Research On Control Strategies Of Modular Multilevel Converter

Modular multilevel converter(MMC)is a novel multilevel topology of voltage source converter.Compared with the traditional voltage source converter,it’s distinct structure make it have many merits,such as the high level,the high quality output voltage,the low switching frequency,the low switching losses,the use of standard power electronic devices as well as the capacity of four quadrant operation and so on.Therefore,the application of MMC in the high-voltage high-power field such as high voltage direct current transmission system,power quality control and etc.This paper firstly introduce the topology,mathematical models and working principles of MMC.Then the control strategies of MMC are introduced in detail.The control strategies include modulation strategy,circulating current suppression strategy and sub-module capacitor voltage balanced strategy.After analyzing the cause of the unbalance of the capacitor voltage in the carrier phase shift modulation strategy,a voltage balanced strategy based on the cycle of driving pulse is proposed,which can achieve system stability and the capacitor voltage balance.Then the 2N+1 level nearest level modulation strategy is introduced(N is the number of each arm of the bridge arm).On the basis of the modulation strategy,an improved MMC topology is proposed in this paper.A traditional sub-module is replaced by three new sub-modules with a capacitance value of two times the traditional sub-module,and the corresponding control strategy is proposed,which can improve output voltage level.By building a MMC simulation platform based on MATLAB/Simulink,the simulation results presented that a voltage balanced strategy based on the cycle of driving pulse can achieve the capacitor voltage balance without the capacitive voltage control.On the basis of new MMC topology,the output voltage level can reach 4N+1,the output harmonic content is reduced to a certain extent,and the power quality is improved.