RESEARCHANDAPPLICATIONOFPROTECTIONTECHNOLOGY

so that fault components at fault point are larger than elsewhere.Normal load current does notinfluence protection based on DPFC principle at all.Fault transient resistance slightly affectsDPFC protection.In many cases,DPFC protection has excellent performances.The first DPFC protection,DPFC directional comparison protection was invented in 1982.In thefollowing years,the whole DPFC protection system was set up successively,DPFC distanceprotection and DPFC current differential protection were developed,at the same time thistechnology migrates from line protection to busbar,transformer and generator protection.Largeamount of real on-site operation records proved that DPFC line,busbar,transformer and generatorprotection are of high security,high reliability and fast operation speed,and play an important rolein improving electric power system security,stability and transmission capacity.1.1.DPFC directional comparison protectionNo matter what type of fault,which fault phase,whether or not a transferring fault,whether or notunder phase discrepancy condition,the phase angle between voltage DPFC and current DPFConly depends on system impedance backward from where the protection installed,and has nothingto do with system electromotive force and fault transient resistance.Line fast protection based onDPFC directional comparison protection can distinguish between positive direction and negativedirection unambiguously.This protection was proved reliable by practice and has been applied toelectric power system widely and successfully [3][5][7].1.2.DPFC distance protectionFault on and near busbar is the most serious fault for electric power system transient state stability.Clearing such kind of fault quickly is key to keep system stability.As to line protection,mainprotection,i.e.pilot protection,cannot trip related breaker quickly enough with the limitation ofsignal transmission time delay.Operation time of distance zone I should not be less than onecycle for the sake of security of itself.DPFC distance protection is a kind of distance protectionusing fault component,and has ultra fast operation speed along with high security.In fact,DPFCdistance protection has set up a record of 3 ms operation time on site.1.3.DPFC current differential protectionCurrent differential protection based on DPFC principle bas been applied to line,busbar,transformer and generator successfully [9][11].RCS-931 ultra high voltage series protection employed DPFC current differential protection withadaptive floating threshold,which can withstand system unbalance and disturbance.Themeasuring element can operate with ultra high speed and high security.The pickup element basedthe same principle has high sensitivity,and yet it will not pick up frequently under nommalcondition even with unbalance and disturbance thanks to floating threshold.