Improvement of differential protection performance by external faults associated with a current transformer saturation

Protective systems require a faithful reproduction of primary current on the Current Transformer (CT) secondary side. Saturation is a common problem in a steel core CT. Current transformer (CT) saturation leads to inaccurate current measurement and, therefore, may cause malfunction of protective relays and control devices that use currents as input signals. This paper have invested an efficient practical way, to avoid the effect of current transformer saturation on differential protection behavior, by adding metal oxide varistor (MOV) element to secondary of current transformer. Simulation study is done by using a program EMTP-ATP, that Differential protection remains stable when a fault occurs outside the protection zone associated with a saturation in a current transformer.