中文摘要：

JIANG Wei is born in Chongqing, China, on Aug 27, 1979. He received the B. Sc. degree in hydroelectricity engineering in 2002, and the M. Sc. degree in 2005, from Xihua University, Chengdu. He is currently pursuing the Ph.D. degree in the school of Electrical Engineering at Southwest Jiaotong University. Currently, be is doing research about transformer DC bias caused by HVDC transmission system. Email：jw_jiangwei@126. com WU Guang-ning （M＇97） is a graduate from e- lectrical engineering of Xi＇ an Jiaotong University with the B. Sc. , M. Sc and Ph.D. degree in 1997. He is a Professor in the School of Electrical Engineering, Southwest Jiaotong University. His research interests include condition monitoring, fault diagnosis and insulation life-span evaluation for electric power equipment. XIAO Hua is a graduate from electrical power system and its automation of ChongQing University with the B. Sc. degree in 2007. She is currently pursuing the M. Sc. degree in the school of Electrical Engineering at Southwest Jiaotong University. She is doing research work about influence of transformer caused by HVDC. HUANG Zhen is a graduate from electrical power system and its automation of Xi＇an Jiaotong University with a B. So. degree in 1991 and from electrical power system and its auto mation from electrical power science research institute of china with a M. Sc. degree in 1996. He is working in Siehuan province electrical power eorporation. He is responsible for dispatching, operation and maintenance of electrical power network. Saleh Yahya Muhammed was born in Lahj, Yemen, on May 1st, 1966. He received B. Sc. degree in Electrical Engineering in 1991 from University of Aden, Yemen. After graduation, he worked as a lecturer in Industrial Technical Institute, Aden from 1992 to 2006. Currently, he is pursuing M. Sc. degree in School of Electrical Engineering in Southwest Jiaotong University, Chengdu. GUAN Lan is a graduate from biology science of Sichuan Agriculture University with the B. Se. degree in 2006. He