A control strategy of a distributed generation unit for seamless transfer between grid connected and islanded modes

In this paper, a control strategy for a distributed generation (DG) unit that provides a seamless transition between grid connected mode and islanded mode is presented. Instead of switching between current control and voltage control modes as commonly proposed in the literature, a voltage control mode is used in both grid connected and standalone modes. The output voltage and frequency are controlled to regulate the reactive and real power injected into the grid, respectively. In the islanded mode, the output voltage and frequency of the DG unit are regulated at their nominal values. Therefore, transition between operation modes is achieved by switching the voltage and frequency references of the same voltage controller. When the grid is interrupted, the DG unit supplies only the load automatically with no abrupt transients in the local load voltage and current, even before the islanding is detected. The principle and implementation of the control strategy to detect the grid conditions and islanding, and to switch seamlessly between the two operating modes are developed. The control strategy is validated through simulations using a detailed switching models in the PSCAD/EMTDC environment.