Implementations of seamless transfer and active islanding detections for microgrid applications

This paper describes experimental implementations of seamless transfer between grid-connected mode and islanded mode of a three-phase utility-interactive voltage source converter (VSC) for microgrid applications. First, a smooth moving from grid-connected mode to islanded mode is accomplished by the islanding detection and the smooth transition. The islanding detection is achieved by the negative-sequence current injection method and quantitative analysis confirms the feasibility of this method. In experimental verifications, this unbalanced current injection is implemented by the decoupled double synchronous reference frame (DDSRF) current controller. In the second phase, a phase compensation scheme of the VSC is studied for grid re-synchronization of the isolated VSC. The operating frequency of the VSC is adjusted according to the angle difference between VSC and utility grid. PSCAD/EMTDC simulator is used to verify theoretical results of both phases. A 2kVA, 220 VAC, 60 Hz laboratory-scaled VSC is also constructed for validating the proposed control scheme. Experimental results agree well with simulation waveforms.