A Fast Blocking Method of Bus Protection when CT Disconnected Based on Braking Current and Zero Sequence Current

Current transformer (CT) disconnection of bus is prone to maloperation of bus differential protection. The existing criteria for CT disconnection do not fully consider the distinction between heavy load high impedance fault and external metal fault transfer to internal high impedance fault. And not fully consider the influence of one-half wiring circulation and unbalanced impedance on CT disconnection zero sequence criterion. Based on the theoretical analysis of the characteristics of braking current variation and zero sequence current distribution in the event of internal fault, this paper puts forward a fast blocking criterion of CT disconnection based on braking current and zero sequence current, and carries out simulation verification. Both the theory and the simulation show that there is a drop of braking current in the event of internal fault. The distribution of zero sequence current in the event of CT disconnection is obviously different from that in the event of internal fault. The criterion proposed in this paper can effectively and reliably distinguish CT disconnection from internal fault.