Key Influencing Factors Analysis Of MMC-HVDC Grid DC Side Fault Current

With decades' continuous development,voltage source converter based HVDC as the third generation of direct current transmission technology has shown broad application prospects in medium and high-power fields.Particularly in renewable energy generation,island power supply,urban power distribution network,static var power compensator,power quality improvement and other important areas,it represents unique advantages.The modular multilevel converter(MMC)as the core of voltage source converter-based DC grid technology(MMC-VSC-DC Grid)will be one of the key technologies for the integration and transmission of multi-energy systems in smart grids.However,in practical engineering scenarios,many tough issues still exist and need to be solved urgently,particularly in rapid isolation of DC side faults.Research on the evolution law of shortcircuit fault on the DC side is the basis for the development of protection strategies.This thesis focuses on the key influencing factors of the DC side short-circuit fault current evolution of the MMC-HVDC grid,explores the influence mechanism of valve topology,system parameters,and provides a reference for MMC-HVDC paremeter design and protection.The major works and initiatives are as follows:(1)According to the basic MMC topology,the mathematical model is derived.The working process and characteristics of different valve structures are explained in detail.The double-ended pseudo-bipolar PSCAD/EMTDC simulation models with three submodule(half-bridge sub-module,full-bridge sub-module,improved hybrid sub-module)topologies are established.The maximum arm fault current detection is embedded to test the DC side fault simulation.(2)The RLC equivalent circuit is adopted to analyze the fault isolation principle of different SM topologies,and the existing SMs are classified according to the arm equivalent capacitor parameter.The analytical expression for the fault current is derived.Meanwhile,based on the fault evaluation index it is concluded that the DC side shortcircuit resistor,smoothing reactor and valve structure are the key factors that have a significant impact on the evolution of the fault current.The fault isolation time influenced by the key factors is studied,especially the smoothing reactor and valve structure.(3)The transient energy analysis method is proposed,and the unified definitions of resistor,inductance,capacitance,and AC-side transient energy are described.Then,the transient energy transfer in different regions(AC side,MMC station and DC side)during fault transients of the double-ended pseudo-bipolar MMC-HVDC is studied in stages.(4)The transient energy transfer diagrams of different valve structures reveal the fault transient energy transfer law,which help propose an improved MMC topology.The fault current evolution characteristic of single-pole ground faults at different fault points in Zhangbei four-terminal DC power grid is studied.