Overvoltage protection, faults and voltage sags

Distribution transformers in rural networks have to cope with frequent lightning induced overvoltages and thus must be protected using spark gaps or surge arresters. The overvoltage protection type has an influence on the system behaviour and on the interruptions and voltage sags experienced by power system customers. In this paper, measurement results showing the effect of overvoltage protection type on established faults in an unearthed medium voltage overhead line network are presented. Detailed results of fault frequencies and fault types are shown. Further, a probability function is established to describe the influence of overvoltage protection type on voltage sags experienced in the network. This latter aspect is important when limiting the effects of interruptions and voltage sags for sensitive customers.