dielectric strength of the insulating gas. Even though the predischarge current hardly exceeds the noise level of approximately 3 A, weak luminosity could be observed in the gap during the first periods of the VFT. This results in a reduced formation time of the discharge channel, which may happen hundreds of microseconds later.