Comparative Study On The Modulation Methods Of Modular Multilevel Converter Based On MOSFET

Low-voltage direct current(LVDC)power grid can improve the efficiency and reliability of power system.The establishment of the LCDC power grid can solves the dilemma of the power supply.Due to the AC loads,the performance of the inverter is greatly significant to the LVDC power system.Compared with the traditional two-level converter,the modular multi-level converter(MMC)based on MOSFET has significant advantages in terms of energy conversion efficiency and voltage change rate.This paper studies the effects of different modulation strategies on the performance of MMC based on MOSFET(MOSFET-MMC)output,the loss of capacitor voltage and the inductance of the bridge arm.Firstly,this paper introduced the topology and working principle of MOSFET-MMC in details,then explains the selection of the submodule topology and the reasons for replacing IGBT with MOSFET.The decoupled model between ac and do side of MOSFET-MMC is built.The DC equivalent circuit explains the control basis of MOSFET-MMC AC output.The AC equivalent circuit explains the source of loop current,and the importance of inhibition of MOSFET-MMC circulation loss was illustrated.Furthermore,the necessity of the capacitor voltage balance demonstrated.Next,this paper explains the selection of submodule capacitance from the perspective of the energy in the bridge arm,and introduces the selection of common bridge arm inductance.The emphasis of this paper is to analyze the performance of MMC in different modulation strategies from different perspectives,which is based on the principle of depth analysis converter operation,the topology of the half bridge sub-module and the traditional capacitor voltage balancing method.With the double Fourier decomposition method and the traditional Fourier decomposition method,Fourier analysis of MOSFET-MMC output voltage are obtained under the action of modulation strategies,which are compared with the simulation results based on MATLAB FFT,then the harmonic characteristics of the output voltage are obtained.Finally,in the same simulation model,the effects of six modulation strategies on the performance of MOSFET-MMC are compared from six points: DC voltage utilization ratio,capacitor voltage fluctuation,the voltage of arm inductance.the capacitance voltage fluctuation,and the bridge inductance partial voltage.Especially the basic components in the capacitor voltage fluctuation and voltage of arm inductance under ideal conditions are analyzed.Under the action of N + 1 type and 2N + 1 type modulation,the mechanism of the interaction between unbalanced voltage and capacitor voltage fluctuation is studied through modulation function.On this basis,the mathematical model of internal virtual electromotive force and the capacitor voltage fluctuation are derived in details.Finally in the simulation experiments,the effect of different modulation strategies on the basic components of three electrical quantities are compared,which are the capacitance of the capacitor,the voltage of arm inductance,and the loop current.In the application of low voltage dc power grid,this paper finally gets the best modulation strategy of MOSFET-MMC.