When planning to interconnect solar photovoltaics (PV) or other distributed generation (DG) to the power grid, utility system planners must consider the risk of unintentional islanding. Islanding occurs when the DG continues to energize a load well after it is disconnected from the utility source. While islanding can be engineered intentionally for reliability and power-quality purposes, islanding that is established accidentally can adversely affect personnel safety and equipment integrity.
But how can we know when the risk of unintentional islanding is negligible — and thereby avoid adding unnecessary study time and cost to getting solar on the grid?
The Sandia National Laboratories report Suggested Guidelines for Assessment of DG Unintentional Islanding Risk lays out a procedure for utility distribution engineers to quickly assess whether additional study of unintentional islanding — which is often done during detailed interconnection studies — is necessary.
“Utilities want to keep their systems secure, so they tend to err on the side of caution — which can lead to conservative and expensive mitigation measures,” said Sandia’s Abraham Ellis, coauthor of the report. “With this report, we aim to reduce the number of cases of unnecessary application of additional protection to mitigate islanding risk, while giving utilities a basis for requesting additional study in cases when it is warranted.”
Field experience with high penetration DG is relatively limited, so distribution engineers are often uncertain as to when supplemental protective measures (beyond those already implemented in the inverter controls) are needed to protect against unintentional islanding. These can include installing costly communications-based protection systems.
The procedure proposed by Ellis and coauthor Michael Ropp identifies cases in which the risk of unintentional islanding is negligible, avoiding the need to take measures that would make PV more expensive than necessary.
“There is a need to ensure that unintentional islanding does not occur with the addition of new DG, but in the great majority of cases, the risk can be ruled out upon inspection,” Ellis said. “The guidelines in this report can help make a technical determination quickly and inexpensively.”
As research efforts supported by the U.S. Department of Energy’s (DOE) SunShot Initiative drive new developments in inverter technology, the field is also anticipating how advanced PV inverter capabilities will affect the risk of unintentional islanding.
“We need to understand this in detail so the industry can be reassured that anti-islanding measures programmed in the inverters will still work as intended, even though inverters may also be expected to tolerate disturbances and support voltage,” Ellis said. “This is a largely unexplored area right now, but these questions need to be answered.”
An additional focus of future work will be to determine if there is a need to update anti-islanding testing procedures in the United States.
“In light of emerging grid support functionality, we need to understand how other countries perform certification testing, identify best practices, and make recommendations for the United States,” Ellis said. “We need to anticipate how inverter standards are changing so we can formulate technical solutions and make recommendations to stakeholders.”