Date Code 20080110 Protection Functions 3-43
SEL-387E Instruction Manual
A5.7
400
A1000
400
A2000
=+ combined secondary current
Note:
Because CTR1<CTR2, the relay uses CTR2 as the common base.
A20
400
A8000
P1PC51 == secondary
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:
I
n
W1 + (I
n
W2 • CTR2/CTR1),
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
A45
200
A3000
200
A6000
=+ combined secondary current
Note:
Because CTR1>CTR2, the relay uses CTR1 as the common base.
A40
200
A8000
P1PC51 == secondary
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 button to force the bit to 1 during element testing. This
saves time if you have chosen electromechanical reset.
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