Research On Current Differential Protection Of Regional Power System With Complex Series Compensation Equipment

With the development of high capacity and long distance in UHV and EHV transmission line,the series compensation device(SC for short)is widely applied to the power system.SC is favourable to the performance of the power system,for providing better transmission capability,increasing of the stability,etc.However the balance of the vectors in transmission line is destroyed by the introduction of SC equipment.The capacitive characteristic of SC may cause the problem like current inversion during fault incepted on such a compensated line to some extent.Furthermore,the risk of fault current reverse will be increasing during the line internal fault with the emergence of these phenomena that the scale expanding,the complex interconnection of power grid,the decrease of the system impedance in the back of the line and the relatively high capacitance impedance.In consequence,the failure of pilot current differential protection in series compensated line will be increasing.Therefore,the influence of SC on the line protection of regional power grid is becoming more and more serious,which makes the relay protection facing new challenges.Based on the principle that differential protection will fail to trip due to lack of sensitivity during current reversed,the effects of SC on line differential protection and the improved principle of differential protection are studied in this paper.It has following several aspects specifically.(1)The characteristics and defining conditions of the reverse current are analyzed for the regional power grid with SC equipment.How SC affects the operation characteristics of relaying with different principles is also discussed.It provides a theoretical principle and method for subsequent research about the influence of SC equipment on line differential protection.Combined with engineering models,the requirement for current inversion has been verified under the environment of EMTDC Engineering.(2)An improved criteria based on the current amplitudes and phases on both sides of series compensated lines are proposed to solve the misstrip of line current differential protection.The reason is analyzed for the reduction of sensitivity calculated by the differential protection at the internal fault.In addition,the operation characteristics of differential protection are studied by focusing on the multiple factors which include different series compensation,power angle difference and transition resistance etc.Research shows that the phases of the two ends of the protected line are not the same,even though the current is reversed.Thus,to improve the sensitivity of protection,the fault current amplitudes and phases are introduced to form a comprehensive restraint criterion.The simulation results based on PSCAD/EMTDC show that the protection sensitivity is obviously increased in the improvement scheme during fault current reversed when there is an internal fault on the line.The scheme has no effect on the reliability of other fault conditions too.(3)For the large power system with multiple series compensator,it's very easy to appear current reverse during internal or external faults in the system.Therefore an improved braking characteristic of wide area current differential protection criterion is proposed.Based on the basic principle of the traditional wide area differential protection,the advantages and disadvantages of the four conventional braking criteria for differential protection in regional large grid system are compared and analyzed theoretically.Further,reasons are obtained about the misstrip of wide area current differential protection under different fault conditions through applying current reverse characteristics in the protected area.An implementation method based on restraint characteristic of wide area current differential protection is used to distinguish the direction of the wide area restraint current.According to Kirchhoff's current law and learning from the improved restraint characteristics mentioned above,current flows in and flows out from the same direction are analogous to the current at both ends of the line.Based on PSCAD/EMTDC simulation method,this thesis validated the effectiveness of the related improved measurement.(4)To verify the improved criterion of wide area differential protection mentioned above,dynamic simulation is used to simulate.