Research On The Key Technology Of Modular Multilevel Converter And Its Extended Application Based On HVDC

The conventional converters are composed of mercury arc valves.Due to many defects in their own performance,their applicable range is greatly limited.With the development of power electronics technology,fully controlled high voltage and high power rate converter emerges,because of its superior performance in terms of volume,power and cost.It has been widely used in various aspects of electrical power production,transmission and distribution.Modular multilevel converter(MMC)was proposed by professor R.Marquart and professor A.Lesnicar from Bundeswehr University in Munich Germany firstly.MMC has unique advantages incomparable to other converters.Once published,MMC structure has attracted extensive attention in new energy generation,high-voltage and high-power converters,electrical power transmission and distribution and other fields.It has become one of the most promising topologies in the field of high voltage and high power converter,especially in the field of flexible electrical power transmission field.Although some achievements has been made in the research on MMC at home and abroad,there are still some problems that need to be further researched and explored to lay the foundation for a large number of engineering applications of MMC.Based on the analysis of previous relevant research results,this thesis conducts in-depth research on MMC from the following aspects:(1)In view of the advantages of MMC,this thesis analyzes the operating principle of MMC firstly,including the characteristics of MMC topology structure,the operating principle of sub-modules and the advantages and disadvantages of the two MMC modulation methods.Then,basic control strategies required by MMC are analyzed,including MMC mathematical model,internal and external loop control strategy,circulating current suppression strategy,and sub-modules capacitor voltage balance strategy,laying a foundation for the subsequent research of MMC.Finally,the effectiveness of the above control strategies are verified by electrical simulation software.(2)Aiming at the configuration problem of the number of redundant submodules in MMC and the control strategy problem of redundant sub-modules,this thesis proposes a configuration method for the number of redundant sub-modules in MMC and a control strategy for redundant sub-modules.Firstly,the topology of MMC and the working principle of sub-modules are studied and analyzed.Based on the objectives of MMC system reliability,the effective utilization rate of redundant submodules,and configuration of the redundant sub-modules in an unbalanced bridge arm(maximum redundant sub-module value),establishing a set of objective functions,and the calculation method of the number of redundant sub-modules of MMC is proposed.Furthermore,the correlation coefficient between the number of redundant MMC submodules and the application scenario is set.According to different application scenarios,the correlation coefficient is determined to make the configuration method of the number of redundant sub-modules of MMC more universal.Finally,the calculated result of redundant sub-modules is combined with the control strategy of redundant sub-module for simulation verification.The simulation results show that the configuration method of the number of redundant sub-modules in MMC system is reasonable and effective,and this configuration method is suitable for the control strategy of redundant sub-modules.The control strategy and the configuration method of the number of redundant sub-modules in MMC can be directly applied to the actual MMC project,which is of great practical significance.(3)In the case of variable frequency operation,when MMC running frequency is too low,the capacitor voltage of MMC sub-modules will have serious fluctuation.The suppression strategy of capacitor voltage fluctuation in MMC sub-modules is proposed,when MMC is running in variable frequency situation.This thesis studies the causes of capacitor voltage fluctuation of sub-modules in the low-frequency situation of MMC firstly.Secondly,based on the traditional method of injecting high frequency components into MMC phase bridge arm circulating current and output voltage to suppress capacitor voltage fluctuation of MMC sub-modules,the specific values of high frequency components to be injected are calculated.Then,the method of injecting high frequency component is analyzed,and the influence of this method on the capacitor voltage fluctuation of MMC sub-modules and the stability of MMC system is analyzed.Then,in order to solve the existing problems,an MMC sub-modules capacitor voltage fluctuation suppression strategy is proposed.Finally,the simulation results show the effectiveness of the strategy.The suppression strategy of capacitor voltage fluctuation of MMC sub-modules in variable frequency situation can be widely applied to the field of variable frequency speed regulating field.Under the premise of ensuring the normal operation of MMC,this suppression strategy reduces the influence of bridge arm circulating current of MMC system and improves the reliability of MMC system.(4)Aiming at the problem of MMC-HVDC system as power supply for grid start-up and recovery.This thesis analyzes the feasibility of MMC-HVDC as the start power source for grid start-up and recovery from three aspects,which are working principle,control strategy and implementation scheme.According to analysis results,designing corresponding MMC-HVDC control strategy,which are MMC-HVDC system start-up control strategy,the over-voltage suppression strategy when starting the long-distance no-load transmission line,the control strategy of starting the generating unit without self-starting capacity,and the cooperative work control strategy of MMCHVDC system and the generating unit.In addition,according to the principle of power grid startup and recovery scheme,a complete power grid recovery process is proposed,which is based on MMC-HVDC system.Finally,the whole power grid startup and recovery process is performed in PSCAD/EMTDC.The simulation results show that the voltage and frequency of the system are kept in a stable range during the whole power grid recovery process,which proves that MMC-HVDC system is feasible and advantageous for power grid start-up and recovery.(5)Setting up MMC physical experiment platform.,the MMC experimental platform is introduced in detail,and the hardware selection of the experimental platform is discussed.In this thesis,the MMC experimental platform is introduced in detail firstly,and the hardware selection of the experimental platform is discussed.Then,analying the control system of MMC experiment platform,and designing the control system of MMC experiment platform.Finally,the established MMC experiment platform is used to verify the control strategy of MMC sub-modules capacitor voltage fluctuation suppression strategy.The experimental results verify the feasibility and effectiveness of this control strategy.