Appropriate Surge Arrester Lead Lengths for Improved Distribution Transformer Protection — Kenyan Case Study

Medium voltage power networks are frequently exposed to overvoltages caused by lightning strikes and other external events. In the Kenyan distribution system, metal oxide surge arresters are widely used for surge protection of secondary distribution transformers. However, lightning strikes are still one of the major causes of secondary transformers failure in the network. A critical aspect in selection and application of surge arrestors is the surge arrestor lead length. The focus of this paper was calculation of the protective ratios of surge arresters with current typical lead lengths installed for surge arresters protecting distribution transformers in 11kV and 33kV networks in the Kenyan distribution system. Comparison was made with the guidelines of IEEE C62.22 on minimum protective ratios. Recommendations on appropriate surge arrester lead lengths that to guarantee the achievement of these minimum protective ratios are given. Recalculations to demonstrate adequacy of the recommendations were also carried out. This will lead to improved protection for distribution transformers against surges in the distribution network.