Research On Low Power Non-isolated Photovoltaic Grid-connected Inverter

Solar energy is the cleanest and the most promising renewable energy, the basic way to use solar energy is light-heat conversion and photovoltaic conversion. Grid-connected PV power generation is one of the main forms in the latter, non-isolated grid-connected PV system has made considerable progress with advantages such as low cost, high efficiency, high reliability. In this paper, single-phase two-stage non-isolated photovoltaic power generation system is the research objects. Several functions of PV inverter system has been studied, also a lot of simulation and experiments has been done.First of all, the physical model and output characteristics of the solar cell are studied in-depth. Simulation model for PV array is built in PSCAD simulation platform with Fortran programming language, simulation results is consistent with the output characteristics of PV array. Secondly, the principle of maximum power point tracking (MPPT) is introduced in this thesis. By comparing several general MPPT algorithms, a new variable step MPPT algorithm has been proposed. To verify the feature of the new MPPT algorithm, a simulation circuit is established with BOOST circuit as the core. Simulation results show that the algorithm has faster tracking speed and much smoother curve. Thirdly, the need for insulation resistance on-line detection of PV inverter is explained. On the base of the analysis of normal detection methods such as balance bridge method, AC detection method, DC leakage current method, an improve threshold method is put forward. Furthermore, prototype hardware circuit and related algorithm are designed. Field experiments show that the new method has the advantage of higher precision and higher reliability. Finally, causes of DC component of PV system and its adverse effects on transformer and the grid are introduced. Then hardware and software method of DC component suppression have been analyzed respectively, an improved virtual capacitor method is proposed, and complete non-isolated PV inverter system simulation circuit has been built in PSCAD platform. Simulation results show that the design of the control algorithm has the characteristics of high quality.