Research On Fault Characteristics And New Protection Principle Of AC System With DC Infeed

With the formation of China’s AC/DC interconnected power grid,the electrical coupling between AC/DC systems is becoming more and more close.The influence of DC feeding on the fault characteristics of AC system has become one of the most concerned problems.Due to the existence of large capacity nonlinear power electronic devices and fast dynamic response of DC control in DC system,the fault current fed by DC is very different from that of traditional AC system,which may lead to incorrect action of AC protection based on power frequency electrical quantity.Therefore,this paper analyzes the transient process of the converter in detail around the AC system fault,studies the influence of DC feeding on the AC side relay protection,and finds the corresponding solutions.The results are as follows:This paper analyzes the adaptability of the existing DC system fault equivalent model.The results show that:in the case of normal operation and fault recovery,the existing models can well describe the operation state of the system;However,the accuracy of the current model is greatly reduced during the system fault,especially in the case of DC side commutation failure.The reason is that it does not consider the transient process of the converter and does not correspond to the actual conditions and DC control strategy.Therefore,this paper proposes a DC closed-loop model based on real-time converter bus voltage and considering converter transient process,which mainly includes three parts:converter model,DC transmission line model and DC control model.The coordination between DC control strategies is analyzed in detail,and the actual system control parameters are given;This paper focuses on the transient process of the converter under various typical commutation failure conditions,obtains the differential expressions of the current and voltage of the converter valve,then obtains the DC current through the DC transmission model,and finally obtains the instantaneous value of the fault current fed by DC,and verifies the correctness of the theoretical analysis through simulation.Based on the analysis of the transient process of DC commutation failure induced by AC system fault at the receiving end,the mechanism of the influence of DC feeding on AC protection is revealed.In order to solve the problem that the DC feeding causes the action current of current differential protection to decrease and the braking current to increase,which causes the protection to refuse to operate,a new incomplete differential protection method is proposed,in which the AC current near the DC side subtracts the DC current and then makes current differential with the opposite side,which fundamentally solves the influence of DC feeding on the protection.Aiming at the problem that the measured impedance of distance protection fluctuates greatly due to DC feeding,a method based on the combination of periodic filtering and cumulative filtering of DC current fluctuation is proposed,which effectively solves the problem of incorrect protection action caused by measured impedance fluctuation.