What is Islanding?
Islanding is where a solar or battery inverter continues to supply an installation even though power from the electric utility is no longer present.
Why do we need Anti-Islanding Protection?
Islanding can be dangerous to utility workers, who may not realize that a circuit is energised, and it may prevent automatic re-connection of devices. For that reason, inverters must detect islanding and immediately stop producing power; this is referred to as anti-islanding protection.
The most common example of islanding is a grid supply that has solar panels attached to it. In the case of a blackout, the solar panels will continue to deliver power as long as the sun is shining. In this case, the supply line is an “island” with power, surrounded by a “sea”, or void of unpowered lines. 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 protection in them.
In intentional islanding, the installation disconnects from the grid, and forces the inverter to power the local circuit. This is often used as a backup system.
How to detect Islanding
Grid-interactive inverters are required to produce AC power that matches the existing power presented on the grid. In particular, the voltage, frequency and synchronise phases of the feed it connects to.
An installation with an array of solar panels and inverters attached convert the varying DC current provided by the panels into AC power that matches the grid supply. If the grid is disconnected, the voltage on the grid line would be expected to drop to zero, a clear indication of a service interruption. However, when the house’s load matches the output of the panels at the instant of the grid interruption, the panels can continue supplying power, which is used up by the house’s load. In this case there is no obvious indication of an interruption.
Even when usage and production are matched, the failure of the grid will result in additional transient signals, there will almost always be a short decrease in line voltage, which will indicate a potential fault. Such events can also be caused by normal events, like the DOL (Direct On-Line) starting of a large electric motor.
Ways to detect islanding without a large number of false trips is the subject of considerable research. Each way has some threshold that needs to be met before a state is considered to be a signal of grid interruption, which leads to a protection blind spot in the range of conditions where a real grid failure will be filtered out. There is a large number of ways of detecting islanding – for more info on the subject, take a look at our article on Islanding Detection Methods here,