3.20
SEL-351A Relay Instruction Manual Date Code 20080213
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Time-Overcurrent Elements
Sometimes SELOGIC control equation torque control settings are set to
provide directional control. See Directional Control Provided by Torque
Control Settings (Not in SEL-351A-1) on page 4.57.
Reset Timing Details (51PT Element Example)
Refer to Figure 3.14.
Any time current I
P
goes above pickup setting 51PP and the phase time-
overcurrent element starts timing, Relay Word bit 51PR (reset indication) =
logical 0. If the phase time-overcurrent element times out on its curve, Relay
Word bit 51PT (curve time-out indication) = logical 1.
Setting 51PRS = Y
If electromechanical reset timing setting 51PRS = Y, the phase time-
overcurrent element reset timing emulates electromechanical reset timing. If
maximum phase current, I
P
, goes above pickup setting 51PP (element is
timing or already timed out) and then current I
P
goes below 51PP, the element
starts to time to reset, emulating electromechanical reset timing. Relay Word
bit 51PR (resetting indication) = logical 1 when the element is fully reset.
Setting 51PRS = N
If reset timing setting 51PRS = N, element 51PT reset timing is a 1-cycle
dropout. If current I
P
goes above pickup setting 51PP (element is timing or
already timed out) and then current I
P
goes below pickup setting 51PP, there is
a 1-cycle delay before the element fully resets. Relay Word bit 51PR (reset
indication) = logical 1 when the element is fully reset.
Operation of Single-Phase Time-Overcurrent Elements (51AT, 51BT, 51CT)
To understand the operation of Figure 3.15, Figure 3.16, and Figure 3.17,
follow the explanation given for Figure 3.14 in the preceding part of Phase
Time-Overcurrent Elements on page 3.16, substituting phase current I
A
(or I
B
,
or I
C
) for maximum phase current I
P
and substituting like settings and Relay
Word bits. The settings ranges and accuracies for the single-phase time-
overcurrent elements settings are the same as the corresponding settings in
Table 3.2.
Courtesy of NationalSwitchgear.com
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