Research On Control Strategy And Stability Of Three-phase AC/DC Converter In Hybrid Microgrid

With the development of economy, fuel and environment crisis become increasingly outstanding. More and more attention has been paid for renewable energy which could be the substitute of fossil energy. Microgrid is a kind of efficient organization forms because of its ability to integrate distributed generation, loads and energy storage. It could operate stable when connected with utility grid. So the study of microgrid has very important academic value and realistic meaning.This paper designs the controller of the three-phase AC/DC converter which connects the AC subgrid and DC subgrid in hybrid microgrid. Different control strategies is used for the converter in grid-connected mode and isolated mode of microgrid. One of the prime reasons which will cause the converter to lose stability is analyzed. Finally, the analysis is verified by MATLAB and dSPACE. The details are as follows:(1) The configuration of laboratory-scale microgrid is presented. The hierarchical control is used for the converters in the microgrid. It contains system level and local level. According to the operational requirements of the microgrid, the local level control strategy is designe firstly. The mathematical model of photovoltaic generation and lithium batteries are built. The control circuits of DC/DC boost converter and bidirectional DC/DC converter are designed. In grid-connected mode and isolated mode of microgrid, the mathematieal models of three-phase AC/DC converter in ABC stationary coordinate and dq rotation coordinate are built and the transfer function of the converter is obtained. Combining feedforward decoupling control method, the closed loop control technology of voltage and current is adopted and the direct current control with a better dynamic performance is used. Then the method of calculating the PI regulator parameters is given.(2) The coordination control strategy at system level is used for the isolated mode of microgrid. It could guarantee the power balance through controlling the output power of DG and load shedding according to the conditions of DG、 battery and loads. The model of hybrid microgrid is built in Matlab/Simulink and the results of different cases in grid-connected mode and isolated mode are analyzed. The results show that, when the hybrid grid operates under varying generation and load conditions, The stable operation of the converters in the hybrid microgrid can be maintained by the control strategy, and the three-phase AC/DC converter converter could switch smoothly between rectifier mode and inverter mode.(3) Experimental platform of three-phase AC/DC converter based on dSPACE is built, The signal acquisition circuit, driving circuit and software protection modle are designed. Real-time control is realized in the Control Desk. The experimental results show that, DC voltage can be maintained stable when converter operates in grid-connected mode, AC voltage can be guaranteed when converter operates in isolated mode, and bidirectional power flow smoothly is achieved.(4) An instability phenomenon of the three-phase AC/DC converter in hybrid microgrid is reported. The consequence of the phenomenon is described and the physical origin is identified. Studies show that AC equivalent series resistance is the prime reason of the phenomenon. The boundary of the resistance is given through the DC working point of the circuit. Finally, the feasibility and validity of the analysis are verified through simulink and experiment.