Research On Power Flow Control And Short-circuit Fault Suppression Strategy Of Multi-terminal DC Grid

The High Voltage Direct Current Transmission based on flexible DC transmission has become an important development direction of the future power grid because of its function of wide-area interconnection of renewable energy and traditional energy.With the continuous development of DC power grid,the problems of power flow control and fault protection have become more prominent.When the number of branches in the DC power grid is greater than the number of converter stations,the problem of insufficient degrees of freedom will occur,and the DC power flow controller(DCPFC)needs to be introduced for line power flow control.When fault occurs in the DC grid,the fault current will increase rapidly,endangering the safe operation of the converter station and the entire system.In DC power grid,power flow control and fault protection problems are usually solved by DC power flow controller,fault current limiter(FCL)and DC circuit breaker(DCCB),respectively.In order to reduce the number of equipment and construction costs,two composite controllers and their control strategies for power flow control and fault current suppression of multi-terminal DC grids are proposed,and related researches have been carried out.Firstly,based on the original power flow control capability of the DC power flow controller,this paper attempts to explore its fault current limiting capability and proposes a composite DC power flow controller with fault current limiting capability.The power flow control part adopts interline DC power flow controller,and the fault current limiting part adopts thyristors to control the inductor input,which realizes current limiting.The topology,control strategy and action process of the proposed controller are studied,and the simulation verification is carried out in the single-ended equivalent system and the four-terminal DC grid.The simulation results show that the controller can control the power flow of two lines during the normal operation,limit the fault current when a pole-to-ground fault occurs on the line,and complete the clearance of the fault current by cooperating with DCCB.The feasibility and effectiveness of the composite device and control strategy are verified.Secondly,a composite DC power flow controller with fault current limiting and circuit breaking capability and its control strategy are proposed.This paper analyzed the structure and operation principle of such a composite device.Then the control strategy under normal working state and line fault state was proposed.The theoretical analysis of the power flow control,current-limiting and breaking processes of the new topology was carried out in stages.In PSCAD/EMTDC,single-terminal converter station and four-terminal DC grids model were built for simulation,which verified the correctness of theoretical analytical calculations.Meanwhile,the simulation results show that the composite device can control the power flow under normal working conditions,suppress the fault current and disconnect the faulty line under fault conditions.Finally,according to the theoretical analysis and single-ended equivalent system simulation results,the parameters of the two composite controllers are analyzed to provide a basis for the parameter design of the controller.The economic analysis of the proposed two composite controllers is carried out.The combination of existing non-composite controllers is selected as a comparison scheme,and simulation experiments are carried out under the same parameters and simulation environment,and the simulation results are compared and analyzed.Based on this,the number of main components of different schemes is calculated,which proves the advantages of the proposed two composite power flow controllers in terms of control performance and economy.