Areva MiCom P120, Technical Manual, Page: 278 / 668

P12x/EN AP/A96 Application Guide
Page 56/80
MiCOM P120/P121/P122/P123
22.3.2.3 Fault De-ionising Time
For high speed autoreclose, the time to de-ionise faults may be the factor the most important
when considering the dead time. This is the time required for the ionised air to disperse
around the fault position so that the insulation level of the air is restored. This time may be
around the following value:
De-ionising time = (10.5 + ((system voltage in kV)/34.5)) / frequency
For 66 kV = 0.25 s (50Hz)
For 132 kV = 0.29 s (50 Hz)
22.3.2.4 Protection Reset
It is essential that the protection fully resets during the dead time, so that correct time
discrimination is maintained after reclose on to a fault. For high speed autoreclose,
instantaneous reset of protection is required.
Typical 11/33kV dead time settings in the UK are as follow:
1st dead time = 5 - 10 seconds
2nd dead time = 30 seconds
3rd dead time = 60 - 100 seconds
4th dead time (uncommon in the UK, however used in South Africa) = 60 - 100 seconds
22.3.3 Reclaim Timer Setting
The following factors influence the choice of the reclaim timer:
•
Supply continuity - Large reclaim times can result in unnecessary lockout for transient
faults.
•
Fault incidence/Past experience - Small reclaim times may be required where there is
a high incidence of lightning strikes to prevent unnecessary lockout for transient faults.
•
Charging time of the spring - For high speed autoreclose, the reclaim time may be set
longer than the spring charging time to ensure that there is sufficient energy in the
circuit breaker to perform a trip-close-trip cycle. For delayed autoreclose, this setting is
of no need as the dead time can be extended by an extra CB healthy check window
time if there is insufficient energy in the CB. If there is insufficient energy after the
check window time the relay will lockout.
•
Switchgear Maintenance - Excessive operation resulting from short reclaim times can
mean shorter maintenance periods. A minimum reclaim time of 5s may be needed to
give sufficient time to the CB to recover after a trip and close before it can perform
another trip-close-trip cycle.
The reclaim time must be long enough to allow any time delayed protection leading to
autoreclose to operate. Failure to do so can cause the autoreclose scheme to reset too soon
and the reactivation of the instantaneous protection.
If that were the case, a permanent fault would look like some transient faults, caused by
continuous autorecloses. Applying a protection against excessive fault frequency lockout is
an additional precaution that can solve this problem.
It is possible to obtain short reclaim times by blocking the reclaim time from the protection
start signals. If short reclaim times are to be used, then the switchgear rating may dictate the
minimum reclaim time. The advantage of a short reclaim time is that there are less lockouts
of the CB, and therefore the number of maintenance visits periods would be reduced.