Research On Balance Control Technique Of The Capacitor Voltage Based On Half-bridge Submodule In An MMC System

Modular Multilevel Converter(MMC)is a type of power conversion device,and has advantages of great output power,high voltage level,small switching loss and strong fault tolerant capability.MMC inserts different number of submodules to generate varying arm voltages and alternating current.However,the capacitor voltages in submodules become different when MMC is operating,leading to the unbalance phenomenon of the capacitor voltages.This phenomenon will lead to the difference of the voltage-stress in submodules,which can even break switches and cause the system failure.Therefore,the balance control of the capacitor voltages in the submodules is significant to ensure the system stable working performance.Based on the MMC system that uses the half-bridge submodule,this research studies the unbalance factors when the Carrier Phase Shift Pulse Width Modulation(CPS-PWM)is used in the system;studies the defects of the balance strategy that is based on the sorting method;and studies the effect of system time delay on the balance and performance of the system.The primary contents in this research are shown below:1)The balance mechanism of the MMC system is studied,and the relations beteween the Delay of Pulse-width Modulation(PWM delay)and frequency ratio are quantitatively analyzed.When the system has a great frequency ratio,the PWM delay can cause harmonics in the modulation wave,which can lead to the difference of the inserted time in submodules and thus leads to the unbalance phenomenon in the capacitor voltages.Conversely,a low frequency ratio can cause the significant fluctuations of the arm currents in one switching period,which leads to the different rates that the capacitor voltages deviates from the normal value and thus lead to the corresponding unbalance phenomenon.Therefore,an optimal method is proposed to determine the appropriate values of PWM delay and frequency ratio,radically suppressing the unbalance phenomenon.Simulations and experiments are eventually provided to verify the theories and methods.2)The balance strategy based on the sorting method causes the increase of the switching frequency,which deteriorates the system working performance.Therefore,the effects of the balance strategy on the switch drives are analyzed,and the indexes to describe the distribution law of switch drives are provided: the instantaneous switching frequency and the average switching frequency.The root cause and amplitude of the increase in the switching frequency are then obtained.The research indicates that the amplitude of the instantaneous switching frequency is determined by the inserted submodule number and sorting results of the capacitor voltages in the submodules.The instantaneous switching frequency in some submodules can reach to a great value when the inserted submodule number and sorting results have great change rate.However,the average switching frequency is irrelevant to the inserted submodule number,and is determined by the change rate of the sorting results of the capacitor voltages.Based on the analysis,the methods are proposed to suppress the increase of the switching frequency.Simulations and experiments are eventually provided to verify the theories and methods.3)The balance strategy based on the sorting method causes the distortion of the alternating current,which deteriorates the system working performance.Therefore,the function of the commands in the balance strategy,the corresponding execution periods and the relations between commands and the execution frequency of the balance strategy are both analyzed.The research indicates that a low execution frequency can lead to the frequency aliasing effect of the inserted submodule number,which is the primary cause that leads to the distortion of the alternating current.Based on the analysis,the distribution of the commands in the balance strategy is optimized,the execution period of the sorting method is decreased,and the execution frequency of the balance strategy is increased,effectively avoiding the frequency aliasing effect and corresponding distortion of the alternating current.Simulations and experiments are eventually provided to verify the theories and methods.4)The system time delay in the MMC system deteriorates the system stability,dynamic performance and stable performance,thus decreasing the effect of the balance control strategy and deteriorating the balance of the capacitor voltages in the submodules.Therefore,the generation process of the system time delay is determined,the system linear model is structured,and the effect of the system time delay is analyzed.The research indicates that the system time delay can lead to the phase decrease in the alternating current loop and circulating loop,and thus causes the decrease of the system stability.To ensure the system stability,the system forcedly decreases the corresponding cut-off frequency to increase the phase margin.However,a low cut-off frequency can deteriorate the system dynamic performance and stable performance.Therefore,an optimal deign method is proposed to determine the parameters in the control system,effectively increasing the system performance and decreasing the effect of the system time delay.Simulations and experiments are eventually provided to verify the theories and methods.This research will promote the organic integration between the MMC system and balance control technique,and accelerate the application of MMC system in the industrial field,including the wind power generation,marine electric propulsion and high voltage direct current transmission.