Research On The Control Method Of Modular Multilevel Converter In Low Frequency Operation

Modular multilevel converter(MMC)has many advantages,such as common DC bus,modular structure,good output harmonic characteristics,four quadrant operation,and can realize the direct grid connection of medium and high voltage.It has been widely used in the fields of HVDC transmission and power transmission.However,due to the complexity of MMC topology,there are still many problems to be solved,such as start-up control,capacitor voltage balancing control and so on.In addition,when MMC is running in low frequency condition,it is also an urgent problem to restrain the violent fluctuation of capacitor voltage of its sub module.This paper first studies the basic working principle of MMC,establishes its time domain mathematical model and its AC and DC side equivalent circuit,and then compares and analyzes the advantages and disadvantages of several common modulation strategies of MMC.In addition,aiming at the problem of large impulse current when MMC starts,a total DC side pre charging control strategy of MMC is studied,which can realize the fast capacitor voltage of upper and lower arm sub modules Fast charging.Aiming at the problem that the capacitor voltage of the sub module of MMC fluctuates violently in low frequency operation,the mechanism of the capacitor voltage fluctuation of the sub module is analyzed from the angle of the instantaneous power of the bridge arm,and the mathematical expression of the fluctuation coefficient is derived,and the conclusion that the fluctuation amplitude is inversely proportional to the operating frequency of MMC is obtained;Then,the working principle of MMC low frequency control strategy double high frequency sinusoidal injection method is studied,and the method to optimize the introduction of circulating current through the bridge arm is optimized;Finally,a low-frequency fluctuation suppression and capacitor voltage balancing control scheme of MMC based on quasi proportional resonance(PR)controller is designed.Aiming at the problem of introducing extra high-frequency common mode voltage into the AC side of MMC by high-frequency injection method,this paper first studies the topology of flyover capacitor modular multilevel converter(FC-MMC),establishes its mathematical model and equivalent circuit,and then proposes a low-frequency control strategy of FC-MMC based on quasi PR controller.This method can suppress the low-frequency fluctuation of sub mode capacitor voltage at the same time,Compared with MMC,the high-frequency injection method has better output common mode voltage characteristics.In addition,compared with FC-MMC,the dual differential mode high-frequency sinusoidal injection method has the advantage of smaller bridge arm current;Finally,a low-frequency fluctuation suppression and capacitor voltage balancing control scheme of FC-MMC based on quasi PR controller is designed.In order to verify the effectiveness of the proposed FC-MMC low-frequency control strategy,a 1.5KW three-phase five level FC-MMC prototype is built in the laboratory.Simulation and experimental results verify the correctness and effectiveness of the proposed control strategy.