Abridgment of grounding banks of transformers with neutral impedances and the resultant transient conditions in the windings

Abstract

The question of grounding transformer bank neutrals through different impedances has recently arisen due to the desire to limit system single-phase and two-phase short-circuit currents. It was found that the use of resistance, only, may be undesirable on account of the high voltage at the neutral in limiting the short-circuit current. The use of inductance, only, may result in high voltages within the transformer and at the neutral due to lightning transients, which necessitates the transformer being fully insulated throughout. Methods using parallel paths with the inductance, (these parallel paths being designed primarily to reduce the lightning transients at the neutral), have been studied and found to limit the transients within the transformers to values approximating those for solidly-grounded neutral which permit the grading of the transformer insulation. The method to be selected depends upon the individual case; but generally, the use of the valve type lightning arrester is the simplest to apply.