Optimal lightning and surge protection for pole mounted distribution transformers

Abstract

A medium voltage (MV) distribution network with overhead lines experiences high overvoltages due to lightning strikes. Such transient overvoltages cause damage to MV equipment such as the pole mounted distribution transformers. Lightning overvoltages and surges produce high energies that exceed the transformers` withstand capability and as a result, they fail which affects the power supply reliability. A number of 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 besides lightning, examining the effectiveness of the currently used transformer lightning and surge protection measures, and then proposing optimal protection measures. These were achieved with 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 protection of the transformers.