Islanding

Islanding is the condition in which a distributed generator (DG) continues to power a location even though electrical grid power is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, and it may prevent automatic re-connection of devices. Additionally, without strict frequency control the balance between load and generation in the islanded circuit is going to be violated, leading to abnormal frequencies and voltages. For those reasons, distributed generators must detect islanding and immediately disconnect from the circuit; this is referred to as anti-islanding.Distributed Resource Unit, IEEE Trans. on Power Electronics, VOL. 23, NO. 1, JANUARY 2008. Islanding is the condition in which a distributed generator (DG) continues to power a location even though electrical grid power is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, and it may prevent automatic re-connection of devices. Additionally, without strict frequency control the balance between load and generation in the islanded circuit is going to be violated, leading to abnormal frequencies and voltages. For those reasons, distributed generators must detect islanding and immediately disconnect from the circuit; this is referred to as anti-islanding. A common example of islanding is a distribution feeder that has solar panels attached to it. In the case of a power outage, the solar panels will continue to deliver power as long as irradiance is sufficient. In this case, the circuit detached by the outage becomes an 'island'. For this reason, solar inverters that are designed to supply power to the grid are generally required to have some sort of automatic anti-islanding circuitry. Some designs, commonly known as a microgrid, allow for intentional islanding. In case of an outage, a microgrid controller disconnects the local circuit from the grid on a dedicated switch and forces the distributed generator(s) to power the entire local load. In the context of nuclear power plants, islanding is an exceptional mode of operation of a nuclear reactor. In this mode, the power plant is disconnected from the grid and cooling systems (especially the pumps) are powered using only the power generated by the reactor itself. For some reactor types, islanding is part of the normal procedure when the power plant disconnects from the grid, in order to quickly recover electricity production. When islanding fails, emergency systems (such as diesel generators) take over. For instance, French nuclear power plants are conducting islanding tests every 4 years. The Chernobyl disaster is a failed islanding test. Electrical inverters are devices that convert direct current (DC) to alternating current (AC). Grid-interactive inverters have the additional requirement that they produce AC power that matches the existing power presented on the grid. In particular, a grid-interactive inverter must match the voltage, frequency and phase of the power line it connects to. There are numerous technical requirements to the accuracy of this tracking.