Schweitzer Engineering SEL-387-0 - Page 107

3.57
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
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:
Equation 3.40
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)
Then
Equation 3.41
Because CTR1<CTR2, the relay uses CTR2 as the common base.
Equation 3.42
The combined secondary current value is less than the 51NC1P setting, so
51NC1 does not assert.
defines a vector quantity.
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
IAW1 IBW1 ICW1++
CTR1
CTR2
--------------- IAW2 IBW2 ICW2+++•
XXXX
30 2A 30.1 122A– 29.5 117A++
100
200
---------•
+7.725 3A7.4 118A– 7.525 124A++
1.071 70.8A=
51NC1P
400 A
200
-------------- 2 A secondary==