Areva MiCOM P441, Техническое руководство

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 127/220
4.5.2 Benefits of Autoreclosure
An analysis of faults on any overhead line network has shown that 80-90% are transient in
nature. Lightning is the most common cause, other possibilities being clashing conductors
and wind blown debris. Such faults can be cleared by the immediate tripping of one or more
circuit breakers to isolate the fault, followed by a reclose cycle for the circuit breakers. As
the faults are generally self clearing ‘non-damage’ faults, a healthy restoration of supply will
result.
The remaining 10 - 20% of faults are either semi-permanent or permanent. A semi-
permanent fault could be caused by a small tree branch falling on the line. The cause of the
fault may not be removed by the immediate tripping of the circuit, but could be burnt
away/thrown clear after several further reclose attempts or “shots”. Thus several time
delayed shots may be required in forest areas.
Permanent faults could be broken conductors, transformer faults or cable faults which must
be located and repaired before the supply can be restored.
In the majority of fault incidents, if the faulty line is immediately tripped out, and time is
allowed for the fault arc to de-ionise, reclosure of the circuit breakers will result in the line
being successfully re-energised, with obvious benefits. The main advantages to be derived
from using autoreclose can be summarised as follows:
•
Minimises interruptions in supply to the consumer;
•
A high speed trip and reclose cycle clears the fault without threatening system
stability.
When considering feeders which are partly overhead line and partly underground cable, any
decision to install auto-reclosing would be influenced by any data known on the frequency of
transient faults. When a significant proportion of the faults are permanent, the advantages of
auto-reclosing are small, particularly since reclosing on to a faulty cable is likely to aggravate
the damage.
At subtransmission and transmission voltages, utilities often employ single pole tripping for
earth faults, leaving circuit breaker poles on the two unfaulted phases closed. High speed
single phase autoreclosure then follows. The advantages and disadvantages of such single
pole trip/reclose cycles are:
•
Synchronising power flows on the unfaulted phases, using the line to maintain
synchronism between remote regions of a relatively weakly interconnected system.
•
However, the capacitive current induced from the healthy phases can increase the
time taken to de-ionise fault arcs.