Design of a Generalized Control Algorithm for Parallel Inverters for Smooth Microgrid Transition Operation

A modified hierarchical control structure for distributed generation (DG) in microgrid is applied in accordance to the recommendations by the IEEE Std. 1676. In the inverter control layer, a generalized control algorithm is developed to enable the DGs to operate in all three modes (i.e., grid-forming, grid-feeding, and grid-supporting modes) with a single control structure, facilitating the seamless transition between the operating modes. This feature is obtained by designing the multiloop controller in the inverter control layer using inverse plant modeling techniques so that the dynamics of the inverter and the $LC$ filter are fully compensated, virtually transforming the controlled DG in the unity gain from the application layer perspective. Thus, effects of disturbances associated with the mode transitions are thus fully eliminated. The effectiveness of the proposed control algorithm is validated by numerical simulations and hardware-in-the-loop experiments. The results show that the DGs can fulfill the tasks defined in the system control layer with fast dynamics, desired accuracy, and good transient behavior.