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Date Code 20000421
Specifications
2-33
SEL-251, -2, -3 Instruction Manual
T
IME
-O
VERCURRENT
E
LEMENT
C
URVE
-T
IMING AND
T
IME
D
ELAY
R
ESET
E
QUATIONS
The following time-overcurrent equations are applicable to the phase, negative-sequence, and
ground/residual time-overcurrent elements.
There are two reset characteristic choices for the phase, negative-sequence, and ground/residual
time-overcurrent elements via the 51RS, 51QRS, and 51NRS settings, respectively. One choice
resets the elements if current drops below pickup for at least one cycle. The other choice (time
delay reset) emulates electromechanical induction disc elements, where the reset time depends on
the time dial setting, the percentage of disc travel, and the amount of current between zero and
pickup. The time delay reset equations (t
r
=...) are given below.
t
M
=
curve operating time in seconds
t
r
=
reset time in seconds (for time delay reset)
TD
=
time dial setting
M
=
multiples of pickup. For curve operating time (
t
M
),
M
>
1.
For reset time (
t
r
),
M
£
1.
Curve 1: Moderately Inverse - see Figure 2.14
t
TD
M
M
=
+
-
é
ë
ê
ù
û
ú
0 157
0 668
1
.
.
t
TD
M
r
=
-
0 949
0 936
.
.
Curve 2: Inverse - see Figure 2.15
t
TD
M
M
=
+
-
é
ë
ê
ù
û
ú
0 180
5 95
1
2
.
.
t
TD
M
r
=
-
0 172
0168
.
.
Curve 3: Very Inverse - see Figure 2.16
t
TD
M
M
=
+
-
é
ë
ê
ù
û
ú
0 0963
3 88
1
2
.
.
t
TD
M
r
=
-
0 246
0 240
.
.
Curve 4: Extremely Inverse - see Figure 2.17
t
TD
M
M
=
+
-
é
ë
ê
ù
û
ú
0 0352
5 67
1
2
.
.
t
TD
M
r
=
-
0 194
0 192
.
.