ABB CIRCUIT SHIELD 87T - Page 20

Transformer
Differential
Relay
IB
7.6
.
1.7
-
4
Page
20
C
.
Harmonic
Restraint
Tests
(
continued
from
page
16
)
2
.
Voltage
-
source
testing
:
A
typical
test
circuit
is
shown
in
Figure
13
a
.
In
this
method
,
the
built
-
in
diode
between
terminals
15
and
16
is
used
to
generate
2
nd
harmonic
content
in
the
current
waveform
,
this
method
.
)
(
Do
not
use
an
electronic
current
source
test
set
with
Set
the
%
Diff
dial
to
15
%
and
the
MinRestraint
dial
to
1.0
.
(
if
these
settings
have
already
been
calibrated
,
mark
the
positions
so
you
can
return
to
the
calibrated
positions
at
the
end
of
this
test
.
)
a
.
Place
the
tap
pins
in
the
2.9
A
tap
.
b
.
Apply
rated
dc
control
voltage
to
the
relay
.
c
.
Set
the
Branch
B
current
at
zero
.
With
the
switch
closed
across
the
diode
,
increase
the
current
in
Branch
A
to
1.0
ampere
.
The
87
T
output
contact
should
close
since
the
input
current
is
all
fundamental
.
d
.
Open
the
contact
across
the
diode
,
thus
causing
significant
2
nd
harmonic
content
in
the
current
waveform
.
The
apparent
current
on
the
Branch
A
ammeter
will
drop
,
but
that
is
proper
.
The
87
T
relay
should
be
restrained
(
output
contact
open
)
,
target
-
reset
button
to
reset
the
target
.
e
.
Do
not
readjust
the
Branch
A
source
.
Actuate
the
increasing
the
fundamental
content
.
f
.
Slowly
raise
the
current
in
Branch
B
The
87
T
will
return
to
the
trip
condition
(
output
contact
closed
,
target
displayed
)
when
the
percentage
of
2
nd
harmonic
falls
below
the
set
value
.
This
value
may
be
read
directly
from
the
curve
in
Figure
13
b
.
Branch
A
current
was
originally
set
to
1.0
A
,
the
vertical
scale
may
be
read
as
the
current
in
Branch
B
.
Since
the
For
the
factory
calibration
of
15
%
nominal
,
the
Branch
B
current
should
be
between
0.8
and
1.0
ampere
for
acceptance
testing
.
The
Branch
B
current
may
be
raised
and
lowered
to
determine
the
"
restrain
"
and
"
unrestrained
"
values
.
Example
:
Assume
the
relay
restrained
with
a
Branch
B
current
of
0.92
amps
.
Reading
the
curve
at
0.92
gives
15
%
2
nd
harmonic
on
the
horizontal
scale
.
internal
potentiometer
R
217
may
be
used
to
adjust
the
value
of
2
nd
harmonic
that
causes
restraint
,
board
layouts
on
page
19
for
the
location
of
R
217
.
Adjustment
:
Refer
to
the
circuit
Note
:
a
change
in
the
restraint
point
may
occur
if
the
polarity
of
the
current
in
winding
1
-
2
is
reversed
.
This
is
due
to
internal
amplifier
offset
.
The
operating
point
should
be
within
the
limits
of
0.8
-
1.0
A
as
given
above
.
D
.
Additional
Tests
for
3
Winding
Relays
:
When
a
3
winding
relay
is
being
tested
,
the
Differential
Characteristic
Test
should
be
repeated
so
that
the
third
input
is
also
tested
.
Referring
to
either
Figure
8
or
Figure
9
as
appropriate
,
the
wire
on
terminal
5
should
be
moved
to
terminal
3
,
and
the
wire
on
terminal
6
moved
to
terminal
4
.
E
.
Note
on
Target
Operation
:
The
relay
’
s
target
may
drop
on
loss
of
dc
control
power
,
even
though
the
relay
has
not
closed
its
output
contact
.
Upon
reapplication
of
control
power
,
the
target
should
be
reset
if
required
.
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