One of the less-discussed issues related to solar photovoltaics is called islanding. This occurs when a PV system tied to the larger grid is cut off from it while still producing electricity. Islanding can pose risk to emergency and repair crews when proper mechanisms are not in place to reduce such incidents. A recently released report “Suggested Guidelines for Assessment of DG Unintentional Islanding Risk” from Sandia National Laboratories discusses how utility engineers can reduce that risk without avoiding solar.
Such islanding can be engineered to allow a system to feed a local load even while not connected to the larger grid, but others are designed to turn off completely if they are islanded. Basically, there are a lot issues at play when designed a system for such incidents. The new guide introduced a method for utility distribution engineers to quickly assess whether additional study of unintentional islanding is needed, Sandia said.
“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 report Coauthor Abraham Ellis. “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.” By reducing such additional study cases, the guide could help utilities add more solar in at a faster pace, while providing additional safety.
“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.”
The report concluded that unintentional islanding can be ruled out in a number of situations. Those include situations where the aggregated nameplate AC rating of all distributed generation systems in a potential island is less than a fraction of the minimum real power load within the potential island, cases in which it is not possible to balance reactive power supply and demand within the potential island and cases in which direct transfer trip is used.
In some other cases, the report recommended that further study can be considered. Those cases include when the potential island contains large capacitors, and is tuned such that the power factor within a potential island is very close to 1.0; when a potential island has a large numbers of inverters or inverters from several different manufacturers; and when a potential island has both inverters and rotating generators.
Ellis recommended further study into how inverters can help with anti-islanding and to make sure they work as intended. “This is a largely unexplored area right now, but these questions need to be answered.” He also recommended investigating international safety standards.
“We need to anticipate how inverter standards are changing so we can formulate technical solutions and make recommendations to stakeholders,” Ellis said. As such an additional focus of Sandia’s future work will determine whether the U.S. needs to update anti-islanding testing procedures.