Supplementary mechanisms for smooth transition between control modes in a microgrid

Performance requirements and/or operational integrity of a microgrid subject to major changes, e.g., transition from a grid-connected to an islanded condition, transfer switch operation within the microgrid, or control communication failure, can necessitate the controllers (or a subset of them) to change their modes of operation, e.g., P/Q mode to V/f mode. Transfer in the control mode can further exacerbate the transients due to the original disturbance. This paper proposes mechanisms for smooth transition between control modes. Emphasis of the reported work is on control mode change subsequent to (1) grid-connected to islanded condition and (2) control communication failure. The proposed mechanisms are based on (1) an observer (or state estimator) and (2) an auxiliary tracking controller. The effectiveness of the proposed mechanisms are investigated based on digital time-domain studies in the PSCAD/EMTDC environment and verified in an RTDS-based hardware-in-the-loop platform.