Key;
Laboratory;
of;
Power;
Equipment;
&;
System;
Security;
and;
New;
Technology;
(Chongqing;
Univer~,ity),;
Chongqing;
400044,;
China;
Grid;
Hunan;
Electric;
Power;
Corporation;
Maintenance;
Company,;
Changsha;
410004,;
China;
Grid;
Hunan;
Electric;
Power;
Corporation,;
Changsha;
410000,;
China;