Research On Strategy Of Three-phase Grid-connected Inverter Based On Improved FCS-MPC

In the new energy photovoltaic power generation grid connection system,the three-phase grid-connected inverter is the core equipment,and its performance is directly related to the power quality of the power grid.Therefore,the research on the algorithm of the three-phase grid-connected inverter has been the focus of research.In order to solve the problems existing in the control algorithm of the three-phase gridconnected inverter algorithm,such as large scale of calculation,delay and current harmonic resonance in the calculation process,two control strategies were proposed to solve the problem:the Finite Control Set(FCS)Model Prediction Control(MPC)based on Optimal Voltage Vector Sequence(OVVS)and the improved FCS-MPC algorithm based on Iterative Calculation of Structural Parameters(ICSP).According to the research literature,Finite Control Set Model Predictive Control(MPC)is an intelligent control method currently applied to three-phase grid-connected inverters.The control method itself has good adaptability,stability,and robustness.This algorithm is different from the traditional PI algorithm.The application of the FCS-MPC algorithm can avoid the parameter setting and pulse width modulation(PWM)control of the PI controller in the traditional PI algorithm.The control process is relatively simple and can effectively suppress the existence of the harmonic waves in output current.optimize the static tracking and dynamic response performance of the control system,and finally obtain the output current with the best power quality.First of all,this thesis introduces the main topology of the three-phase gridconnected inverter,and then establishes the dynamic equations of output current of the three-phase grid-connected inverter under the three-phase stationary abc coordinate system,the two-phase stationary αβ coordinate system,and the two-phase rotating dq coordinate system.Subsequently,the algorithm of the current source three-phase gridconnected inverter is studied in depth.The principle of feedforward decoupling algorithm based on closed-loop current and the design and modulation of PI controller are analyzed.In order to solve the problem of poor static tracking performance of the output current in the traditional current closed-loop algorithm,distortion and overshoot in the waveform,and the slow dynamic response,a model prediction current algorithm is proposed to replace the traditional PI current closed-loop algorithm.According to the advantages of the FCS-MPC algorithm summarized in the reference,the FCS-MPC algorithm can replace the PI controller and modulation link in the current closed-loop feedforward decoupling algorithm,effectively improving the system’s steady-state tracking and dynamics responsive performance.Secondly,this thesis is based on the actual application of the traditional FCS-MPC algorithm,it was found that there are calculation delay problems and current oscillation problems in the control process when the traditional MPC algorithm is used to control the output current.These problems prevent the output current from achieving stability,fast tracking and dynamic response.In order to solve the above problems,an FCS-MPC algorithm based on Optimal Voltage Vector Sequence control is proposed.The two sampling period prediction values are used to compensate the tracking error caused by the delay problem.In addition,the control algorithm changes from one voltage vector to the use of two adjacent voltage vectors during the sampling period,and then the two and zero voltage vectors act together on the control circuit to eliminate current oscillations.In order to reduce the switching frequency,the voltage vectors are sorted in order of action to obtain the OVVS to achieve optimal voltage vector control.Subsequently,a simulation model was established in the Matlab/Simulink simulation platform to verify the feasibility of the proposed algorithm.The comparison of several simulation results led to the conclusion that the steady-state performance and power quality of the FCS-MPC algorithm were good.Finally,in order to further simplify the calculation and improve the dynamic response performance of the algorithm,a control algorithm based on ICSP-FCS-MPC is proposed.The control target is still the inverter output current.Iterative calculations are carried out in batches and updates for the structural parameters R,L,Kd,and Kq in the control system,thereby improving the performance of the control system during step response and optimizing the dynamic performance of the control system.The results obtained using Matlab/Simulink simulation and experimental platform show that the algorithm has a good steady-state tracking effect and can achieve error-free tracking control performance.For the ramp function,due to the second-order model established in the algorithm,the control system can realize the error-free tracking of the harmonic function.For the step response,the algorithm responds quickly with almost no overshoot,and there is only a short-term three-phase imbalance in the transition phase.Therefore,the static and dynamic characteristics of the algorithm are further optimized.