Research On Three - Phase Voltage - Type Photovoltaic Grid - Connected Control System

With increasing fossil energy crisis and environmental pollution, the solar energy, as a type of renewable energy sources, has great potential and advantages in response to environmental pollution and energy structure optimization. In recent years, the governments have issued policies to encourage and support the development of photovoltaic industry. Therefore, it’s necessary to do some research on photovoltaic (PV) power generation technology. The research object of this paper is the PV grid-connected generation system, it mainly explored some key problems in the three-phase PV grid-connected generation system, such as the control strategy of the maximum power point tracking (MPPT) and PV-grid inverter control method, the main research contents are as follows:Firstly, the MPPT control method of photovoltaic cells is studied. The equivalent mathematical model of photovoltaic cells is set up, the principle and significance of MPPT is introduced and several classical MPPT methods are analyzed. An improved algorithm which consists of current perturbation algorithm and adaptive control algorithm, is proposed to solve the disadvantage of slow dynamic response and strong oscillation which are existed in perturbation-observation method under sudden changes in the irradiance. The current disturbance size related to the irradiance and the operating range of adaptive control of this method is derived in detail, a boost circuit is designed to realize the proposed algorithm, and a small-signal model is established to analyze the stability of system, which can improve the energy efficiency of system and make full use of the PV array.Secondly, the control method of PV grid-connected inverter is studied. The topological structure, the design of relevant parameters and performance analysis of three-phase PV grid-connected inverter are introduced. The mathematical model of three-phase PV grid-connected inverter based on the LC filter is established in two-phase (d-q) rotating coordinate system.The decoupling control of cross-coupling term in the model is realized. The principle and realization of space vector pulse width modulation (SVPWM) technology, three phase phase-locked loop (PLL) technology and double-closed loop vector control method based on the grid voltage orientation is analyzed systematically, and a improving double-closed loop vector control strategy is proposed. Where the fuzzy control method is introduced to fully adapt to the nonlinear and time-varying characteristics of the PV control system itself, and a fuzzy PI controller which can meet the performance requirements is designed to improve the anti-interference ability and robustness of the system.Based on MATLAB/Simulink simulation platform, the simulation model of three-phase PV grid-connected control system is built to study the MPPT, PLL phase sequence test and three-phase PV grid-connected experiment respectively. The simulation experiments results show that the tracking performance of the proposed MPPT algorithm is better than traditional perturbation-observation method under sudden changes in the irradiance, and phase sequence detection based on PLL has high efficiency and reliability. Furthermore, the whole three-phase PV system can not only successfully realize the grid-connected, but also the improved intelligent grid control algorithm has fast dynamic respons and strong anti-interference ability. Finally, in order to achieve the user-friendly target, the GUI is designed, which formed the software package with the function of demonstration effect.