DuringtwoyearsoddperiodfromDec.1987toFeb.1990,therehadbeenrepeatedlypollution-flashoverfailuresonour500kVtransmissionlines,resultinginawidescopeblackout.Therewereeleven500kVlinesaltogetherinNorth-east,EastandNorthChinapowergrids,wherepollution-flashoverfailureshappenedoneafteranotheron36transmissionlinetowers(poles).Since500kVtransmissionlinesarethemaintrunksofhighvoltage