Up: Sediver
Sediver Catalog 1970
44 of 136
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Protection by horn gaps
in medium voltage systems
The distribution of electricity to the greatest possible quantity of
end-users entails in the rural areas the development of medium voltage
nets, the length of which-- compared to the number of consumers-- is
high. It is therefore of the utmost importance to reduce the price
of the construction and of operating of these lines.
This price can be considerably reduced by using on the medium voltage
grids automatic devices for the detection and elimination of faults.
Spark gaps are an important part of this equipment and help saving
money by avoiding human intervention on the occasion of failure.
Usual faults
The various faults which can occur on a net can be classified in:
- Self extinguishing faults: they disappear in a very short
time without releasing the interrupting relays.
- Temporary faults: they entail a very short switch-off of
the line (generally 6/10 to 10/10 of one second).
- Semi-permanent faults: they entail one or more switch-off
of about ten seconds.
- Permanent faults: they require the help of the maintenance team.
90% of the troubles occur amongst the first three classifications.
It is of the utmost interest, in order to reduce the maintenance costs,
to eliminate them automatically without requiring any human intervention.
This is why a protection by switchgears has been realised and is commonly
used in France, with fast and slow reclosure automatic programme. It
is obvious that this system can work only if, behind the switchgear,
the protection apparatus which-- after use-- necessitate human
intervention, disappear (this is mainly the case of fuses). This is
precisely where the use of horn-gaps is of great interest.
Protection against Lightning
Lightning-arresters are expensive, and their technical advantages are
more theoretical than real. A direct lightning stroke entails currents
which might exceed the capacity of a lightning-arrester, and therefore
might cause its deterioration. Practically a total protection cannot
be obtained against direct lightning stroke. The main quality of the
variable resistance valve-type lightning-arresters as well as of the
ejector-type arresters is to switch off the fault follow current, and
this would be interesting only if the insulation level of the line
were high-- which is generally not the case in medium voltage systems,
especially if they are built with concrete poles.
In the nets with a low insulation level, lightnings cause often
simultaneous flashes in various points, and the action of a
lightning-arrester does not prevent the development of the arcs in
other places and does not avoid the breaker to operate. Actually,
in view of the nature of the variable resistance valve type
lightning-arresters (switching off their fault follow current in
less than one cycle), as well as of the use ejector type lightning-arresters,
the follow current does not discharge through the arresters (it would
actually destroy them) whilst it discharges through horn-gaps.
To sum up, horn gaps which do not ensure in principle the same
excellent protection as lightning-arresters have practically quite a
sufficient efficiency.
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