Research On Small-scale Stand-Alone AC Photovoltaic System

As the shortage of conventional fossil energy and environmental pollution problem depravation,countries all over the world pay more attention to the development and utilization of renewableenergies。As one of the exploitation means of solar energy, photovoltaic(PV) power generation doesgreat signification in weakening energy crisis and environmental pollution,as a result, it has become aresearch hotspot。In recent years, each country conducts extensive and intensive research inlarge-scale grid-connected PV systems, however, medium and small-scale stand-alone PV systemsalso have bright foreground.This paper takes the small-scale manageable AC stand-alone PV system for the research objectand a PV system which sets PV cells and battery as the main power source and energy storageequipment respectively is designed. The system consists of PV cells, battery, unidirectional DC/DCcoverter, bidirectional DC/DC converter, full-bridge inverter and AC load. The operational modes andcontrol strategies of the system are studied in this paper.For a purpose of easier analysis of PV cells, an engineering non-linear mathematical model ofthe PV cells is concluded in according to the four characteristic parameters (open circuit outputvoltage Voc, short circuit output current Isc, maximum power point voltage Vmand maximum powerpoint current Im) on the basis of the equivalent circuit model of the PV cell. The mathematical modelis proved to be feasible through the simulation model which is constructed in Matlab/Simulink. Therealization of MPPT is a very important issue to the PV cells, so several common MPPT methods areanalized and compared in this paper, while advantages and disadvantages of these methods arepointed out.The stand-alone system proposed in this paper has four differnet operational modes underdifferent PV cells and battery conditions. The working statuses of the converters in the system underdifferent operational modes are discussed. The unidirectional DC/DC converter or the bidirectionalDC/DC converter both have three different working modes, while the full-bridge inverter have twoworking modes. In order to make the three converters coordinate to work, the energy managementstrategy of the system is set up. The key function of the strategy is controlling the three converters inthe system work in the corresponding modes in referring to the different statuses of PV cells andbattery, which causes the two power sources (PV cells and battery) coordinate to work, so the stablepower supply for the load is assured. The two DC/DC converters in the system both have two various working modes when they arenot shut down. So two distinct controllers are prepared for either of them, in order to regulate theparameters of the controllers independently and increase the flexibility of the system.At last, the simulation and hardware experiment of the stand-alone PV system are excuted toanalysis the system states under different operational modes. The results of the simulation and theexperiment concide with the theoretical analysis, the coincidence proves the feasibility of the controlstrategies.