Effects on Short Circuit Level of PV Grid-Connected Systems under Unintentional Islanding

Most distribution networks are designed to operate in radial mode without having distributed generation (DG) penetration into the networks. The increasing DG penetration has resulted in bi-directional power flow that affects the operation of conventional feeder protection. This paper focuses on the fault contribution of a distributed generation, specifically PV grid-connected system in an event of unintentional islanding. An experimental setup is created using a 6 kW off-the-shelf grid-connected inverter to emulate the event of a fault occurred on a distribution feeder. A hardware setup is prepared according to the IEEE standard for realistic results. The experimental results have shown that, without violating the specified criteria in the standard, the grid-connected inverter takes times to detect an islanding condition and stop energizing. Based on the obtained results, a simple model is adopted to capture and explained the system behavior under unintentional islanding. While different models can adequately explain the system behaviors, they results in different short circuit levels when incorporated into a power flow study. This observation introduces the possibility of selecting and/or upgrading protective devices in a distribution system.