Research On Coordination Control Strategy For DC Micro Grid With Two DC Buses

Compared with AC micro grids, DC micro grids are of simpler construction and easier control strategy. Besides, there are no reactive power and frequency issues in DC micro grids which make it more convenient to connect with PV systems, energy storage systems and DC loads directly. In view of the above advantages, DC micro grids have attracted more and more attentions from researchers at home and abroad.At present, domestic and international research on DC micro grids most concentrated on the single bus structure, in which all the DC micro sources and DC loads are connected to DC bus through DC/DC converters, resulting in low efficiency of the whole system. In order to improve the operating efficiency of the system, the DC micro grids should provide different voltage levels for different voltage requirements to reduce the use of converters. In this thesis, a novel DC micro grid with two DC buses is proposed, which can provide two different voltage levels for different DC loads.First, the mathematical model of PV cells is developed, and the characteristic curves are analyzed. Based on the analysis results, the control strategy of PV modules is studied. Then, the MPPT control is introduced, and the variable step incremental conductance method is chosen as the MPPT control in DC micro grid system, and the simulation results are verified.Second, the energy storage system is mainly discussed. The working principle, mathematical model and charging and discharging characteristics of lithium battery and super capacitor are analyzed; by analyzing the charging and discharging characteristics of each energy storage unit, combined with the control requirements of the DC micro grid system, the hybrid energy storage system is selected as energy storage unit in DC micro grid; the connection between the energy storage system and the DC bus is analyzed, and the different operation and control modes of the bidirectional DC/DC converters are discussed.Third, the structure, power flow and control strategy of DC micro grid with two DC buses are analyzed. The proposed DC micro grid consists of two different voltage levels of DC micro grids which are connected with a bidirectional DC/DC converter. The hybrid energy storage system with super capacitor and battery is used in the DC micro grids, and the coordination control strategy is designed to optimize the operation of the system and extend the service life of the battery. Based on the droop characteristics of bidirectional converter, the droop control of Buck/Boost converter is proposed to achieve a power balance in the whole system.Fourth, the DC micro grid with two DC buses experimental platform including the PV unit, HESS, DC loads and Buck/Boost bidirectional converter is constructed. And the proposed coordination control strategy is verified by experiments. Experimental results show that the power flow between the high and low voltage DC bus can be achieved through the control of the bidirectional converter, thus ensuring the stable operation of the system. Compared with single bus DC micro grid, the proposed two buses DC micro grid has more reliability and flexibility.