Basler BE1-CDS220, Instruction Manual

Page: 461 / 476

BE1-CDS220 Settings Calculations D-3
For delta-connected CTs:
V
BG
is a function of the proportion of positive-sequence to zero-sequence currents,
but may be approximated by the same equation (for worst case).
Neglecting R
R
, use R
L
from Table D-1.
HIGH
∗
LOW
V 6 . 13 ) 7 . 0 )( 5 . 19 ( V
BG
= =
V 0 . 98 } 7 . 0 ) 2 ){( 70 ( V
BG
= =
∗
Since a phase-to-ground fault looks like a phase-to-phase fault on the delta side of a
delta/wye transformer, each CT has to carry only one times the one way lead burden.
Step 4. Determine the effective CT accuracy class (V
CE
):
( )( )
Ratio Maximum
use in Turns CT of Number Accuracy Base
V
CE
=






=
N
N
V
A
C
HIGH LOW
200
120
60
(400) V
CE
=






=
3 . 533
240
160
) 800 ( V
CE
=






=
Step 5. Determine the saturation factor (S
F
):
V
B
is the largest of the burden voltages calculated in steps 2 and 3.
CE
B
F
V
V
S
=
HIGH LOW
11 . 0
200
4 . 22
S
F
= =
18 . 0
3 . 533
0 . 98
S
F
= =
NOTE
Maximum recommended S
F
= 0.5. Larger saturation factors will make the relay
insecure for external faults. Possible solutions are to increase the CT quality,
adjust the CT tap connections to increase effective accuracy class, or to use
internal phase compensation instead of delta connecting CTs.