9.1.2.3 Voltage dependent time delay
Since the fundamental frequency in a power system is the same all over the system,
except some deviations during power oscillations, another criterion is needed to decide,
where to take actions, based on low frequency. In many applications the voltage level
is very suitable, and in most cases is load shedding preferable in areas with low
voltage. Therefore, a voltage dependent time delay has been introduced, to make sure
that load shedding, or other actions, take place at the right location. At constant
voltage, V, the voltage dependent time delay is calculated according to equation
147.
At non-constant voltage, the actual time delay is integrated in a similar way as for the
inverse time characteristic for the undervoltage and overvoltage functions.
( )
_ _ _
Exponent
V VMin
t t MaxTripDelay t MinTripDelay t MinTripDelay
VNom VMin
-
= × - +
-
é ù
ê ú
ë û
EQUATION1559 V1 EN (Equation 147)
where:
t is the voltage dependent time delay (at constant voltage),
V is the measured voltage
Exponent is a setting,
VMin, VNom are voltage settings corresponding to
t_MaxTripDelay, t_MinTripDelay are time settings.
The inverse time characteristics are shown in figure 281, for:
VMin
= 90%
= 100%
t_MaxTrip
Delay
= 1.0 s
t_MinTripD
elay
= 0.0 s
Exponent = 0, 1, 2, 3 and 4
1MRK505222-UUS C Section 9
Frequency protection
553
Technical reference manual
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