| The rapid development of power electronic devices and technology promotes the progress of voltage source converter based DC transmission.Compared with AC transmission system,DC transmission system has the advantages of fast regulation speed and high stability,and has gradually developed into a multi terminal DC transmission system including multiple power transmitting and receiving terminals and its final form——DC power grid.Compared with the traditional two terminal DC transmission,DC grid has the advantages of high reliability and good redundancy.It is an effective technical scheme to solve the interconnection of asynchronous grid and large-scale renewable energy grid connection.However,DC grid still has two technical problems at this stage: flexible power flow regulation and fault current suppression,which limits its engineering popularization and application.As the core equipment of DC grid,DC power flow controller is mainly used to regulate the power flow distribution in the system,and its fault current limiting potential can be explored through in-depth analysis.Therefore,the research on DC power flow controller with fault current limiting capability has great scientific research significance and engineering application value for solving the problems of power flow control and fault current limiting in DC grid.In this paper,relying on the national key R&D program "Current Limiting Principle and Topology of DC Grid Devices" [2018YFB904603],the power flow regulation principle and fault current limiting method of the fault current limiting type DC power flow controller suitable for DC grid are studied.It involves cascade topology,power flow regulation and power balance control method,voltage reduction current limiting method and parameter optimization design of dual-interline and multi-interline DC power flow controller with fault current limiting function.The main work is as follows:(1)In the research background of DC grid,the basic principle,topology structure and fault current limiting scheme of DC power flow controller are investigated and analyzed.The basic characteristics of DC transmission system are introduced,the development trend of DC system from two terminals and multiple terminals to DC grid is summarized,and the two technical problems of DC power flow control and fault current limitation are expounded.On this basis,the power flow control principle of DC grid and the classification of DC power flow controller are introduced,and the most promising interline DC power flow controller is selected for indepth research.The basic classification of interline DC power flow controller is summarized,the structural characteristics of different topologies are compared,and the modular cascaded interline DC power flow controller suitable for high voltage and high power DC system is analyzed in detail.In addition,aiming at the problem of fault current limiting in DC power grid,this paper introduces the existing current limiting schemes,expounds the current limiting potential and self-protection mechanism of interline DC power flow controller,and determines the basic research direction——interline DC power flow controller with fault current limiting function.(2)Aiming at the demand of power flow control and fault current limiting between two lines,a dual-interline fault current limiting DC power flow controller is proposed.The topology of dual-interline DC power flow controller is given.Its topology has the advantages of modularization and expansibility,which can be applied to high-voltage and high-power DC system to realize the reasonable distribution of power between two lines.A time-sharing control method of voltage and current corresponding to this topology is proposed,which can realize the independent control of line current and capacitor voltage.The operation characteristics of dual-interline DC power flow controller under DC short-circuit fault condition are analyzed,the current limiting method by reducing voltage and overvoltage bypass protection strategy are proposed,and the system and internal factors affecting the current limiting capacity are studied.The simulation model is established in PLECS software,and the results show that the topology is effective under power flow control,power step and fault current limiting conditions.(3)Aiming at the requirements of power flow control and fault current limiting between multiple lines,a multi-interline fault current limiting DC power flow controller is proposed.The topology adopts modular structure,which can not only increase the output voltage by expanding the number of sub modules,but also increase the number of ports by expanding the number of bridge arms,so as to realize the flexible regulation of multiple line currents and fault current suppression.By establishing the equivalent circuit model,the power flow control and fault current limiting principle of multi-interline DC power flow controller are analyzed.The power balance mechanism of multi-interline DC power flow controller is studied,and a hierarchical power balance control method is proposed.A three terminal DC transmission system is built in the simulation software to verify the working principle and control method of multi-interline DC power flow controller.(4)Aiming at the dynamic simulation platform of six terminal DC power grid in the national key R & D program,a fault current limiting dual-interline DC power flow controller suitable for 500 V / 10 A DC system is designed and developed.Firstly,the structure and parameters of the experimental device are introduced,and the capacitance parameters,power switch and driving circuit of the dual-interline DC power flow controller are designed and selected in detail.On this basis,the sampling hardware circuit,protection hardware circuit and main controller hardware circuit of the control system are designed.The sampling,PI control,PWM modulation and communication modules are designed in the FPGA control system,and the corresponding programs are developed.The DC system is built in the laboratory to verify the power flow control and fault current limiting functions of the proposed power flow controller.Finally,the device is applied to the six terminal DC power grid dynamic simulation platform to prove the effectiveness of the DC power flow controller in the DC power grid system. |
