Study On Control Strategies Of Double-mode Photovoltaic Inverter

Because of the world energy crisis and environmental deterioration, it is becoming the primary task for human being to exploit the renewable and green energy sources for the sake of sustainable development. With the technological progress of solar cell and power electronics, the photovoltaic technology and industry is not only an important supplement of current energy consumption, but also the potential main energy source in the future. Recently, the photovoltaic system constantly develops with the tendency of lower cost, higher efficiency and higher power density. The main application form of solar energy will be the grid-connected photovoltaic system.The main task of this paper is to conduct a research on the control strategy of a double-mode photovoltaic inverter, i.e., inverter with grid-connected and stand-alone modes. This kind of inverter is characterized by fine reliability and higher utilization ratio of renewable energy. The application form of photovoltaic power generation and the classification of the grid-connected inverters are summarized in this paper. Taking reliability, efficiency and cost into consideration, an inverter with two-stage full-bridge structure was chosen to be studied. The topology of this structure is usually used in small scale grid-connected inverter.The single polarity SPWM control method was selected to generate control signals of inverter in this paper with the analysis of various kinds of current control methods. According to particular cases of photovoltaic system, different control strategies were applied under different modes. Under the grid-connected mode, the load voltage is determined by the grid. The inverter works as a current source, and the output current is directly controlled by double current closed-loop control strategy. Under the stand-alone mode, the inverter works as a voltage source, and its voltage is controlled by current-voltage closed-loop control strategy. Simulations for single-phase and three-phase inverters working under the two modes respectively were conducted with MATLAB Simulink. On the basis of instantaneous reactive power theory, a software phase-locked loop applied on three-phase circuit was proposed, and simulation results verified its validity.When the double-mode inverter switches between the two modes, it is easy to cause surge and interference to loads, electrical network and power sources. So it is necessary to reduce this influence by adopting some effective control scheme of mode-switching. The switching processes between modes of stand-alone and grid-connected with different switching sequence were studied in detail in this paper. A seamless switching scheme between the two modes was presented. Simulations for switching process of single-phase and three-phase inverters between the two modes using this scheme were conducted, and the results proved its feasibilityControl circuit based on DSP (TMS320F2812) was introduced. Several parts of the hardware designs are analyzed and some of the software flow charts are given.