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CARRIER RELAYING
which
must
be investigated.
has a minimu� operating value in excess of the
minimum required to operate the receiver relay
so that an indication of impending trouble can
be obtained before actual failure occurs.
A
three-pole
single-throw
from
a
common
handle
carrier
trip
and
out-of-step
shown in Figure 1.
The
"Carrier
On-off"
and opening
carrier supervision and permits the type HZ or
HZM relays to operate in the conventional step
-zone manner.
The
switch
HRK
types
or HRP
ground relay and ground pro
tection
is
available
relays.
Out-of-Step-Protection
It
is
often desirable to prevent the oper
ation
of relays during out-of-step conditions
so
that
the system can be separated at loca
tions
where synchronizing equipment is avail
able.
The
carrier relaying system provides a
means
of
preventing
step conditions
without
ty to trip for internal faults occuring during
out-of-step
conditions.
difference
between a three phase fault and an
out-of-step condition is that a fault suddenly
reduces
the voltage and increases the current
whereas
during the approach of an out-of-step
condition, the voltage and current changes are
comparatively gradual.
The type HZM relays are usually set to oper
ate
on faults, but not on load or synchroniz
ing surge swings from which the system can re
cover.
However,
when the system goes out-of
step,
the type HZM relays operate to separate
the
system.
To provide out-of-step blocking
in
the Type HZM Carrier scheme, an additional
blocking
relay
is required, such as the Type
TS0-3 out-of-step
blocking relay described in
I.L. 41-418.4.
In
the
type HZ carrier scheme, out-of-step
blocking is provided by using the type RSN re
lay instead of the type RS relay.
6
The alarm relay
switch operated
is
connected
in the
circuits, as
switch
is marked
it
removes
also
removes the
through back-up ground
tripping during out-of
impairing the abili
One
fundamental
In
this
scheme,
the distance elements all
operate simultaneously for a three phase fault
if
they
are
to operate at all, while during
out-of-step the Z3 operates first, followed by
Z2
and then Zl.
As the system returns toward
the "i· n -phase"
position,
the opposite order: that is, Zl, Z2, Z3.
To
prevent
tripping during out-of-step, it
is necessary to arrange for the closure of the
three
contacts A, B, C,
relay
back
contact, RRB, to operate an addi
tional
blocking
relay
cuit.
This
blocking relay must have a slight
time-delay
so that
it does not open the trip
circuit before tripping on a three phase fault
can
occur.
On the
other hand, it must open
the
trip circuit during an out-of-step condi
tion before
the second element, Z2 is operat
ed.
The out-of-step blocking contact is designa
ted as X2, and
is connected
cuit as
shown
in Figure
it
are three contacts, A, B, C, which are the
back
contacts on the auxiliary switches A, B,
operated
by
the Z3 carrier starting con
C,
tacts
of
the distance relays thru the auxil
iary
contactor
switches, GSA, CSB,
The
make
contacts
of
series
with the back contact, RRB, of the re
ceiver blocking relay,
P,
of a pendulum type time-delay relay, whose
lower
contacts
make and energize the coil of
of the X2 blocking relay.
three
of the Z3 carrier start contacts close,
the
back
contacts,
A,
trip circuit after a 3 to 4 cycle delay.
contact X2,
opens by virtue of all three make
contacts,
A, B, and C, closing through RRB to
energize the P coil and in turn, the
If
the electrical center is inside the pro
tected
line section, and in other cases where
the
two
voltage
sources
phase,
the directional and impedance elements
at
each end of the line will be closed.
stops
carrier
(previously started
contact) .
This
energizes RRT to allow the contact RRB
the elements
reset
and for the receiver
to open the trip cir
in the trip cir
1 .
In parallel with
and esc.
these switches are in
and energize the coil,
Every time that all
B, C, and X2, open the
Back
X2 coil.
appear 180° out of
This
by the Z3
.., ... /'
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