UNWANTED ISLAND MAINTAINING IN LV GRIDS: ANALYSIS AND POSSIBLE SOLUTIONS. A STUDY BASED ON THE REAL-TIME DIGITAL SIMULATOR

Islanding refers to the condition in which one or m ore distributed generators continue to power a section of the network even though grid power from the electric u tility is no longer present. Unwanted islanding can be danger ous to utility workers, who may not realize that a circuit is still powered. Furthermore, because the present electrica l systems are not designed to work under these condit ion, the unwanted isle is uncontrolled with reference to protection system in case of fault, rated values of voltage and frequency. Finally, reclosing the circuit onto an active island may cause problems with the utility's equipm ent. For that reason, a smart grid must prevent islandin g or detect it and immediately clear the situation. In this work the unwanted island generation and maintaining are analyzed for a 4-wires LV grid c by a mix of generators and loads. Both single-phase and three-phase generators are considered, in order to have an unbalanced grid with realistic topology. Analysis i s performed using the Real-Time Digital Simulator (RT DS) installed in the Enel Test Center of Milan. The purpose of this work is to evaluate the real ri sk of islanding, analyzing different scenarios and taking into account transitory phenomena and capability limits of generators.