ABB CIRCUIT SHIELD 87T - Page 23

Transformer
Differential
Relay
IB
7.6
.
1.7
-
4
Page
23
Three
-
Winding
Sample
Settings
Calculation
(
continued
from
page
12
)
:
Tertiary
Primary
Secondary
7
.
Select
tap
values
1
.
558
Closest
ratios
are
1.56
(
5.0
/
3.2
)
and
1.55
(
4.5
/
2.9
)
but
Ai
2
looking
at
the
second
requirement
:
Ai
3
to
satisfy
both
Ai
2
and
Ai
3
select
tap
2.9
for
the
primary
side
.
This
also
automatically
satisfies
A
2 3
3.003
requires
the
use
of
taps
8.7
/
2.9
=
tertiary
/
primary
;
therefore
,
8.7
/
4.5
1.93
.
1.928
:
4.5
A
8.7
A
Tap
values
are
then
2.9
A
(
Note
:
in
some
cases
it
will
be
necessary
to
select
different
ct
ratios
and
recalculate
to
obtain
a
workable
combination
of
tap
settings
,
difficult
cases
it
may
be
necessary
to
use
a
set
of
external
auxiliary
ct
’
s
to
adjust
one
of
the
current
inputs
to
a
workable
value
.
)
In
2.79
A
2.32
A
8
.
Relay
currents
at
max
load
currents
Continuous
rating
of
relay
tap
(
Table
2
)
1
.
79
A
17
A
ok
10
A
ok
10
A
ok
(
1.933
/
1.928
)
/
1.928
0.3
%
(
1
.
555
-
1
.
55
)
/
1
.
55
0.3
%
(
3.003
-
3.00
)
/
3.00
0.0
%
9
.
Mismatches
M
2
3
Mi
2
Mi
3
10
.
Select
percentage
differential
setting
:
Mismatch
is
less
than
15
%
,
so
use
a
relay
setting
of
25
%
.
11
.
Select
the
instantaneous
setting
:
_
High
-
side
relay
current
at
self
-
cooled
rating
=
(
30000
/
(
161
*
y
/
3
)
*
5
/
400
*
>
/
T
=
2.32
A
8
(
x
tap
)
.
8
x
1.2
=
9.6
(
x
tap
)
23.2
/
2.9
(
2.32
*
10
)
/
(
primary
side
tap
)
Multiply
by
1.2
for
margin
:
(
Factory
setting
of
10
x
tap
is
ok
)
12
.
CT
performance
check
CT
secondary
currents
at
max
fault
18.9
A
Effective
C
ratings
of
ct
’
s
267
58.2
A
29.4
A
167
200
Maximum
allowable
burdens
1.4
ohms
3.4
ohms
7.0
ohms
Maximum
one
-
way
lead
resistance
(
assuming
relay
is
the
only
connected
device
)
1.0
ohms
0.59
ohms
2.0
ohms
13
.
Minimum
operating
current
Select
setting
to
give
minimum
operating
current
of
30
%
relative
to
full
load
current
:
Primary
side
relay
current
at
max
load
=
3.1
A
Primary
side
relay
tap
2.9
A
30
%
x
3.1
A
/
2.9
A
32
%
of
tap
.
32
*
(
100
+
D
)
/
D
=
.
32
*
(
125
)
/
25
(
M i n
Restraint
setting
)
1
.
6
R
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