GE GCX51A - Page 13

GEI
-
98328
Assume
that
it
is
desired
to
use
GCX
51
B
relays
to
protect
the
two
-
terminal
,
5
mile
,
69
KV
transmission
line
shown
in
Figure
9
.
Maximum
load
current
=
450
amps
CT
Ratio
-
600
/
5
PT
Ratio
-
69
,
000
/
115
Zsec
=
Zprim
X
CT
Ratio
PI
Ratio
120
=
Zsec
=
5
(
0.14
+
j
0.80
)
Z
80
°
0.14
+
j
0.80
ohms
50
U
Zsec
=
0.813
Select
the
standard
-
reach
relay
.
From
a
system
fault
study
,
determine
the
minimum
secondary
fault
current
at
breaker
#
1
for
a
three
-
phase
fault
at
bus
B
.
Now
determine
the
minimum
secondary
fault
current
at
breaker
#
2
for
a
three
-
phase
fault
at
bus
A
.
Both
these
values
should
exceed
5
amperes
for
the
relay
selected
above
,
when
set
on
the
0.5
ohm
base
reach
tap
that
is
determined
below
.
The
percent
tap
setting
for
the
first
-
zone
reactance
unit
is
obtained
from
the
following
equation
:
(
Xmin
)
x
100
T
=
XL
where
:
T
=
#
1
tap
setting
in
percent
Xmin
=
Basic
reach
tap
setting
of
reactance
unit
X
[
_
=
Desired
reactance
reach
in
secondary
phase
-
to
-
neutral
ohms
If
we
assume
that
it
is
desired
to
set
the
first
zone
for
90
%
of
the
distance
to
the
remote
terminal
,
then
:
X
|
_
=
0.9
(
0.80
)
=
0.72
Secondary
ohms
For
this
ohmic
reach
use
a
0.5
ohm
basic
reach
tap
setting
.
Thus
,
Xmin
=
0.50
and
T
=
0
^
50
.
x
100
=
69
<
4
%
Since
the
69.4
%
calculated
from
the
equation
above
is
not
an
integral
number
,
use
the
next
highest
#
1
tap
,
which
is
70
%
.
To
achieve
this
setting
the
tap
lead
from
the
lower
No
.
1
position
would
be
connected
to
the
70
%
point
and
the
tap
lead
from
the
upper
No
.
1
position
would
be
connected
to
the
0
%
point
.
The
second
-
zone
must
be
set
to
reach
beyond
the
far
terminal
.
Assume
that
after
due
consideration
it
is
desired
to
set
the
second
-
zone
reactance
reach
for
1.5
secondary
ohms
.
13
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