3-44 Protection Functions Date Code 20080110
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:
()
)
2ICW2IBW2IAW
2CTR
1CTR
•1ICW1IBW1IAW +++++
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∠2° A IAW2 = 1545∠3° A
IBW1 = 3010∠–122° A IBW2 = 1480∠–118° A
ICW1 = 2950∠117° A ICW2 = 1505∠124° A
Secondary Currents
IAW1 = 30∠2° A IAW2 = 7.725∠3° A
IBW1 = 30.1∠–122° A IBW2 = 7.4∠–118° A
ICW1 = 29.5∠117° A ICW2 = 7.525∠124° A
Then,
()
()
A124525.7A1184.7A3725.7
200
100
•A1175.29A1221.30A230
°∠+°−∠+°∠+
°∠+°−∠+°∠
A8.70071.1 °∠=
Note:
Because CTR1<CTR2, the relay uses CTR2 as the common base.
A2
200
A400
P1NC51 == secondary
The combined secondary current value is less than the 51NC1P setting, so
51NC1 does not assert.
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