Research On Control Strategy Of Modular Multilevel Converter

With the development of power electronics technology,the application of multilevel converter in high voltage and high power situation has been paid more and more attention.Modularized multilevel converter,as a new multilevel topology structure,has many advantages such as high integration,easy expansion and low switching loss,and has become a hot research topic in power electronics in the world.In this paper,the control strategy of MMC is studied and analyzed.Firstly,the topology structure and working principle of modular multilevel converter are analyzed in detail,and the mathematical model under two coordinate systems is derived.The parameters of the main circuit are designed and the specific formula of parameter selection is given.Three common modulation strategies,carrier phase-shift modulation,closest level approximation modulation and carrier cascade modulation,are studied respectively.Secondly,the mechanism of capacitor voltage fluctuation of submodule is analyzed theoretically.The capacitor voltage fluctuation of submodule will seriously affect the safe and stable operation of modularized multilevel converter.Aiming at the problems of high switching frequency and high converter loss in traditional submodule capacitor voltage equalization algorithm,an improved submodule capacitor voltage equalization algorithm based on Max-Min function is proposed in this paper,and the effectiveness of this method is verified by simulation comparison.Thirdly,the mechanism of interphase circulation is analyzed theoretically.The circulation will distort the current waveform of the bridge arm and increase the system loss,so it is necessary to suppress the circulation.The circulation contains a large number of even harmonic components,of which the double harmonic component has the highest content,so the suppression of the circulation is mainly to suppress the double harmonic component.For the circulation components on the bridge arm,a loop suppression strategy is proposed based on Proportional-Integral-Resonant control,which can effectively suppress the double frequency component of the loop,reduce the distortion of the bridge arm current and reduce the capacitor voltage fluctuation of small modules.Compared with the circulation suppression strategy based on Proportional-Integral Control,this strategy does not require coordinate transformation and interphase decoupling.Finally,the layered control principle of the MMC-HVDC transmission system and the design of the inner and outer loop controllers are studied,and the control mode of the MMC converter stations at both ends is determined.The simulation model of MMC-HVDC is built in Matlab/Simulink,which verifies that the control strategy of MMC-HVDC transmission system adopted in this paper can meet the corresponding control requirements and ensure the safe and stable operation of the system at all times.