High-power Motor Drive System Based On Modular Multilevel Converters

In recent years,the modular multilevel converter(MMC)has received extensive attention from academia and industry due to its advantages of modularity,easy to increase the number of levels and having the DC bus.At the same time,it is considered to have a good application prospect in high-power motor drive occasions.However,in addition to the above advantages,the suppression of capacitor voltage fluctuations at low frequencies is a major challenge when the MMC is used in high-power motor drive occasions.Meanwhile,since the MMC contains multiple sub-modules and each sub-module contains multiple power devices,the loss generated under the normal operation of the MMC-based high-power motor drive system is also a problem that cannot be ignored.Therefore,it is significant to study capacitor voltage fluctuation suppression and loss optimization in the case of MMC driving motors.In this paper,the MMC-based high-power motor drive system is taken as the research object.Firstly,the double sine wave injection method based on circulating current compensation and the square wave plus quasi-square wave injection method based on circulating current compensation are proposed to solve the problem of capacitor voltage fluctuation suppression,which can realize capacitor voltage fluctuation suppression under the circulating current error.Secondly,the multi-objective loss optimization strategy based on double-frequency circulating current injection and the multi-objective loss optimization strategy based on double-frequency plus quadruple-frequency circulating current injection are proposed to solve the problem of loss optimization,which can reduce the maximum loss of power devices in sub-modules without increasing the total sub-module losses.The specific research contents and innovative results are as follows:(1)The topology structure of the MMC-based motor drive system is introduced and the mathematical model and vector control strategy of the three-phase PMSM are introduced.Then,the working principle of the three-phase MMC is introduced and the mathematical model of the three-phase MMC is established.Further,the control strategy of the three-phase MMC system is introduced in detail,including phase-shifted-carrier PWM strategy,MMC capacitor voltage balance control,MMC double-frequency circulating current suppression and MMC system control.(2)The traditional double sine wave injection method and the traditional square wave plus quasi-square wave injection method under the circulating current error are analyzed in detail,and it is concluded that both of them cannot suppress the capacitor voltage fluctuation under the influence of the circulating current error.On this basis,a double sine wave injection method based on circulating current compensation and a square wave plus quasi-square wave injection method based on circulating current compensation are proposed respectively,where the reference of the circulating current is closed-loop compensated by arm instantaneous power control to suppress the capacitor voltage fluctuation under the circulating current error.(3)The calculation model of conduction loss and switching loss of each power device in the half-bridge sub-module is deduced in detail,and the loss model of the MMC sub-module is established.Then,the loss of the motor drive system under normal operation is analyzed,and it is concluded that the loss of each power device in the sub-module is very different.On this basis,a multi-objective loss optimization strategy based on double frequency circulating current injection and a multi-objective loss optimization strategy based on double frequency plus quadruple frequency circulating current injection are proposed,where optimal circulating currents are injected to reduce the maximum loss of power devices in sub-modules without increasing the total sub-module losses.(4)The effectiveness of the proposed capacitor voltage fluctuation suppression strategy and multi-objective loss optimization strategy is verified by the simulation software PLECS and the experimental platform built in the laboratory.