Protection requirements in distribution systems with high penetration of DG and possibility of intentional islanding

Distribution systems are generally radial in nature, characterized by a single source feeding a number of downstream networks whose protection scheme has been designed assuming in the system unidirectional power flows. Difficulties to strengthen the transmission network by means of new overhead transmission lines because of their great unpopularity ('not in my back-yard' attitude), are leading to satisfy the ever increasing of load demand by connecting DG to distribution feeders rather than expanding transmission facilities. The presence of DG with the injection of active and reactive power alters the power flow distribution and, in turn, affects the network voltage profile. The impact of DG operation on protection performance might be critical because after connecting distributed generation, part of the system may no longer be radial, thus coordination might not longer hold. Since feeders consist, in some cases, of long overhead lines (e.g. in rural areas), they are characterized by high outage rates. In recent years, the islanding operation of portions of a distribution grid has become an important topic, widely discussed in the academic and industrial communities. In the event of failure of the main power supply, the islanded operation may improve the quality and continuity of the power supply. This operation mode is encouraged by the small sized electrical power generation technologies and mostly in order to assure power supply in case of black-out. If the DGs have sufficient capacity to sustain the loads connected to that portion of distribution system with embedded generation, a temporary autonomous island my be created. One of the goals of forming a “microgrid” is to provide a back-up to loads. In order to apply this concept, a number of problems must be solved. First of all the protection system must provide protective devices capable to work correctly both in the grid connected mode and in islanded mode. In this paper we explore the possibility of applying new protection schemes by pointing out traditional and innovative techniques for maintaining coordination and by developing methods to assure protection for an islanded portion of the system supplied by DGs.