is received but locally 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, permissive schemes are inherently faster and have better security
1
against false tripping than a blocking scheme, since tripping 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 a received signal,
its dependability
2
is 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"
1
The ability to block operation in case of an external fault.
2
The ability to operate in case of an internal fault.
1MRS759142 F
Protection related functions
REX640
Technical Manual
1259