The Research Of Cascade Rectifier Based On Sliding Mode Control

In recent years,cascaded H bridge multilevel converters have been widely used.But the control of the whole bridge and the balance control of the DC side voltage of each bridge is very complicated because of the nonlinear and non minimum phase system characteristics of the cascade H bridge rectifier.This article takes the rectifier stage of the the cascaded H bridge multilevel converter as the research object.For cascaded H-bridge rectifier with unity power factor transport problems,DC side voltage balance control and dynamic performance were studied,through the analysis of the topological structure and working principle of the single-phase two cascade rectifier stage.Firstly,this article analyzes the working mode of cascade H-bridge rectifier,the mathematical model based on the switching function and duty ratio are established respectively,and the restrictive conditions for the stable operation of the system are researched;for the existence issues of large overshoot and disturbance voltage fluctuation,slow recovery in the traditional PI control system,the sliding mode control strategy is transplanted to the control of cascade rectifier because of its characteristics of the rapid response and strong anti-interference;the appropriate state variables is selected according to the control objectives and the sliding surface function of the stable operation of the system is obtained,realizing the control of voltage and unit power factor of the system.For the DC side voltage balance control problems of cascade rectifier,a balance control method based on two-dimensional modulation is used,an appropriate modulation track is chosen dynamically to adjust the active power of each H bridge absorbed from the grid,in order to the DC voltages of the H bridges tend to be balanced;and the restrictive conditions of load unbalance are further discussed.Finally,the theoretical simulation analysis is completed in the MATLAB /Simulink,and in the effects of disturbances,the simulation waveforms of the traditional PI and sliding mode control are compared;and a semi-physical experiment platform based on RT-lab is built up;the feasibility and effectiveness of this paper proposed voltage balancing control algorithm strategy are verified by the simulation results andthe experiment results.