This thesis presents a control strategy for islanded (autonomous) operation of an electronically-coupled Distributed Generation (DG) unit and its local load. In a grid-connected mode of operation, the grid dominantly dictates frequency and voltage at the Point of Common Coupling (PCC) and the DG unit controls its exchanged real and reactive power components with the grid, using the conventional control strategy based on dq-current components. Subsequent to an islanding detection and confirmation, the dq-current controller is disabled and the proposed robust controller is activated. The proposed controller utilizes (i) an internal oscillator for frequency control and (ii) a feedback control system to regulate the island voltage. Despite load parameters uncertainties and unbalanced load conditions, the proposed controller guarantees robust stability and pre-specified performance criteria, e.g. fast transient response and zero steady-state error.

To provide smooth transition from a grid-connected mode to an autonomous (islanded) mode, a fast and accurate islanding detection method is also proposed and investigated in this thesis. The islanding detection method is based on injecting a negative-sequence current through the Voltage Sourced Converter (VSC) controller of a DG unit and detecting and quantifying the corresponding negative-sequence voltage at the PCC, as the islanding indicator, by means of a Unified Three-phase Signal Processor (UTSP). Performances of the islanding detection method and the proposed robust controller are demonstrated based on digital time-domain simulation studies in the PSCAD/EMTDC software, and validated based on a laboratory scale experimental setup.