Research On The DC Fault Protection And Control Strategy In Modular Multi-level High Voltage Direct Current Transmission System

Voltage source converter based on high voltage direct current(VSC-HVDC)is a new and flexible system of power transmission.HVDC based on voltage-source converter can efficiently solve the issues of grid-connected and absorptive of large-scale clean energy,which also provide support for global interconnect technology of energy development.Voltage source converter is an important part of VSC-HVDC.Among that,modular multilevel converter(Modular Multilevel Converter,MMC)has many advantages as output levels,the high quality of output waveform and low switching frequency of the device due to its structural features.Multilevel high voltage direct current transmission system(MMC-HVDC)which is based on MMC module has incomparable advantages comparing with traditional flexible DC transmission.The paper focuses on researching MMC-HVDC,and mainly studies on the topology of MMC,modulation strategy,sub-module capacitor voltage balance,faults protection and control of DC and so on.First,the author makes a detailed introduction to MMC of the work principle,the sub-module(SM)structure of modular multilevel converter,sub-module and three-phase MMC operating principle.Two modulation strategies(PWM modulation and trapezoidal wave modulation)which are suitable for MMC are analyzed in this thesis.Also,the author selects two common modulation strategies(CPS-SPWM and NLM)to analyze its work principle and apply range,and finds a good method of capacitor voltage balance according to different modulation strategies.What's more,the basic units of the main circuit parameters selection of MMC is explained in this thesis.Second,the author introduces the control system of MMC-HVDC,establishes the mathematical model of MMC,and designs the outer ring of voltage and inner current controller;valve control applies to state-level approximation of modulation,carrier phase-shifting strategy and applicable modules of capacitor voltage balance sorting algorithm.At last,author establishes MMC-HVDC system model and verifies the credibility and effectiveness of the controller.Last,taking the traditional half-bridge type HBMMC as a researched object,the author builds transmission system model and analyzes its faults of DC.The researcher proposes clean DC faults based on full bridge type FBMMC aiming at the problem that HBMMC can not eliminate DC side fault.Besides,the author analyzes its DC fault clear mechanism and further combines full bridge sub-module(FBSM)with half bridge sub-module(HBSM)to form the sub-module mixed type MMC in order to research its clear DC fault mechanism and optimize mixed type MMC topology parameter design.By making a comparative analysis of typical module MMC simulation model,the author finds that HBMMC can not effectively eliminate the DC fault.FBMMC and sub-module mix MMC can realize self-clean of DC fault depending on converters without AC breaker.Also self-prevent sub-module can block the DC fault current.