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2.2 Overvoltage and Undervoltage Protection
2.2.1
Phase Overvoltage Protection
GRD150 provides two independent phase overvoltage elements with programmable
dropoff/pickup(DO/PU) ratio. OV1 is programmable for inverse time (IDMT) or definite time
(DT) operation. OV2 has definite time characteristic only.
Note: OV1 element that has inverse time or definite time characteristics is discriminated with OV1-I
Figure 2.2.1 shows the characteristic of overvoltage elements.
2.2.1.1 Inverse Time Overvoltage Protection
The inverse time overvoltage protection element OV1-I has the IDMT characteristic defined by
equation (1):
=
t
TMS
where:
t = operating time for constant voltage V (seconds),
V = energising voltage (V),
Vs = overvoltage setting (V),
TMS = time multiplier setting.
The IDMT characteristic is illustrated in Figure 2.2.2.
Definite time reset
The definite time resetting characteristic is applied to the OV1 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 falls below 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 exceeds the setting for a transient period without causing tripping,
then resetting is delayed for a user-definable period. When the energising voltage falls below the
or OV1-D respectively.
Figure 2.2.1
⎡
⎤
⎢
1
⎥
×
( )
⎢
⎥
−
⎢
V
⎥
1
⎣
⎦
Vs
Pickup
Dropoff
0
Characteristic of Overvoltage Elements
(1)
⎯
⎯
74
6 F 2 S 0 8 4 2
V
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