January 2016 Page 3–27
Chapter 3. Applications
3
3.6.4 Direct Transfer Trip Channel
Considerations
The channel and its terminal equipment are major
factors in the proper operation of the direct trans-
fer-trip system. The DTT channel must neither fail
to provide a correct trip signal nor provide a false
signal.
While other types of modulation are possible,
FSK equipment offers the best compromise
between noise rejection capability and equipment
complexity. Two frequencies are usually transmit-
ted in an FSK system: the “guard” frequency is
transmitted during non-trip conditions and the
“trip” frequency is transmitted when a breaker trip
is required. Because a signal is always present, the
FSK system will allow the channel to be continu-
ously monitored. Continuous channel monitoring
is necessary in a direct trip system, because break-
er 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 application 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 chan-
nel 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 illustrat-
ed in Figures 3–19 & 3–20.
Bi-Directional Direct
Transfer Trip Channel
––
DTTDTT
52a52a
52
TC
52
TC
++
Shunt Reactor
Protection
87.50/51.63, etc.
Figure 3–18. Direct Transfer Trip for Shunt Reactor Protection
To Next Page
Loading...
Need help?
General