ABB CIRCUIT SHIELD 87T - Page 11

Transformer
Differential
Relay
IB
7.6
.
1.7
-
4
Page
11
Example
:
SETTINGS CALCULATION
The
following
transformer
is
assumed for
this
example
:
12
/
16
/
20
MVA
OA
/
FA
/
FA
,
115
kv
delta
,
13.8
kv
wye
,
8.5
%
impedance
,
on
-
load
tap
changer
range
+
/
-
10
%
.
Low
. ..
S
.
ide
13.8
kv
wye
)
Hl
9
b
„
SJ
.
de
_
X
1
L
§
_
Jsy
_
jdeltal
1
.
Maximum
load
current
(
at
20
MVA
)
:
IH
=
20000
/
(
115
*
/
3
)
=
100.4
A
II
=
20000
/
(
13.8
*
y
/
3
)
837
A
2
.
Maximum
through
fault
currents
(
assuming
infinite
bus
)
:
IH
F
12000
/
(
13.8
*
V
3
*
0.085
)
5907
A
12000
/
(
115
*
^
3
*
0.085
)
709
A
IL F
3
.
Choose
ct
ratios
:
=
100
:
5
multiratio
1200
:
5
set
on
1000
:
5
tap
.
CT
secondary
currents
at
maximum
through
fault
:
IH
F
s
(
5907
/
1000
)
*
5
=
29.5
A
(
ok
)
(
709
/
100
)
*
5
=
35.5
A
(
ok
)
ILFS
4
.
Load
currents
on
ct
secondary
side
at
maximum
transformer
rating
(
20
MVA
)
:
IH
S
(
837
/
1000
)
*
5
=
4.19
A
(
100.4
/
100
)
*
5
II
s
5.0
2
A
5
.
Relay
currents
at
maximum
load
current
:
High
-
side
ct
secondaries
in
wye
IH
P
Low
side
ct
secondaries
in
delta
II
R
4.19
*
s
/
3
7.26
A
5.02
A
6
.
Ratio
of
relay
currents
:
1
arger
/
sma
11
er
=
7.26
/
5.02
low
side
/
high
side
1
.
446
A
7
.
Select
taps
from
Table
1
:
Closest
ratio
is
1.41
using
taps
4.5
/
3.2
.
Assign
taps
in
proper
relationship
as
calculated
in
step
6
:
Larger
value for low
side
ct
’
s
=
4.5
A
tap
.
Smaller
value for
high
side
ct
’
s
-
3.2
A
tap
.
(
see
notes
on
Figure
2
page
7
for
physical
relationship
of
of
tap
settings
to
input
terminals
)
8
.
Check
continuous
ratings
of
taps
from
Table
2
:
Both
taps
selected
are
rated
10
A
continuous
-
ok
.
9
.
Calculate
the
mismatch
:
1
(
1.41
-
1
.
446
)
|
/
1.41
|
(
T
—
A
)
|
/
(
smaller
of
T
or
A
)
0.036
/
1.41
M
0.025 2.5
%
.
10
.
Select
percentage
differential
setting
(
slope
)
:
S
=
M
+
LTC
=
2.5
+
10
=
12.5
%
.
Use
25
%
setting
.
11
.
Select
the
instantaneous
setting
:
High
-
side
relay
current
at
self
-
cooled
rating
(
12000
/
(
115
*
y
/
3
)
*
5
/
100
=
3.01
A
30.1
/
3.2 9.4
(
x
tap
)
1 1 . 3
(
x
tap
)
.
(
3.01
*
10
)
/
(
high
side
tap
)
M u l t i p l y
by
1 . 2
for
margin
=
9.4
*
1.2
Calibrate
relay
to
g i v e
instantaneous
setting
11
x
tap
.
12
.
CT
performance
check
:
Determine
the
approximate
lead
lengths
and
w i r e
size
used for the
ct
secondary
connections
.
Determine
the
one
way
lead
resistance
,
and the burden of
any
other
devices
connected
.
For
t h i s
example
we assume
the
high
-
side
ct
’
s
(
100
:
5
)
are
ANSI
class
C
200
and
the
low
-
side
ct
’
s
(
1200
:
5
multiratio
)
are
C
400
.
I L F S
29.5
A
IH
F
s
35.5
A
Effective
C
rating
of low
side
ct
=
400
*
1000
/
1200
=
333
Maximum
allowable
burdens
are
:
RH
S
(
333
v
/
2
)
/
29.5
A
(
200
v
/
2
)
/
35.5
A
5.64
ohms
2.82
ohms
RL
S
(
example
continues
on
next
page
)
Courtesy of NationalSwitchgear.com