Research And Improvement On Output Voltage Balance Control Strategy Of Cascaded H-bridge Rectifier

In recent years,cascaded H-bridge rectifier has been widely concerned in the field of high-voltage and high-power because of its highly modularity,scalability,and easy to achieve redundant design and modular design.The output voltage balance control is the main research question of the cascade H-bridge rectifier.At present,the output voltage balance control strategies of the cascade H-bridge rectifier can be divided into three categories: voltage sequencing balance strategy,carrier modulation balance strategy,and pulse compensation balance strategy.Among them,the pulse compensation balance strategy is widely used because of its intuitive principle,simple control and easy expansion.However,the current research on the pulse compensation balance strategy is mainly to improve the control algorithm,there are few studies on the dynamic characteristics of the cascaded H-bridge rectifiers when the load is switched over a large range.In fact,when the load of the cascaded H-bridge rectifier is switched over a large range,the output voltage will instantly change,resulting in a large unbalanced voltage between stages,and there is a long adjustment time,which will affect the safe and stable operation of the system in serious cases.Therefore,the focus of this paper is to improve the dynamic characteristics of the cascaded H-bridge rectifier when the load is switched over a large range.Firstly,the working state of the single-phase PWM rectifier that constitutes the cascaded H-bridge rectifier is analyzed under two modulations.The multi-carrier SPWM modulation strategy is introduced,and the mathematical model of the cascaded H-bridge rectifier is given.Based on this,the factors affecting the output voltage balance of the cascaded H-bridge rectifier are analyzed.Secondly,in order to analyze the dynamic characteristics of the traditional proportional pulse compensation voltage balance strategy when the load is switched over a large range,the overall double closed-loop control block diagram is first given,and the characteristics of the voltage outer-loop controller and the current-inner loop controller are analyzed.Then,the simulation model and experimental platform are used to verify the overall double closed-loop control strategy.Based on this,the traditional proportional pulse compensation voltage balance strategy is modeled,and the design formulas of the voltage balance controller are given.Simulation results and experimental results prove the correctness and effectiveness of the voltage balance strategy.However,when the load of any stage is switched over a large range,the dynamic characteristics are poor,there is a large unbalanced voltage between stages and the adjustment time is long.Finally,for the problem of poor dynamic characteristics of the traditional proportional pulse compensation voltage balance strategy when the load is switched over a large range,three improved voltage balance strategies are proposed.The first is the voltage balance strategy based on the square feedback of the output voltage,which takes the square of the output voltage as the feedback quantity,equivalently introduces the variable proportion link,and indirectly realizes the fast tracking of the output voltage.The second is the voltage balance strategy based on the ripple voltage feedforward,which uses the relationship between the output current and the ripple voltage to feedforward the ripple voltage,thereby canceling out the disturbance of the output current to the voltage balance link.The third is the voltage balance strategy based on the fuzzy adaptive PI control,which introduces the fuzzy control technology into the voltage balance link,and uses the proportional coefficient and integral coefficient that are dynamically adjusted in real time to improve the adaptive ability of the PI controller.Simulation results and experimental results show that all three improved voltage balance strategies can effectively improve the dynamic characteristics when the load of the cascade H-bridge rectifier is switched over a large range.Through comparative analysis,it can be seen that the voltage balance strategy based on the square feedback of the output voltage is the least difficult to achieve and the dynamic characteristics of the system are optimal.