Research On Control Strategy Of PV Inverter Based On 3-D Coordinate System

In recent years, with the increasing depletion of conventional fossil energy sources, photovoltaic power generation has become a major part of today’s energy, for its rich, clean, easy to use and other advantages. This paper focus on photovoltaic grid-connected inverter, including its mathematical modeling, modulation algorithm, grid synchronization technology, the DC voltage control strategy parameters optimization designing, AC current control strategiesof harmonic and unbalanced voltage susppression, based on three-dimensional rectangluar coordinate system.Research on three-phase inverter modulation wave based on the space rectangular coordinate system is presented in this paper, and the concept and characteristics of 3-D space modulation wave vector and new research methods is presented as well. Analytical expressions of SPWM and SVPWM can be obtained with modulation wave vector, which makes the modulation method, control strategy and the main circuit model can be analyzed and modeled in the unified form. The judge condition for over modulation is presented as well. This new method is correct, clear, intuitive and easy to calculate.For PV grid-connected inverter grid synchronization, several typical lock method have been compared. The proposed method concludes GDSC method filteredaccording to the actual existence of the grid harmonic components, positive and negative sequence component decomposing method with the elliptical trajectory and synchronous reference frame phase-locked loop. The proposed method reduces the computational complexity and improves system stability with fast response.A new detection method for three-phase voltage based on 3D-space normal vector is presented in this paper. This new method has some advantages such as quick detection, simple calculation and adaptability for unbalanced voltage. Simulation and experiment is given and the results show that the method has very good speed and accuracy for detection of unbalanced three-phase voltage.The Ⅰ-Ⅴ curve of PV panels is non-linear and affected by the changing light intensity, which requires high control performance of DC voltage loop. This paper presents an optimized control parameters design strategy of DC voltage loop. Firstly, model of PV panel and inverter are established and through comparison and research of kinds of DC voltage-loop control strategies based on ITAE and overshoot. A new optimal DC voltage control strategy is presented in this paper, which implements linear control with best ITAE and overshoot in various conditions, and its control performance in the condition of dynamic and stable light intensity is improved as well.PV grid-connected inverter is effected by the unbalanced and harmonic grid voltage, sampling circuit and other interference. The harmonic injected into grid must meet the industry and national standards, and therefore, it’s important to reduce the inverter current harmonic. Based on two-frequency Bode diagram, this paper has researched traditional harmonic susppression strategies, and using the frequency shift to reform transfer functions.Complex inertial controller is proposed control current harmonics. The proposed method has discussed how to improve the dynamic performance under steady state conditions and dynamic conditions with the changes of current reference and grid voltage, and how to improve the control response speed in the condition of enough stability performance.