A power island, often simply called an island, is an energized section of circuits separate from the larger system. For an island to remain energized, local load must be supplied by matching output of local generation. An example of islanding would be a remote area that loses connection to the greater grid due to a downed transmission line but continues to operate using local generation such as backup generators.
In most cases, islanding is not routinely practiced by utilities in developed countries except in temporary situations after a widespread outage. However, islanding can happen accidently when a circuit experiences an outage and a customer running a backup generator has not installed proper interconnection equipment to isolate their internal wiring from the distribution grid. Crews working to restore power can be put in danger if islands of energized lines exist where the crews expect to find de-energized circuits.
To provide acceptable voltage, frequency, and power quality, islanded circuits must be managed locally. Failure to stay within acceptable tolerances can result in damage to customer equipment.
An example of acceptable islanding is a microgrid. A microgrid is a subset of a larger grid that is designed to be operated either interconnected to the grid or islanded from the grid. When islanded, the microgrid uses local supply resources and a master microgrid controller to ensure acceptable power quality.