Islanding is the intentional or unintentional division of an synchronous interconnected power grid into individual disconnected regions with their own power generation and distinct frequency. Intentional islanding is often performed as a defence in depth to mitigate a cascading blackout. If one island collapses, it will not take neighboring islands with it. For example, nuclear power plants have safety-critical cooling systems that are typically powered from the general grid. The coolant loops typically lie on a separate circuit that can also operate off reactor power or emergency diesel generators if the grid collapses. Grid designs that lend themselves to islanding near the customer level are commonly referred to as microgrids. In a power outage, the microgrid controller disconnects the local circuit from the grid on a dedicated switch and forces any online distributed generators to power the local load. Unintentional islanding is a dangerous condition that may induce severe stress on the generator, as the generator must match any changes in electrical load alone. If not properly communicated to power line workers, an unintentional island can also present a risk of electrical shock. Unlike unpowered wires, islands require special techniques to reconnect to the larger grid, because the alternating current they carry is not in phase. For these reasons, solar inverters that are designed to supply power to the grid are generally required to have some sort of automatic anti-islanding circuitry, which shorts out the panels rather than continuing to power the unintentional island. Methods that detect islands without a large number of false positives constitute the subject of considerable research. Each method has some threshold that needs to be crossed before a condition is considered to be a signal of grid interruption, which leads to a "non-detection zone" (NDZ), the range of conditions where a real grid failure will be filtered out. For this reason, before field deployment, grid-interactive inverters are typically tested by reproducing at their output terminals specific grid conditions and evaluating the effectiveness of the anti-islanding methods in detecting island conditions.
Intentional islanding Intentional islanding divides an electrical network into fragments with adequate power generation in each fragment to supply that fragment's loads. In practice, balancing generation and load in each fragment is difficult, and often the formation of islands requires temporarily shedding load. Synchronous generators may not deliver sufficient reactive power to prevent severe transients during fault-induced island formation, and any inverters must switch from constant-current to constant-voltage control. Intentional islanding can be used after a blackout and during the black start process to restore power to isolated parts of the grid. Assuming P≠NP, no good cut set criterion exists to implement islanding. Polynomial-time approximations exist, but finding the exactly optimal divisions can be computationally infeasible. However, islanding localizes any failures to the containing island, preventing failures from spreading. In general, blackout statistics follow a power law, such that fragmenting a network increases the probability of blackouts, but reduces the total amount of unsatisfied electricity demand. Islanding reduces the economic efficiency of the wholesale power market, and is typically a last resort applied when the grid is known to be unstable but has not yet collapsed. In particular, islanding improves resilience to threats with known time but not location, such as terrorist attacks, military strikes on electrical infrastructure, or extreme weather events.
Home islanding Following the 2019 California power shutoffs, there was a rise in interest in the possibility of operating a house's electrical grid as an island. While typical distributed generation systems are too small to power all appliances in a home simultaneously, it is possible for them to manage critical household power needs through traditional load-frequency control. Modules installed in series between the generator and large loads, like air conditioners and electric ovens, measure the island power frequency and perform automatic load shedding as the inverter nears overload.
Detection methods Automatically detecting an island is the subject of considerable research. These can be performed passively, looking for transient events on the grid; or actively, by creating small instances of those transient events that will be negligible on a large grid but detectable on a small one. Active methods may be performed by local generators or "upstream" at the utility level. Many passive methods rely on the inherent stress of operating an island. Each device in the island comprises a much larger proportion of the total load, such that the voltage and frequency changes as devices are added or removed are likely to be much larger than in normal grid conditions. However, the difference is not so large as to prevent identification errors, and voltage and frequency shifts are generally used along with other signals. The active analogue of voltage and frequency shift detection attempts to measure the overall impedance fed by the inverter. When the circuit is grid-connected, there is almost no voltage response to slight variations in inverter current; but an island will observe a change in voltage. In principle, this technique has a vanishingly small NDZ, but in practice the grid is not always an infinitely-stiff voltage source, especially if multiple inverters attempt to measure impedance simultaneously. Unlike the shifts, a random circuit is highly unlikely to have a characteristic frequency matching standard grid power. However, many devices, like televisions, deliberately synchronize to the grid frequency. Motors, in particular, may be able to stabilize circuit frequency close to the grid standard as they "wind down". At the utility level, protective relays designed to isolate a portion of the grid can also switch in high impedance components, such that an islanded distributed generator will necessarily overload and shut down. This practice, however, relies on the expensive widespread provision of high-impedance devices. Alternatively, anti-islanding circuitry can rely on out-of-band signals. For example, utilities can send a shut-down signal through power line carrier communications or a telephony hookup.
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