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| Shielding Angle |
When lightning strikes a phase
conductor, no other object shares in carrying the lightning current.
Most flashes to an unprotected phase conductor are therefore capable
of producing flashovers.
OHGWs may intercept the stroke and
shunt the current to the ground through the tower impedance and
footing resistance if they are properly located. The resultant
voltages across the transmission-line insulation, and the likelihood
of flashover are substantially reduced.
Shield wires or static wires are
normally employed on transmission and distribution lines, and are
located high above the phase conductors to shield the phase
conductors from direct lightning strokes.
The shield wires intercept most direct
strokes and allow them to be conducted harmlessly to ground, that is
providing low resistance path to ground.
On distribution systems, the grounded
neutral conductor sometimes is located above the conductors to act as
shield conductors.
One important task of transmission-line
designers is to locate the OHGWs. Well-planned geometry will reduce
the probability of lightning striking the phase conductors to an
acceptable level.
The proper placement of the OHGW around
the phase conductors is usually deÞned by the shielding angle.
The shielding angle is negative if the OHGWs are horizontally
disposed outside the phase conductors.
Before about 1951, a shielding angle of
30° was usually employed for transmission lines. This produced
acceptable lightning performance on existing lines of voltages up to
230 kV.
In the mid 1950s, 345 kV lines were
introduced and tall double-circuit lines were constructed. The
lightning performances of these lines were considerably worse than
expected.
After extensive theoretical, field, and
laboratory investigations, a general agreement was reached that the
usual 30° shielding angle should be decreased as the height of
the transmission-line structures increased.
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