Research On Leakage Current Suppression Of Photovoltaic Medium Voltage Modular Multilevel Inverter

At present,the consumption of primary energy is increasing and the environmental damage is becoming more and more serious all over the world.Photovoltaic and other new energy sources have attracted more and more attention,which have the characteristics of environmental protection and safety,and have a wide range of raw materials,and have been paid more and more attention and applied.In recent years,the installed proportion of photovoltaic power generation is increasing,and the main forms are large-scale centralized power generation and medium-high voltage long-distance transmission,so as to reduce the stress of converters and transmission lines and reduce the cost.Traditional inverters can’t meet the requirements of voltage level upgrading,so multilevel converters came into being.Among them,the converter represented by Modular Multilevel Converter(MMC)is widely used in such emerging fields as flexible DC transmission.However,the leakage current of the system will affect the quality of the output current,and even endanger personal safety.In this paper,a new modulation strategy for photovoltaic inverter under medium and high voltage conditions is adopted to suppress leakage current,which is of great significance in improving system efficiency and safe operation.Firstly,according to the development of medium and high voltage photovoltaic inverters at home and abroad and the research status of leakage current suppression,this project is determined.The circuit topology,working principle,mathematical model and traditional carrier modulation strategy of three-phase three-level MMC are introduced,and the causes of leakage current in MMC and common suppression methods are analyzed.Secondly,aiming at the problem that the traditional scheme can’t effectively suppress the leakage current,this paper puts forward the suppression method and a new single-carrier modulation method.Through a series of logical operations on the driving signal,the circuit works in certain working states.Then,the simulation verifies the feasibility of the new single-carrier modulation strategy.The high-frequency fluctuation of parasitic capacitor voltage and the high-frequency change of common-mode voltage are suppressed,and the leakage current suppression effect is obvious.At last,the proposed new modulation scheme is verified by experiments,and a hardware-in-the-loop(HIL)experimental platform is built,which is implemented on Star Sim circuit(MT 6020 Solver)and Star Sim HIL.A control circuit based on digital and logical operation is designed for steady-state and transient experimental verification.The effect of leakage current suppression is consistent with the theory,and the results prove the feasibility and correctness of this scheme.