ABB REL670 2.2 IEC, Applications Manual

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16.8 Direct transfer trip logic
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16.8.1 Application
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The main purpose of the direct transfer trip (DTT) scheme is to provide a local
criterion check on receiving a transfer trip signal from remote end before tripping
the local end CB. A typical application for this scheme is a power transformer
directly connected, without circuit breaker, to the feeding line. Suppose that an
internal symmetrical or non-symmetrical transformer fault appears within the
protective area of the transformer differential protection. The line protection will,
in some cases, not recognize the fault. The transformer differential protection
operates for the internal fault and initiates a trip of the secondary side circuit
breaker. It also sends the carrier signal to the remote line end in order to open the
line circuit breaker.
~
I
DIFF
>
Load
DTT
CS CR TRIP
TRIP
VT1
CT3 CT2 CT1
Power
Transformer
Line
X
source
Source
en03000120.vsd
IEC03000120 V1 EN-US
Figure 378:
Usually carrier receive (CR) signal trips the line circuit breaker directly in normal
direct transfer trip scheme (DTT) but in such cases security would be
compromised, due to the risk of a false communication signal. A false CR signal
could unnecessarily trip the line. Therefore, a local criterion is used, to provide an
additional trip criterion, at the same location as the line circuit breaker. The local
criterion must detect the abnormal conditions at the end of the protected line and
transformer and permit the CR signal to trip the circuit breaker.
Another application is a line connected shunt reactor, where the reactor is solidly
connected to the line. Shunt reactors are generally protected by differential
protection, which operates the local line circuit breaker and sends a transfer trip
command to the remote line end.
The line protection in the remote end is much less sensitive than the differential
protection and will only operate for low impedance reactor faults very close to the
high voltage terminal. To avoid frequent line trips at the local end due to false
transfer trip signals, a local criterion check is required to be added at the local end.
The trip signal from local criterion will ensure the fault at the remote end and
release the trip signal to the local side circuit breaker. The local criterion must
detect the abnormal conditions and permit the CR signal to trip the circuit breaker.
1MRK 506 369-UEN B
Section 16
Scheme communication
Line distance protection REL670 2.2 IEC 727
Application manual