Seamless Mode Transition of a VSI in a 3-Phase Standalone Microgrid

During transition of an incoming Voltage Source Inverter(VSI) from current to voltage-controlled mode in a standalone micro-grid, any instantaneous mismatch between these mode reference voltages causes a significant current transient which could lead to cascaded tripping of the operating units. Reported mode transition strategies either focus on grid-interactive systems or do not provide any insight about appropriate handling of the mode reference voltages during this transition. This paper proposes a compensation technique to the input voltage reference that includes synthesizing a current reference for equal power sharing, a synchronization loop and a proportional controller for equalization of the mode reference voltages. Mathematical analysis is presented in this paper to show that the aggregate of proposed compensations can eliminate any instantaneous mismatch between the mode reference voltages, thus allowing for zero transient mode transition. The proposed method is validated by experimental results obtained from a 22.5 kVA, 3-phase standalone micro-grid, comprising 3 VSI-s of identical rating connected in parallel.