Islanding

Islanding is when a generator such as PV keeps powering load through utility lines after it has been cut off from the utility source. An island that was planned ahead to keep service running is an intentional island. One that was not planned is an unintentional island, and these are normally prohibited because they can damage equipment, lower power quality and endanger utility workers. A sustained unintentional island can form only if PV output and local load match closely in both real and reactive power when the upstream switch opens. If they do not match, voltage or frequency quickly drifts out of range and the inverter can detect the loss of mains. Detection methods can be passive, active (for example impedance detection or positive frequency feedback), or communications-based, such as direct transfer trip. Most commercial inverters combine impedance detection with positive feedback. Sandia suggests a sustained island is not physically possible when the total PV nameplate rating in the potential island is below 2/3 of the minimum daytime feeder load. Ride-through functions such as low-voltage ride-through make islands more likely to form, because low voltage can no longer be used to detect an island. Inverter islanding detection is designed to find balanced islands, not faults, so it should not be relied on as fault protection.

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• sentence 2: support check: partial - u32 supports the preplanned versus unintentional distinction, the typical prohibition, and the equipment and worker hazards, and u19 supports the power-quality concern, but no passage says the prohibition is because of power quality.
• sentence 5: support check: partial - u15 gives the 2/3 fraction of minimum daylight feeder load for DG/PV nameplate, but it does not attribute it to Sandia, and it is described as a conservative statement. -->