Research On Voltage Equalization And Circulating Current Suppression Method Of Modular Multilevel Converter

As a frontier subject in power field,power electronics technology is widely used in the research of HVDC,flexible AC transmission,new energy generation and grid connection,reactive power compensation and uninterrupted power supply.In the context of the current global energy Internet construction,research on the development of high-voltage and high-power inverters and their control strategies have received widespread attention from scholars at home and abroad.Modular multilevel converter(MMC)is a new type of topology converter that can replace traditional voltage source converters.With its advantages of high modularity,simple control circuit and strong extensibility,MMC is widely used in the latest HVDC projects.In this paper,the methods of modulation voltage equalization and circulating current suppression of MMC are mainly studied.Firstly,the topology and modulation operation principle of MMC and its sub-modules are analyzed.The output characteristics and operating characteristic equations of MMC are derived based on the average model.According to the operating characteristics of MMC,the principle of internal circulation in modular multi-level structure and the equalization problem of sub-modules in modulation control under two level modes are analyzed.The research direction of submodule voltage equalization and circulating current suppression for MMC control is clarified.According to the requirement of stable operation of sub-module voltage,the coordination relationship between submodule capacitor voltage equalization and MMC modulation strategy is studied,and the step-wave modulation at 2N+1 level is improved.Two kinds of capacitor voltage equalization methods for MMC sub-modules are systematically studied,and the effect of modulation changing output level on voltage equalization is also studied.The effectiveness of each voltage equalization strategy is verified by simulation.On the basis of uniform equalization of voltage,Top K sorting algorithm is introduced,and the hardware circuit implementation scheme of Top K algorithm is designed by means of voltage controlled oscillator(VCO),which can effectively improve the sorting efficiency.In addition,the triggering condition of sorting link is improved by introducing the algorithm frequency reduction strategy,and the switching frequency and loss of sub-modules in the process of voltage equalization are reduced to improve the efficiency of MMC.The principle of MMC circulating current suppression is deeply studied.An adaptive hysteresis interval circulating current suppression method is proposed by optimizing the control strategy.The hysteresis interval permissible range is adjusted adaptively according to the voltage offset of each phase bridge arm sub-module and the calculation results of the circulating current equation.The contradiction between the effect of circulating current suppression and the switching frequency is effectively balanced.By building a two-terminal active MMC-HVDC system,the adaptability and reliability of the improved control strategy of equalizing voltage and circulating current suppression proposed in this paper are verified from the system control level.Finally,simulation models are built to verify the feasibility of modulation strategy,Top K sorting,algorithm frequency-lowering and voltage equalization strategy,and hysteresis interval adaptive circulating current suppression method.The simulation results are compared with those of traditional methods to verify the effectiveness of the above theories and methods.