Common-mode Voltage Suppression Of Modular Multilevel Converters Based On Optimized Modulation Methods

Modular multilevel converters(MMCs)have received widespread attention in the fields of high-voltage direct current(HVDC)transmission systems,motor drive systems,and grid-connected photovoltaic systems,since they have advantages of superior harmonic performance,high efficiency and scalability.At the same time,the MMC will generate high dv/dt and di/dt due to the high-speed operation of switch devices,and cause the problem of high-frequency fluctuations of common-mode(CM)voltage under commonly used modulation methods.Thus,the CM voltage becomes a crucial source of electromagnetic interference(EMI)affecting the normal operation of the secondary equipment of the power grid and the MMC itself.In addition,in the motor drive systems,the CM voltage problem of MMC will accelerate the deterioration of the insulation of motor windings damaging the motor bearings,cause the mis-operations of zero-sequence current relays,and even endanger the lives of people who touch the motor shell.In this thesis,a series of optimized modulation methods are proposed for the CM voltage problem of dual-star structured MMCs under various conventional modulation methods.The commonly used modulation methods covered in this thesis include not only the multi-carrier modulation methods applicable to the MMC with low number of levels(generally less than 21 levels),but also the nearest level modulation method(NLM)applicable to its high number of levels.The main research contents and innovation points are as follows.First,this thesis deduces the expression of CM voltage,which shows that the CM voltage is proportional to the difference between the total number of sub-modules(SMs)inserted into three lower arms and that of three upper arms.Then through simulation,it is concluded that under three commonly used modulation methods of MMC,the difference mentioned above changes in a high frequency,and the CM voltage changes stepwise accordingly.The amplitude of each step is 1/3 of the capacitor voltage of the SM under NLM,and 1/6 of that under CLS-SPWM and CPS-SPWM.Then,since CLS-SPWM and NLM are usually used together with the sorting algorithm to achieve capacitor voltage balancing,it is convenient to directly recalculate the number of SMs inserted into the arms.Therefore,this thesis analyzes the number of SMs inserted into the arms for the five-level MMC in segments and proposes a six-segment CLS-SPWM(S~2 CLS-SPWM)and a six-segment NLM(S~2 NLM),which can eliminate high-frequency fluctuations of the CM voltage in the MMC.After that,a pulse sequential connection CPS-SPWM(PSC-CPS-SPWM)method is proposed to eliminate CM voltage steps of MMC by analyzing and optimizing the CPS-SPWM.The correctness and feasibility of the proposed optimized modulation method are verified on the simulation platform.In addition,since PSC-CPS-SPWM will reduce the maximum modulation ratio of MMCs to a certain extent,this thesis introduces chaos theory into CPS-SPWM and a chaotic CPS-SPWM method is proposed to reduce peaks of the CM voltage at the frequency with the largest spectral amplitude and its multiples.The correctness of the chaotic CPS-SPWM is verified by simulation.Finally,the experimental platform based on MMC with Si C MOSFETs as the power switching devices is built,and its hardware and software are designed.Then the optimized modulation methods proposed in this thesis are verified by this experimental platform.In order to better present the characteristics of optimized modulation methods proposed in this thesis,a comparison of these optimized modulation methods is summarized at the end.