Instantaneous (unrestraint) unit setting – Basler Electric BE1-87T User Manual
Page 88
4-46
BE1-87T Installation
9171300990 Rev V
Neglecting R
R
, use R
W
and R
L
from Figure 4-37:
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
):
HIGH
LOW
Step 16. Determine the saturation factor (S
F
):
VB is the largest of the burden voltages calculated in steps 13 and 14.
HIGH
LOW
11
.
0
200
4
.
22
=
=
F
S
18
.
0
3
.
533
0
.
98
=
=
F
S
NOTE
Maximum Recommended SF = 0.5.
Larger saturation factors will make the relay insecure for external faults. The only solution is to increase
the CT quality.
Step 17. Determine the maximum external fault multiple (I
E
).
Instantaneous (Unrestraint) Unit Setting
•
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
V
V
N
N
CE
C
A
=
=
(Base Accuracy) (Number of CT Turns in Use)
Maximum Ratio
( )
V
CE
=
=
(400) 300
600
200
V
CE
=
=
800 800
2000
5333
(
)
.
S
F
V
B
V
CE
=