Date Code 20020129
Differential, Restricted Earth Fault,
3-43
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
The relay determines whether to assert Relay Word bit 51NC1 by selecting the greater of the two
CT ratios, using this ratio as a common base in calculating the combined secondary current and
then comparing this combined secondary current against the 51NC1P pickup setting. This pickup
setting is a secondary quantity that the relay calculates by dividing the primary current pickup by
the greater of the two CT ratios.
For CTR1<CTR2, the relay performs the following operation on the secondary quantities it
receives from the CTs:
(
)
(
)
2
ICW
2
IBW
2
IAW
2
CTR
1
CTR
•
1
ICW
1
IBW
1
IAW
+
+
+
+
+
Note:
XXXX defines a vector quantity.
The following example illustrates the equivalent operation on the primary quantities entering the
CTs:
Assume
CTR1 = 500/5 =
100
CTR2 = 1000/5 =
200
Pickup =
400 A (primary)
Primary Currents
IAW1 = 3000
µ
2
A
IAW2
=
1545
µ
3
A
IBW1 = 3010
µ
–122
A
IBW2
=
1480
µ
–118
A
ICW1 = 2950
µ
117
A
ICW2
=
1505
µ
124
A
Secondary Currents
IAW1 = 30
µ
2
A
IAW2
=
7.725
µ
3
A
IBW1 = 30.1
µ
–122
A
IBW2
=
7.4
µ
–118
A
ICW1 = 29.5
µ
117
A
ICW2
=
7.525
µ
124
A
Then,
(
)
(
)
A
124
525
.
7
A
118
4
.
7
A
3
725
.
7
200
100
•
A
117
5
.
29
A
122
1
.
30
A
2
30
µ
+
-
µ
+
µ
+
µ
+
-
µ
+
µ
A
8
.
70
071
.
1
µ
=
Note:
Because CTR1<CTR2, the relay uses CTR2 as the common base.
A
2
200
A
400
P
1
NC
51
=
=
secondary
The combined secondary current value is less than the 51NC1P setting, so
51NC1 does not assert.
