Circulating Current Analysis And Suppress Of A Hybrid AC-DC Microgrids Connected By MMC

In recent years, in the background of environment polluted, energy shortage, the traditional power supply in remote areas exposed the increasingly serious drawbacks, while the development of microgrids solved this problems well. It is one of the key technologies in the future, which is to maximize the output of the distributed generation system and to improve the reliability and power quality of the power supply system.Aiming at the problems that exist in the structure of AC,DC and hybrid AC-DC microgrids,a hybrid AC-DC microgrids connected by modular multilevel converter is studied in this thesis. The circulating current in the system of hybrid AC-DC microgrids connected by MMC is mainly studied.Firstly, introduced three kinds of microgrids structure of AC, DC and hybrid microgrids,as well as compared with the respective advantages and disadvantages of AC, DC and hybrid microgrids. Analyzed the superiority of the new generation MMC over the traditional structure converter and mathematical model, working principle and modulation of MMC. At the same time, several main micro-sources in the microgrids are introduced.Then, the specifc structure of the hybrid AC-DC microgrids connected by MMC is given. The passage of circulating current between multiple micro source inverters and between MSI and MMC are analysed in the subnet of AC microgrids, drawn out that the circulating current is mainly due to the respective output voltage exist in the vector difference. At the same time, through the simulation model to verify the amplitude difference, phase difference effect on the circulating current. At the same time, it is analysed that the series equivalent micro sources output voltage is not equal to the MMC DC side voltage will cause the circulating current in the subnet of DC microgrids as well.Finally, in order to suppress the circulating current of the subnet of AC microgrids and the subnet of DC microgrids, adopted the droop control, voltage and current double closed-loop control, MMC sub module voltage control and slave controller. at the same time, built a hybrid AC-DC microgrids model which containing the photovoltaic, wind micro sources, super capacitor and MMC, the simulation result verified the suppression effect of the circulating current of a hybrid AC-DC micro grid connected by MMC.