Investigation On Resonant Current Control Of Three-Phase Grid-Connected Converters

Three-phase voltage-source converters are widely used in grid-connected systems due to its simple structure and reliable operation.To achieve efficient operation of grid-connected systems,the improvement in the control of grid-connected converters is a key.The current control strategy is a basic part of the entire control system,which will directly affect the currents fed into the grid.However,due to the inaccuracy of modeling and discretization error,the dynamic of resonant control is difficult to be improved.In addition,the errors in voltage feedforward terms can be regarded as the disturbances in the current loop,which will lead to distortion,unbalance and DC component in the output currents of the converter.Therefore,this paper investigates the improvement in resonance control performance of grid-connected voltage-source converters.In this paper,DC offset and scaling error in grid-voltage measurement are briefly introduced with the mathematical models of errors in voltage-feedforward terms established respectively in stationary and synchronous rotating coordinates.Based on that,this paper presents the mathematical models of proportional integral and proportional resonance control in stationary and synchronous rotating coordinates respectively,and analyzes the disturbance-suppression performance of the current loop.In order to solve the inaccurate modeling in the design of traditional resonant controllers,the zero-order-hold discretization model with control delay is established on the basis of analyzing the characteristics of discrete control systems.On this basis,the state-space method is introduced into the design of resonant controllers,and the state-space resonant controller is proposed correspondingly.In order to further improve the dynamic response of the resonant control,the model predictive method is introduced into the design of the resonant control,and the predictive resonant control is then proposed.In order to improve the disturbance-suppression ability of the current loop,this paper analyzes the characteristics of two disturbance-suppression structures,and proposes a current-loop disturbance-suppression strategy based on the double-loop control structure.Based on the analysis of the characteristics of repetitive controller and multiple resonant controllers,a harmonic-suppression controller based on the improved polynomial internal model is proposed.In addition,to solve the problem that DC offset of grid-voltage measurement brings DC components into the AC side of converters,a DC-current-suppression controller is designed using the dual-loop control structure.To solve the problem that delay may cause degradation in the performance of phase-locked loop(PLL),this paper presents a PLL with delay compensation ability with the characteristics of PLL are analyzed.In order to reduce the discretization error of PLL,the traditional discretization method is analyzed and an improved state-space discretization method is then given.