Basler Electric BE1-87T - Manual (Page 132)

A-4 BE1-87T Setting Notes 9171300990 Rev V
Setting Note 4
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.
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 0.5 to
2 (plot shows
10 × I
T
).
S
F
87T
D2751-18
02-03-98
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