The Research On The Sub-module Voltage Balanced Control Strategy Of MMC-HVDC Based On Model Prediction Control

In recent years,with the rapid economic development and continuous innovation in the field of power electronics automation,There has been a growth in the demand for electricity in the society.However,from the perspective of energy distribution in China,the distribution of new energy sources,such as wind power and photovoltaic power generation,is not only distributed more dispersedly,but also farther away from the electricity area.Besides,there is a kind of imbalance of the power generation and consumption between the eastern and western regions.Most of the clean energy is located in remote areas in the west.However,large electricity users are mostly distributed in the eastern provinces.In order to solve these imbalances between supply and demand,long-distance and large-capacity transmission systems are getting more and more attention.Therefore,an efficient and flexible DC transmission system has emerged.As a promising transmission method,flexible DC transmission has great advantages in the field of renewable energy generation and grid connection,and island power supply.Having the advantages of high efficiency,low harmonic,modular design and easy cascade,the modular multilevel converter(MMC)has become the most potential method in long-distance HVDC transmission systems and has been widely used in the field of high voltage and large capacity energy conversion.This thesis takes the two-terminal MMC-HVDC system as the research object,and it takes an elaborate description of three kinds of topological structure,working principle,mathematical model of MMC sub-module,and it studied the sub-module capacitance voltage equalization,internal circulation suppression and system control strategy improvements.Through the comparison and analysis of the advantages and disadvantages of traditional control strategies and sub-module capacitance voltage equalization algorithms,in order to improve the dynamic response speed of the MMC-HVDC based on the modular multi-level converter,a novel method which combines a model predictive control(MPC)of MMC-HVDC system with improved sub-module voltage balanced control strategy is proposed.The method,which utilizes the prediction model,feedback correction and rolling optimization to obtain the optimal voltage control,overcomes the difficulties in the traditional way of setting PI parameters of the internal loop current controller and the outer loop controller and tackles the problem of low dynamic response.Aiming at the problem of voltage balanced of sub-module capacitance,the nearest level approximation modulation(NLM)equalization method based on ordering algorithm is adopted torealize effectively the balance.Finally,a 21-level MMC-HVDC system simulation model is built on the PSCAD-EMTDC software platform.The simulation results indicate the feasibility and effectiveness of the control strategy.