Zos
2
SEC.
OHM
MIN.
Fig.
3.
Out-of-Step Blocking Operations.
spectively, so that any time a fault is detected in the
trip direction the "stop" lead is tied to minus to
prevent transmission of a blocking signal. Thus, the
stop lead has precedence over the start lead. If, for
example, the channel is being used for voice com
munication at the instant of an internal fault, the
stop circuit will interrupt transmission to permit
tripping, provided that either ZP or
D0
operates.
A blocking signal is required from breaker
B,
fig. 1, during an external fault because the phase or
ground tripping units would otherwise trip at breaker
A.
At
breaker
B
either the 10s or the
ZS
contact
opens to put positive voltage on the start lead; since
neither the CSP nor the CSG contacts close at
B
the stop lead is not energized, and, therefore, a
blocking signal is transmitted.
Receiver Relay Comparison Circuits
Receiver relay, RR, has an RRH holding coil and
an RRT tripping coil as shown in Fig.
2.
RRH is
energized by the receiver output when a blocking
signal is transmitted; RRT coil is energized when
ever the local tripping relay contacts close in an
attempt to tri p the breaker. RRH coil current holds
the RRP and RRG contacts open; RRT coil current
closes the RRP and RRG contacts in the absence of
RRH coil current.
This is the key point, which
produces the comparison function -the force pro
duced by RRH current is stronger than the opposing
forc e produced by RRT current, so that the RRP and
RRG contacts will be closed by RRT current only
when no blocking signal is received.
4
In Fig.
1,
ZP or
D0
+
1
0
contacts close at breaker
A whether the fault is internal, or external to the
right of breaker
B;
in either case, CSP or CSG is
energized and RRT, in turn, is energized. For the
external fault only, the transmitter at
B
sends a
blocking signal, which produces a receiver output at
A, which energizes the RRH coil to prevent RRP and
RRG contacts from closing; thus, tripping is pre
vented.
Conversely, for an ihternal fault, tripping
units operate at both stations to energize CSP or
CSG contacts to prevent transmission of a blocking
signal; another set of CSP
/
CSG contacts energize t he
RRT coil. Since neither station receives a blocking
signal, both RRH coils are deenergized; both RR
relays operate to close their RRP and RRG contacts
to permit either ZP or
D0
+
10 contacts to energize
the trip coil.
A reactor
is
shown in series with the RRT coil
in fig.
2.
This reactor is used only with audio tone
or frequency shift carrier channels, where the channel
delay time is abnormally large. This reactor delays
RRT coil current buildup long emmgh to compensate
for the RRH coil current buildup time occasioned by
the channel delay.
otherwise undesired tripping
might occur before a blocking signal can be received.
Operation will now be summarized.
External Phase Fault Operation
Refer to Table I, which tabulates external phase
fault functioning at the top. The external fault is to
the right of breaker
B
as shown in the upper left of
Table I.
At breaker
B,
ZS contact opens and the ZP con
tact remains open,. so that CSP is not energized; the
start lead is at positive and the stop lead is open,
so the transmitter sends a blocking signal. Tripping
does not occur because ZP is open.
At breaker A, ZP contact closes, energizing CSP.
CSP contact energizes RRT; however, RRP contact
is held open by RRH current.
Internal Phase Fault
Operation is the same at both stations. Refer to
Table I. ZS does not operate, since it is set to look
into the external system. ZP contact closes, ener
gizing CSP. CSP contact closes to apply negative to
the stop lead, preventing blocking signal transmission.
Another CSP contact energizes RRT; since there is
no signal t6 produce RRH coil current, RRT coil
current closes RRP, permitting ZP to trip the breaker.
I '
I
I,
jl
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