The Control Strategy Of Power Conversion Device In DC Micro Grid

The demand of energy is increasing with the development of social economy,and against the backdrop of this,the distributed generation technology is developing rapidly and therefore plenty of distributed generations are put into the micro grid.In the meantime,DC micro grid emerges in the wake of the developing trend of direct current supply in urban areas.Compared with the AC micro-grid,the DC micro grid has the merits of low energy consumption,no consideration of frequency and phase,and simple wiring.However,the application of DC micro-grid is still in demonstration stage,and in order to improve its reliability of the power supply,it is necessary to study extensively the coordination control of DC micro-grid.Many aspects of DC micro-grid are introduced in this thesis,including its related concepts,merits compared with traditional DC micro-grid and research status at home and abroad.Additionally,the network form and operation mode of DC micro grid are discussed on the basis of studying three kinds of topology structure of DC micro-grid.The research objectives of the thesis are divided into three levels based on the above analysis.Firstly,the modeling and control methods of each unit in DC micro-grid are studied.Distributed generation units,such as photovoltaic and wind power,work in the most powerful or constant pressure modes.The energy storage unit can charge and discharge according to the operation condition of the micro-grid.The parallel converter can guarantee the energy exchange between the micro-grid and the big power grid.The operation of load converter is controlled by means of down-power operation instead of removal in order to ensure the reliability of DC micro-grid.The research in this level involves approximately the control methods of all current transformation devices in the DC micro-grid,which lays a theoretical foundation for the operation control of DC micro-grid.Secondly,the problem of voltage mismatch often happens when the transformers of each unit are connected to the DC bus,and therefore Dc transformer is served as the transfer station in this thesis when each unit connecting to the DC bus.As a key component in DC micro-grid,DC transformer has its own problems in the application.In this paper,the more efficient control strategy is proposed respectively to solve the problems of excessive current and low commutation efficiency in the starting and changing process of the DC transformer.Finally,further research is conducted on the coordinated control strategy for the whole operation of DC micro grid based on the above study of the control method of power converter device in order to improve the reliability of DC micro-grid and ensure the stable and efficient operation.The operation status of DC micro grid in isolated island mode and grid mode is analyzed respectively,and a unified coordinated control strategy for the isolated island and grid mode is proposed.With the strategy,the DC micro grid can switch quickly in different modes and automatically skip the fault state,which improves the reliability of DC micro grid.The simulation model of each unit is established in Matlab/Simulink for the control strategy of the power conversion device studied in this thesis.Firstly,the soft start control strategy and the reversing control strategy of DC transformer are verified by simulation and then the operation mode of DC micro-grid is simulated and analyzed based on the model of each unit.The above simulation results show that the proposed control strategy can effectively enhance the efficiency of the DC transformer,and ensure the efficient operation and reliability of power supply of DC micro grid.