Research On Fault Pass-through And Control Strategy In AC Side Of Modular Multilevel 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(VSC)provides a method for solving problems such as the large-scale clean energy grid connection,and it also provides a pioneering practice for the construction of the global energy internet.Voltage source converters are mainly divided into two-level converters,three-level converters and modular multi-level converters(MMC),of which the modular multi-level converter has a large number of output levels.The advantages of high output waveform quality,relatively low switching frequency of power electronic devices,and small switching losses have gradually become the focus of research on VSC-HVDC.This paper takes the HVDC transmission system based on modular multi-level converter as the research object.It mainly studies the topological structure of MMC,modulation strategy,circulation suppression,sub-module capacitor voltage balance control,AC unbalanced fault traverse control and other issuesFirstly,the author makes a detailed introduction to the topology structure of the modular multilevel converter,the operating principle of the sub-module(SM)and the working principle of the three-phase MMC.Two modulation strategies commonly used in MMC are analyzed,including pulse width modulation(PWM)in carrier phase-shift width modulation and nearest-level approximation modulation(NLM)in step-wave modulation,and the applicable ranges of these two modulation strategies are analyzed.At the same time,the mechanism of the circulation flow in MMC is also analyzed,and a method to suppress the circulation is proposed.Secondly,the author introduces the control system of MMC-HVDC.It is divided into system control,converter station control and converter valve control,and establishes the mathematical model of MMC.Based on this,the MMC controller is designed and the improved control strategies based on nonlinear current control is proposed.Analyzing the sub-module capacitance voltage fluctuation mechanism,and proposing the sub-module capacitor voltage equalization strategy based on sequencing.Finally,a 21-level MMC-HVDC system simulation model is built.The effectiveness and feasibility of the improved control strategy was verified by the simulation resultsFinally,taking the unbalanced fault on the AC side of the MMC-HVDC as the research object,analyzing the characteristics of the unbalanced fault on the AC side,it is concluded that the mathematical model of the unbalanced fault on the AC side of the MMC-HVDC consists of two independent positive sequence systems and negative sequence systems that do not affect each other.Two unbalanced fault traversal control strategies on the AC side are introduced Building a 21-level MMC-HVDC simulation model.Two single-phase ground faults and two-phase short-circuit faults on the AC side are used for simulation analysis.According to the simulation results,compared with the general control strategy,the improved control strategy based on the nonlinear current control responds faster when facing the unbalanced fault on the AC side and can reduce the overcurrent during the fault.