Get your ROCOF right!

  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.
ROCOF protection relies on the fact that once a generator is islanded, its output frequency will no longer be locked to that of the network, but will change rapidly to a frequency determined by its own characteristics and those of the loads it is still supplying. It is this change in frequency that is detected by ROCOF protection devices.
ROCOF protection conforms to engineering requirements such as G59/3 in the UK and standards such as ANSI 81R in the USA. There are strict guidelines for ROCOF settings and unless these are optimised, the protection may unnecessarily trip a generator when problems occur with the power network, and the resulting loss of capacity may make the problems worse.

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