ABB RELION REX640 Technical Manual page 1157

1MRS759142 C
REX640
Technical Manual
is less secure than a permissive scheme because it trips for the external faults within
the reach of the overreaching local zone if the communication channel is out of service
and no block signal is received.
Inadequate speed or low dependability of the communication channel can cause
spurious tripping for external faults.
To secure that the blocking signal arrives before the local overreaching zone trips, the
trip is delayed by the Coordination Time setting. This setting must be set longer than
the sum of the maximal transmission time of the channel and the start and response
times of the protection relays sending and receiving the CS_BLOCKING signal. A
security margin of at least 10 ms should be considered. The time delay Carrier Min
Dur for prolonging the CS_BLOCKING signal is set to "0".
Setting guidelines for the blocking scheme DCB
Scheme type = "Blocking"
Coordination Time = "0.1" s (10 ms + maximal transmission time)
Carrier Min Dur = "0" s
Directional comparison unblocking scheme DCUB
Metallic communication paths unfavorably affected by fault-generated noise or by
other electrical phenomena, such as lightning, are not suitable for conventional
permissive schemes that rely on the signal transmitted during a fault in the protected
feeder. Principally, the communication media must be properly shielded or otherwise
designed to provide an adequate performance during such conditions.
The unblocking scheme can be used to overcome the lower dependability of
permissive schemes. The function is used in older, less reliable communication,
where the signal has to be sent through the primary fault. The unblocking function
uses a carrier guard signal CRG which must always be present, even when no CR_PRM
signal is received. Due to the absence of the CRG signal during the security time, the
Loss of carrier time setting is used as a CR_PRM signal. This also enables a permissive
scheme to operate when the communication channel is temporarily lost during an
internal fault.
Setting guidelines for the unblocking scheme DCUB
The unblocking function is configured with the Unblock Mode setting.
• Unblock Mode = "Off": The unblocking function is disabled.
• Unblock Mode = "Permanent": If the CRG signal disappears for a longer time than
defined in the Loss of carrier time setting, a logical carrier received signal CRL is
generated. The CRL signal remains active until the CRG signal is present. The
communication failure is not signalled by the LCG output. The Loss of carrier
time setting is set to "35" ms.
• Unblock Mode = "Time window": If the CRG signal disappears for a longer time
period than the Loss of carrier time setting, a CRL signal is generated. The CRL
signal remains active for 150 ms, after which a communication failure is signalled
Protection related functions
Section 5
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