Grounding Banks of Transformers with Neutral Impedances and the Resultant Transient Conditions in the Windings

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. This limitation has been required to ease the duty on circuit breakers and assist in maintaining system stability. The use of resistance, inductance, and various combinations has been studied as to the effect on the short-circuit current and particularly upon lightning transients within the transformer windings and at the neutral. 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 that the transformer be 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.