Effect of Excitation on Lightning Surges in Transformer Windings.

Impluse voltage tests to which transformers are subjected before commissioning are carried out on unexcited units, whereas an operating transformer is exposed to voltage surges while it is excited. The higher peak value of a chopped pulse wave, substantiated in the standard, among others, by the superposition of overvoltages due to the pulse voltage and an alternating voltage is to-day questioned as regards UHV transformers. For better understanding the problem, an approximate analysis of the effect of transformer excitation upon the insulation hazard due to lightning surges is presented in a slightly different approach to that adopted in the literature.