Basler Electric BE1-87T - Manual (Page 79)

9171300990 Rev V BE1-87T Installation 4-37
Step 10. Determine the CT mismatch (M
N
):
LOW TERTIARY −
=
) ( (
−
)
=
( )
−
( )
=
=
MN
II
T
T
100
3
2 3
2
100
1822
7 55
6 0
2 5
100
2
0
(the smaller of the above)
(the smaller of the above)
2.41-2.40
.
.
.
.
.40
.4%
Step 11. Determine the total mismatch (M
T
):
Add the maximum CT mismatch M
N
(based on the power transformer in the neutral tap
position) to the total permissible tap excursion from neutral. In this example, a ± 10% load tap
change (LTC) must be accommodated. Therefore:
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 (I
F3
for three-phase, and
I
FG
for line-to-ground). Refer again to Figure 4-35 for this example. The maximum fault current is
recorded for each set of terminals for all combinations of external faults.
Verify CT Performance
HIGH TERTIARY LOW
Step 13. Determine the worst case burden voltage for a three-phase fault (V
B3
).
• For wye-connected CTs:
) (
3 3 R L F B
R R I V + =
• For delta-connected CTs, based on a three-phase fault (refer to Appendix A, Setting Note 2):
) R 3 R 3 R ( I V
R L 3 F 3 B
+ + =
W
Where:
I
F3
= determined in Step 12 R
R
= relay resistance in ohms (< 0.05 ohm)
R
W
= winding burden
R
L
= one-way lead resistance in ohms
Neglecting R
R
, use R
L
from Figure 4-35:
HIGH LOW −
=
(
−
) )
=
−
=
=
M
I
I
T
T
N
100
100
6 04
7 55 2 0
2 5
100
0 80
0%
1
3 1
3
(the smaller of the above)
0.80- 0.80
.
. .
.
.
(
(the smaller of the above)
( ) ) (
HIGH TERTIARY −
=
−
=
−
=
=
M
I
I
T
T
N
100
100
18 22
6 04
6 0
2 0
100
3 0
0 6%
2
1 2
1
(the smaller of the above)
3.02-3.0
.
.
.
.
.
.
( ) ) (
( ) ) (
(the smaller of the above)
M M
T N
= + LTC
M
T
= + = 0 6 10 10 6 % . .
IF3
3360
120
=
= 28 A IF3
13
600
=
=
,280
22 A
IF3
8400
240
=
= 35 A
IFG =
=
3375
120
28 A
IFG =
=
9000
240
38 A