Research On The Control Strategies Of Microgrid With Photovoltaic And Energy-storage Systems

ABSTRACT:With the increasing of human demand for energy and the increasingly decline of the coal, oil, natural gas and so on, people have realized the importance of the development of renewable energy. The key technology of using of renewable energy is how to integrate renewable energy into the power grid, and the microgrid technology can just solve this difficult problem. Put the distributed power and load as a whole, forming a controllable small grid, which can work both in the island mode and in the grid mode, which is called microgrid. Microgrid includes DC microgrid, AC microgrid and AC/DC microgrid, and the AC microgrid is connected to large power grid using AC bus, which conforms to AC electricity custom. Moreover, the AC loads don’t need specialized inverter. Microgrid has two working states, namely:grid-connected and islanding. The smooth-switch between two working states that "plug and play" of microgrid is the key point of microgrid research.The research object of this thesis is the AC microgrid which is consists of photovoltaic panels, batteries and load. Combining the structure of the microgrid and its integrated control objectives, it designs the system control strategy under grid-connected state and islanding state. In the grid-connected state, it uses the large power grid to maintain voltage and frequency of microgrid. PVs work in the maximum power point tracking mode, and batteries work in the P/Q control mode. When the grid is abnormal, the system can switch to island mode automatically. In this thesis, by analyzing of power matching, excess and shortage of microgrid, the different control strategies of switching between grid-connected and islanding are proposed. In the islanding state, PVs can work in the maximum power point tracking mode or the constant power mode, batteries work in the V/f control mode. When the system received grid connecting signal and prepared to parallel in grid, it should start synchronous grid-connected controller to make voltage, phase and frequency of the system maintain synchronization with large power grid, then closing the synchronizing switch to finish grid connecting.The thesis used the simulation model and experimental platform of the AC microgrid with photovoltaic and storage system, through increasing or decreasing the load at AC bus terminals and changing the state of the synchronizing switch to detect the stability and reliability of the system. The simulation results show that this control strategy realizes the smooth transition between the two kinds of states, which meets the requirements of the system. An experiment platform based on LabVIEW PC interface, which is easy to operate the simulation platform, has been designed.