ABB CIRCUIT SHIELD 87T - Page 5

Transformer
Differential
Relay
IB
7.6
.
1.7
-
4
Page
5
APPLICATION
DATA
The
ABB
Circuit
-
Shield
Type
87
T
provides
high
-
speed
differential
protection
for
power
transformers
.
The
Type
87
T
is
a
single
-
phase
relay
.
A
set
of
three
relays
is
required
for
the
pro
-
tection
of
one
power
transformer
.
The
relay
is
offered
in
models
for
the
protection
of
two
winding
and
three
winding
transformers
.
The
relay
has
a
linear
percentage
differential
characteristic
in
order
to
accomodate
ct
ratio
mismatch
,
the
operation
of
on
-
load
tap
changers
,
and
ct
errors
at
high
fault
currents
external
to
the
protected
zone
.
Controls
are
provided
on
the
front
panel
to
adjust
the
slope
(
15
-
40
%
)
,
and
the
minimum
operate
current
.
A
second
-
harmonic
restraint
unit
prevents
relay
operation
on
transformer
magnetizing
inrush
current
.
content
,
but
may
be
adjusted
if
required
.
This
unit
is
factory
calibrated
to
restrain
on
15
%
second
harmonic
An
unrestrained
instantaneous
unit
which
operates
on
the
magnitude
of
the
difference
current
is
provided
to
back
up
the
percentage
differential
unit
.
The
instantaneous
unit
is
factory
set
at
10
x
tap
,
but
may
be
adjusted
(
8
-
20
x
)
in
the
field
.
The
relay
has
ratio
correcting
taps
,
therefore
,
external
auxiliary
ct
’
s
will
not
The
high
continuous
rating
of
each
tap
gives
the
user
more
The
low
burden
of
the
relay
(
see
table
3
)
provides
usually
be
required
,
flexibility
in
selecting
taps
,
improved
ct
performance
.
A
procedure
for
calculating
the
relay
settings
for
a
particular
application
follows
on
page
10
.
MOTE
t
DIMENSIONS
ARE
-
^
5
^
-
MM
8.36
213
6.56
6.66
1.19
175
167
30.2
t
PANEL
r
tt
16
o
4.68
3
124
£
J
2.44
62
L
?
o
FRONT
VIEW
(
4
)
^
|
-
0
IA
HOLES
D
.
O
SIDE
VIEW
*
T
•
n
7
n
•
*
n
4
n
3
n
2
o
>
0
0
[
0
i
3.69
0
0
0
0
|
PANEL
CUTOUT
94
0
6
T
T
0
0
0
0
0
0
0
0
1.64
£
IS
13
ii
Uio
i
«
H
12
“
47
~
iias
.
9
*
52.4
i
DRAMfOUT
TEST
CASE
STUD
NUMBERS
(
BACK
VIEW
)
3.19
81.0
6.62
168
Figure
1
:
Relay
Outline
and
Panel
Drilling
Courtesy of NationalSwitchgear.com