GE GCX51A - Page 23

GEI
-
98328
From
the
table
it
is
seen
that
the
angle
of
the
impedance
of
the
6
ohm
tap
is
86
°
.
Therefore
:
zzRelay
=
2
2
,
5
pg
OD
cos
(
86
-
0
)
=
5.22
ohms
The
calibration
curve
for
the
portable
test
reactor
should
again
be
referred
to
in
order
to
determine
exact
reactance
of
the
6
ohm
tap
at
the
current
level
being
used
.
For
the
purpose
of
this
illustration
,
assume
that
the
reactance
is
6.2
ohms
.
impedance
of
this
tap
may
be
calculated
as
follows
:
ZL
=
xL
/
cos
4
=
Since
the
angle
of
the
impedance
of
the
6
ohm
tap
is
86
°
,
the
6.2
=
6.23
ohms
997
b
*
From
this
calculation
it
is
seen
that
the
reactance
and
the
impedance
may
be
assumed
the
same
for
this
particular
reactor
tap
.
Actually
,
the
difference
need
only
be
taken
into
account
on
the
reactor
3
,
2
,
1
and
0.5
ohm
taps
.
The
test
-
box
autotransformer
tap
setting
required
to
close
the
MHO
unit
contact
with
the
fault
switch
closed
can
now
be
determined
as
follows
:
5.22
=
84.3
%
%
Tap
=
6.2
If
the
ohmic
pickup
of
the
MHO
unit
checks
correctly
according
to
the
above
,
the
chances
are
that
the
angle
of
the
characteristic
is
correct
.
The
angle
may
,
however
,
be
very
easily
checked
by
using
the
calibrated
test
resistor
in
combination
with
various
reactor
taps
.
The
calibrated
test
resistor
taps
are
pre
-
set
in
such
a
manner
that
when
used
with
12
and
6
ohm
taps
of
the
specified
test
reactor
,
impedance
at
60
°
and
30
°
respectively
will
be
available
for
checking
the
MHO
-
unit
reach
at
the
60
°
and
30
°
positions
.
The
MHO
-
unit
ohmic
reach
at
the
zero
-
degree
(
0
°
)
position
may
be
checked
by
using
the
calibrated
test
resistor
alone
at
the
line
impedance
.
The
calibrated
test
resistor
is
supplied
with
a
data
sheet
,
which
gives
the
exact
impedance
and
angle
for
each
of
the
combinations
available
.
The
test
-
box
autotransformer
percent
tap
for
pickup
at
a
particular
angle
is
given
by
:
2
(
2.5
)
cos
(
60
-
a
)
(
E
2
Tap
X
)
Z
|
_
(
100
)
%
Tap
=
where
:
a
=
angle
of
test
impedance
Z
|
_
Z
|
_
=
the
60
°
,
30
°
,
or
0
°
impedance
value
taken
from
the
calibrated
resistor
data
sheet
=
MHO
unit
restraint
tap
setting
As
in
the
previous
tests
the
source
impedance
should
be
readjusted
in
each
instance
to
maintain
approximately
the
same
fault
-
current
level
for
each
check
point
.
When
checking
the
angle
of
maximum
reach
of
the
MHO
unit
as
indicated
above
,
there
are
two
factors
to
keep
in
mind
that
affect
the
accuracy
of
the
results
.
23
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