Research On Common Mode Voltage Minimization Of MMC Motor Drive System Based On High Frequency Link Interconnection

Modular multilevel converter(MMC)is suitable for medium-voltage motor drive systems under carrier phase shift sinusoidal pulse width modulation(CPS-SPWM),which has two types of common-mode voltage(CMV)problems,that is,system CMV(SCMV)and arms CMV(ACMV).The two kinds of CMV problems are more serious when the motor runs at low speed,which endangers the safety operation of the system.In order to analyze the generate mechanism of the two types of CMVS and suppress them,this paper takes switching function and submodule(SM)capacitor voltage as the starting point for mechanism analysis,proposes a scheme combining topology improvement and modulation mode optimization,and further studies it.Firstly,the basic model of the conventional MMC motor drive system is analyzed.According to the expressions of the two types of CMV,the generate mechanism of the two types of CMV is related to the submodule capacitor voltage and PWM modulation,and the expression of the submodule capacitor voltage is further derived according to the basic model of the MMC.The PWM frequency domain model of arm is modeled and analyzed by double Fourier integral method.Secondly,the harmonic characteristics of the two kinds of CMV are analyzed when the modulation modes are different.The modulation strategies of N+1 level and 2N+1level under CPS-SPWM modulation are analyzed,the characteristics of two types of CMVS of MMC under two different modulation modes of CPS-SPWM is analyzed,and further the characteristics of two types of CMVS under N+1 level and 2N+1 level modulation is analyzed when the number of submodules is different in parity.Then,considering the optimization of the topology,a kind of high frequency link interconnection MMC(HFLI-MMC)was proposed,which realizes the suppression of the low-frequency fluctuation of the capacitor voltage of submodules through three-phase fluctuating current coupling.Thus,the low frequency component of SCMV and ACMV are inhibited.Pulse optimization control of PCS-PWM(POC-CPS-SPWM)is proposed to ensure that the number of input submodules of the upper and lower bridge arms of MMC is the same at any time,thus further suppress the high frequency component of SCMV.Finally,the two types of CMV are minimized.Finally,according to the proposed topology improvement and modulation optimization scheme,the experimental platform of HFLI-MMC based on dspace control was built,and the suppression effect of HFLI-MMC topology and POC-CPS-SPWM scheme on the two types of CMV was verified.