Research On Model Predictive Control Of Three-phase PWM Converter Based On Sliding Mode Theory

Three-phase voltage PWM converter has been widely studied because of its excellent performance such as bidirectional power flow,controllable power factor,low harmonic current and stable controllable dc-link voltage.In the traditional model predictive control method,for the current inner-loop,the input current under the action of 8 voltage vectors at k+1 time is predicted by the discrete mathematical model of the converter,and the value function is defined to evaluate the eight prediction results.Finally,the switching state corresponding to the optimal voltage vector is selected as the control signal of the k+1 time power converter.For the voltage outer-loop,the PI controller is used to generate the reference value of active current.However,in this traditional model predictive control,the current inner-loop prediction algorithm is more complex,and the operation time is time-consuming.The voltage outer-loop PI controller will cause larger overshoot or undershoot longer stability time and larger steady state error under disturbance.In order to reduce the time consuming of the current inner-loop model prediction algorithm,and overcome the shortcomings of PI controller.In this paper,a simplified control method combining current inner-loop voltage predictive control with voltage outer-loop sliding-mode control is presented.Firstly,the working principle of three-phase voltage source PWM converter is given and its mathematical model is built.The mathematical models in ?? and dq coordinate system are obtained by coordinate transformation.Then the sliding-mode variable structure control theory is briefly analyzed,and the design method of sliding-mode variable structure is given.Secondly,based on the current internal-loop model current predictive control,the voltage predictive value function is constructed.The voltage vector which minimizes the error between the actual voltage value and the reference value is directly selected as the optimal voltage vector,and the corresponding switching state is used as the output of the controller to realize the control of the power switch tube.Then,in the voltage outer-loop control,the active current expression based on sliding-mode control is derived based on the dc-link voltage relationship in dq coordinate system.In order to improve the problem of large output jitter and slow approaching speed of sliding-mode controller designed by traditional exponential reaching law,an improved approach law is proposed.Compared with the traditional sliding-mode controller,the results show that the proposed sliding-mode controller designed by the improved approach law can obviously restrain the output jitter and accelerate the approach speed.Finally,the control strategy is simulated and analyzed by building model in Matlab/Simulink and hardware experiment platform.The results show that the proposed control method simplifies the current inner-loop model prediction algorithm,overcomes the shortcomings of the traditional model prediction voltage outer-loop PI controller,and suppresses the influence of the unknown changes such as the dc-link voltage change and the output power demand mutation.The feasibility and effectiveness of the proposed control strategy are verified.