A Control Strategy of Microgrid-Connected System Based on VSG

In a microgrid-connected system, under the condition of the unbalance of the grid, the output power of the virtual synchronous generator (VSG) will oscillate at twice the grid frequency, and the three-phase output currents will produce the unbalanced fluctuations, which will seriously affect the stable operation and safe of the power system. In this paper, a control strategy of a microgrid-connected system based on VSG is proposed. Firstly, the control command expressions under different control targets are obtained by analyzing the operation of VSG under the unbalanced grid voltages. And then, an improved VSG control structure is designed based on the traditional VSG control structure. Finally, the negative sequence current obtained by the control structure is added to the positive sequence current obtained by the traditional VSG control system to gain the SVPWM (space vector pulse width modulation) control signals, thus implementing the control of the microgrid-connected system. The results obtained show that the strategy can effectively improve the output power quality of the system under the condition of unbalanced grid voltage, and does not depend on the type and cause of the unbalance of the grid voltages, and the voltage source characteristics of VSG are not changed. Furthermore, the control system has fast response speed and both the structure and the instruction current calculation method are simple and efficient.