Switching surges with transformer load-ratio control contactors

THE INVESTIGATION described in this paper was undertaken to determine the magnitude and characteristics of the abnormal voltage transients accompanying the operation of load ratio control contactors on power transformers. As test results definitely established the fact that currents may be abruptly ruptured at other than normal current zero, accompanied by very severe over-voltages, the main purpose of the paper was directed to this phenomena, both from the experimental and theoretical viewpoint. It is believed that the subject will prove interesting to anyone concerned with switching surges in general, especially in view of the fact that certain aspects of the phenomena appear not to have been presented heretofore. Thus, the effect of current interruption at other than normal frequency current zero is gone into in detail; general equations on recurrent restriking are given; the extremely interesting possibility of restriking so as to practically discontinue the voltage transient is described; and the paradox of an increase in recovery voltage when shunt capacitance is increased is explained. These points are illustrated and supported by oscillographic evidence.