Permissive Overreaching Transfer Trip Systems; Permissive And Non-Permissive Underreaching Transfer Trip Systems - Ametek UPLC CU44-VER04 Applications Manual

You may conserve frequency spectrum by using a
narrow band frequency shift carrier, but at the
expense of channel speed.
Another consideration is an open breaker situa-
tion. When the remote breaker is open for an
extended period of time, the relay system must be
able to trip. The remote relay system sends a trip
signal when detecting a remote open breaker. If
this remote signal is received for 1,000 ms (1 sec)
or longer, the carrier receiver logic interprets this
as an open breaker and allows the local end to trip
whenever the local relays detect a fault.
An older system (STU unblock) is shown in
Figure 3–3.
3.1.2 Permissive Overreaching
Transfer Trip Systems
Overreaching transfer trip systems require a chan-
nel signal to trip, and are used with a frequency-
shift audio tone, modulated on a communication
channel (e.g., public or private telephone lines).
These systems are generally not used with power-
line carriers. There are, however, successful appli-
cations of power-line carrier on POTT schemes
where parallel lines allow for cross-coupling of
the carrier signal.
G
Protected Line
1
Breaker 2 Trip Fault Detector (P
Figure 3–4. Basic Logic Diagrams for Directional Comparison Unblocking.
May 2012
Breaker 1 Trip Fault Detectors (P 1 )
F I
2
Power Line Carrier
Channel f (G to H)
1
Power Line Carrier
Channel f (H to G)
2
)
2
Solid State Logic (per Terminal)
P
AND
Unblock
(See Figure 2-1)
Note: (X) Normally 4 Ms.
Chapter 3. Applications
3.1.3 Permissive and Non-
Permissive Underreaching
Transfer Trip Systems
For overreaching systems, the directional phase
and ground trip fault detectors (P) must be set to
overlap within the transmission line and not over-
reach any terminals (see Figure 3–5).
That is, at least one trip fault detector (P) must
operate for all internal faults, and none should
operate for any external fault. In practice, distance
relays are normally required for both ground faults
and phase faults, although directional instanta-
neous ground-overcurrent relays might meet these
requirements in some cases.
Though it is the least complex, the non-permissive
system is rarely used because of the high potential
for false outputs from the channel, which would
cause incorrect tripping. If a non-permissive sys-
tem is used, the channel considerations should be
as described later for direct trip systems. The sys-
tem is made permissive by the additional set of
phase and ground overreaching fault detectors
(FD), which must operate for all internal faults
(see Figure 3–5).
Contact Logic (per Terminal)
H
F
E
Timer
X
Trip
O
P
Key Transmitter
to Unblock
RR
RR
Channel
Signal
Receiver
(F 1 at H,
Trip
F at G)
Coil
2
52a
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