Risk of unintentional islanding in LV distribution networks with inverter-based DGs

The number of small power Distributed Generation (DG) units connected to the distribution networks has increased particularly for photovoltaic (PV) applications. According to recently issued Italian standards, each DG unit is allowed to participate in the active and reactive power regulation, using specifically defined P-f and Q-V droop characteristics. Furthermore, these standards have introduced wider frequency and voltage thresholds allowed during normal operation of DG units. Therefore the risk of unintentional islanding operations is expected to increase, due to the possibility of the DG units to supply the loads with a wider voltage and frequency range without the protections interventions. In this paper, the impact of the loads´ voltage and frequency dependence on the isolated grid´s frequency and voltage deviation, during unintentional islanding operations, has been considered. The system has been modeled and analyzed through detailed dynamics simulations. The results show that, using a classical grid-connected inverter control system, the loads´ voltage and frequency dependence may play a crucial role in determining the unintentional islanding operation and possible failure of the protections intervention.