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2.9.2
Undervoltage Protection
GRL100 provides four independent undervoltage elements for phase and earth fault protection.
UVS1 and UVS2 are used for phase fault protection, and UVG1 and UVG2 for earth fault
protection. UVS1 and UVG1 are programmable for inverse time (IDMT) or definite time (DT)
operation. UVS2 and UVG2 have definite time characteristic only.
UVS1 and UVG1 undervoltage protection elements have an 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 2.9.2.1.
The UVS2 and UVG2 elements are used for definite time undervoltage protection.
Definite time reset
The definite time resetting characteristic is applied to the UVS1 and UVG1 elements 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.
(2)
97
6 F 2 S 0 8 5 0
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