Flashover mechanism of high voltage outdoor terminations with silicone rubber hollow core insulators

One major design criterion for high voltage outdoor terminations is the lighting impulse level. In case the terminations do not meet the required flashover voltage, any transient voltage in the field may exceed this level and lead to a complete breakdown of the insulation system. The trend towards more compact cable accessories but at the same time with higher requirements on the insulation system leads to increased challenges for outdoor terminations. The flashover process of outdoor terminations is shown in this paper and includes an explanation of the physical effects leading to a streamer discharge. In general, the streamer discharge mechanism is explained and secondly, it is applied to the electrical field distribution of a typical 145 kV outdoor termination. Further on, a concept for the determination of the inception voltage of the impulse flashover voltage is shown. In this context, different streamer propagations are discussed and exemplified. The paper proceeds with the simulation of the streamer propagation of the initially discussed 145 kV outdoor termination. It will be shown how simulation tools can be used to determine the streamer inception and streamer propagation. Subsequently, the simulated results are compared with test results which were collected during the lightning impulse test with 1,2/50 mus impulse voltage form with both negative and positive polarity. This gathered knowledge allows to design out-door terminations with composite hollow core insulators and predict the impulse voltage performance, especially when different creepage lengths of the hollow core insulator come into use.