GE GCX51A - Page 49

GEI
-
98328
The
family
of
curves
in
Figure
1
-
3
provide
the
information
to
determine
how
much
less
the
safe
ratio
of
ZMOAOU
can
be
»
Curves
are
provided
for
several
ratios
of
fault
current
IF
to
load
current
I
|
_
.
The
two
sets
of
curves
in
Figures
I
-
2
and
1
-
3
may
be
used
as
described
in
the
following
worked
example
.
Assume
a
transmission
line
having
a
secondary
impedance
of
0.813
ohms
at
an
angle
of
80
°
.
ohms
.
For
a
90
%
reach
,
the
first
-
zone
OHM
unit
(
Xou
)
would
be
approximately
0.72
Assume
further
that
Xs
as
determined
from
a
study
is
1.5
secondary
ohms
.
Thus
:
XS
/
XOU
=
1.5
/
0.72
=
2.1
Refer
to
Figure
1
-
2
and
select
the
curve
for
the
80
°
line
angle
.
Find
the
point
on
this
curve
corresponding
to
Xs
/
Xou
=
2.1
.
This
yields
a
value
of
about
9.3
for
XsA
0
U
»
and
since
%
0
U
=
0.72
,
the
maximum
setting
of
ZMO
=
9.3
(
0.72
)
=
6.7
ohms
.
This
is
on
the
basis
of
no
load
current
and
must
be
further
corrected
as
determined
by
the
ratio
of
fault
current
to
load
current
as
shown
in
Figure
1
-
3
.
If
the
ratio
of
the
fault
current
to
maximum
load
current
is
5
,
find
the
point
on
the
curve
for
this
ratio
corresponding
to
Xs
/
Xpu
=
2.1
.
This
yields
a
value
of
0.83
,
and
the
maximum
permissible
Mho
unit
setting
is
thus
0.83
(
6.7
)
or
5.57
ohms
.
Since
the
curves
in
Figures
1
-
2
and
1
-
3
do
not
include
margin
,
it
is
suggested
that
at
least
10
%
margin
be
taken
.
Therefore
,
the
maximum
reach
setting
of
the
Mho
unit
for
this
example
should
be
5.57
-
0.56
=
5.01
ohms
.
60
°
/
MAXIMUM
TORQUE
ANGLE
OF
MHO
UNIT
2
nd
ZONE
OHM
UNIT
1
st
ZONE
OHM
UNIT
J
~
~
90
°
-
-
P
Qi
,
-
PROTECTED
LINE
QT
-
OHM
UNIT
SETTING
-
1
st
ZONE
(
X
.
J
OS
-
SYSTEM
IMPEDANCE
|
XS
)
Figure
1
-
1
.
(
0178
A
8101
-
0
)
R
-
X
Diagram
Illustrating
Response
of
GCX
51
Relay
to
Phase
-
to
-
Phase
Fault
on
Adjacent
Phase
Pair
49
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