Research On Protective Relay Scheme For 220kV Double Circuit Lines On The Same Tower

As a important electric power transmission modle ,the double circuit lines on the same tower have been used increased, because it may enhance the transmission capacity of each way, reduce used ground and save cost. There are 120 kinds of faults occurred in double circuit lines, for the existence of the cross country faults. It is very complex when we calculate and analyze the faults occurred in double circuit lines, so it is very significant to study the character of cross country faults in double circuit lines and bring forth a protection scheme used in double circuit lines.There is mutual-induction between the phases alsobetween the lines in double circuit lines, thus the protective relay used in single circuit lines will act incorrectly .There are several special problems when the cross country faults are occurred in double circuit lines, for example, the two normal same phase current may not be equal when the faults occurred in single circuit lines or the cross country faults occurred between the different phases ;the two fault same phase current may not be equal, when the cross country fault occurred in the operation between two lines. These special problems have greatly effect on the correct action of relay, so the six-sequence component theory is presented, eliminating the mutual-induction is its virtuality. We can get the sequence current and sequence voltage of double circuit lines when its mathematics model was associated with the boundary condition of the faults. Then we analyzed the special problems in double circuit lines using the theory above.The results of analysis will support the protection scheme used in double circuit lines later.The fault phase selection is a key factor whether theprotective relay will act correctly or not. The research on the behavior of the phase current difference suddenly-change and the sequence current elements under various cross country faults in double circuit lines is presented. The result of the research indicated that error phase may be selected by above two elements under such faults conditions. Then the phasor condition between the phase currents under cross country faults in double circuit lines are analyzed. A new fault phase identification method is proposed, which is based on comparing the difference of the fault voltage judging the phasor condition between the fault currents .The fault phase selector of new protection in thesis is integrated by three components mentioned above.Finally a 220kV protective relay scheme is proposed which is used in double circuit lines. The fast major protection is the pilot distance relay and zero sequence direction relay; the fast I section protection is consisted of the main frequency variation distance selector; the reserved protection is consistedof three steps phases and ground distance relay and twodelayed zero frequency direction overflowing relay; the fault phase selector of new protection in this scheme is integrated by three components in double circuit lines ; finally the theory of the protective relay and the action performance are analyzed.