Research On The Control Method Of A Single-phase Modular Multilevel Converter

In recent years,modular multilevel converter(MMC)has been widely concerned by scholars at home and abroad for its many advantages,such as high modular topology structure,low system loss,low switching device voltage withstand requirement,high reliability,less harmonic content of output waveform and easy to realize redundant control.In this paper,the single-phase modular multilevel converter is taken as the research object,focusing on the modulation strategy,sub-modules capacitor pre-charging control method,sub-module capacitor voltage balancing control method and circulating current control method.The main research contents are as follows.Firstly,the circuit topology and working principle of single-phase MMC are analyzed in detail,the equivalent circuit and its mathematical model of single-phase MMC are established,the principle of three modulation strategies commonly used in MMC are studied,and the realization methods and application occasions of various modulation strategies are discussed.Secondly,the possible consequence and harm caused by unbalanced capacitance and voltage of sub-module of the MMC are described.The pre-charging control method of sub-module capacitor is studied before the MMC works normally.The method of sub-module capacitor voltage balancing control based on closed-loop control and the method of direct balancing control based on sub-module capacitor voltage sequencing are analyzed.Aiming at the shortcomings of the direct balancing control method based on sub-module capacitor voltage sequencing that makes the controller computationally intensive and the operating frequency of the switching device fast,an improved sub-module capacitor voltage balancing control method is presented in this paper.Then,the mechanism of single-phase MMC circulating current is analyzed,the mathematical model is established,the circulating current is calculated,and the quasi-proportional resonance control strategy is adopted to control the circulating current.Furthermore,a single-phase MMC circuit experiment platform with ARM+FPGA as the control core is built.The power circuit and the control system are modularly designed.The control system includes the main controller circuit,the auxiliary controller circuit,the sampling circuit,the PWM fiber transmission circuit,and the bus interface circuit.At the same time,the software program is designed for MMC control method.Finally,the simulations and experiments are used to verify the sub-module capacitor voltage balancing control method under different modulation strategies.The method of direct balancing control based on sub-module capacitor voltage sequencing and the method of improved sub-module capacitor voltage balancing control are compared and analyzed.The circulating current control strategy is proved.And the feasibility and effectiveness of the control strategy for the single-phase MMC is verified.