Influence of triple-junction on sparkover voltage for a series gap of arrester

Considering the latest development of advanced information society, a stable supply of electric power should be maintained. Lightning surge arrester can contribute to reduce electric failures due to overvoltage from lightning. Since 1970's, zinc oxide (ZnO) element with superior non-linear voltage (V) - current (I) characteristics has been widely installed. Even in Japan, the surge arrester with a series gap has been installed to distribution systems for reducing the electrical failures. As mentioned above, the ZnO element has a superior nonlinearity of V-I characteristic, which can achieve suppress of overvoltage energy. Furthermore, the ZnO surge arrester with a series gap can interrupt the following current, immediately. The surge arrester with a series gap is composed of a simple gap electrode, a ceramic spacer for fixing the gap of the electrodes, and ZnO elements. The upper and the bottom of the ceramic spacer face metallic electrodes; therefore there are triple-junction points between the ceramic spacer, the metallic electrode, and gas. It is well known that electric field strength near the triple-junction point becomes high because of a triple-junction effect. It is also considered that the triple-junction point becomes a source of primary electrons by field emission due to the high electric field. Therefore, the triple-junction effect may influence the discharge characteristics so that the investigation will contribute to the further stable operation of a surge arrester with a series gap. In this study, we carried out experiments concerning how the triple-junction effect influences discharge characteristics such as breakdown voltage and discharge ignition. We first varied the distance between the discharge axis and the triple-junction point and then evaluated the discharge ignition time. Additionally, several ceramic materials with various relative permittivity were used. Based on the results, we evaluated the optimum distance between the discharge axis and the triple-junction point, which would be useful for design of a surge arrester with a series gap.