1. School of Electrical Engineering, Chongqing University, Chongqing 400044, P. R. China
2. State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, P. R. China
3. Sinopec Chongqing SVW Chemical Co., Ltd, Chongqing 400044, P. R. China