In recent years, digitized underfrequency relays, which sense not only
the frequency deviation, but also the rate of change of frequency,
have ken designed and built [2,3]. The advantage of knowing the rate
of change of frequency is that it is then possible to predict
whether a critical frequency drop will occur before it occurs,
providin better selectivity [6]. In areas where generation exds
the loat the problem is not as severe, since it is always possible to
reduce generation, whereas for excess load, it is not possible to
increase generation beyond the capability of the source.