Modeling And Controller Design For Photovoltaic Grid-connected Inverters

Grid-connected PV system is currently a hot research issue. Grid-connected inverte is a critical component of grid-connected PV system, so the study on it has both theoretical and practical significance. Status of research on the PV grid-connected inverter is analyzed. This article mainly focuse on modeling the current loop and voltage of single phase grid-connected inverter, modeling the current loop of three phase grid-connected inveter based on two-phase stationary axis, grid current control methods, and maximum power point tracking and islanding effect. system and grid current control technology. Experimental platform is built to verify the theory.For single-phase grid-connected inverter, a boost DC-DC converter is employed as the first stage to implement maximum power point tracking(MPPT) function and second stage with a single inductor filter is used to implement grid current injection. Compared with the commonly used MPPT method, variable step size perturbation and observation method is chosen in this article. The models of current loop and voltage loop are built with Proportion-Integration(PI) control; the modeling process of DC voltage loop is analyzed in detail and compared with commonly used control scheme; because of the double-frequency ripples in the DC bus capacitor voltage, a voltage feedforward scheme is designed in order to improve steady-state and dynamic performance of the output waveform, which leads to relatively less control complexity and makes voltage disturbance effectively suppressed. The Islanding effect is also analyzed. Passive and active detection methods are combined to achieve islanding protection in the experiment.For three-phase grid-connected inverter, a three-wire half-bridge single-stage topology is used. Three-phase PLL are introduced. The characteristics of LCL filter are analyzed. Due to the presence of resonance peaks in LCL filter, damping is necessary to ensure system stable. Compared with common passive damping methods and active damping methods, series resistance in the filter capacitor branch is used to achieve passive damping to simplify the design of system. LCL filter parameters are designed in detail. LCL filter based three-phase single-stage grid-connected inverter is modeled in two-phase stationary axis, and its control system is designed. Combined PI compensation and repetitive control, a composite control scheme is proposed, which can retain the advantages of both PI control and repeat control, and improve system performance.After the analysis of the principles, the simulation is done based on MATLAB/simulink, and experimental prototype platform of single-phase and three-phase grid-connected inverter are built respectively. The results are analyzed.