Research On Control Strategy For Modular Multilevel Converter

In recent years,with the rapid development of power electronics technology,multilevel converters have been widely used in high voltage and high power applications,and new multilevel converter topologies are emerging one after another.Modular Multilevel Converter(MMC)has attracted much attention since it was proposed in 2001.With its many advantages such as modular structure and flexible expansion,it has been widely used in the field of high-voltage and even ultra-high voltage power transmission.However,MMC are still limited by their own topological structure in certain specific applications,which leads to problems such circulating currents and capacitor voltages imbalance.In response to the above problems,this paper takes MMC as the research object,and carries out research and analysis on its main control strategies.Firstly,the paper discuss the main circuit topology of MMC,the common structure of sub-modules and working principle.Construct the mathematical model,and derive two simplified equivalent models.The common modulation strategies are analyzed,and the control effects of carrier phase shifted sinusoidal pulse width modulation strategy in two different level modes are verified by simulation.This paper focuses on the comparison between the closed-loop control of capacitor voltage and the voltage sorting control strategy.Aiming at the switching loss problem of traditional sorting control,an optimized sorting control to improve the switching frequency and switching loss is studied and verifies by simulation comparison.Secondly,through the analysis of the circulation generation mechanism,a steady-state mathematical model of circulation is established.The circulation suppressing strategy based on PI controller is studied for the negative sequence double frequency circulation that accounts for the main component.To solve the limitations of the application of PI controller and meet the control requirements of MMC with nonlinearity and strong coupling,the paper proposes a circulation suppressing strategy based on fractional order PI~?.It designs fractional order control of MMC circulation suppressing using frequency design method and coordinate vector method,and realize the rational approximation and discretization of the fractional integral operator using Oustaloup filtering and Tustin discretization.And the simulation shows that the circulating current suppressing strategy based on fractional order PI~?has better control performance.Thirdly,the finite control set model predictive control(FCS-MPC)is analyzed for the problems of multiple MMC control targets and strong coupling.To solve heavy calculation problem of traditional FCS-MPC strategy,the paper studies fast optimized model predictive cntrol strategy based on NLM(NLM-FOMPC).On the basis of adding voltage sorting control to reduce the prediction of control variables,the paper introduces NLM to achieve rolling optimization is no longer affected the number of modules is limited.Simulation verifies the control performance of the NLM-FOMPC strategy.Finally,the RTDS(Real Time Digital Simulator,RTDS)experimental platform is introduced,and the MMC control system model is built.The simulation results verify the feasibility and effectiveness of the relevant control strategies.There are 87 figures,5 tables and 96 references in this thesis.