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

9171300990 Rev V BE1-87T Installation 4-45
Verify CT Performance
NOTE
This procedure uses the ANSI accuracy class method. See Appendix A,
Setting Note 7 for more information.
Step 12. Determine the maximum CT secondary fault current for external faults at F (I
F3
for three-phase,
and I
FG
for single-phase). Refer again to Figure 4-37 for this example:
HIGH LOW
IF3
1916
60
=
= 32 A IF3
10603
160
=
= 66 A
IFG =
=
1171
60
19.5 A
IFG =
=
11231
160
70 A
Step 13. Determine the worst case CT burden voltage for a three-phase fault (V
B3
).
•
For wye-connected CTs:
V
B3
= I
F3
(R
L
+ R
R
)
• For delta-connected CTs, for three-phase fault:
V
B3
= I
F3
(R
W
+ 3R
L
+ 3R
R
)
Note that the wye connection produces a lower burden on the CTs (see Appendix A, Note 2).
Where:
I
F3
= determined in Step 12 R
L
= one-way lead resistance in ohms
R
W
= winding burden R
R
= relay resistance in ohms (< 0.05 ohm)
Neglecting R
R
, use R
W
and R
L
from Figure 4-37:
HIGH LOW
VB3 (32)(0.7) =
= 22.4 V
VB3 (66)(0.7)
46.2 V
=
=
Step 14. Determine the worst case burden voltage for a line-to-ground fault (V
BG
).
• For wye-connected CTs:
V
BG
= I
FG
(2R
L
+ R
R
)
Where:
I
FG
= determined in Step 12
R
L
= one-way lead resistance in ohms
R
R
= relay resistance in ohms
• 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).