Research On Key Techniques Of Low-power Photovoltaic Generation System

So far the research is mainly focused on the medium and high power photovoltaic generation system. As the relevant government policies support and application of building integrated PV project, low power photovoltaic generation system is especially concerned. There are some techniques in common between low power and high power photovoltaic generation system. However, the present circuit topologies and corresponding control strategies applied in high power photovoltaic is not suitable for low power photovoltaic generation system because of different power levels. Hence, it is necessary to research on key techniques of low power photovoltaic generation system. The main contents are as follows:Firstly, the optimize structures for low power photovoltaic generation system and relevant key techniques are discussed. Why the present structure of PV array is so inefficient is studied. Efficiency is most important factor for all of the photovoltaic generation system, especially for the low power level system. Three structures which are suitable for low power photovoltaic generation system and corresponding control strategies are proposed. PV modules are decoupled by DC/DC optimizer in first structure and second structure, and PV module are decoupled by micro inverter in the third structure. Each PV module is decoupled with each other in the above three structures, so every module can harvest maximum power with the result of high generating efficiency. Finally, the differences of DC/DC optimizers are compared in first structure and the second structure.Secondly, the current control strategy in DC/AC inverter which is the post-stage of first structure or second structure is discussed. The disadvantage of PR current controller is analyzed, and it’s suggested that PR controller is single degree of freedom and can’t balance the performance between tracking ability and anti-disturbance ability. R-P controller, modified R-P controller and RP-RP controller are proposed based on the control theory of degree of freedom. A novel single phase DC/AC inverter without PLL circuit is proposed by adopting RP-RP controller.Thirdly, MIC system with quasi DC bus structure is focus on. The flyback converter MIC system with active clamp circuit and interleaving technique is designed in this paper. Large signal model, second order approximation model and forth order precise model are obtained. The predictive control algorithm to calculate the duty cycle in the steady state and active damping method by adopting capacitor voltage feedback are proposed. Based on the above key techniques, current loop and current-sharing loop are well designed. Thus the grid current phase is synchronized with grid voltage and the current flowing into two interleaving converter are equal.Fourthly, grid synchronization is the common issue for grid-connected system. A canonical small signal model of PLL circuit is proposed and some key specifications are analyzed, as well as the effect of noise on PLL circuit.A series of key techniques suitable for low power photovoltaic generation system are proposed. All the theory result are simulated by commercial software, such as Matlab, Psim and PSpice; also Buck-Boost DC/DC optimizer, Boost DC/DC optimizer, 5kW single phase grid-connected inverter and 200 W MIC system are developed to verify the conclusion.