Study on a Mal-opertaion case of differential protection due to the interaction between magnetizing inrush and sympathetic inrush

Sympathetic inrush is one of the most important topics for relay protection, which has adverse effects on transformer protection. This paper takes a local maloperation case as an example to analyze the principle of the interaction between magnetizing inrush (M.I) and sympathetic inrush (S.I) and discusses the decay characteristic of magnetizing inrush in high impedance transformer. By means of introducing a new concept of acting factor to describe the decay velocity of magnetizing inrush in a quantitative way, it is found that sympathetic inrush can affect magnetizing inrush decay rate significantly even when the sympathetic inrush is small. The interaction between magnetizing inrush (M.I) and sympathetic inrush (S.I) is the main reason for the mal-operation. In the end, countermeasures are put forward to avoid the maloperation and a large number of MATLAB simulations prove the countermeasures to be reasonable.