Research On Double Closed-loop Sliding Mode Control Of Three-phase Voltage-Type PWM Rectifier

The traditional uncontrolled rectification and phase-controlled rectification have low power factor,slow dynamic response,and large current harmonics on the grid side,which injects a large amount of harmonic interference into the power system and reduces the power quality.The three-phase voltage type PWM rectifier has the characteristics of single power factor operation,small current harmonics on the grid side,bidirectional flow of energy,adjustable output voltage,etc.These superior performances make it suitable for wind power generation,superconducting energy storage and active filtering.It has a wide range of applications with reactive power compensation and motor frequency control.However,the three-phase voltage type PWM rectifier still has a problem that the anti-interference ability is not strong and the robustness is poor.Therefore,research on robust control of three-phase voltage-type PWM rectifiers is of great significance for improving power quality.Based on the topology of three-phase voltage-type PWM rectifier,this thesis presents the equivalent current diagram and analyzes the working principle of the rectifier.The mathematical model of PWM rectifier based on three-phase static abc coordinate system is established by using equivalent circuit diagram.The mathematical model and equivalent circuit based on synchronous rotating dq coordinate system are given by coordinate transformation.Then two different double closed-loop control strategies are designed for the three-phase voltage-type PWM rectifier.Simulations are compared through MATLAB platform to analyze the advantages and disadvantages of the two different control strategies.Firstly,the three-phase phase-locked loop is used to complete the conversion of the threephase voltage-type PWM rectifier from the stationary ab coordinate system to the synchronous rotating dq coordinate system,and two different dual closed-loop controllers are designed.The first one is that,the current inner loop is controlled by PI decoupling,and the outer loop is controlled by PI.The second one is that,the current inner loop adopts sliding mode variable structure control(SMC),and the voltage outer loop adopts PI control.In order to improve the voltage utilization of the three-phase voltage-type PWM rectifier,both control strategies use the SVPWM modulation algorithm.Secondly,due to the discontinuous positive and negative functions in the sliding mode variable structure control law,the phenomenon of "shake" arises.Because the three-phase voltage type PWM rectifier adopts double closed loop control,the "jitter" of the current inner loop increases the instability of the voltage outer loop.In this thesis,the "boundary layer" method is used to replace the non-continuous positive and negative functions with continuous saturation function to form "sliding mode pseudo-variable structure control".Based on the traditional constant velocity approach rate strategy,exponential rate is added.The near rate,the "boundary layer" method combined with the exponential approach rate weakens the "shake" and improves the stability of the double closed loop structure.Finally,a comparative simulation of the two different double closed-loop control strategies is performed on the MATLAB platform.The simulation results show that the SMC-PI double closed-loop control has better dynamic response performance than the PI double-closed loop control from three aspects: the start-up of the three-phase voltage-type PWM rectifier to the stability,the sudden load change and the voltage setpoint change.It is able to run continuously with a single power factor when the load changes and the given voltage changes.At the end of the thesis,the prototype system framework of three-phase voltage source pwm rectifier is proposed.