Characteristics and development mechanisms of partial discharge in SF ₆ gas under impulse voltages

It is well known that impulse voltages strongly affect the reliability of gas-insulated switchgear (GIS) under actual operational conditions. The alternating-current (ac) withstand voltage test and its corresponding partial discharge (PD) measurement are inadequate for the detection of all the hidden flaws in GIS. Such inadequacy can be compensated by additional impulse withstand voltage test and its PD detection measurement. However, the bulkiness and inefficiency of aperiodic impulse voltage generator make it unsuitable for on-site application. Therefore, in practical tests, oscillating impulse voltage (OIV) is generally adopted instead of aperiodic impulse voltage. In this paper, we measure the PD inception voltage (PDIV) and the breakdown characteristics of a needle-plate gap in SF₆ gas under four types of impulse voltages by electrical and optical methods. The 50% probability PDIV is analyzed as a function of gas pressure. The physical mechanisms of the PD under the four impulse voltages are discussed. The PD characteristics under ac voltage are investigated for comparison. Experimental results reveal that the PD development process under the two aperiodic impulse voltages was mainly dominated by the precursor mechanism which consists of streamer and leader stages, while the PD propagation under the two OIVs was affected by the displacement current produced by oscillation. The difference of the PD characteristics under the aperiodic impulse voltage and OIV, and ac voltage are also presented.