ABB RELION REX640 Technical Manual page 1168

Section 5
Protection related functions
[3] The ability to block operation in case of an external fault
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Applying RESCPSCH requires a communication channel capable of transmitting an
"On" or "Off" signal in each direction. To enable fast tripping, the most important
requirement for communication is the communication speed.
The performance of RESCPSCH is directly related to the communication channel
speed, and to the security against false or lost signals. Therefore, dedicated
communication channels are recommended. With short distances of up to a few
kilometers, a typical communication media is a simple pilot wire based on auxiliary
power. With distances of up to 150 km, fibre-optic cables using digital data
transmission can be used. To avoid false signals that could cause unwanted operation,
the security of the communication channel should be emphasized. Also, the
dependability of the communication channel should be considered to ensure that the
signals are reliably transmitted during power system faults.
RESCPSCH supports five communication schemes.
• Direct underreaching transfer trip DUTT
• Permissive underreaching transfer trip PUTT
• Permissive overreaching transfer trip POTT
• Directional comparison blocking scheme DCB
• Directional comparison unblocking scheme DCUB
Depending on whether the communication channel is used for sending a block or a trip
signal, the communication schemes can be divided into blocking schemes and
permissive schemes.
In permissive schemes, the trip signals of the directional residual overcurrent
protection are interchanged between the terminals to receive a permission to trip
during an internal fault. The tripping of the local terminal depends on both the starting
of its own forward-looking function and the received signal from the opposite
terminal. In the underreaching scheme, no signals are sent during an external fault. A
trip signal is sent to the opposite terminal during an external fault if the fault is seen 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 the signal 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
against false tripping than a blocking scheme, since tripping in an external fault is not
1MRS759142 C
[3]
REX640
Technical Manual