GE GCX51A - Other Checks and Tests

GE
1
-
98328
First
,
when
checking
the
MHO
unit
at
angles
of
more
than
30
°
off
the
maximum
reach
position
,
the
error
becomes
relatively
large
with
phase
-
angle
error
.
This
is
apparent
from
Figure
13
where
it
is
seen
,
for
example
,
at
the
zero
-
degree
(
0
°
)
position
that
a
two
or
three
degree
(
2
°
or
3
°
)
error
in
phase
angle
will
cause
a
considerable
apparent
error
in
reach
.
Secondly
,
the
effect of
the
control
spring
should
be
considered
,
since
the
MHO
unit
can
only
have
a
perfectly
circular
characteristic
when
the control
spring
torque
is
negligible
.
For
any
normal level
of
polarizing voltage
,
the control
spring
may
be
neglected
,
but
in
testing
the
unit
as
indicated above
it
may
be
necessary
to
reduce
the
test
-
box
autotransformer
tap
setting
to
a
point
where
the
voltage
supplied
to
the
unit
may
be
relatively
low
.
This
reduces
the
torque
level
,
since
the
polarizing
as
well
as
the
restraint
will
be
low
,
with
the result
being
that the
control
-
spring
torque
will
no
longer
be
negligible
.
The
result
of
the
control
spring
at
low
polarizing voltages
is
to
cause
the
reach
of
the
MHO
unit
to
be somewhat reduced
.
In
order
to
see
the
effect of
the
errors
mentioned above
in
their
true
proportion
,
it
is
suggested
that
the
reach characteristic
,
determined
by
test
,
be
plotted
on
an
R
-
X
impedance
diagram
as
typified
by
Figure
5
.
It
is obvious
that the
apparent
errors
in
reach
,
resulting
from
phase
-
angle
error
at
angles
well
off
the
maximum
reach
position
,
occur
in
a
region
where
the
fault
impedance
vector
will
not
lie
.
As
for
the
error
introduced
by
spring
torque
,
it
should
be
noted
that the
MHO
unit
is
not
the
measuring
unit
for
the
primary
protection
but
rather
is
only
a
directional
unit
.
Therefore
,
its
directional
response
is
the
most
important
consideration
.
For
third
-
zone
backup
protection
,
the
MHO
unit
is
the
measuring
unit
,
but
for
remote
faults
the
voltage
at
the
relay
is
not
apt
to
be
low
,
and
furthermore
the
accuracy
of
a
third
-
zone
backup
unit is
not
as
important
as
that
of
a
first
-
zone
unit
.
In
addition
to
the above
tests
on
the
MHO
unit
,
it
may
also be
checked
for
directional action
with
the
test
-
box
circuit
as
shown
in
Figure
12
.
The
fault
resistor
,
R
,
may
be
zero
,
the
test
reactor
should
be
set
on
the
0.5
ohm
tap
.
With
the
test
-
box
switch closed
,
the
MHO
unit
contact
should
remain
closed
for
a
current
range
of
6
-
60
amperes
with
2
%
rated
voltage
applied
.
A
voltmeter
should be
used
to
read the
voltage
at
studs
17
-
18
of
the
relay
.
The
voltage
over
the
6
-
60
ampere
-
current
range
is
adjustable
by
means
of
the
test
-
box
autotransformer
tap
switches
.
When
the
connections
are
changed
to
the
reverse
position
,
the
MHO
unit
contacts
should
remain
open
for
the
same
conditions
as
described
above
.
c
.
Other
Checks
and Tests
In
addition
to
the
calibration
checks
on
the
OHM
and
MHO
units
as
described
above
,
it
is
desirable
to
make the
following
general
tests
:
Check
that
the
voltage
-
selection
link
for
the
OX
transfer
unit is
in
the
correct
position
for
the
DC
voltage
to
be
used
(
48
,
125
,
or
250
V
)
.
Check
that
the
OX
unit is
picking
up
at
80
%
of
nominal rated
voltage
,
as
determined
by
the
link
position
,
by
applying
a
variable
DC
voltage
between
studs
12
and
13
.
1
.
This
can
be
Check
that
the
target
seal
-
in
unit is
operating
at
tap
value
,
checked
by
means
of
the
connections
in
Figure
12
if
the
zone
-
1
signal
circuit
is loaded
with
the
proper
by
-
pass
resistor
to
draw
a
current
of
approximately
tap
value
and
the
OHM
and
MHO
contacts
(
and
OC
unit
if
present
)
are
closed
by
hand
.
Be
sure
the
seal
-
in
unit
tap
screw
is
left
in
the
required position
(
0.6
or
2.0
amps
)
for
the
application
.
2
.
If
it
is
24
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