  The paper describes a distributed under-frequency load shedding and load
restoration scheme, that exploits frequency and Rate-of-Change-of-Frequency
(ROCOF) measurements produced by Phasor Measurement Units (PMUs) as detectors
of a large contingency. The paper further discusses the appropriateness of
using the synchrophasor model for estimating ROCOF during fast transients
following a severe generation-load imbalance. The performance of the proposed
relaying scheme is compared with a frequency-only based strategy, by means of a
real-time digital simulator implementing the time-domain full-replica model of
the IEEE 39 bus system.
