Research On Coordinated Control Strategy Of DC Microgrid

As a new type of power system, DC microgrid can utilize the DC output distributed generation effectively and provide reliable power supply for equipments tend to use DC current, which matches the development needs of clean energy and smart grid in the future, thus, it has been widely concerned and researched. Under the backgroud, this thesis makes a deep research of operation control for DC microgrid system. In order to optimize solar energy utilization and realize system stability,coordinated control strategy for DC microgrid is studied with emphasis.Firstly, the overall structure of DC microgrid and system components are introduced. Based on the analysis which is focused on the engineering mathematics model of PV cell, the output characteristics of PV cell is studied by simulation results.The whole simulation model for the system is constructed in Matlab/Simulink, which provides a verification platform for the research of operation coordinated control of DC microgrid.Secondly, the structure of interface control circuit and operation control modes of photovoltaic system(PVS), energy storage system(ESS) and grid connected system(GCS) are deeply analyzed in this thesis. Hence, control modes of each unit system are studied with emphasis on the control principle, how to realize, where to apply and its control action. The simulation results show that each unit system control strategy is feasible and effective when it is used in overall operation coordinated control of the system.Then, multi-mode operation coordinated control strategy of DC microgrid system is deeply studied. According to the different DC bus voltage controller, 9operation modes are designed. For each mode, the overall operation coordinated control strategy which can coordinate the operation control of each unit system is proposed after detailed analyzing the operating state of system, which is based on the comprehensive consideration of the state of power grid, the energy relationship of system, the operating limits of battery energy storage and grid connected converter.The simulation results verify that the control scheme can maintain both the stability of DC bus voltage and the balance of system energy, thus to realize safe and reliable operation of DC microgrid system.Finally, this thesis makes a deep study of LVRT coordinated control scheme for DC microgrid. The overall control method combined the coordinated control for photovoltaic/battery/loads(PBL) with the coordinated control for active and reactive power is put forward, which is based on the energy situation under different operating conditions and the LVRT control requirements. During low voltage of the system, thecoordinated control for PBL can maintain the DC-link voltage constant, balance the system power and optimal utilization of solar energy. Meanwhile, according to the grid voltage change level, the coordinated control for active and reactive power can provide reactive power to support grid-side voltage recovery and avid overcurrent of grid-side converter output. The effectiveness of proposed control scheme is verified by the simulation results.