Schweitzer Engineering SEL-387-0, Instruction Manual

Page: 106 / 556

3.56
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
Then, converting the observed primary values to secondary values, we have
Equation 3.35
Because CTR1<CTR2, the relay uses CTR2 as the common base.
Equation 3.36
The combined secondary current is less than the 51PC1P setting, so 51PC1
does not assert.
For CTR1>CTR2, the relay performs the following operation on the
secondary quantities it receives from the CTs:
Equation 3.37
where n = Phase A, Phase B, or Phase C.
The following example illustrates the equivalent operation on the primary
quantities entering the CTs. Assume:
CTR1 = 1000/5 = 200
CTR2 = 500/5 = 100
I n W1 = 3000 A (primary)
I n W2 = 6000 A (primary)
Pickup = 8000 A (primary)
where n = Phase A, Phase B, or Phase C.
Then, converting the observed primary values to secondary values, we have
Equation 3.38
Because CTR1>CTR2, the relay uses CTR1 as the common base.
Equation 3.39
The combined secondary current value is greater than the 51PC1P setting, so
51PC1 asserts and the time overcurrent curve begins timing.
The settings defining the curve in this case are 51PC1P for the pickup setting,
51PC1C for the particular curve equation, 51PC1TD for the time dial, and
51PC1RS for the curve reset.
Curve time-out causes the Relay Word bit 51PC1T to assert. When the current
drops below pickup, 51PC1 deasserts and the element resets according to the
setting 51PC1RS. At the completion of reset, Relay Word bit 51PC1R asserts.
This bit normally is at logic state 1, when the element is at rest during normal
system operation. You can use the
TAR command to verify the state of the bit.
You can use the Level 2 serial port command RES or the front-panel RESET51
function under the
{OTHER}
pushbutton to force the bit to 1 during element
testing. This saves time if you have chosen electromechanical reset.
2000 A
400
-----------------
1000 A
400
----------------- +
7.5 A combined secondary current
=
51PC1P
8000 A
400
-----------------
20 A secondary
= =
InW1 InW2
CTR2
CTR1
--------------- •
 
 
+
6000 A
200
-----------------
3000 A
200
----------------- +
45 A combined secondary current
=
51PC1P
8000 A
200
-----------------
40 A secondary
= =