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TB XRW1-4 04.01 E
A reliable criterion of detecting mains failure is the
measurement of the rate of change of frequency df/dt.
Precondition for this is a load flow via the mains cou-
pling point. At mains failure the load flow changing
then spontaneously leads to an increasing or decreas-
ing frequency in the grid island. At active power deficit
of the internal power station a linear drop of the fre-
quency occurs and a linear increase occurs at power
excess this assumption is valid within a time window of
some 100 milliseconds, because regulator effects be-
come here not marked out yet. Typical frequency gra-
dients during application of "mains decoupling" are in
the range of 0.5 Hz/s up to over 2 Hz/s. The
XRW1-
4
detects the instantaneous frequency gradient df/dt of
each mains voltage period in an interval of one half
period each. Through multiple evaluation of the fre-
quency gradient in sequence the continuity of the direc-
tional change (sign of the frequency gradient) is de-
termined. Because of this special measuring procedure
a high safety in tripping and thus a high stabilty against
transient processes, e.g. switching procedure are
reached. The total switching off time at mains failure is
usually approx. 100 ms (T = 4) depending on the set-
ting.
