Research On The Stabality Control Technology Of PV Grid-connected Inverter Under Weak Grid

In recent years, with the growing energy crisis and environmental issues, as aclean and renewable energy solar energy has developed rapidly. Photovoltaic(PV)power generation system is usually used to solve the needs of remote areas.However, in remote areas, islands and the grid end, the grid is usually designed forrelatively small loads and its electricity supply mostly is medium and low voltagelevel and the grid presents the weak grid characteristics. The short-circuit capacityand high grid impedance characteristic of weak grid can have negative impacts onthe control performance, power quality and stability of PV grid-connected system.Therefore this paper studies the PV grid-connected inverter under weak grid fromthe points of grid synchronization, stability control method and so on, proposesconcrete measures to improve the control strategy and performance of inverter andensures that the PV grid-connected system can operate steady under weak grid.Herein the two-stage topology is used in the PV grid-connected system, basedon this firstly the system model is established, the inverter output impedance modelis derived and the impacts of various parameters on it is analyzed; secondly theimproving impedance-based criterion for current source system is introduced, andthen the impacts of different grid conditons on the inverter are studied and it isfound that grid impedance increase could reduce the system stability. Based on thisvirtual reactance control method is proposed to improve the system stability; Thenaiming at the drawbacks of zero crossing detection synchronization method underweak grid, the frequency locked loop based on second-order integrator(SOGI-FLL)technique is proposed to achieve the inverter reliable and steady grid-connectionunder weak grid.Finally, a single-phase PV grid inverter experimental platform based on thecontrol chip of TMS320F2812is built, the hardware circuit and software controlalgorithms are designed, and experiments on this platform are completed. Theexperimental results verify the correctness and validity of the virtual inductancecontrol method and grid synchronization method which accord with the theoreticalanalysis.