ABB CIRCUIT SHIELD 87T - Page 4

Transformer
Differential
Relay
IB
7.6
.
1.7
-
4
Page
4
MINIMUM
RESTRAINT
(
Minimum
Operating
Current
)
:
The
minimum
operating
current
is
set
by
the
right
-
hand
dial
on
the
front
panel
,
which
is
continuously
adjustable
.
The
minimum
operating
current
value
is
determined
by
the
following
relationship
between
the
restraint
dial
setting
,
the
percent
differential
setting
,
and
the
current
tap
setting
:
(
D
/
(
100
+
D
)
)
x
R
x
T
Im
D
=
%
differential
setting
R
=
adj
min
restraint
setting
T
=
tap
setting
Im
=
minimum
operating
current
in
amperes
where
For
example
,
with
D
=
25
%
,
R
=
2.3
and
T
=
5
A
:
(
25
/
125
)
x
2.3
x
5
46
%
X
5
A
2.3
A
Im
effect
of
varying
the
Minimum
Restraint
adjustment
is
graphically
illustrated
in
The
Figure
7
and
tabulated
in
this
chart
:
Restraint
Dial
Setting
Minimum
Operate
Current
Percent
Differential
Setting
30
%
of
tap
1
.
05
40
%
1
.
5
43
%
65
%
2.3
2 1
%
25
%
1
.
05
30
%
1
.
5
45
%
2.3
1
.
05
1 4
%
15
%
1
.
5
20
%
30
%
2.3
The
setting
that
provides
a
30
-
40
%
minimum
operating
current
would
be
typical
for
most
applications
.
INSTANTANEOUS
:
The
instantaneous
element
operates
directly
on
magnitude
of
the
measured
current
difference
.
This
internal
adjustment
is
factory
set
at
10
x
tap
,
but
may
be
adjusted
from
8
to
20
x
as
may
be
required
by
the
application
.
This
element
must
be
set
by
test
.
Refer
to
the
section
on
Testing
for
the
calibration
procedure
.
HARMONIC
RESTRAINT
:
The
harmonic
restraint
unit
blocks
relay
operation
on
transformer
energization
by
detection
of
the
second
harmonic
content
of
the
inrush
current
.
This
element
is
factory
set
to
restrain
on
15
%
second
harmonic
,
which
is
appropriate
for
most
applications
.
An
internal
calibration
adjustment
provides
a
range
of
approximately
10
-
20
%
,
and
may
be
set
by
test
.
Refer
to
the
section
on
Testing
for
the
calibration
procedure
.
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