Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-73
AP
4.1.3.3 Dead timer setting
High speed auto-reclose may be required to maintain stability on a network with two or more
power sources. For high speed auto-reclose the system disturbance time should be
minimized by using fast protection, <30 ms, such as distance or feeder differential protection
and fast circuit breakers <60 ms. For stability between two sources a system dead time of
≤300 ms may typically be required. The minimum system
dead time considering just the CB
is the trip mechanism reset time plus the CB closing time.
Minimum relay
dead time settings are governed primarily by two factors:
Time taken for de-ionization of the fault path
Circuit breaker characteristics
Also it is essential that the protection fully resets during the dead time, so that correct time
discrimination will be maintained after reclosure onto a fault. For high speed auto-reclose
instantaneous reset of protection is required.
For highly interconnected
systems synchronism is unlikely to be lost by the tripping out of a
single line. Here the best policy may be to adopt longer dead times, to allow time for power
swings on the system resulting from the fault to settle.
4.1.3.4 Follower time setting (P544 and P546 only)
In the ap
plication of auto-reclosing to a feeder terminal switched by two circuit breakers, the
P544 and P546 provide the necessary control for both circuit breakers. The two circuit
breakers are normally arranged to reclose sequentially with one designated leader circuit
breaker reclosing after a set dead time followed, if the leader circuit breaker remains closed,
by the second, follower, circuit breaker after a further delay, the follower time.
The follower time is provided to prevent un-necessary operation of the follower circuit
breaker. The follower time should be set sufficiently long as to avoid an un-necessary
closure of the follower circuit breaker where conditions are such that it would be required to
trip again.
After expiry of the dead time, the leader circuit breaker will attempt a reclosure. The
minimum value of the follower time should allow sufficient time for the auto-reclosure of the
lead circuit breaker to be considered successful. Consider a worst case where the phase
differential elements are unavailable and back-up protection is being provided by the
distance elements. Take the case of reclosure onto a dead line with a persistent fault at the
remote end of the line. Local end protection (Time delayed Back up protection, like distance
Z2 element) may detect this fault after set time delay (typically > 200 ms), time must be
allowed for the leader circuit breaker to re-trip (50 - 100 ms), and a safety margin needs to
be added so that typically, the minimum follower time could be around 500 ms.
If the reclosure of the lead circuit breaker is successful, the follower circuit breaker can be
allowed to reclose. Delaying the reclosure of the follower circuit breaker will allow any
transients to decay before the switching and if the transient decay figure is known, can be
used to determine a minimum follower time value. The larger of the two values can then be
used as the minimum follower time.
Note: Since the follower circuit breaker should only be reclosed if the system
is healthy, and, since in a dual circuit breaker scheme where the
system is healthy the follower circuit breaker acts more like a bus
coupler, there is no real requirement for fast switching and a time
delay in excess of 1 s might be appropriate. Indeed, in the P544 and
P546 and P546 default follower time is chosen as 5 s and this can
comfortably be applied to most applications.
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