Research On Commutation Failure Suppression Strategy And Fault Recovery Method For UHVDC System Under Hierarchical Connection

With the continuous growth of new energy installed capacity in the three north area and the increasing load demand in the eastern coastal area,ultra-high voltage direct current(UHVDC)system under hierarchical connection has become an important technical means to improve the voltage support capacity of multi infeed DC system and guide the reasonable distribution of power flow.Because the UHVDC system under hierarchical connection uses semi controlled thyristor as the converter unit,the inverter station is very sensitive to the voltage change of AC system.If the AC bus grounding fault occurs,it will increase the risk of inverter side converter commutation failure and threaten the safe and stable operation of AC power grid.Therefore,this paper focuses on the inverter side commutation failure and commutation failure recovery of UHVDC system under hierarchical connection.Firstly,the basic structure and control method of UHVDC system under hierarchical connection are introduced in detail,and the response characteristics of inverter side converter to commutation voltage change and commutation failure recovery characteristics are simulated and analyzed.The commutation failure mechanism of fault layer and non fault layer converter and the interaction law of controller causing subsequent commutation failure are obtained,which lays a theoretical foundation for optimizing system transient characteristics and fault recovery characteristics.Secondly,aiming at the problem of simultaneous commutation failure of high and low side inverters,a coordinated control strategy is proposed based on the dominant factors of commutation failure and the commutation voltage time area theory.The proposed strategy takes the decrease of AC voltage as the dominant factor of commutation failure of fault layer converter and the increase of DC current as the dominant factor of commutation failure of non fault layer converter,which can suppress the simultaneous commutation failure of high and low side converters in UHVDC System under hierarchical connection.Then,based on the above coordinated control strategy,a dynamic current margin control method is proposed to solve the problem of controller interaction law in the process of commutation failure recovery,so as to clarify the current control objective in the process of fault recovery,and then suppress the subsequent commutation failure to improve the system recovery characteristicsFinally,simulation results show that the above control strategy can effectively suppress high and low end converter commutation failure and subsequent commutation failure,and improve the transient characteristics and fault recovery characteristics of the system.