DC corona discharge from floating particle in low pressure SF₆

The final objective of this research project is clarification of the particle-triggered corona mechanism in SF₆ gas-insulated electrical equipment as well as development of high precision diagnostic method of foreign metallic particle in GIS insulated by SF₆ gas using the electrical signal due to corona discharge. The following research steps are conducting to achieve our goal: 1) clarification of particle-triggered corona phenomena in air which is well-understood corona mechanism, 2) clarification of particle-triggered corona phenomena under dc and ac applied voltage in SF ₆ gas at low pressure in which a stable corona occurred, and 3) clarification of the pressure effect on the particle-triggered corona mechanism. This study is the second step of our research series research, which focuses on characteristics and the mechanism of particle-triggered corona discharge in SF₆ gas on the basis of measurements of corona current pulses at a pressure range of 30 kPa to 50 kPa under dc applied voltage. The identification of positive and negative corona pulses in SF₆ gas from the waveform, amplitude and repetition rate of corona current pulses is difficult; thus, current and light pulses of coronas from the both tips of particle were measured simultaneously to discriminate the polarity of corona current pulses. Experiments showed that near the corona onset voltage, positive corona accompanied pulse current but negative corona tended to be glow-like corona. As the applied voltage was increased, negative corona changed into streamer type accompanying strong current pulse. Positive corona pulses maintained a small amplitude with an instantaneously high repetition rate, even if the field enhancement on the positive tip of particle occurred due to the positive-ion inflow to the particle produced by negative corona. On the basis of current pulse measurements, the mechanism of particle-triggered corona was discussed. Moreover, corona activity and breakdown voltage were investigated by changing the particle position. This study confirmed that the active corona development and fall in breakdown voltage were due to the effect of intermittent short-circuit arc in the minor gap between particle and electrode when the particle was in the vicinity of the electrode.