Research On Low Frequency Operation Model Predictive Control Of MMC

Modular Multilevel Converter(MMC)is applied in the field of variable frequency drive.When the motor is in the start up or low frequency operation,the submodule capacitor voltage fluctuation increases sharply,which seriously endangers the stable and reliable operation of MMC system.This thesis mainly studies the submodule capacitor voltage fluctuation under low frequency conditions and the low frequency control strategy of MMC based on model predictive control.Firstly,this thesis studies the topology structure and the working principle of MMC,the modulation strategy suitable for multilevel converters,the capacitor voltage equalization control strategy and the circulation suppression strategy of MMC.Then the control effect and output performanceof the modulation,the voltage equalization and the circulation control strategies are verified by simulation.Aiming at the problem that the submodule capacitor voltage of MMC fluctuates violently under low frequency condition,the capacitor voltage fluctuation formula is derived,and the conclusion that the submodule capacitor voltage fluctuation range is inversely proportional to the output frequency and proportional to the output current amplitude is obtained.The principle of low frequency suppression strategy based on high frequency signal injection method is studied.This method can effectively suppress the low frequency fluctuation of capacitor voltage,but will inject extra high frequency voltage to the output side.Star-channel MMC can avoid the problem of increasing common mode voltage on the output side of traditional low frequency suppression strategy by constructing an interphase high-frequency component path.Then,the above low frequency suppression strategy and its optimization scheme are studied and analyzed and verified by simulation.Aiming at the problems such as complex parameter setting and large calculation amount of the low frequency suppression strategy using PI regulator,a low frequency suppression strategy of MMC based on deadbeat current predictive control is proposed.This method obtains the optimal voltage modulation signal of the bridge arm in the current period by calculating the reference value of AC output current and circulation,and establishing the voltage prediction model of bridge arm with high frequency common-mode voltage.The control strategy of star MMC based on deadbeat current predictive control is further studied.The low frequency suppression strategy based on deadbeat current prediction does not require traversal optimization of the objective function,and eliminates most of the cascade PI controller.It has low computational cost,fast dynamic response,and can realize fixed frequency control of MMC.In order to verify the feasibility of the designed control strategy applied in the field of motor drive,the MMC-induction motor control system based on the deadbeat current predictive control was established.The simulation results show that the proposed control strategy can effectively suppress the low frequency fluctuation of the submodule capacitor voltage,drive the motor smoothly,and the system has good low frequency operation performance.Finally,the above control strategies are verified by real time simulation experiment.The results show that the low frequency suppression strategy of MMC based on deadbeat current predictive control can effectively suppress submodule capacitor voltage fluctuation under low frequency conditions,and improve the low frequency operation capability of the system.