IntroductionInrecent10years,duetofastdevelopingofeconomyinChina,seriouspowershortageoccurs.Thesystemfrequencyoftenfelltoabnormalvaluesinmostnetworks.Forthesecurityofpowersystems,forcedswitchoffoffeedersfromthedistributionsubstationoftenhappendtorecovernormalfrequency.Infact,thespeedofdevelopmentofChinesepowerindustryisveryhighinthisperiod.Theyearlyaverageincreasingrateofgeneratingcapacitywasabout8%.Theyearlynewlyinstalledca-pacitywasmorethan9000MWinrecent4years.Buttheannual
