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CARRIER RELAYING
positive
by
the
operating
result, plate current
G
tube, as shown
at
in Figure 2.
cycle plate-current pulses
primary winding
of the
secondary voltage
(Figure 2, H) is
to provide a steady relay operating force, and
the direct voltage
so obtained causes current
to flow to operate the relay R (Figure 2,
During
an
external
detectors pick up, carrier is transmitted from
the two ends
of the
line
cycles
as
shown at
C'
This change occurs because the
end of the line
has reversed with
the current at the opposite end.
rent entering the line is in phase with
current leaving it, the
each receiver
will
provide
continuous restraining voltage (Figure 2,
This voltage in series with the operating vol
tage (2, B') gives a resultant value which
always more negative than the
the relay tube (Figure 2, F') .
plate current
flows
(Figure 2,
relay does not operate.
The phase position of the line current theo
retically
may
fall
anywhere between the in
phase position and the 180 degree out-of-phase
position.
Figure
shows the characteristics
3
of
the
relay over this entire range.
phase position changes from in-phase (external
fault)
to
out-of-phase (internal fault) , the
blocks
of
the
distant
with respect to the blocks of
phase
position
local carrier, the cwo extremes being shown in
Figure
2, C
and D,
Figure 2, C' and D'. Be
tween these t' d O extremes
the
relay tube will
be blocked over more and
more
of
the
cycle, and the rms value of the
A-C component
of the
crease.
This
will
result in a decrease of the cur
rent in relay R as shown
angle relay-current relationship Figure
adjusting
the sensitivity, the relay R can be
set
to
operate
when
currents
at
the two ends of the line becomes
any
desired
value,
such
voltage.
As
a
will flow in the
relay
The
half
flow
through
the
transformer
T.
The
rectified
I) .
fault,
when the fault
on alternate half
and D'
in Figure 2.
current at one
respect to
If the cur-
the
rectified carrier
at
a
substantially
E' ) .
is
cutoff bias
of
As a result no
G')
and
the
As the
carrier
change
in
the plate current of
plate current will
de-
on the typical phase
By
3.
the
angle between the
35 degrees, as
as
shown.
The preceding explanation is based on single
frequency operation
of the carrier equipment.
That
is,
the
transmitters
both
ends
of
the line are tuned to the same
carrier frequency.
It should
ever, from a comparison
that carrier
is
transmitted
the
half-cycles
when
voltage present.
Consequently,
necessary to receive the local carrier signal,
since the relay-tube plate current always will
be zero during these half-cycles.
that
two-frequency
operation of the
transmitters and receivers is possible without
complication,
if
it
auxiliary
function
which requires a two-fre
quency
channel.
In
two-frequency operation,
the transmitter at one end of the line and the
receiver
at the opposite end are tun�d to one
carrier
frequency, and the other
and receiver
are
tuned to a
quency.
This
permits
mission
and
reception in both directions
would be needed for telemetering in one direc
tion
and
load control in the opposite direc
tion.
This relaying
system will operate correctly
for faults with an uneven distribution of cur
rent
from opposite
line terminals.
an
internal fault
which picks up both
detectors
(FDl and FD2)
(#1)
but only
FDl at the
(#2).
Tripping cannot occur at station #2 be
cause FD2 is not
picked up.
up of FDl will transmit the half-cycle
of carrier of the proper
will
still
allow
high-speed
station #1.
After the breaker at station #1 has tripped,
redistribution
of the
up FD2
at station #2
trip
in
a
high-speed
operation.
It
is
desirable
to check periodically the
condition
of the carrier set to determine its
ability to send
and receive a carrier signal.
and receivers at
be noted,
how
of Figure 2, B and C,
locally only on
there
is no operating
it
is never
This means
carrier
is
desired to use some
transmitter
different
fre-
simultaneous
trans-
as
Consider
fault
at one line terminal
opposite
terminal
However, picking
blocks
phase position which
tripping
at
fault current may pick
case it
in which
will
sequential
tripping
5
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