Cases of unusual mal-operation of differential protection for transformer

generator and transmission line in operation due to adjacent transformer switching-on have seriously threatened the safe and stable operation of the power grid. An investigation is made on the causes of mal-operation by analyzing the main factors

such as magnetizing inrush

sympathetic inrush

and current transformer saturation

including field recorded fault data and simulation results. It is pointed out that the current transformer transient or current transformer local transient saturation induced by the long-lasting aperiodic component from the magnetizing inrush current is the major cause of mal-operation. To solve this problem

the harmonic restraining criterion and the improved ratio restrained characteristic of current differential protection are proposed to avoid this sort of mal-operation. The research results are verified by field recorded fault data and simulation results. This work is supported by National Natural Science Foundation of China(No. 51277084

No. 51177058)and China Southern Power Grid Company Limited(No. K-GD2012-324).