On the Requirements of Current Transformers in Differential Protection of Transformers at Electric Power Stations

Abstract

The problem of picking protective current transformers (CT) in order to ensure proper functioning of differential protection in transient regimes is examined. It is proposed that the accuracy limit factor K_lim referenced to the nominal current of a power transformer be used as the main parameter in choosing CT. Amethod is presented for calculating K_lim as a function of the time to saturation t_s, the decay time constant of the aperiodic component of short-circuit (SC) current, and the residual induction and current of external SC. It is shown that increasing K_lim results in smaller CT errors in the time interval of the saturated state of a magnetic circuit. It is pointed out that the effect of screening must be taken into account in finding K_lim for CT which do not have a proper primary winding and are arranged in phase with screened conductors.