ABB RELION REX640 Technical Manual page 1172

Section 5
Protection related functions
1166
Carrier Min Dur = "0.1" s
Carrier Min Dur = "0" s (in case of parallel lines and current reversal logic)
Directional comparison blocking scheme DCB
In the blocking scheme, the reverse-looking residual overcurrent function is used for
sending a block signal to the remote end to block the overreaching forward-looking
residual overcurrent function.
The blocking scheme is very dependable because it operates for faults anywhere on
the protected feeder, even if the communication channel is out of service. Conversely,
it is less secure than a permissive scheme because it trips for the external faults within
the reach of the overreaching local function 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 function 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_PRM signal. A
security margin of at least 10 ms should be considered. The time delay Carrier Min
Dur for prolonging the CS_PRM signal is set to "0".
Setting guidelines for the blocking scheme
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 adversely affected by fault-generated noise or by other
electrical phenomena, such as lightning, are not suitable for conventional permissive
schemes that rely on signal transmission 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
1MRS759142 C
REX640
Technical Manual