Research On Dual-mode Mode Predictive Control For A Three-phase LCL Grid-connected Inverter

In recent years,with the gradual increase in the installed capacity of renewable energy connected to the grid,the three-phase LCL-type inverter,which serves as the interface between renewable energy and the power grid,has received widespread attention and research.Research on control strategies for grid-connected inverters is of great significance in ensuring the stable operation of the system.It is a crucial technical challenge to improve the control strategies of grid-connected inverters and enhance their robust control performance from multiple perspectives.This paper focuses on the research of robust predictive control and dual-mode switching principles for current-source and voltage-source mode grid-connected inverters.The main research areas include: modelfree current prediction control for LCL-type grid-connected inverters,design of an improved model-free controller based on sliding mode observer,theoretical investigation of parameter-free sliding mode predictive control for current-source mode and voltagesource mode grid-connected inverters,design of a weighted sliding mode predictive control scheme for grid-connected inverters,and design of a dual-mode switching scheme for grid-connected inverters in weak grid conditionsFirstly,taking the three-phase LCL-type grid-connected inverter as the research object,under the model predictive control(MPC)method,the impact of parameter mismatches between the filter inductance and capacitance on the accuracy of grid current control is analyzed by establishing the expression for prediction control error of grid current.To address the issues caused by parameter mismatches,a model-free direct prediction control method for grid current is proposed using the theory of hyper-local modeling.Furthermore,algebraic identification techniques and a collective disturbance observer based on sliding mode observer are employed to estimate the key variables online,thereby reducing the influence of parameter mismatches on grid current control.Secondly,to thoroughly eliminate the influence of model parameters on grid current control,a model-free sliding mode predictive current control method for three-phase LCL grid-connected inverters is proposed based on the principles of sliding mode predictive control.The key aspect of this method is the establishment of a novel model-free sliding mode predictive cost function.This method is not affected by any model parameters and achieves model-free robust predictive control of grid current.Thirdly,addressing the high dependency on circuit parameters in conventional model predictive control(MPC)for voltage-source mode grid-connected inverters,a model-free predictive capacitance voltage control method for LC-filtered grid-connected inverters is proposed based on sliding mode predictive control theory.Compared to conventional capacitance voltage model predictive control,this method achieves capacitance voltage predictive control without requiring any model parameters.Furthermore,to improve waveform quality,a weighted sliding mode predictive control scheme is designed,reducing the harmonic content of the filter capacitance voltage.Fourthly,considering the complementary characteristics of current-source mode and voltage-source mode,in order to improve the stability of grid-connected inverters under weak grid conditions,a dual-mode switching control strategy based on model predictive control is proposed.The basic principles of dual-mode switching based on model predictive control are analyzed in the paper,and a dual-mode switching predictive control scheme(including amplitude switching and phase switching)is designed to achieve smooth transition from current-source mode to voltage-source mode,ensuring the stable operation of the grid-connected inverter.The stability of the LCL-type grid-connected inverter under impedance fluctuations is verified through simulation platforms.Finally,on the hardware platform of three-phase LCL grid-connected inverter,the effectiveness of the proposed model-free predictive control and parameter-free sliding mode predictive control for grid-connected inverter is verified.