Research On MPPT Techniques For Grid-connected Photovoltaic Inverters

Worldwide energy shortage and environmental pollution have constrained the development of society, so we have to search, exploit and use new alternative energy sources. For this reason, solar energy has attracted more and more attention. Nowadays, one important research topic of solar energy application is PV grid-connected power generation, while the kernel of the PV power generation system is grid-connected inverter.In this thesis, the circuit structure and control strategy of a two-stage transformerless grid-connected PV inverter are introduced first, and then the MPPT technologies are researched and analyzed in depth. Then a 3kVA prototype is built up, and the experimental results verified the correctness of the theory. The main contents of this thesis are as follows:1) At first, the principle and output characteristics of solar cells are analyzed. Then two methods to calculate the solar cell's parameters are given in detail, and then two different simulation models of solar cells under Matlab have been built. Besides, the comparison between experimental data and simulation data also has been done. In a word, all these provide the basis for photovoltaic system simulation, theoretical research and system design.2) At the same time, some current topologies and control methods of the grid-connected inverter are analyzed and compared. Further more, the operation principle, control strategy and system structure of the 3kVA two-stage transformerless PV grid-connected inverter are also introduced in detail.3) In addition, the principles, characteristics and applications of several common MPPT methods are analyzed. As we all know, a serious limitation of the Perturb and Observe (P&O) MPPT method is that it can get confused and track in wrong direction during rapidly changing irradiation. In order to overcome this problem, a improved dP-P&O method has been proposed. Finally, the simulation and experimental results show that the dP-P&O method has good dynamic and static performance, and the performance is improved effectively in the case of rapidly changing irradiation.