GE GCX51A - Page 22

GE
1
-
98328
percent tap
setting
for
pickup
is determined
.
This
difference
results
from
the
fact
that
,
unlike
the
OHM
unit
,
the
impedance
pickup
characteristic
of
the
MHO
unit
on an
R
-
X
diagram
is
a
circle
passing through
the
origin
.
The diameter
of
this circle
is
the reach
of
the
unit
at
its
angle
of
maximum
torque
,
which is
normally
60
°
with
respect
to
the
R
axis
.
Since
the
reactance
of
the
test reactor
may
be
very
accurately
determined
from
its calibration
curve
,
it
is
desirable
to
check
relay
pickup
with
the
fault
reactor
alone
,
due
account
being
taken
of
the
angular
difference
between
the
line
reactance
,
X
[
_
,
and
relay
angle
of
maximum
reach
.
As
in
the
case
of
the
OHM
unit
,
the line
reactance
selected
,
X
(
_
,
should be
the
test
reactor
tap
nearest
above
twice the
MHO
unit reach
,
with
account
being
taken
of
the
difference
in
angle
of
the
test
reactor
tap
impedance
and the
relay
angle
of
maximum
reach
.
From
Figure
13
it
is
seen
that
twice the
relay
reach
at
the
angle
of
the
test
reactor
impedance
is
:
2
ZRelay
=
2
E
^
m
|
n
cos
(
p
-
e
)
where
:
0
=
angle
of
test
reactor
impedance
9
=
MHO
unit
angle
of
maximum reach
The
test
-
box
autotransformer
percent
tap
for
MHO
unit
pickup
is
then
given
by
the
equation
:
2
ZRelay
(
100
)
%
Tap
=
ZL
As
an
illustration
of
the above
,
the
example
in
the
section
on
CALCULATION OF
RELAY
SETTINGS
will
again
be
used
.
In
this
example
the
E
2
tap
setting
was
calculated
to
be
86
%
,
and
since
a
standard reach
relay
was
used the basic
minimum
reach
of
the
MHO
unit
is
2.5
ohms
at
the
60
°
angle
of
maximum
torque
.
In
determining
the
test reactor
tap
setting
to
use
,
it
can
be
assumed
as
an
approximation
that the
angle
0
of
the
test reactor
impedance
is
80
°
.
Based
on
this
assumption
,
twice
the
relay
reach
at
the
angle
of
the
test
reactor
will
be
:
2.5
x
100
2
ZRelay
=
2
cos
(
80
-
60
)
=
5.6
ohms
86
Twice
the
relay
Therefore
the
6
ohm
tap
of
the
test reactor
should
be
used
,
reach
at
the
angle
of
test reactor
impedance
should
be
recalculated
,
using
the
actual
angle
of
the
reactor
tap
impedance
rather than the
assumed
80
°
.
XIII
shows
the
angles
for
reach
of
the
reactor
taps
.
Table
TABLE XIII
COS
4
>
-
60
TAP
ANGLE
m
0.883
0.891
0.899
0.906
0.921
0.934
0.951
24
12
87
86
6
3
85
83
2
81
1
78
0.5
22
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