Bumpless transfer for distributed generation units with mode-dependent controllers

Distributed generation units (DGUs) in grid-connected microgrids are typically controlled as current sources. Nevertheless, when the microgrids are isolated, some DGUs must switch their control law to operate as voltage sources. Switching between controllers implies unwanted transients at the DGUs output power due to mismatches between controller's outputs, challenging both voltage and frequency transients and potentially leading to instability. This work proposes a bumpless-transfer scheme based on sliding-mode reference conditioning for the DGUs to achieve a smooth transition between grid-connected and island modes in electrical microgrids. By guaranteeing the establishment of a sliding regime, the suggested bumpless algorithm reduces to zero the mismatch between the DGU controllers' outputs when the switch is performed. In this manner, jumps in the control action and the associated unwanted transients are avoided regardless of when the control law switches. Results show good and robust performance in the face of both islanding and reconnection processes.