Up: Sediver
Sediver Catalog 1970
46 of 136
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4. Use of the horn gaps
To benefit by the greatest extension of the arc allowed by the shape
of the horns, horn gaps are designed for outdoor use: the arc can go up
more than 1 m above the horns, and the indoor use-- which is not
recommended-- would require special protections, especially a separation
wall to avoid that the arc reaches the neighbouring phases.
The use of a strain string as support of the horn gap settles the
problem, both for indoor and pole-mounted transformers.
Birds have much greater difficulty in perching on leaning horns than
on horizontal shanks. In order to avoid the fast flight of a bird
between horns, it is recommended to use a horn gap with a double
interval, equipped with an "anti-bird-knife" located between the horns.
The adjustment of the width between horns of a horn gap depends on the
maximum impulse voltage which is expected, i.e. actually at the level
of the withstand impulse voltage of the installation which is to be
protected.
In order to allow for altitude, the widths are to be increased by 1%
each 100 m above 500 m.
It is customary-- and compulsory in France-- to connect the earthed
electrode of the horn gaps to the other earthed parts of poles and
hardware.
Protection by horn gaps is not confined to the medium voltage/low
voltage transformers on poles, or in substations. It is also
recommended for connections between aerial lines and underground cables,
and especially for medium voltage insulated conductors emanating from
a MV/LV substation.
Conclusion
Several hundred thousands of SEDIVER protecting horn gaps are presently
in service on overhead medium voltage distribution systems in France and
foreign countries (chiefly Belgium and Finland). They have been
standardised by ÉLECTRICITÉ DE FRANCE and Electric Companies
in view of the numerous advantages:
- Price more than ten times lower than that of other protecting
apparatuses (lightning arresters) for a similar efficiency.
- Anchorage string included in this price.
- Very high resistance to atmospheric factors.
- Uninterrupted service when flashover in many cases.
- Quick Self-Extinction property.
- Heavy admissible current.
- Fault-follow currents localised on the horns.
Systems with directly earthed neutral and equipped with switchgear
with fast and slow reclosure automatic programme obtain the maximum
efficiency from protecting horn gaps, but those are still available
with other systems.
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