Optimal lightning and surge protection of pole mounted secondary distribution transformers

Abstract

Lightning overvoltages and surges produce high energies that exceed the pole-mounted distribution transformers` withstand capability and as a result, they fail which affects the power supply reliability. Several methods have been employed by utilities to reduce transformer failure rates due to lightning such as the use of overhead earth wires on overhead lines, surge arresters, spark gaps among others but the failure rates of these distribution transformers are still alarming. Appropriate transformer protection against such failures is therefore key and thus, this project aims at investigating other causes of transformer failure, examining the effectiveness of the currently used transformer protection measures, and then proposing optimal protection measures. This was achieved with a review of various literature, surveys and field visits, interviews and inquiries from different distribution network operators, and EMTP software simulations. The use of filtered surge arresters or line surge arresters at 2 or 3 pole spans among the proposed measures provided optimal protection of the transformers. Ensuring proper protection and functioning of the surge arresters through reduction of their energy absorption is the best way forward to the protection of the transformers.