|Protection of overground transmission lines|
Because of its form,
the overhead electric-power lines are sort of objects that are pretty
well exhibited to risk of lightning hits. Lightning that hit the overhead
electric-power lines could make a higher voltage than insulators on overhead
transmission lines may stand and then will come to skipping and short
circuit. Current bow made on that way will not turn off for a long time
because the voltage of phase conductor is enough for its maintaining.
In praxis, with protecting rope are covered all
transmission lines over 30 kV. Mostly, there are two protecting ropes.
When lightning hits the protecting rope there could come to large drop
of voltage while passing through pylon and grounded conductor. In that
case phase conductors, which are on nominal voltage, would have a voltage
much smaller than part of pylon they are hooked on. The insulation between
pylon and conductor wouldn't stand that, and "recurrent skip"
would appear. Such skipping could permanently damage the insulation and
throw the line out of operation for a long time.
There is one more device for decreasing that overvoltage
named protecting sparklet. It is set parallel to insulator chain where
are phase conductors and it's insulating the phase conductors from transmission-line
pylon. The voltage of skipping on sparklet is smaller than insulation
of transmission line could stand. The main function is to move the arch
originated from recurrent ship on insulator chain and that way it protects
the insulator from damage. The sparklets are usually installed on the
transmission lines over 110 kV but sometimes on the nets of lower voltages,
on especially critical places. Its function is also to make the more favourably
distribution of electrical field around insulators chain and on that way
it protects from different kinds of discharge (corona).