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Implementation of Multiple-Slack-Terminal DC Microgrids for Smooth Transitions Between Grid-Tied and Islanded States

Bidirectional interlinking converter (BIC) is normally configured as the slack terminal to regulate system bus voltage for dc microgrid operation in grid-tied state. In case of utility grid fault, BIC is disabled and the system changes to operate in islanded state. Localized battery energy storage (BES) voltage regulation mode (VRM) operation is activated to maintain system power balance. Operating mode changes of BES and BIC during islanding and reconnection might result in system voltage fluctuations and undesirable inrush current. Detailed transition procedure, which is crucial, has seldom been discussed in literature. In this paper, multiple-slack-terminal dc microgrid, in which both BIC and BES operate in VRM with droop control, is implemented to ensure smooth transitions between grid-tied and islanded states. BES operating mode change is eliminated, and thus, system bus voltage is actively regulated throughout the transitions. Contactor is installed at BIC dc side to decouple BIC operating mode change and grid connection. A counter is used to ensure bus voltages at both sides of contactor have been matched stably to minimize inrush current. To enhance system control accuracy, multilevel energy management system is carried out. A lab-scale dc microgrid has been developed for experimental verifications.