GE GCX51A - Page 48

GEI
-
98328
APPENDIX I
In
the
application
of
GCX
51
phase
relays
,
one
relay
is
used
to
provide
phase
fault
protection
for each
pair
of
phases
.
Thus
one
relay
is
used
for
phases
a
-
b
,
a
second
relay
for
phases
b
-
c
,
and
a
third
relay
for
phases
c
-
a
,
or a
total of three
relays
,
all of
which
will
operate
for
three
-
phase
faults
within
the
set
reach
.
The further
limitation
on
the
MHO
unit
reach
,
mentioned
in
the
APPLICATION
and
CALCULATION
OF
SETTINGS
sections
,
is
experienced
on
phase
-
to
-
phase
faults and
involves
one
of the
two
relays
associated
with
the unfaulted
phases
rather
than
the
relay
protecting
the
faulted
phases
.
For
example
,
if
we assume a
phase
b
-
c
fault
,
it
is
the
phase
a
-
b
relay
which
may
tend
to
overreach
.
Referring
to
the
R
-
X
diagram
of
Figure
1
-
1
,
OL
represents
the
protected
circuit with
the
relays
located
at
0
,
and
0
T
represents
the
reach of the
first
-
zone
OHM
unit
setting
.
For
a
phase
b
-
c
fault
at
the
first
-
zone
setting
(
T
)
,
the
phase
b
-
c
relay
will
properly
see an
impedance
0
T
.
However
,
the
phase
a
-
b
relay
will
see
an
impedance
originating
at
0
and
terminating
somewhere
along
the
line
TP
,
depending
on
system
conditions
.
If
this
impedance
happens
to
be
OPi
,
it
will
fall
within
the
first
zone
characteristic
of
the
phase
a
-
b
relay
.
This is
,
in
effect
,
overreaching
.
If
the
impedance
happens
to
be
0
P
2
,
it
will
fall
outside
the
MHO
characteristic
of
the
a
-
b
relay
and
there
will
be
no
tripping
of
that
relay
.
In
order
to
prevent
the
relay
associated with
the
unfaulted
phase
from
overreaching
as
described
above
,
it is
necessary
to
limit
the reach
setting
of
the
MHO
unit
.
In
order
to
do
this
,
it
is
necessary
to
know where
along
the
TP
line
the
impedance
seen
by
the
unfaulted
phase
relay
terminates
.
Referring
to
Figure
1
-
1
and
with
everything
plotted
in
terms
of
secondary
ohms
,
the
secondary
impedance
TP
is
equal
to
(
V
3
)
(
ST
)
,
where
ST
is
the
vector
sum
of
the
system
impedance
behind
the
relay
(
OS
)
plus
the
impedance
0
T
,
all
in
secondary
ohms
.
The
system
impedance
is
plotted
as a
pure
reactance
in
order
to
obtain
conservative
results
.
The
system
impedance
behind
the
relay
in
secondary
ohms
(
Xs
)
may
be
obtained
as
follows
:
Assume
a
three
-
phase
fault
at
the
relay
terminals
of
the
protected
line
and
determine
the
maximum
fault
current
I
'
30
supplied
through
the
relay
terminal
in
secondary
amperes
with
the
remote
breaker
open
.
XS
=
1
30
Once
TP
is
plotted
on
the
R
-
X
diagram
,
a
MHO
circle
may
be
drawn
so
that the
impedance
,
OP
,
seen
by
the
unfaulted
phase
relays
falls
outside
the
circle
.
In
order
to
simplify
the
investigation
,
the
curves
of
Figure
1
-
2
and
1
-
3
have been
computed
.
The
family
of
curves
for
various line
angles
in
Figure
1
-
2
provides
the
means
of
determining
the
permissible
Mho
unit
setting
(
ZMO
)
as
a
multiple
of
the
zone
1
reactance
unit
setting
(
Xou
)
in
terms
of the
ratio
of
Xs
/
Xou
.
where
Xs
is
the
system
impedance
behind
the
relay
location
.
The
curves
in
Figure
1
-
2
are on
the
basis
of
no
load
flow
in
the
protected
line
.
Load
flow
into
the
protected
line
at
the
relay
location
tends
to
aggravate
the
situation
represented
by
the
plot
in
Figure
1
-
2
.
In
other words
,
with
load
present
the
permissible
ratio
of
ZMO
/
XOU
will
be
less
than
the
curves
in
Figure
1
-
2
indicate
.
Then
,
48
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