GE GCX51A - MHO Unit; OHM Unit

6
E
1
-
98328
The
primary
purpose
of the
MHO
unit
in
the
type
-
GCX
51
relays
is
to
provide
the
directional
discrimination
that
is
necessary
since
the
OHM
unit
is
inherently
nondirectional
.
The
MHO
unit
directional
characteristic is
such that
it
will
operate
correctly
for
either
forward
or reverse
faults
at
voltages
down
to
1
%
of
rated
voltage
over a
current
range
of
5
to
60
amperes
.
A
secondary
purpose
of
the
MHO
unit is
to
measure
fault
impedance
for
the
third
zone
of
protection
.
At
reduced
voltage
,
the
ohmic
value
at
which
the
MHO
unit
will
operate
may
be
somewhat
lower
than
its
calculated
value
.
This
"
pullback
"
or
reduction
in
reach
is
shown
in
Figure
6
for
the
1.0
ohm
MHO
unit
used
in
the
short
-
reach
relay
,
or
in
Figure
7
for
the
2.5
ohm
MHO
unit
used
in
the
standard
-
reach
relay
.
The
percentage
of
relay
reach
for
a
constant
tap
setting
is
expressed
as
a
function
of the three
-
phase
fault
current
,
I
30
,
for
various ohmic
reach
settings
.
The
MHO
unit
will
operate
for all
points
to
the
right
of
the
curves
.
The
static
curves
of
Figures
6
and
7
were
determined
by
tests
performed
with
no
voltage
supplied
to
the
relay
before the
fault
was
applied
.
The
dynamic
curves were
obtained
with
full
rated
voltage
of
120
volts
supplied
to
the
relay
before
the fault
was
applied
.
These
dynamic
curves
illustrate
the
effect
of
the
MHO
unit
memory
action
,
which
maintains
the
polarizing
voltage
on
the
unit
for
a
few
cycles
after
the
inception
of the
fault
.
This
memory
action
is
particularly
effective
at
low
voltage
levels
where
it
enables
the
MHO
unit
to
operate
for
low
fault
currents
.
This
can
be
most
forcefully
illustrated
for
a zero
-
voltage
fault
by
referring
to
Figure
7
.
A
zero
-
voltage
fault
must
be
right
at
the
relay
bus
and
therefore
,
to
protect
for
this
fault
,
it is
imperative
that
the
relay
reach
zero
percent
(
0
%
)
of
its
setting
.
Figure
7
shows
that the
MHO
unit
,
under
static
conditions
,
will
not
see
a
fault
at
zero
percent
(
0
%
)
of the
relay
setting
regardless
of
the
tap
setting
.
However
,
under
dynamic
conditions
when the
memory
action is
effective
,
Figure
7
shows that
a
2.5
-
ohm
MHO
unit
with
a
100
%
tap
setting
will
pick
up
if
I
30
is
greater
than
2.5
A
.
This is
of
course
a
marginal
condition
.
The
minimum
fault
currents
considered
safe
are
listed
in
Table
I
in
the
APPLICATION
section
.
OHM
Unit
The
OHM
unit
impedance
characteristic
when
represented
on
the
R
-
X
diagram
(
Figure
5
)
is
a
straight
line
parallel
with
the
R
axis
.
The
unit
will
operate
for
fault
impedances
lying
below
its
characteristic
,
and
hence
is
non
-
directional
.
During
normal
conditions
when
load
is
being
transmitted
over
the
protected
line
,
the
voltage
and
current
supplied
to
the
unit
present
an
impedance
that
lies
close
to
the
R
axis since
load
will
be
very
near
unity
power
factor
in
contrast
with
the
reactive
KVA
that
flows
during
fault
conditions
.
An
impedance
near
the
R
axis
will
lie
below the
OHM
unit
characteristic
(
see
Figure
5
)
and hence
the
OHM
unit
contact
will
be
closed
.
This
will
cause
no
trouble
,
however
,
since
the
directional
MHO
unit
contact
will
not
be
closed
for
this
condition
(
see
Figure
3
)
.
The
basic
minimum
reach
of
the
OHM
unit
as
listed
in
Table
II
under
RATINGS
is
obtained
when
the
restraint
tap
leads
are on
100
%
.
The
ohmic
reach
can
be extended
by
setting
the
restraint
tap
leads
on
a
lower
percentage
position
on
the
tap
block
.
The
setting
of the
two
tap
leads marked
No
.
1
determines
the
reach
of
the
instantaneous
or
first
zone
,
and
the
setting
of
the
two
tap
leads marked
No
.
2
determines
the reach of the
intermediate
or
second
zone
.
8
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