Direct Power Control Algorithm For Single-Phase Cascaded H-Bridge Rectifiers

With the increase of China’s high-speed railway operation mileage and the development of the traction drive technology,many scholars carry out the research on non-power frequency traction transformers to achieve the lightweight design of high-speed trains,where the cascaded power electronic traction transformer is normally used to solve the lightweight for power frequency transformers.As the input stage of cascaded power electronic traction transformer,the safe and smooth operations of cascaded H-bridge rectifier are of the important significance for successfully executing the power devices.In order to improve the dynamic performance of CHBRs and solve the fluctuation problem of reactive power when the grid-side equivalent inductance parameters are perturbed,a modified direct power control structure is proposed in the thesis.On the basis of the nominal power model,PI controller and H_∞loop shaping controller are tuned respectively.Therefore,modified direct power based on PI controller control algorithm and modified power control based on H_∞-LS controller strategy are optimated.The research contents and the experimental results in the thesis are introduced as follows.Firstly,the running principle of the topology of the cascaded H-bridge rectifier is analyzed and its mathematical model is bulit.Theαβstationary coordinate system is constructed according to the second-order generated integral algorithm,and then the direct-axis quadrature-axis coordinate system is designed through the coordinate transformation,thus obtaining a voltage relationship in the dq rotating coordinate system.Combined with the instantaneous power calculation formula,the power feedforward control structure of the cascaded H-bridge rectifier is designed.Secondly,an modified direct power control structure is designed to further improve the dynamic performance in view of the need to perform dq/αβcoordinate transformation when calculating the modulated input signal for power feedforward control.The advantages of the structure are:(a)it can delete a phase-locked loop to avoid calculating the grid-side voltage frequency and phase,and(b)it can omit coordinate transformation when calculating the modulated input signal.Based on the nominal power model,the executing steps for the PI controller are described in detail.Next,in order to suppress the fluctuation of the reactive power of the cascaded H-bridge rectifier under the perturbation of the equivalent inductance parameter on the grid side,an H_∞loop shaping controller is designed based on the modified power control structure.According to the loop shaping steps and the influence of compensator parameter on the amplitude-frequency characteristics of the power loop transfer function of the cascaded H-bridge rectifier,the H_∞loop shaping controller is tuned.Finally,the power feedforward control,PI-MDP control algorithm and H_∞-LS MPC strategy are verified and compared in Matlab/Simulink simulation software,Micro Lab Box real-time simulation system and low-power experimental platform.The PI-MDP control algorithm effectively improves the dynamic performance of the cascaded H-bridge rectifier with small overshoot.The reactive power of the cascaded H-bridge rectifier based on H_∞-LS MPC strategy control cannot fluctuate significantly when the grid-side equivalent inductance parameter suddenly changes from the nominal value to 140%times the nominal value.