Control Strategy Research Of Multi-functional Grid-connected Inverter In Microgird

The energy crisis and environment pollution are becoming more aggravated around the world, so it is an inevitable trend that taking full advantage of renewable power (such as photovoltaic, wind et al.) generation. As an effective measure of accommodating distributed generation, microgrid has attracted more and more attention. Notably, the grid-connected inverter in microgrid is a key plant, which has similar structure with power quality devices and flexile control method to realize active control of power quality. The research of multi-functional grid-connected inverter control strategy is meaningful for improving adaptive ability of inverter in weak grid, stability and reliability of microgird operation. A thorough study of synchronization phase, unbalanced grid and harmonic compensation control method will be carried out based on multi-functional grid-connected inverter. The main work and innovation of this paper are shown as follow:Based on the analysis of mathematic model and dynamic performance of synchronization reference frame, the decoupling methods of positive and negative sequence based on second-order generalized integrator and moving average filter are studied respectively. The structure, parameters selecting and dynamic performance of this two filters are presented in detail, according to these, then the synchronization reference frames based on dq synchronously rotating frame are designed. The simulation results have shown the two method could detect information of sample grid voltages precisely, and eliminate the negative effects of unbalanced grid and harmonics. Notably, the method based on moving average filter has the simpler structure, the faster response speed, and the stronger harmonic suppression ability.In order to improve the adaption abilities of grid connected inverter, an integrated control for grid-connected inverter is studied to control the active and reactive power of positive, negative sequence under fundamental components, and the harmonic comprehensively. First the mathematical model of inverter is built, and we put forward to a current united control method under the positive sequence dq rotating coordinates to combine the positive and negative sequence currents and improve the response behavior of system. At restraining harmonics point, an integrated method is proposed to reduce the number of controllers and make complexity into smaller, which lets the 6k±1 characteristics harmonics transfer to 6k harmonics under the dq rotating coordinates. Then after the adding of fundamental, second order ripple and 6k harmonics, the integrated control is realized by PR controller. Finally, the experiments are carried out and the results verify the effectiveness of the proposed method.A rated power of 50kW multi-functional grid-connected inverter is developed, and the hardware, control system and software design are introduced in further. The experiments show that the proposed control method can restrain the second-order harmonic in dc link and compensate the harmonics of non-liner loads effectively, which improves the power quality of microgrid.