4-46
BE1-87T Installation
9171300990 Rev R
Neglecting
R
R
, use
R
W
and
R
L
from Figure 4-35:
HIGH
∗
LOW
V
BG
=
=
19.5(0.7)
13.6 V
VBG
=
=
70 (2(0.7))
98.0 V
NOTE
∗
Since a phase-to-ground fault looks like a phase-to-phase fault on the delta side of a
delta/wye transformer, each CT only has to carry one times the one way lead burden.
Step 15.
Determine the effective CT accuracy class (
V
CE
):
V
V
N
N
CE
C
A
=
=
(Base Accuracy) (Number of CT Turns in Use)
Maximum Ratio
( )
HIGH
LOW
V
CE
=
=
(400)
300
600
200
V
CE
=
=
800
800
2000
533 3
(
)
.
Step 16.
Determine the saturation factor (
S
F
):
VB is the largest of the burden voltages calculated in steps 13 and 14.
S
F
V
B
V
CE
=
HIGH
LOW
11
.
0
200
4
.
22
=
=
F
S
18
.
0
3
.
533
0
.
98
=
=
F
S
NOTE
Maximum Recommended
S
F
= 0.5.
Larger saturation factors will make the relay insecure for external faults. The only solution is to increase
the CT quality.
Instantaneous (Unrestraint) Unit Setting
Step 17.
Determine the maximum external fault multiple (
I
E
).
•
For wye-connected CTs and with WYE jumpers on Analog Board #2, shown in Figure 2-
4:
I
I
T
E
F
=
=
Maximum Relay Fault Current
Corresponding Tap
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