Current Transformer Excitation Under Transit Conditions

Tests of circuit breakers, relays, and similar devices in which current transformers have been used to step down the large currents involved to values measurable by the oscillograph, have shown peculiar errors in the records during the transient portion of the current wave. The purpose of this paper is to show that the abnormal excitation of the magnetic circuit of the current transformer required during transient conditions is the cause of these errors, and to point out how the errors may be minimized sufficiently to be negligible. Mathematical expressions are also derived to make it possible to predetermine the ability of a transformer to handle transients correctly and to guide in the design of transformers for this service.