Suggested Guidelines for Assessment of DG Unintentional Islanding Risk

As increasing numbers of Distributed Generation (DG) systems are connected to utility systems, distribution engineers are becoming increasingly interested in evaluating the risk of unintentional islanding. Utilities desire to keep their systems secure, while not imposing unreasonable burdens on customers wishing to connect DG. However, utility experience with these systems is still relatively sparse, so distribution engineers often are uncertain as to when additional protective measures, such as direct transfer trip, are needed to avoid unintentional islanding. Utilities tend to err on the side of caution, which in some cases may lead to the unnecessary requirement of additional protection. The purpose of this document is to provide distribution engineers with guidance on when additional measures or a more in-depth evaluation may be prudent. The guide also describes situations in which utilities may be able to ascertain that the risk of an unintentional island is extremely low and no additional mitigation or study are needed. The goal is to reduce the unnecessary application of additional protection for DG interconnection. While the content applies to any DG, this document has a focus on photovoltaic (PV) installations.