Control Strategy For Three-phase PWM Rectifier With Active Power Decoupling Circuit Under Unbalanced Grid Voltages

Three-phase pulse width modulation(PWM)rectifiers are widely used in various application,including electric drive,power quality control,and renewable energy generation system,due to their sinusoidal grid-connected current,high power factor,adjustable DC voltage,and bidirectional energy transmission.The unbalanced power grid often occurs in power system,which may threaten the safe and stable operation of PWM rectifiers.The conventional three-phase PWM rectifier cannot simultaneously achieve satisfactory AC control and DC performance,including symmetrical and sinusoidal three-phase current and stable DC voltage without double-line-frequency ripple,due to its limited control degrees of freedom.Therefore,a new topology of three-phase PWM rectifier based on active power decoupling(APD)circuit is proposed in this paper.In addition,two control strategies for the designed rectifier topology are presented,including direct current control(DCC)and model predictive control(MPC).Firstly,the mathematical models of the three-phase PWM rectifier main circuit and the APD circuit are established.Consequently,the power equations of AC and DC sides for three-phase PWM rectifier under unbalanced grid voltage are derived.In addition,the problem of the limited system control degrees of freedom is analyzed and the necessity of introducing APD circuit is elaborated.Furthermore,the principle that APD circuit can achieve the satisfactory control performance of AC and DC sides simultaneously under unbalanced power grid is thoroughly discussed.The DCC and MPC strategies for the main circuit and APD circuit of three-phase PWM rectifier are proposed from the two aspects of independent control and unified control.The DCC strategy is designed in synchronous rotating frame.The main circuit works in the control mode of AC-side negative sequence currents elimination,and the APD circuit works in the control mode of DC voltage regulation without double-line-frequency ripple.The scheme can achieve AC-side and DC-side control performance simultaneously under unbalanced voltage sag condition.In order to improve the integration of control algorithm,a unified control strategy based on MPC for the main circuit and APD circuit of PWM rectifier is proposed.In this scheme,the control objectives of AC side and DC side are included in the same cost function,which simplifies the system implementation.Since the scheme is designed in the stationary reference frame,the control performance of AC and DC sides can be achieved under unbalanced voltage sag condition and unbalanced phase condition.In addition,the 3kVA experimental platform of the PWM rectifier based on APD circuit is established,and the main circuit,APD circuit,control circuit,and detecting circuits are designed and tested,which provide the basis for the further experiments.The simulation results show that the presented three-phase PWM rectifier with active power decoupling circuit and its control strategies can achieve high power factor operation(the power factor of each phase is above 0.95,and the current THD is less than 5%),negative sequence currents elimination to obtain the three-phase symmetrical currents and dc voltage regulation without double-line-frequency ripple under various unbalanced conditions,including one-phase voltage sag,two-phase voltage sag,and unbalanced phase condition,which improves the operation performance and stability of PWM rectifier under complicated operating conditions.