Schweitzer Engineering SEL-387-0 - Page 108

3.58
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
For CTR1>CTR2, the relay performs the following operation on the
secondary quantities it receives from the CTs:
Equation 3.43
The following example illustrates the equivalent operation on the primary
quantities entering the CTs. Assume:
CTR1 = 1000/5 = 200
CTR2 = 500/5 = 100
Pickup = 400 A (primary)
Then
Equation 3.44
Because CTR1 > CTR2, the relay uses CTR1 as the common base.
Equation 3.45
The combined secondary current value is greater than the 51NC1P setting, so
51NC1 asserts and the time overcurrent curve begins timing.
As with the phase current element, four settings define the curve: 51NC1P is
the pickup setting, 51NC1C defines the curve equation, 51NC1TD defines the
time dial, and 51NC1RS determines how the curve resets.
defines a vector quantity.
Primary Currents
IAW1 = 30005° A
IAW2 = 15453° A
IBW1 = 3010–123° A
IBW2 = 1480–118° A
ICW1 = 2950115° A
ICW2 = 1505124° A
Secondary Currents
IAW1 = 155° A
IAW2 = 15.45 3° A
IBW1 = 15.05–123° A
IBW2 = 14.80–118° A
ICW1 = 14.75115° A
ICW2 = 15.05124° A
IAW1 IBW1 ICW1++IAW2 IBW2 ICW2+++
CTR2
CTR1
---------------•
XXXX
15 5A 15.05 123A– 14.75 115A++
+15.45 3A 14.80 118A– 15.05 124A++
100
200
---------•
2.1164 76A 0.11270 73A+= 2.230 76A=
51NC1P
400 A
200
-------------- 2 A secondary==