The Research Of Capacitor Balance Control Strategy For High Voltage And Large Power MMC-STATCOM

With the rapid growth of China's economy and the sharp increase in electricity demand,the construction of the smart grid based on the ultrahigh voltage power grid and the coordinated development of all levels of power grid to realize electric power supply and power resource allocation in large scale and long distance transmission is necessary.In order to ensure the quality of electric power,the high voltage and high power reactive power compensation device is put forward to provide sufficient reactive power.Modular multilevel converter shares the advantages of high modularization,low output harmonic content and so on,which can be conveniently applied to the occasions of high voltage and high power.Moreover,when it is applied in power quality management,it overcomes the disadvantage of the traditional STATCOM in boosting and increasing capacity,and has broad prospects.However,with the increase of the voltage level and the number of sub modules,the existing capacitor voltage balancing method based on sorting share weakness like large computation,frequent sorting,high switching losses et al,which will consume huge computing resources of the controller,increase investment in hardware and share some important problems such as frequent sorting and large switching losses.Therefore,how to reduce sorting calculation and switching frequency has always been a scholarly research priority.Firstly,this thesis analyzes the topology,basic principle of modular level converter and the reactive power compensation mechanism of STATCOM,so that the working mechanism of MMC applied to reactive power compensation is obtained-Then,according to the working principle of MMC-STATCOM,the mathematical model of DC and AC side is established,and the relationship between input and output is deduced,on this basis,the control strategy is obtained.Secondly,in depth analysis of the causes of the sub module capacitor voltage imbalance,the NLM modulation method and capacitance voltage balancing strategy based on the sorting algorithm are introduced,and simulation results verify that the analysis of the causes of the sub module capacitor voltage imbalance is correct and the voltage imbalance strategy is effective.Thirdly,by analyzing the problems of the existing voltage balancing method,this thesis optimize Merge sort as assemble sort,drop the number of operations to n in once sorting,then put forward the re-rank rules to reduce the computation and switching losses,eliminate the reverse caused by discreteness of capacitance parameters to get the correct order afterward,then an improved merge sort capacitor voltage balancing method comes into being.Finally,after studying the method of building the MMC-STATCOM system model in the MATLAB/SIMULINK environment,the 21-level model is built to verify the Improved capacitor voltage balance algorithm.By compared with traditional voltage balance strategies in five aspects:voltage deviation,sorting calculation,ranking frequency,switching frequency and the actual simulation time,the simulation result indicate that the improved voltage balance strategy based on merge sort can both reduce the computation budget and the switching frequency on the premise of maintaining good voltage balance,which verifies the correctness and efficiency of the improved voltage balance strategy.At the end of this paper,the main circuit parameters of MMC-STATCOM system and some hardware and software are briefly designed,which will lay the foundation for future work.