Embedded or distributed generation schemes, where a local generator is connected directly to the distribution network, are becoming widespread. The operators of these schemes face an important challenge: how to ensure that the system behaves safely and predictably if the local generator becomes isolated from the network – a condition known as islanding.
There are numerous potential hazards associated with islanding, not the least of which is that engineers working to restore the network connection may not realise the system is still powered. Another hazard is that the generator may continue to supply local loads but, without support from the network, this may result in it being heavily overloaded.
The solution usually adopted to address these hazards is to immediately shut down the islanded generator, but this can only be done if a fast and dependable way of detecting islanding is available. Many approaches are possible, but one that is widely used is rate of change of frequency (ROCOF) protection, which has established a reputation for responding faster and more reliably than alternative protection techniques.