Islanding

Islanding refers to the condition of a distributed generation (DG) generator continuing to power a location even though power from the electric utility is no longer present. Consider for example a building that has a solar panels that feed power back to the electrical grid; in case of a power blackout, if the solar panels continue to power the building, the building becomes an "island" with power surrounded by a "sea" of unpowered buildings.

Islanding can be dangerous to utility workers, who may not realize that the building is still powered even though there's no power from the grid. For that reason, distributed generators must detect islanding and immediately stop producing power.

In intentional islanding, the customer disconnects the building from the grid, and forces the distributed generator to power the building.

Technical discussion

Islanding of inverter-connected PV-generator systems means any situation where thesource of power from the network operator’s distribution system is disconnected from thenetwork section in which the generator is connected, and one or more inverters maintain asupply to that section of the distribution system or consumer’s installation. The situation may cause an electrical shock hazard to service personnel operating on the islanding network section while it has been supposedly shut down through separating it from the main power station.

Detection methods

Detecting the absence of power from the grid is complicated by two items:
* The distributed generator itself is a source of power whose voltage is by definition identical to the voltage from the grid, so it is hard to distinguish the two
* A nearby motor may continue to spin and act as a generator, creating a frequency similar to the original line frequency (50 or 60 Hz). That may be also true if the load in the building forms a resonant circuit at the line frequency. Islanding may be detected passively, actively or by utility notification:
* Passive detection is done by detecting that the line voltage and frequency are no longer within certain limits.
** One principle is that, once the grid is no longer establishing the line voltage, that voltage will change (it will usually drop, but not necessarily).
** The other principle is that, even if a motor is creating a line frequency, it will slow down, and therefore do so at lower frequency than the standard line frequency.
* Active detection is done by purposely adding a disturbance to the line, and seeing its effect
** The principle is that the grid offers essentially a zero ohm impedance. In the absence of the grid, the impedance that is seen is the load in the building, which is substantially higher.
* For utility notification, the utility knows when it removes power to the building, and tells the inverter to stop generating power

Criticism

There is discussion in the field about whether islanding is indeed a significant hazard. A study in the Netherlands has suggested that waiting for 60 seconds after disconnecting a network from the main power station should be sufficient time so that statistical power and load fluctutations in the islanding network section will cause shutdown functionality of the PV generators to trip and shut down the network section, thus ending the islanding condition. [http://www.iea-pvps.org/products/download/rep5_07.pdf]

References

* [http://www.sandia.gov/ Sandia National Laboratories] , [http://www.osti.gov/bridge/servlets/purl/806700-Tz59N8/native/806700.PDF Evaluation of Islanding Detection Methods for Utility-Interactive Inverters in Photovoltaic Systems]
* [http://hybrids-plus.com/ Hybrids Plus] [http://hybrids-plus.com/pmwiki/index.php?n=Ext.IEEE1547 IEEE 1547] , IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, in plain English
* UL 1741: Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources