Adaptability Analysis Of Current Differential Protection For AC Transmission Lines Connected To LCC-HVDC Inverter Station And Its Improvement

In recent years,LCC-HVDC(Line Commutated Converter based High Voltage Direct Current)technology has developed and matured,and the AC/DC(Alternative current/Direct current)interconnection system has been formed gradually with the construction of HVDC projects in China.In the AC/DC interconnection system,faults of the AC system side will cause the commutation failure of the inverter station to occur.And new fault features will occur,which will cause the incorrect action of current differential protection.Therefore,it is of great significance to study the performance of current differential protection under the AC/DC interconnection environment.This paper mainly focuses on the adaptability analysis of current differential protection for AC transmission lines connected to LCC-HVDC inverter station and its improvement.In the AC/DC interconnection system,the fault of the AC power grid in the receiving end is likely to cause the DC system commutation failure.Based on the dynamic phasor theory and the switch function method,and combined with the commutation process of DC system,the transient phase characteristics of equivalent power frequency current are obtained under three types of working status of inverter station,including no commutation failure,single commutation failure,and continuous commutation failure.Based on the equivalent model of AC/DC interconnection system,phase characteristics of AC system power-frequency variation current are analyzed.The result shows that the phase characteristics of equivalent power frequency current are different from the pure AC system,which has effect on the performance of current differential protection.According to the phase characteristics of the equivalent power-frequency variation current of AC system,by deducing expressions of the current differential protection in AC/DC interconnection system based on superposition principle,the effect of three different AC transmission line structures connected to inverter station,including single transmission line,several single transmission lines and double transmission lines,on current differential protection is analyzed in detail.The analysis results indicate that current differential protection may reject acting when faults occur under the circumstance of AC/DC interconnected operation.In the AC/DC interconnection system,the differential protections incline to reject acting when faults occur.Therefore,phase related current pilot protection as an auxiliary criterion is proposed to solve this problem.The phase related current pilot protection is of high sensitivity in high resistance earth fault,which can ensure the correct and reliable action of the protection.