Research On Submodule Capacitance Reduction Method Of Modular Multilevel Converter And Related Problems

Modular Multilevel Converter(MMC)is widely used in high-voltage and high-power applications.It has become a research hotspot in recent years.However,the MMC may consist of up to hundreds of submodules and each submodule equipped with submodule capacitors.The submodule capacitors are responsible for the low frequency ripple power,so the capacitance value is very large.A large number of submodule capacitors with large capacitance value greatly reduce the power density of the MMC.Therefore,the research on the method of reducing the capacitance value of submodules and its related problems is of great significance for reducing the volume and cost of the MMC.This paper analyzes working modes of the MMC and gives the model.Based on the model of the MMC,the expected steady-state performance and dynamic performance,the control parameters of MMC output voltage outer loop are designed.Inductor current inner loop and circulating current loop are also designed.The submodule capacitor voltage equalization algorithm with low execution frequency is introduced and compared with the traditional submodule capacitor voltage equalization algorithm.The closed-loop control method of injecting the second-harmonic circulating current based on instantaneous power balance is studied.The design method of submodule capacitance under this control method is given.In the submodule capacitance,this control method and the suppression method of the second-harmonic circulating current are compared.It is concluded that the closed-loop control method of injecting the second-harmonic circulating current based on instantaneous power balance can greatly reduce the capacitance value.In order to solve the problem that the current stress of the switching device is increased when the control method of injecting the second-harmonic circulating current based on instantaneous power balance is used,a method of injecting the second-harmonic circulating current based on the feedback of the fluctuation of submodule capacitor voltage is proposed to reduce the current stress.And the design method of the control parameters in this method is proposed.However,the above method requires that submodule capacitors are equipped with voltage sensors.If there are no submodule capacitor voltage sensors,the above method is not applicable.Therefore,in order to reduce the current stress of the power device without the submodule capacitor voltage sensors,the method of injecting the second-harmonic circulating current based on real-time calculation is proposed.Finally,an MMC prototype is designed and built.The experimental results show that the MMC prototype has a good performance.The experimental results also show that the proposed design method of submodule capacitance,the method of injecting the second-harmonic circulating current based on the feedback of the fluctuation of submodule capacitor voltage,and the method of injecting the second-harmonic circulating current based on real-time calculation are effective.