Operation of microgrid at constant frequency with a standby backup grid-forming generator

Grid-forming generator is of paramount importance in the operation of an islanded microgrid. A fault in it without proper remedy could lead to collapse of the whole microgrid system. In this paper, besides a main grid-forming generator, a standby backup grid-forming generator is adopted in the system to prevent this from happening. Under no fault condition, the main grid-forming generator takes de-coupled PQ control by outputting only real power demanded by the system and transferring system-demanded reactive power to and from its accompanying reactive power compensator while the standby grid-forming generator works as a grid-supporting generator and outputs a small amount of real power and nearly zero reactive power. Two cases have been studied. In the first case the main and standby backup grid-forming generators work side by side with hardwired communication link while in the second case, two are remotely located and has no communication link. When a fault occurs to the main grid-forming generator and forces its disconnection from the grid, the backup grid-forming generator takes over the grid-forming role and the whole system can continue to operate smoothly. Comparatively transitions of control overtake in the first case is almost seamless while in the second case there is a detection time delay which could be shortened by combining voltage variation detection with frequency drift detection. Compared with conventional frequency droop control, the system operates at constant frequency to reduce operation complexity.