ABB RELION REX640 Technical Manual page 1245

1MRS759142 F
in the forward direction and the overreaching scheme is used. However, in case of
external faults, tripping is always blocked because either no signal is received as the
opposite terminal sees a reverse fault or because the signal is received but the fault
is seen in the reverse direction. In either case, the blocking is not dependent on the
received signal so a lost communication channel does not result in false operation.
In the blocking scheme, a blocking signal is sent to the opposite terminal if the
fault is locally seen in the reverse direction, that is, during an external fault. Thus,
the blocking is dependent on the received signal, and there is typically a need to
delay the tripping of the terminal receiving the blocking signal. This delay depends,
for example, on the response times of the communication channel and terminals.
During an internal fault, there is no signal transmission between the terminals, so
the tripping does not depend on the received signal from the opposite terminal.
In conclusion, compared to the blocking schemes, the permissive schemes are
inherently faster and have better security
in an external fault is not 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 the received signal, its dependability
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, counting from the beginning of the interruption, or for 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 DSOCPSCH. 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 zone to trip the local breaker
and to transfer the trip signal to a remote terminal. The remote terminal operates
immediately based on the received transfer trip signal, without any additional
conditions. This scheme is very simple, but the 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 after 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 an underreaching zone, or by the tripping signal from an external
protection relay, such as transformer or reactor protection. At the remote end,
the received signal initiates a 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
be set to 10...30 ms, depending on the type of the communication channel. The
1
The ability to block operation in case of an external fault.
2
The ability to operate in case of an internal fault.
REX640
Technical Manual
Protection related functions
1
against false tripping, since tripping
2
is lower than that of the blocking scheme.
Coordination Time setting, which should
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