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S3 PROTECTION
Positive sequence undervoltage (27_1)
NOTE:
6–106
The relay has one Positive Sequence Undervoltage element per protection group. The input
for this element is the computed positive sequence voltage based on measured three-
phase voltages at relay terminals. The time delay characteristic can be programmed as
either definite time, or inverse time. A minimum operating voltage level is programmable to
prevent undesired operation before voltage becomes available.
•
Undervoltage Protection: For voltage sensitive loads, such as induction motors, a
drop in voltage will result in an increase in the drawn current, which may cause
dangerous overheating in the motor. The undervoltage protection feature can be used
to either cause a trip or generate an alarm when the voltage drops below a specified
voltage setting for a specified time delay.
•
Permissive Functions: The undervoltage feature may be used to block the functioning
of external devices by operating an output relay, when the voltage falls below the
specified voltage setting. Note that all internal features that are inhibited by an
undervoltage condition, such as underfrequency and overfrequency, have their own
inhibit functions independent of the undervoltage protection features.
•
Source Transfer Schemes: In the event of an undervoltage, a transfer signal may be
generated to transfer a load from its normal source to a standby or emergency power
source.
The undervoltage elements can be programmed to have an inverse time delay
characteristic. The undervoltage delay setpoint defines a family of curves as shown below.
The operating time is given by:
Where:
T = Operating Time
D = Undervoltage Delay setpoint
V = Voltage as a fraction of the nominal VT Secondary Voltage
V
= Pickup Level
pu
At 0% of pickup, the operating time equals the Undervoltage Delay setpoint.
Figure 6-47: Inverse time undervoltage curves
20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
The following path is available using the keypad. For instructions on how to use the
keypad, please refer to Chapter 3 - Working with the Keypad.
PATH:
SETPOINTS > S3 PROTECTION > S3 SETPOINT GROUP 1(2) > POSITIVE SEQ UV 1(2)
D
T =
1- V/V
pu
= .0
0
10
20 30
40
50
60
70
% of V pickup
350 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
.0 1.0
80 90 100 110
Eq. 14
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