Study on complex inrush caused by transformer energization and its influence on current differential protection

Several cases of unusual mal-operation of the current differential protection for transformer, generator and transmission line already in service, due to the adjacent transformer switching-on, have been reported. The complex inrush current, including the magnetizing inrush current and the sympathetic inrush current, is considered to be relevant with this sort of mal-operation. However, the detail reason still means further verification. In this paper, the main factors, including magnetizing in-rush, sympathetic inrush, and CT saturation are analyzed in detail. It is disclosed that the CT transient saturation or CT local transient saturation, caused by the long-lasting aperiodic component from the magnetizing inrush current, leads to the mal-operation. To solve this problem, the countermeasure is proposed, including applying the harmonic restraint criterion and the improved the ratio restraint characteristic. The field recorded fault data and the simulation results prove the correctness of analysis conclusion and the effectiveness of the proposed improving methods.