Hitachi Relion 670 Series Applications Manual page 286

Section 13
Scheme communication
To secure that the send signal will arrive before the zone used in the communication scheme will trip,
the trip is released first after the time delay tCoord has elapsed. The setting of tCoord must be set
longer than the maximal transmission time of the channel. A security margin of at least 10 ms should
be considered.
The timer tSendMin for prolonging the send signal is proposed to set to zero.
Z rev
A
OR
IEC09000015 V2 EN-US
Figure 136:
OR: Overreaching
CR: Communication signal received
CS: Communication signal send
Z rev : Reverse zone
A
13.1.2.2 Permissive schemes
In permissive schemes, the permission to trip is sent from the local end to the remote end(s), when
the protection at the local end has detected a fault on the protected object. The received signal(s) is
combined with an overreaching zone and gives an instantaneous trip if the received signal is present
during the time the chosen zone has detected a fault.
Either end may send a permissive (or command) signal to trip to the other end(s), and the
teleprotection equipment needs to be able to receive while transmitting.
A general requirement on permissive schemes is that it shall be fast and secure.
If the sending signal(s) is issued by underreaching or overreaching zone, it is divided into a
permissive underreach or permissive overreach scheme.
Permissive underreaching scheme
Permissive underreaching scheme is not suitable to use on short line length due to difficulties for
distance protection measurement in general to distinguish between internal and external faults in
those applications.
The underreaching zones at the local and remote end(s) must overlap in reach to prevent a gap
between the protection zones where faults would not be detected. If the underreaching zone do not
meet the required sensitivity due to for instance fault infeed from the remote end, a blocking or
permissive overreaching scheme should be considered.
The received signal (CR) must be received when the overreaching zone is activated to achieve an
instantaneous trip. In some cases, due to the fault current distribution, the overreaching zone can
operate only after the fault has been cleared at the terminal nearest to the fault. There is a certain
risk that in case of a trip from an independent tripping zone, the zone issuing the send signal (CS)
resets before the overreaching zone has started at the remote terminal. To assure a sufficient
280
A
B
CS
Z rev
A
A
Principle of blocking scheme
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1MRK506375-UEN Rev. N
B
TRIP = OR + tCoord+ CR
B B
IEC09000015_2_en.vsd
Railway application RER670
Application manual
M16866-33 v4
M16866-53 v4