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.
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