Research On The Strategy Of Clearing And Recovering DC Line Fault In AC/DC System

Due to the flexible and fast control characteristics,High voltage direct current transmission project has been widely used in China.While promoting the overall utilization of energy resources in China,its complex and changeable electrical characteristics also pose a great threat to the safe and stable operation of power grid.As one of the most common faults in DC system,DC line fault will undoubtedly bring new challenges to the protection and reclosing strategy based on traditional AC characteristics.Therefore,this paper focuses on the scene of DC line fault in AC/DC system,and studies the restart strategy of DC line.The strategy of adaptive restart is put forward to judge the failure state of the system before restart.The simulation results verify the correctness of the strategy proposed in this article.The achievements of this paper are summarized as follows:Aiming at the problem that the existing DC restart strategy lacks the process of identifying the nature of the fault before the restart operation,it is possible that the fault still exists at the restart time,which leads to the restart failure and brings great impact to the system,a DC adaptive restart strategy based on the recovery voltage amplitude is proposed.Through the analysis of the establishment process of the recovery voltage at the outlet of the inverter station,it is found that the recovery voltage is related to the system structure parameters,the trigger angle of the inverter station and the fault condition.On this basis,the expression of inverter outlet voltage including fault location,transition resistance and fault duration is obtained analytically.The recovery voltage amplitude is used to judge whether the fault disappears or not,and then the DC adaptive restart strategy based on the recovery voltage amplitude is constructed.Aiming at the problem that the DC adaptive restart strategy based on the recovery voltage amplitude takes a long time and is prone to misjudgment in special cases,this paper proposes to use the shorter recovery voltage gradient to judge whether the fault exists or not,so as to realize the DC adaptive restart.Based on the analytical expression of recovery voltage,the expression of recovery voltage gradient is obtained.It is found that the main factors affecting the recovery voltage gradient are the size of transition resistance,fault location and fault duration.On this basis,the transient/permanent fault identification criterion is constructed to realize adaptive restart of DC lines by using the characteristics that the voltage gradient at the outlet of inverter station decreases monotonously and tends to zero quickly in case of permanent fault,and the gradient value will decrease sharply after the transient fault disappears.Aiming at the problem that the DC voltage measurement device is easily interfered by external factors and the measurement value is inaccurate,which leads to the unreliable judgment results of the above two strategies,a DC adaptive restart strategy based on the rate of change of DC current is proposed.When the permanent fault occurs,the current change rate is almost zero;when the transient fault occurs,the current change rate will change greatly at the moment when the fault point disappears.It is found that the current change rate is almost independent of the fault parameters when the transient fault disappears.As long as the system parameters remain constant and the sampling time is determined,a stable change rate can be obtained.