ABB REL650 series, Applications Manual

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circuit breaker in another substation. Both examples involve a transfer trip scheme, to
which a local low current criterion is added, to avoid unwanted trips, caused by false
transfer trip signals.
Power transformer, directly connected to the feeding line
The main purpose of UC2PTUC (37) is to provide a local criterion, which is added to a
received transfer trip signal, in order to increase the security of the overall tripping
functionality. A typical application for this function is a power transformer directly
connected to the feeding line, as shown in figure
113 .
~
37
Load
LDD
CS CR TRIP
TRIP
VT1
CT3 CT2 CT1
Power
Transformer
Line
X
source
Source
ANSI11000091_1_en.vsd
ANSI11000091 V1 EN
Figure 113: Power transformer, directly connected to the feeding line
Suppose that an internal non-symmetrical transformer fault appears within the protective
area of the transformer differential protection. In most cases, the line protection will not
recognize the fault. The transformer differential protection operates for the internal fault
and initiates a trip of the secondary side circuit breaker. It addition to that, it sends the
carrier signal CS to the remote line end, in order to open the line circuit breaker in that
substation.
The carrier receive (CR) signal could trip the line circuit breaker directly, according to a
so called direct transfer trip scheme (DTT), but in such cases security would be
compromised, due to the bad quality of the communication link. A false CR signal could
unnecessarily trip the line. Therefore, a local detection device (LDD) is used, to provide
an additional trip criterion, at the same location as the line circuit breaker. The LDD must
detect the abnormal conditions at the end of the protected line and transformer and permit
the CR signal to trip the circuit breaker. The current in at least one of the phases at the
sending end of the line decreases, when the differential protection of the transformer trips
the circuit breaker on the secondary side, and the breaker contacts open. This means, that
an undercurrent function, properly set, could provide a good criterion to increase the
security of the protection for the line and the transformer, with basically unchanged
dependability.
Line connected shunt reactor
The main purpose of UC2PTUC (37) is to provide a local criterion, which is added to a
received transfer trip signal, in order to increase the security of the overall tripping
1MRK 506 334-UUS A
Section 7
Current protection
225
Application manual