A B
OR
A
OR
B
OR
A
CS
A
TRIP
B
= OR
B
+ CR
B ,
OR
B
+ T2
IEC09000014-1-en.vsd
IEC09000014 V1 EN
Figure 370: Principle of Permissive overreaching scheme
OR: Overreaching
CR: Communication signal received
CS: Communication signal send
T2: Timer step 2
Unblocking scheme
Metallic communication paths adversely affected by fault generated noise may not be
suitable for conventional permissive schemes that rely on a signal transmitted during a
protected line fault. With power line carrier for example, the communication signal may
be attenuated by the fault, especially when the fault is close to the line end, thereby
disabling the communication channel.
To overcome the lower dependability in permissive schemes, an unblocking function can
be used. Use this function at older, less reliable, power-line carrier (PLC) communication,
where the signal has to be sent through the primary fault. The unblocking function uses a
guard signal CR_GUARD, which must always be present, even when no CR signal is
received. The absence of the CR_GUARD signal during the security time is used as a CR
signal. This also enables a permissive scheme to operate when the line fault blocks the
signal transmission. Set the tSecurity to 35 ms.
16.1.2.4 Intertrip scheme
In some power system applications, there is a need to trip the remote end breaker
immediately from local protections. This applies for instance when transformers or
reactors are connected to the system without circuit-breakers or for remote tripping
following operation of breaker failure protection.
1MRK 506 369-UUS - Section 16
Scheme communication
Line distance protection REL670 2.2 ANSI 753
Application manual
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