Diverse Submodule Voltage Modular Multilevel Converter And Improved Modulation Method

Modular multilevel converter(MMC),featuring in high modularity,excellent expansibility,simple redundancy program,no directly-cascaded semiconductor switches and easily-conducting capacitor voltage control,is drawing growing attentions from both the industry and academic circles.In the medium voltage applications,the MMC modulated with nearest level modulation(NLM)will generate AC voltage with high distortion because of the low submodule number per arm.This paper designs a diverse submodule voltage(DSV)MMC and the suitable improved modulation scheme to decrease the AC voltage distortion without utilizing the pulse width modulation.Each arm of the DSV MMC includes N half-bridge full-voltage(FV)submodules(SMs)and one full-bridge half-voltage(HV)SM,whose average capacitor voltage is half of the FV SM.Comparing with the half-bridge MMC with N SMs per arm,the DSV MMC increases the levels of arm voltage and inner electromotive force from N+1 to 2N+1 by adding a HV SM per arm.The AC voltage distortion is decreased.The specific formulas of switching-in numbers of HV SM and FV SMs per arm is concluded.By adding the biases x,y in these formulas,the DSV MMC causes the difference between stepping time of upper and lower arm voltage.The levels of the inner electromotive force are increased from 2N+1 to 4N+1 and the AC voltage distortion is furtherly decreased.Compared with the half-bridge MMC modulated with NLM,the total harmonic distortion(THD)of AC voltage is decreased by approximately 61.1%.In order to figure out the optimized x,y,with which,the DSV MMC can produce the AC voltage with lowest THD,the effect of x,y on the arm voltage is analyzed.It is concluded that there are three kinds of arm voltage in the whole range of x,y.The basic characters of these arm voltage are analyzed and the responding boundary conditions are calculated.Based on these conditions,the optimized x,y of the three kinds of arm voltages are calculated respectively.Only when the arm voltage is the second kind of arm voltage,there are optimized x,y.They satisfy x=-0.25,-0.375≤y≤0.125 or x=0.25,-0.125≤y≤0.375.The effects of x,y on the difference between stepping time of upper and lower arm voltage are analyzed when x,y are the optimized.The AC voltage error caused by the stepping time difference and the specific generating principle are revealed.The total voltage across the whole phase of DSV MMC are centered on NUC(UC represents the average capacitor voltage of FV SM)when x=±0.25.Based on it,the controlling scheme of the stepping time difference is proposed to suppress the AC voltage error.Moreover,the instability of HV SM capacitor voltage is revealed and the controlling scheme utilizing hysteresis control and coordination of FV SM is designed to keep the HV SM capacitor voltage without affecting the total arm voltage.Synthesizing these controlling schemes,the overall program of the improved modulation scheme is summarized.Based on the single phase DSV MMC hardware platform and the MATLAB/SIMULINK software model,the feasibility and advantages of the DSV MMC and its improved modulation scheme are verified.