df/dt Protection for Generators

The df/dt protection is used to identify abnormal changes in system frequency and take remedial actions in order to prevent generator overload and the resulting blackout.  The df/dt operates faster than ordinary under frequency relays as it is able to predict the under frequency much earlier.   

df/dt relays are also known as ROCOF relays (Rate Of Change of Frequency Relays) The df/dt is usually wired to a load shedding system which trips select breakers to isolate loads. 

The df/dt functions by measuring the rate of change of frequency.  When the frequency changes too fast, it is an indicator of a forthcoming under frequency.  The setting of the df/dt relay is in Frequency/Time in seconds. e.g. 0.3Hz/second or 0.4 Hz/.5 seconds.

Some manufacturers provide a more reliable setting involves specifying two frequency set points and the time taken for the frequency to cross the two limits.  For instance, a df/dt relays can be programmed to operate if the system frequency crosses 48.5 Hz and 48 Hz in 0.4 seconds