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.
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