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Page 2–12 December 1996
TCF–10B System Manual
Technologies, Inc.
system, because breaker tripping is not supervised
by any local relays.
As noise in the channel increases, a point is
reached where there is a high probability of false
tripping. The level of noise at which the channel
becomes unreliable must be determined by tests.
Signal-to-noise ratio monitors must then be
included with any direct trip channel, to block
possible false tripping. It is important, however,
not to get the noise monitors any more sensitive
than required, since their operation will prevent
tripping.
There are three important aspects to the applica-
tion of FSK channels to direct trip systems:
channel bandwidth, dual channel systems, and
channel protection.
Although faults should be cleared in the shortest
possible time, speed is not the only criterion for
selecting equipment.
It is important to use the
narrowest bandwidth equipment possible. A wide
bandwidth channel may give the desired speed,
but more noise enters the system. Thus, the
channel will block tripping sooner than a narrower
bandwidth channel with the same received signal
level. A wideband channel will consequently not
be as dependable as a narrower channel under
equal receive-level conditions.
A dual channel system is recommended for direct
trip applications. Two FSK channels should be
used in series, so that both must trip before the
breaker is tripped. Many tests have indicated that
dual channels improve the security of the direct
trip system by several orders of magnitude. Use of
a dual channel system has very little effect on
dependability, even if both channels are on the
same transmission medium.
If you want to increase the dependability, you can
modify the dual channel transfer trip scheme to
allow a single channel trip when there is failure of
the other channel. A typical Dual Channel
Throwover to Single Channel Scheme is illus-
trated in Figure 2-12.
2.3 Special Considerations
The TCF–10B frequency-shift equipment can
operate in either the two- or three-frequency
mode, but ordinarily operates as a two-frequency
system. The three basic frequencies are as follows
(see Figure 2-13):
f
C
Center frequency
f
H
High-frequency, is a frequency shift (
,
f)
above f
C
f
L
Low-frequency, is a frequency shift (
,
f)
below f
C
LOC*
CH-1
52a
*Loss of Channel,
Relay Normally Energized
TRIP
CH-2
52
TC
LOC*
CH-2
TRIP
CH-1
+
–
Figure 2–12.
Dual Channel Transfer Trip with Throwover
to Single Channel on Loss of One Channel.
f
L
(f
C
–
∆
f) f
H
(f
C
+
∆
f) f
C
Figure 2–13. TCF–10B 3-Frequency System.