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Partial discharge current pulses in SF6 and the effect of superposition of their radiometric measurement

The practical advantages of employing non-contact radio frequency (RF) methods for detecting partial discharges (PDs) in high voltage equipment have led to significant effort being focused on the diagnosis of electrical plants using RF techniques. This has particularly been the case for gas insulated substations, which use sulphur hexafluoride (SF6) as an insulating medium. One of the most important challenges facing RF diagnostics is the problem of relating the RF emissions to some measure of severity of the PD. Previous work has established that the amplitude or energy of RF signals radiated from a PD source is strongly dependent on the rate of change of current in the PD pulse. In this paper, measurements of PD current pulses in SF6 are presented for a point-plane configuration using an extremely wide bandwidth (13 GHz) measurement system. By this means, PD pulse shapes have been recorded with better resolution than has previously been possible and rise times have been measured with a high degree of accuracy. The results show a considerable variation in pulse shape, with the minimum rise time measured being 35 ps. With this high time-domain resolution, we have been able to distinguish features within the PD pulses that will affect the energy of the radiated RF signal. In particular, the current pulses tend to occur in bursts of up to ten individual pulses in as little as 1 ns, which will excite multiple RF signals in rapid succession. The effect of superposition of RF waveforms has been investigated by studying the variation in detected RF energy with respect to the time delay between PD pulses. It was found that when two PDs occur within a short period (< 150 ns) the combined energy of the resulting RF pulse has the potential to vary by ±30% of that resulting from two equivalent PD pulses with a wider pulse spacing (≫ 150 ns). In terms of a practical monitoring system concerned with order-of-magnitude variations; this is not considered to pose a major problem for the RF technique.