ABB Relion 670 series - Direct Transfer Trip Logic

16.8 Direct transfer trip logic
16.8.1 Application
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
CSCR TRIP
TRIP
VT1
CT3CT2CT1
Power
Transformer
Line
X
source
Source
en03000120.vsd
IEC03000120 V1 EN
Figure 380:
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.
1MRK 506 369-UUS - Section 16
Scheme communication
Line distance protection REL670 2.2 ANSI 775
Application manual