| In recent years, our country has been facing the threat of energy shortages. To solve the serious environmental problems, the development of new energy and solutions for a series of questions about new energy generation connected to the grid seem urgently. Micro-grid has been proposed to avoid the impact on grid caused by distributed energy, which can provide CCHP(Cooling heating and power) for load within the region, also can be run in parallel with the grid. DC micro-grid is a kind of micro-grid with DC transmission mode, and is composited by micro-power, power electronics interface, energy storage and load, transmission by DC mode.In this paper, we discuss and research on the main electronic converters in DC micro-grid. First in the case of unbalanced load the inverter can provide access to zero-sequence current by increasing the fourth leg on the basis of three-phase inverter, the three-phase four-leg inverter can be used in various unbalanced occasions and nonlinear loads. Secondly, the process needed to be smoothly and friendly when the DC micro-grid access to the AC power grid as a DC micro-sources. Grid-connected-inverter under control of virtual synchronous generator can adjust to participate to adjustment automatically, improving the access quality of interface while achieving distributed power connected to grid, it can also eliminate the voltage fluctuations caused by a large number of local load cut or vote in the course of access to the grid. After DC micro-grid connected to grid successfully, grid connected inverter can inhibit the voltage distortion of large grid, and maintain the stability of load-side voltage. Finally, A flexible control method of DC/DC converter based on virtual DC generator is proposed for the charging problem of the most common DC load —electric vehicle in DC micro-grid. In the constant voltage charging mode, the DC/DC converter under control of virtual DC generator can maintain the charging voltage by stabilizing the load voltage from affection of the disturbance occurred by DC bus voltage. It can also eliminate the affect of DC bus' s fluctuations and make the output current in the constant current charging mode. The inertia of DC generator enables the load side voltage, current changes to fluctuate gently, thus improving the stability of electric vehicle charging effectively.This paper built a simulation model of a DC micro-grid in PSCAD/EMTDC, the model includes three-phase four-leg inverter module with unbalanced load, DC/DC converter module with load under virtual DC generator control, and interface modules based on virtual synchronous generator. And this paper verified the feasibility of virtual synchronous generator and virtual DC generator control strategy and the validity of neutral line for unbalanced load module through simulation and experiment, laid the foundation for further study of DC micro-grid. |
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