Research On Modulation Methods For Converters Gradationally Controlled Modular Multilevel Converter

At present,clean energy is being developed and used on a large worldwide.The state advocates the construction of a "new power system with new energy as the mainstay" to achieve the "dual carbon" goal of "carbon peak,carbon neutrality".Flexible DC transmission technology is crucial in the grid connection and large-capacity longdistance transmission of new energy due to its advantages of flexibility and reliability.As an important part of the DC transmission system,modular multilevel converter(MMC)has been widely used in flexible DC transmission because of its advantages of modular structure,low harmonic content,and high fault handling ability.However,in the case of higher voltage level,MMC requires more sub-modules,and the converter station has a large volume and weight,occupied area,and high investment cost.This thesis takes this as the background to study reducing the number of sub-modules while ensuring that the output voltage of the inverter has more levels and less harmonic content.The main research works carried out are as follows.First,Gradationally Controlled Modular Multilevel Converter(GC-MMC)and Multi-modular Gradationally Controlled Modular Multilevel Converter(MGC-MMC)are analyzed,and the modulation strategy and the method of equalizing the capacitor voltage of the sub-module are studied,namely two-round modulation.The solution of re-selecting the combinations of status numbers at certain intervals is proposed for the problem of large fluctuations of sub-module capacitance voltage due to multiple combinations of status numbers at zero level.Second,the method of using nearest level pulse width modulation(NL-PWM)for GC-MMC is proposed by adjusting the ratio of the voltage rating of the sub-module capacitor.The number of output voltage levels is more than that using nearest level modulation(NLM),reducing the maximum ratio of sub-module capacitor voltages.The superiority of applying NL-PWM for GC-MMC is illustrated by simulating and comparing MMC.Third,the method of applying NL-PWM is proposed for MGC-MMC.The level number of the output voltage is effectively improved,and the effectiveness of the proposed method is verified by building a simulation model.Meanwhile,a simulation model of the two-terminal DC transmission system is established.The simulation results show that the MGC-MMC with NL-PWM can quickly respond to and follow the given active and reactive power.Fourth,for the sub-module contained in MGC-MMC is the full-bridge sub-module and the two-round modulation has good applicability,nearest multi-Level modulation(NMLM)is proposed,which increases the number of output voltage levels and the amplitude of the output phase voltage,so that it crosses the limit of half DC voltage and expands the operating range of the converter.Its effectiveness is verified by building a simulation model for simulation research.