Research Of Control Strategies For Microgrid Inverter Based On Virtral Synchronous Generator

Distributed generation (DG) has been a hot research topic in the world in recent years, which can improve the security and reliability of power system. By mimicing the characteristics of synchronous generator, the micro-grid inverter based on the virtual synchronous generators (VSG) can connect to the grid. Compared with the traditional control methods, VSG can improve the compatibility and stability between the grid and DG, and can adapt to a variety of operating system modes. In this paper, the inverter of 3-phase and 4-leg topology was researched combined a researching topic. The main contents of this paper are as followed:Mathematical modeling of the VSG. Analyzing the mathematical models of synchronous motors and the three-phase four-leg inverter, the mathematical modeling of the VSG was got by the use of the equations of motion of the rotor synchronous, motor flux equation, and electromagnetic transient relationships. The active-frequency controller and the reactive-voltage controller were designed and optimized, which could operate the three-phase four-leg inverter to mimic a synchronous generator.The synchronization control method based on virtual synchronous generator was researched. Two different ways of synchronization and grid-connected solutions (the difference was whether to add a phase locked loop) were designed from the principle of grid connected control of synchronous generator, the advantages and disadvantages of which was analysised. Using the way without lock Grid-phase loop control can make the VSG connect to the grid smoothly.The behaviors of the VSG under different operation modes of the microgrid were analysised. The characteristics of the power output under grid-connected operation and island operation were researched. In order to achieve optimal control in different operating modes, different control strategies were designed.The control loop of bottom was designd. To improve the quality of voltage, the bottom control loop of voltage and current was designed based on the quasi proportional resonant controller. The three-dimensional space vector modulation algorithm producing driving waveform for four brige arms was researched and achieved by simulation and experiment.The experimental platform was set up. In order to verify the correctness of the control scheme, the simulation platform of the VSG in islanded, grid-connected, and synchronization operation system were set up in Matlab/Simulink. At the same time, a experiment platform of 10kVA virtual synchronous generator was built in the lab.The simulation and experimental results show that the design in this paper can make the VSG connect to the grid smoothly and operate steadily under different operating modes. The output of the voltage and current harmonic content were very small. The system can reflect the characteristics of synchronous generator with variable frequency and variable voltage and electromagnetic transient process.