Mechanisms for Inception of DC and 60-Hz AC Corona in SF6

Using a pulse counting technique, inceptions of positive and negative point-plane corona in SF6 under dc and 60-Hz ac conditions were measured. Effects of gas pressure, UV-radiation, and point electrode size on differences between ac and dc, and between positive and negative inceptions were investigated. Inceptions were also calculated using the streamer criterion. Agreement was obtained with measured negative inceptions for both ac and dc conditions, but not with positive inceptions. The growth in the active-electron initiation volume with applied voltage was calculated and used to explain the observed polarity effect. The magnitude of the polarity effect is predictably reduced, either by irradiating the gap or by increasing the diameter of the point electrode. The difference between ac and dc positive inceptions is attributed to the enhancement of avalanche-initiating electron production by the residual ion space charge from negative corona in the previous half-cycle. Without radiation, it is hypothesized that electron initiation for positive corona occurs mainly via field-enhanced collisional detachment of negative ions, the probability of which depends on the degree of field nonuniformity in the gap. For negative corona, initiation is assumed to occur via field emission from the point electrode. The presence of radiation enhances initiation in the positive case by photodetachment of negative ions, and in the negative case by photoelectric emission.