| High-voltage flexible DC networks are an effective means of solving the problem of largescale access and consumption of renewable energy,and are one of the important development directions for future power systems.However,its development also makes the problems of power system tidal current control and fault protection more prominent.If the number of DC branches is higher than the number of converter stations,there is the problem of insufficient degrees of freedom,which may lead to line overloads or underloads due to unregulated power.In addition,when a short-circuit fault occurs in the DC network,the instantaneous fault current can seriously jeopardise the safe and stable operation of the converter station and the grid due to the lack of a DC current crossing zero point and the extremely high rise rate of the fault current.In DC networks,tidal current control and fault protection are usually handled by different devices,including DC Power Flow Controller(DCPFC),Fault Current Limiter(FCL)and DC Circuit Breaker(DCCB).DCCB).In order to solve the problem of multi-terminal DC network current control and fault current suppression,as well as to improve the device economy and equipment integration,two composite devices and their control strategies are proposed in this paper.Flow control and fault protection in DC grids are typically addressed by utilizing DC Power Flow Controller(DCPFC),Fault Current Limiter(FCL),and DC Circuit Breaker(DCCB)individually.This article proposes two composite devices and corresponding control strategies suitable for flow control and fault current suppression in multi-terminal DC grids,which can reduce the number of devices and lower construction costs.First of all,the operation principle and the topological structure of the Modular Multilevel Converter(MMC)are analyzed.On this basis,the fault characteristics of flexible DC systems under short-circuit conditions are analysed in depth and fault models are built on a simulation platform to verify the fault opening effect of hybrid DCCBs,providing a theoretical basis for the development and modification of current limiting devices and circuit breakers in power systems.A two-port inductively coupled fault current limiter with tidal current control is proposed.In the tidal current control section,an inter-line DC tidal current controller is used for regulation,while the current limiter section uses IGBT switching to control inductive access to suppress shortcircuit currents.The proposed fault current limiter topology,control strategy and workflow are analysed and investigated,and a four-terminal DC network equivalent model is developed to simulate and validate the method.The simulation results of this topology confirm that it can control the currents of two lines under normal operation and can perform fault current limiting in the event of a single pole earth fault on one line.Also,the topology can be used in combination with DCCB to clear the fault current,thus effectively verifying the feasibility of the proposed fault current limiter and its control strategy.A multi-port inductive coupling type DC circuit breaker with flow control and limiting functions is proposed.The structure and working mechanism of this composite device are studied,and the control methods of the device under normal operation and fault conditions are proposed.On this basis,the flow control,fault current limiting and breaking processes of the new topology structure are further analyzed.In this study,a four-terminal DC grid model is created using MATLAB/SIMULINK for simulation purposes.The simulation results indicate that the composite device can effectively control the flow of power during normal operation and suppress fault current while also cutting off the faulty line during line faults.Moreover,the composite device is suitable for multi-port output,thus enabling device integration and reducing investment costs.The correctness and effectiveness of the research content are verified. |
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