The Research Of Capacitor Charge Balance Control Strategy On Three-phase Voltage PWM Rectifier

With the development of wind turbines and new energy vehicles,the energy conversion system imposes high power density,high reliability,stable steady state performance and fast dynamic performance on the rectifier.PWM rectifier has the advantages of two-way flow of energy,controllable power factor and output voltage,low-harmonic current as well,which make it suitable for back to back PWM converter technology of wind turbines and new energy vehicles.This paper is devoted to improving the dynamic performance of the output DC voltage of a three-phase PWM rectifier,and then comparing it with the dynamic performance of the traditional control strategies with voltage PI loop.In this paper,the capacitor charge balance control strategy is proposed to improve the dynamic response of the output voltage,which is always affected by the PI parameters in both voltage oriented control and direct power control.Capacitor charge balance control can improve the dynamic performance of output voltage in DC converter system,torque impulse balance control can improve the dynamic performance of out speed in permanent magnet motor system.This article respectively analyzes the waveforms of output voltage and the active power under the sudden change of the load resistance condition.The charge of the capacitor is constant after the increased load,and combined with the idea of torque impulse balance control in motor system,the ideal waveforms of output voltage and active power based on capacitor charge balance control in dynamic process can be derived.The system can reach the steady state only by experience once increase and decrease process,and the combination of optimal vectors can be count using this strategy.This control strategy can realize fast tracking of DC-side output voltage during dynamic process and optimize the dynamic performance of three-phase voltage PWM rectifier.Comparison of dynamic performance between capacitor charge balance control strategy and model predictive control is investigated.Compared to direct power control,model predictive control replaces traditional switching vector tables with objective functions,therefore,achieving the fastest dynamic adjustment of the inner power loop.The capacitor charge balance control strategy is also model predictive control in essence.Different from traditional predictive control,the traditional predictive control still includes the voltage loop PI regulator.Due to the linear output characteristics of the PI parameters,and the inertia constant of the three-phase rectifier system is usually small,the controlled quantity usually tracks the given value of the PI output very quickly in a control period.So the avtive power in current component of the rectifier has an oscillation process in the dynamic process of the adjacent control period,making the control process fail to achieve the fastest adjustment process.The capacitor charge balance control strategy is designed for controlling both of voltage loop and power loop of three-phase rectifier system.In the steady state,the traditional direct power control is still used,but after the DC-side load is changed,the system is controlled by capacitor charge balance control.The control algorithm combines the mathematical model of the three-phase rectifier with the sudden load to obtain a duty cycle that can increase and then decrease the output active power.The output voltage only needs to experience once decrease process and once rise process to reach steady state.Compared with the traditional control strategy with voltage PI loop,the capacitor charge balance control has faster dynamic response speed.Finally,the control strategies carried out respectively based on 3-phase VSR,a digital control system based on DSP and a three-phase half-bridge power converter are designed,and the effectiveness and feasibility of this control strategy are verified by comparisons.