This chapter presents a method for operating an islanded microgrid at a constant frequency. The proposed method uses de-coupled PQ control plus real power reference generation based on voltage variation to control the grid-forming generator and grid-supporting generators. Its effectiveness has been validated by a three-phase microgrid system where there is one grid-forming generator, one grid-supporting, and one grid-feeding generator. The grid-forming generator produces its own voltage reference with a constant frequency of 50 Hz, while the grid-supporting and grid-feeding generators take the voltage as a reference at their respective coupling point with the microgrid. It is found that the grid-forming and grid-supporting generators work collaboratively to keep voltages at each bus around the rated value. For a practical microgrid, it is necessary to determine the location and sizing of each grid-supporting generator in order to keep the voltage profile within specification under all operating conditions. To achieve these two purposes and also to reduce the computational demand of modeling and to shorten simulation time, a single-phase equivalent microgrid has been adopted in this research. Such approach is useful for the design of a practical microgrid.
Part of the book: Development and Integration of Microgrids