A-4
BE1-87T Setting Notes
9171300990 Rev R
SETTING NOTE 4
S
F
87T
D2751-18
02-03-98
1) Find the maximum pu fault current for external faults (ie).
2) Assume that one of the input CTs saturates to 70%.
3) Set unrestraint pickup >.7 X IE.
I
FH
I
FH
/R
CL
0.3I
FL
/R
CL
R
CL
-
+
I
OP
TRIP
UNRESTRAINT
MPU SETTING
T
H
T
L
0 3
.
×
×
I
R
T
FL
CC
L
I
R
T
FH
CH
H
×
R
CH
I
FL
D2751-19
03-12-98
Figure A-4. Unrestraint Tap Setting
SETTING NOTE 5
Slope Margin
The slope formula accounts for an operating current bias due to magnetizing current in one winding. The
magnetizing current is assumed to be 4% of the OA rating. Step 20 is used to scale this magnetizing ratio
to the corresponding ratio of relay operate current: 4 x
I
T
%.
The formula further assumes a conservative margin corresponding to an operation current of 23% of tap.
The actual margin varies with the tap settings, as given by the following equation:
(
)
Margin
=
− × −
+ ×
35 4
3
35
I
M
S
T
T
The margin variations for different tap settings (
I
T
) can be evaluated with this equation. The following plots
show the calculated slope and the resulting margin for
M
T
varying from 1 to 11% and
I
T
varying from .5 to
2 (plot shows
10
×
I
T
).
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