Research On Low Frequency Operation Control Strategy Of Modular Multilevel Converter

Compared with traditional multilevel converter,modular multilevel converter(MMC)has the characteristics of high modularity,good output characteristics,easy to expand and so on.At the same time,with the increasing demand for high-voltage and high-power transmission,the application field of MMC is further expanded.However,when MMC is applied in the field of transmission,especially in the motor speed regulation,there will inevitably be low-frequency conditions in the motor start-up process,and the capacitor voltage will have a large low-frequency pulsation.Therefore,how to reduce the voltage fluctuation of sub-module capacitor under low frequency condition has become the core problem.In this paper,the star-channel modular multilevel converter(SCMMC)is the research object,focusing on its low-frequency control strategy,analyzing the characteristics of the structure to prevent the AC output side from generating common mode voltage when using the low-frequency control strategy of high-frequency injection method,and aiming at the topology,a low complexity two-layer control theory is proposed to realize the normal start-up of starchannel MMC with zero common mode voltage.This paper studies the traditional MMC fundamental frequency operation control strategy,mainly including sub-module unified pulse width modulated(SUPWM),carrier phase shifting PWM,The advantages and disadvantages of two modulation strategies and two voltage sharing algorithms are analyzed,and a five level MMC simulation model is built to verify the corresponding modulation strategy and voltage sharing algorithm.This paper studies the traditional MMC fundamental frequency operation control strategy,mainly including sub module unified pulse width modulated(supwm),carrier phase shifting PWM(CPS-PWM).The advantages and disadvantages of two modulation strategies and two voltage sharing algorithms are analyzed,and a five level MMC simulation model is built to verify the corresponding modulation strategy and voltage sharing algorithm.In the traditional MMC topology,the fluctuation mechanism of sub-module capacitor voltage is analyzed,and the voltage fluctuation of sub-module capacitor voltage under low frequency condition is analyzed.This paper introduces a lowfrequency operation control strategy based on high-frequency injection method of supwm modulation,which speeds up the energy exchange frequency between submodules,and makes the capacitor voltage of sub-modules keep constant under the lowfrequency condition of MMC.At the same time,for the removal of high-frequency signal,a high-frequency circulating current limiting function method is designed to optimize the high-frequency signal injection method,so that the injected highfrequency signal always exists in the motor start-up process,and it can be removed stably after reaching the power frequency to ensure the smooth start of the motor.On the basis of optimizing the high frequency signal injection method,this paper analyzes that the AC output side will produce large common mode voltage,which will affect the normal operation of the motor.In order to realize zero common mode voltage(CMV)motor start-up,a low complexity strategy called carrier phase shifting submodule unified pulse width modulated(CPS-SUPWM)is proposed based on starchannel MMC.CPS-SUPWM adopts the double-layer control theory,which has the characteristics of keeping the control complexity unchanged with the increase of the number of sub-modules.It realizes the MMC zero common mode voltage with motor starting.The simulation results show that the control strategy is effective and feasible.According to the control strategy and the corresponding MMC structure,hardware in the loop and hardware in the loop experiments are carried out to verify the proposed control algorithm.The experimental results show that the proposed control strategy is effective and feasible in MMC at low frequency condition.