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2.2.2
Phase Undervoltage Protection
GRD150 provides two independent phase undervoltage elements. UV1 programmable for inverse
time (IDMT) or definite time (DT) operation. UV2 has definite time characteristic only.
Note: UV1 element that has inverse time or definite time characteristics is discriminated with UV1-I
or UV1-D respectively.
Figure 2.2.5 shows the characteristic of undervoltage elements.
Figure 2.2.5
2.2.2.1 Inverse Time Undervoltage Protection
The inverse time undervoltage protection element UV1-I has the IDMT characteristic defined by
equation (2):
⎡
⎢
1
=
×
t
TMS
( )
⎢
−
⎢
V
1
⎣
Vs
where:
t = operating time for constant voltage V (seconds),
V = energising voltage (V),
Vs = undervoltage setting (V),
TMS = time multiplier setting.
The IDMT characteristic is illustrated in Figure 2.2.6.
Definite time reset
The definite time resetting characteristic is applied to the UV1 element when the inverse time
delay is used.
If definite time resetting is selected, and the delay period is set to instantaneous, then no
intentional delay is added. As soon as the energising voltage rises above the reset threshold, the
element returns to its reset condition.
If the delay period is set to some value in seconds, then an intentional delay is added to the reset
period. If the energising voltage is below the undercurrent setting for a transient period without
causing tripping, then resetting is delayed for a user-definable period. When the energising
voltage rises above the reset threshold, the integral state (the point towards operation that it has
travelled) of the timing function (IDMT) is held for that period.
This does not apply following a trip operation, in which case resetting is always instantaneous.
0
Characteristic of Undervoltage Elements
⎤
⎥
(2)
⎥
⎥
⎦
⎯
⎯
78
V
6 F 2 S 0 8 4 2
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