| Modular multilevel converters based on high-voltage DC transmission multi-terminal systems for wind farms offer greater flexibility and energy exchange between multiple regions.The biggest advantage of HVDC transmission is that when the transmission distance is long,the relative cost is lower than that of the AC system.At present,modular multi-level converters have been widely used in the field of high-voltage direct current transmission,as a commutation device for the power transmitting end and the receiving end.However,the main problem faced by modular multilevel converters is the submodule capacitor voltage fluctuations during low frequency operation.In order to improve the reliability of modular multilevel converters,the dissertation studies the improved calculation method of modular multilevel converter loss and the study of bridge arm circulation suppression.The main contents of the dissertation include:Summarizes the current research hotspots and application hotspots of wind power,HVDC and modular multi-level converters,summarizes the development status and main modulation methods of modular multi-level converters,and analyses their respective advantages and disadvantages.Inverter control technology is studied to solve the problem of high capacitor voltage of sub-module in high-power electric drive and grid-connected stage of current modular multi-level converter in wind power generation,and theoretical analysis is carried out.A space vector pulse width modulation(SVPWM)algorithm for three-phase AC/DC converter based on multi-module converter is proposed.SVPWM has many advantages over traditional sinusoidal triangular pulse width modulation algorithms.The dissertation proposes an SVPWM algorithm that is not affected by voltage levels and can be extended to any sub-model.The mathematical model of the model is proposed.The converter is controlled by a closed-loop vector control algorithm of a rotating D-Q coordinate system.In view of voltage fluctuation,the dissertation proposes a wave suppression method based on high-frequency circulating current closed-loop injection,which is to inject high-frequency common-mode voltage into the MMC AC side and inject the same frequency circulation to construct a fast channel for energy exchange between the upper and lower arms.Finally,the submodule capacitor voltage fluctuation is suppressed.In the Matlab/Simulink,the MMC-based converter is simulated,and the fluctuation suppression method based on high-frequency circulating-loop closed-loop injection is applied.The simulation results are given.Research shows that the SVPWM algorithm and there are the wave suppression method in the dissertation effective for suppressing the module voltage fluctuation. |
PDF Full Text Request