ABB RELION REX640 Technical Manual page 1169

1MRS759142 C
[4] The ability to operate in case of an internal fault
REX640
Technical Manual
possible in case of a channel interruption. In a blocking scheme, a lost communication
channel and a simultaneous external fault may lead to a false operation of the
protection if the communication channel is not supervised. On the other hand, as the
fast trip of the permissive scheme depends on a received signal, its dependability
lower than that of the blocking scheme.
The unblocking scheme enhances the dependability of the permissive scheme. If the
communication channel is interrupted and, simultaneously, a fault occurs in the
forward direction, tripping is still possible either during a fixed time interval after the
beginning of the interruption or as long as the communication channel is lost. This is
achieved by connecting the supervision output of the implemented communication
functionality to the dedicated carrier guard signal input of RESCPSCH. In general, the
unblocking scheme provides better security than the blocking scheme because
tripping in external faults is only possible if the fault occurs within the fixed time
interval after the beginning of the channel interruption.
The direct transfer trip scheme uses the underreaching function to trip the local
breaker and to transfer the trip signal to the remote terminal. The remote terminal
operates immediately based on the received transfer trip signal, without any additional
conditions. This scheme is very simple, but its security is low as a spurious signal
results in false operation of the protection.
Direct underreaching transfer trip DUTT
In some applications, there is a need to trip the remote end breaker immediately
because of fault detection by the local measurements. This applies, for example, when
the transformers or reactors are connected to the system without circuit breakers, or
for remote tripping following the operation of the local breaker failure protection
(CBFP).
In the direct intertrip scheme (DUTT), the carrier send signal CS_INTER_TR is
initiated by the underreaching or the tripping signal of an external protection relay,
such as transformer or reactor protection. At the remote end, the received signal
initiates the breaker trip immediately without any further local protection criteria. To
limit the risk of an unwanted trip due to spurious sending of signals, the trip can be
delayed by the Coordination time setting, which should be set to 10...30 ms,
depending on the type of the communication channel. The communication channel
operating in direct intertripping applications should be secure and dependable.
Setting guidelines for the intertrip scheme DUTT
Scheme type = "Intertrip"
Coordination time = "0.050" s (10 ms + maximal transmission time)
Carrier Min Dur = "0.1" s
Protection related functions
Section 5
[4]
is
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