Evaluation of Lightning Overvoltage Protection Schemes for Pole-Mounted Distribution Transformers

Abstract

Direct lightning strikes to medium voltage (MV) overhead lines cause insulation flashovers in great majority. Generally, the pole-mounted distribution transformers are protected by spark gaps and surge arresters at the MV terminals. In this paper, Alternative Transients Program-Electromagnetic Transients Program (ATP-EMTP) simulations have been performed to evaluate the performance of different lightning protection schemes for the pole-mounted distribution transformers. Both first and subsequent stroke currents were considered to estimate the lightning performance of a typical MV unearthed network. The results indicate that the effect of subsequent stroke current is more severe at the low voltage (LV) terminals of the transformer. In case of spark gap protected transformers, the operation of spark gaps produce steep-front overvoltages. As a result, high frequency overvoltages transfer from MV to LV terminals of the distribution transformer that are directly connected to the consumer premises. The steepness of the overvoltages can be mitigated by installing a series filtering element at the MV side of the distribution transformer. Furthermore, surge arrester can be employed to smoothly limit the transient overvoltage at the MV terminals of the transformer in spite of the higher protection cost. Thus, low rating surge arresters in conjunction with spark gaps can be also used to mitigate the lightning overvoltages. Finally, it is recommended that higher energy class surge arrester should be employed for this combined spark gap and surge arrester protection scheme.