ABB CIRCUIT SHIELD 87T - Page 3

Transformer
Differential
Relay
IB
7.6
.
1.7
-
4
Page
3
Connections
:
Typical
connection
diagrams
for
two
and
three
Internal
connections
in
Figure
2
.
winding
transformers
are
shown
in
Figures
3
and
4
.
Special
care
should be taken
to
connect
the dc control
power
in
the
proper
polarity
.
These
relays
have
a
metal front
panel
which is
connected
through
printed
circuit
board
runs
to
a
rear
terminal
marked
"
G
"
.
In
all
applications
this
terminal
should
be
wired
to
ground
.
The
87
T
relay
includes
an
external
resistor which is
supplied
mounted
to
the
case
and
wired
between
terminals
9
and
operation
.
This resistor
must
be connected
for
proper
10
.
Note
:
Connections
are
critical
in
this
application
.
Most
problems
experienced
during
commissioning
are
due
to
incorrect connections
:
interchanged
phases
,
or
ct
polari
-
ties
,
or
incorrect
make
-
up
of
the delta
ct
connections
.
Using
the
transformer
nameplate
drawing
as
the
reference
,
a
phasing
-
check should be made
to
be
sure
that
the
ct
secondary
connections
are
made
up
properly properly
for the
particular
transformer
winding configuration
.
In
case
of
difficulty during
commissioning
,
a
plug
-
in
test
unit is
available
on
loan
from
the
factory
as
an
aid
in
analyzing
connection
problems
;
however
,
significant
load
current
must
be
flowing
to
use
this
device
.
3
.
SETTINGS
Procedures
for
calculating
settings
for
a
particular
installation
are
given
in
the
Application
section
.
CURRENT TAPS
:
The
ct
secondary
currents
seen
by
the
relay
are
usually unequal
due
to
the
primary
to
secondary
ct
’
s
on
the
primary
and
secondary
sides
of
the transformer
.
ratio
of
the
transformer
being
protected
and the
choice
of
ratios
for
the
The
tap
settings
are
selected
to
equalize
the
input
currents
seen
by
the
differ
-
ential
measurement
unit
of
the
relay
.
The
tap
values and
ratios
provided
are
shown
in
Table
1
.
Setting
calculations
will
determine
the
tap
selection
required
for the
high
-
side
and
low
-
side
ct
inputs
.
It
is
important
to
identify
which
input
terminals
of
the
relay
are
connected
to
the
high
-
side
ct
’
s
and
to
the
low
side
ct
’
s
.
these
Then
when
making
settings
,
to
relate
the
front
panel
tap
to
the
input
current
terminals it
affects
:
the left
side
tap
for
input
terminals
1
-
2
,
the
right
side
tap
for
terminals
5
-
6
,
and
the
center
tap
(
on
3
winding
units
)
for
input
terminals
3
-
4
.
%
DIFFERENTIAL
:
The
ABB
Circuit
-
Shield
Type
87
T
relay
operates
when
its
input
currents
differ
in
magnitude
(
vectorial
1
y
)
.
The
operating
point
occurs
when the
difference
,
expressed
as
a
percentage
of the smaller
,
exceeds the
setting
.
The
percentage
differential
or
"
slope
"
setting
is
made
with
the
left
-
hand
front
panel
dial
.
illustrated
in
the
graphs
of
Figures
6
and
7
.
the
operate
current
relative
to
the
tap
value
,
This
continuously adjustable
control allows
settings
from
15
to
40
%
,
and
is
In
these
plots
the
vertical
axis
is
Mo
=
(
Ii
/
T
1
)
-
(
I
2
/
T
2
)
and
the
horizontal
axis is
the
smaller
restraint
current
relative
to
tap
value
MR
=
I
2
/
T
2
The
Percent
Differential is
D
=
MO
/
MR
where
i
nto
and
i
nto
11
is
the
1
arger
restraint
current
T
1
the
tap
value
,
12
is
the smaller
restraint
current
T
2
the
tap
value
.
Note
:
MR
=
R
when
I
2
/
T
2
<
R
where
R
(
adjustable
)
Minimum
Restraint
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