Transient potency factor of protective current transformer¿a new standard for transient behaviour

The correct performance of protective current transformer during transient is very important to ensure dependable service of the protective equipment. The existing method of measuring the accuracy of a current transformer for accuracy limit primary current; which is a multiple of normal load current ; does not take into account the time constant of aperiodic component of primary fault current. The increase in speed of operation of high speed relays and switch gear now calls for a very short time for the fault detection by the relay apparatus. Times of the order of 20 to 40 ms are not uncommon. In such a short time, the transient currents and fluxes in a current transformer do not reach steady state values and as such application of steady state standard in such case do not give accurate results and may frequently cause maloperations of the connected relay equipment. As far as the author is aware, no standard for transient performance of current transformers have been adopted. This paper presents a new standard for indicating the transient potentialities of a current transformer.