State;
Key;
Lab;
for;
Electrical;
Insulation;
and;
Apparatus,;
Xi'an;
Jiaotong;
University,;
Xi'an;
710049,;
China;
School;
of;
Electrical;
and;
Information;
Engineering,;
Hunan;
University,;
Changsha;
410082,;
China;
Technische;
Universitaet;
Darmstadt,;
64283;
Darmstadt,;
Germany;
Dept.;
of;
Electrical;
Engineering;
and;
Computer;
Science,;
University;
of;
Manitoba,;
Winnipeg,;
Canada;