Schweitzer Engineering Laboratories SEL-251 - Page 47

Date Code 20000421 Specifications 2-33
SEL-251, -2, -3 Instruction Manual
TIME-OVERCURRENT ELEMENT CURVE-TIMING AND TIME DELAY RESET
EQUATIONS
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
0157
0668
1
.
.
t
TD
M
r
=
-0949 0 936. .
Curve 2: Inverse - see Figure 2.15
t TD
M
M
= +
-
é
ë
ê
ù
û
ú
0180
595
1
2
.
.
t
TD
M
r
=
-0172 0168. .
Curve 3: Very Inverse - see Figure 2.16
t TD
M
M
= +
-
é
ë
ê
ù
û
ú
00963
388
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
00352
567
1
2
.
.
t
TD
M
r
=
-0194 0192. .