Determination of streamer inception and flashover at curved insulation surfaces with embedded electrodes

In the literature, several approaches are discussed for the calculation of streamer inception, streamer propagation, and flashover at insulation surfaces in a gaseous ambient. The most simple concepts use the three parameters critical electric field, effective ionization coefficient and mean breakdown gradient of the ambient gas. In gas insulated substations pluggable cable connections with a cast resin socket with embedded electrodes are frequently used. In this contribution the partial discharge inception voltage as well as the flashover characteristics for alternating and lightning impulse voltage applications of both polarities are determined for a broad range of medium voltage sockets in ambient air. The results show that the critical polarity of the lightning voltage depends on the exact geometry of the socket. An engineering focused approach for the calculation of the partial discharge inception voltage and the minimum breakdown voltage is implemented. The results of the calculations and the experiments are compared. The calculations predict the measured values very well and explain the detected flashover behavior. With the obtained knowledge from this engineering focused approach it is possible to enhance retrofit solutions for (especially medium voltage) gas-insulated substations. On the other hand, an approach, just taking the mentioned gas parameter as input, delivers a useful tool for the calculation of flashover voltages in other insulating gases.