Optimization Control Strategy for Islanded Parallel Virtual Synchronous Generators

Virtual Synchronous Generators (VSGs) can reduce the frequency and power oscillation in the grid. For parallel multi-VSGs in island microgrid, the differences of equivalent output impedance and line impedance affect the orderly sharing of reactive power and restraining of circulating current greatly. The power sharing and circulating current characteristics of VSGs are analyzed in this paper. To solve this problem, an optimization control method for parallel VSGs is proposed, which included the inner voltage and current loop and the outer reactive power loop. In the inner voltage and current loop, a virtual complex impedance including a resistive component and an inductive component is introduced. It reduces the coupling between active power and reactive power, and it reduces the disorderly distribution of power due to impedance differences. In the reactive power loop, the output voltage feedback and integrator links are adopted, which reduce the deviation of output voltage and restrain circulating current. The effects on the equivalent output impedance in the different control parameters and virtual complex impedance are analyzed, the effects on system stability in the different resistive components of virtual complex impedance are analyzed, and the proper parameters are selected. Simulation and experimental results show the correctness and validity of the proposed control method.