Up: Sediver
Sediver Catalog 1970
45 of 136
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Horn gaps
1. Principles and characteristics
The components are one hot-live electrode and one earthed electrode,
offering a V-shaped gap path in air. The characteristics of a horn
gap are:
- Minimum flashover voltage: i.e. the lowest voltage at which
flash occurs across the gap under power frequency: for the outdoor
type the wet value will be considered.
- Maximum Impulse Flashover Voltage (1/50 wave) or 100% Flashover
Voltage: this is the lowest voltage (peak value) for which it is
sure that each impulse will cause a flashover.
Conditions to ensure protection
In order to ensure uninterrupted service, flashovers should not occur
further to a switching surge. As a matter of fact this question of the
switching overvoltages is practically never raised in medium voltage
systems.
It is necessary that the horn gap ensures a protection against the
lightning overvoltages. It is necessary in this respect that the
striking be sufficiently fast and occurs at a voltage sufficiently
inferior to the impulse withstand level of the transformer which must
be protected. In fact, the protection level of a horn gap varies
with the steepness of the wave. Experience shows that the protection
is efficient for a maximum impulse flashover voltage of about 75% of
the insulation level of the transformer.
Horn gaps can, in certain cases, have the advantage of collecting
fault follow currents
In case of several simultaneous flashovers, the shortest arc will
probably keep alone the follow current and extinguish the other arcs.
Horn gaps consequently extinguish flashovers on insulator strings.
We have already seen that lightning-arresters are naturally
inefficient in this respect.
2. Arc withstand and admissible current
With the use of a horn gap, the arc moves quickly by running vertically
from the point where it is initiated, and the iron electrodes are
practically not deteriorated: their dimensions are calculated so that
spreading off of small metallic particles fall far from the insulating
supports; and, for ten operations of 1 second with a 1000 A discharge,
the metallisation of the insulating supports is practically nil.
For the same reason, the gap is practically not altered after flashover.
The increase of the maximum impulse flashover voltage after 10 operations
of 1 sec. is always lower than 10%.
The current to be considered is the fault follow current between phases,
in case of simultaneous flashovers on two or three horn gaps, as this
can evidently happen.
Horn gaps can withstand an intensity which is independent from their
regulation. The arc does not remain at the point where it is initiated,
it runs alongside the horns. The limit of the admissible current is
high and reaches at least 1000 A during 1 sec.
3. Possibilities of quick self-extinction
This quick extinction is obtained with a horn gap for a follow current
of about 10 A in a system with insulated neutral, and with an intensity
which is much higher in a system with earthed neutral through a coil.
This extinction is therefore obtained without switching off, as with a
lightning arrester.
For high intensities of about 500 to 1000 A, self-extinction is generally
obtained when the arc has spread over sufficiently (from a half to one
second). If the up the line protection relays lag within these values,
it is possible to avoid interruptions in service.
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