Study On The Small Photovoltaic System And Its Key Issues

As the source of traditional energy dwindling fast with a corresponding rise in cost, many countries are paying attention to renewable energy. The photovoltaic (PV) energy becomes a promising alternative source. PV power systems aer easy to design, install, no noise, longer life and little maintance. Because of high investment of PV system, it has great significance to study PV system diagnosis, maximum power point tracking (MPPT), new grid-connected inverter topology and its control method.On the base of mathematical model, the output characteristics of PV module in normal and shadow background are simulated. With the proposed sampling circuit and PC-based monitor software, the output characteristics of PV module in different shadows are studied. Based on the analysis of simulation and experiment results, the output characteristics at normal condition with different shadow effect situations are compared. Then, the principles of PV output characters in shadow problem are summarized, and the PV array under shadow condition mathematic model is built.The causes of PV array fault points under fault condition are studied and the output characteristics of PV string under shadow situation are simulated using MATLAB software. On the base of studying simulation results, a new type of PV string fault diagnosis method is purposed. Hardware experiment had been implemented, and experimental results were analyzed. The results verify the effectiveness of the method. PV array output characteristics under fault conditions are simulated, the rule of change is summarized. Fault diagnosis method based on branch current and PV module voltage detection is studied, and optimal voltage sensor placement based on covering all weight nodes is proposed. Experimental results are presented to validate the proposed method. At last, a multi layer PV array fault diagnosis and fault PV array manage method are proposed.Through analysis PV mathematic model and PV output power & duty-ratio relationship, a novel two stage variable step-size maximum power point tracking control strategy had been proposed. By simulation model, four MPPT methods are compared, and simulation results indicate two stage variable step-size has quick respond and good stability. Hardware experiment had been set up, and the validity of the proposed method is verified by experiment results. PV array output characteristics under shadowed conditions have multi local maximum power point. This paper proposes a new global MPPT method under non-uniform condition. The change rule of voltage-differential coefficient curve feature is summarized and equivalent area is introduced to located real maximum power point interval, and multi local MPPT method is developed. Simulation and experimental results are presented to validate the proposed method. Furthermore, on the base of PV array fault diagnosis method, this paper also proposes MPPT strategy under shadow effected condition.Based on two types of step-up DC-DC converter topologies, this paper proposes a type of single input double output high step-up converter which combined coupled inductor with Luo converter. Work condition of new topology is researched, continuous conduction mode and discontinuous conduction mode is studied. Because of low output voltage of PV array, a step-up multi-level grid-connected photovoltaic (PV) inverter is proposed, which is composed by high voltage gain DC-DC converter, three level DC-DC converter and line frequency inverter. In order to get high quality output current, multi-stage hysteresis control is developed which can bring down Mosfet frequency and output current total harmonic distortion (THD). Simulation model is built using MATLAB to proof the proposed topology and control method, 600W prototype which uses TMS320LF2812 as controller is also set up, the simulation and experimental results verify the proposed system correction.