x/EN AP/Na7
-28 MiCOM P74
5.3 Central processing unit (P741)
A central instruction for a reduced operation mode of the busbar protection on two levels can
be applied selectively zone by zone.
5.3.1 Busbar protection (87BB) Blocked
The busbar protection is monitored only (not protected by 87BB) (i.e. all BB protection trips
are inhibited and measurements are enabled). All other protection remains in service and
trips can still be issued for genuine CBF conditions.
5.3.2 87BB Blocked & 50BF disabled
In this mode, both the busbar and circuit breaker fail conditions are monitored but all trips are
inhibited.
Additionally, all protection functions are disabled when the system is awaiting configuration
downloads (topology is missing).
5.4 System operation under failed communications situation
With each start or reboot of the CU, all the zones are set to 87BB and 50BF disabled mode
as described above. They will remain in this mode until all peripheral units are recognised
as being in service and synchronised. (PU CONF & STATUS/PU in service).
If a PU that was considered to be out of service but suddenly communicates with the CU, the
CU automatically places all zones to a waiting system configuration mode while waiting for
an input from the user to either assign the PU in service or disconnect additional PUs.
During operation, if the communication with a PU is broken during a very short time, the
zone connected to the CT of the non-communicating PU is temporarily suspended. If the
communication is restored, the differential protection is restored for the zone. On the other
hand, if the break in communications persists longer than permitted as given by the PU Error
Timer, the zone protection is suspended.
If the PU error self-reset mode is disabled, for the reinstatement of the zone the user must
intervene:
• If communication is restored the user must reset the alarm by the same command to
reset PU Error (PU CONF & STATUS -> Reset PU Error)
If the PU error self-reset mode is enabled, the reinstatement of the zone will be automatically
done after the set timer delay.
On the PU, an alarm will indicate loss of communication with the CU.
On the CU, an alarm will indicate that one or more PUs are no longer synchronised.
In the PU CONF & STATUS column, it is possible to view the list of synchronised PU (PU
connected) after having altered the list of PU in service (PU in service).
If at the time of the initial start-up, the topology of the substation was implemented including
futures (for example 15 PU including 6 extensions) it is possible to boot the system only
activating the existing 9 PUs in the cell PU in service. When the futures 6 PUs are
connected, it will be sufficient to connect them and indicate that they are now in service in
the CU menu columns.
Note : 'loss of communication with the CU' alarm can be disabled by disable the
event filter bit (PSL_DDB_ELEMENT_252 - bit 28 of DBM_PARAMETERS).
This means when the event filter bit is disabled, the alarm will be disabled
as well.
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