GE GCX51A - OHM Unit Transfer Auxiliary

GEI
-
98328
TABLE
V
Overall
Time
Curves
for
Relay
Basic
Min
.
Reach
(
MHO
Unit
)
Volts
Prior
to
Fault
Figure
#
Relay
ITU
T
9
OTT
20
0.1
0
120
21
Short
Reach
0.2
0
22
0.2
120
23
0.4
0
24
0.4
75
5775
0.25
170
0
26
120
27
0.5
Standard
Reach
28
0
0.5
120
1.0
29
30
1.0
0
Vernier
Adjustment
for
Low
Tap
Settings
The
input
leads
are
normally
set
at
100
%
but
with
a
high
secondary
-
1
ine
reactance
,
where
the
No
.
1
tap
leads
would
be
set
at
a
low
percentage
,
the
input
connections
may
be varied
by
a
vernier
method
to
obtain
a
closer
setting
,
example
,
if
the
desired
first
zone
reach
is
1.2
ohms and
the basic
minimum
r
-
ach
of
the
OHM
unit is
1.0
ohm
,
with
the
input
on
100
%
the
output
tap
setting
would
be
100
/
1.2
or
83.3
%
,
which
can
be
set
within
0.4
%
.
However
,
if
the
desired
first
zone
reach
were
9.5
ohms
,
the
output
setting
would
be
100
/
9.5
,
or
10.55
%
.
The
nearest
output
tap
would
be
11
%
,
which
is
4
%
off
the
desired
value
.
To
correct
this
,
the
input
leads
can
be
shifted
to
95
%
,
in
which
case
the
output
setting
would
be
95
/
9.5
,
or
10
%
,
which
of
course
can
be
set
exactly
.
For
OHM
Unit
Transfer
Auxiliary
The
OHM
unit
transfer
auxiliary
,
OX
,
is
a
telephone
-
type
relay
whose
coil
and
contacts
are
shown
on
the
right
in
the
internal
-
connection
diagram
of
Figure
8
.
The
unit is mounted
at
the
top
of
the
relay
and
is
used
to
change
the
setting
of
the
OHM
unit
to
provide
a
second
step
of
transmission
-
line
protection
,
controlled
by
the
Type
RPM
or
SAM
timing
relay
as
shown
by
the
external connections
diagram
of
Figure
3
.
The
normally
-
closed
contacts
of
the
transfer
auxiliary
provide
the
circuit
for
instantaneous
tripping
used
for
faults
in
the
first
step
of
line
protection
.
auxiliary
changes
the
setting
of
the
OHM
unit
by
switching
to
the
No
.
2
taps
on
the
autotransformer
,
from
which
a
smaller
potential
is
supplied
to
the
unit
potential
restraint
windings
.
This
extends
the
ohmic
reach
of
the
OHM
unit
and enables
it
to
operate
for
faults
in
the
second
zone
of
transmission
-
line
protection
.
Its
operation
is
If
the
fault is
beyond
the
first
zone
of
protection
,
the
transfer
10
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