Lightning Protection for Distribution Transformers

Usual methods of connecting arresters for the protection of distribution transformers are often inadequate. In service some transformers still flash over although the arrester, in itself, is capable of protecting the transformer with a large margin of safety. Surge current flowing to ground through the ground lead of the arrester gives rise to inductive drop and resistance drop voltages. These voltages added to that permitted by the arrester can be great enough to flash over the transformer bushings to secondary neutral. By interconnecting the ground lead of the arrester and the transformer secondary neutral, the voltage across the transformer insulation is limited to that permitted by the arrester alone. Tests have been made to determine the conditions of surge required to duplicate flashovers that occur in practise. Under such conditions measurements were made of the voltages appearing on a representative secondary circuit when bushing flashovers are permitted, and when flashover is prevented by interconnecting the arrester ground and the transformer secondary neutral.