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What is the application of rate of change of frequency (ROCOF) protection df/dt (81R) in protection relay.

Rate of change of frequency (ROCOF or df/dt) function is used for fast load shedding, to speed up operation time in over- and under-frequency situations and to detect loss of grid. For example a centralized dedicated load shedding relay can be omitted and replaced with distributed load shedding, if all outgoing feeders are equipped with protection devices e.g. VAMP devices.
A special application for ROCOF is to detect loss of grid (loss of mains, islanding). The more the remaining load differs from the load before the loss of grid, the better the ROCOF function detects the situation.

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