On the requirements of current transformers in differential protection of transformers at electric power stations

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 Klim referenced to the nominal current of a power transformer be used as the main parameter in choosing CT. A method is presented for calculating Klim as a function of the time to saturation ts, 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 Klim 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 Klim for CT which do not have a proper primary winding and are arranged in phase with screened conductors. © 2017 Springer Science + Business Media New York.