0
Fig shows the critical lightning current steepness Ill,cr decreases versus grounding resistance Rg
for a line with INS160 insulators. It can be clearly seen that as the grounding resistance increases, the critical lightning current steepness Ill,cr decreases.
Fig. 8
The number of lightning outages n o caused by direct lightning strokes (DLS) on conductors of an unprotected line can be estimated by
the following equation
n0=Ndls
P( I
l
)P
arc
(1-P
rc
) …………….…..(1)
Where NDLS is the number of
direct lightning stroke(DLS) on a line; P (I
l
) is the probability of lightning current likely to cause flashovers of the line insulation; Parc
is the probability of a power are caused by an impulse flashover an insulator; and Prc
is the probability of successful line breakers enclosures.
It is shown that the steepness and not the magnitude of lightning current Il l Il is the important factor in the performance if a LFA protected line thus(1) can be written in the
following form.
n0
’
= Ndls
p( I’l,cr)P
arc
(1-P
rc
).
Where n|0 is the number of lightning outages on an LFA protected line caused by direct lightning strokes on the phase conductors and P (Il,cr
) is probability of a lightning current with steepness greater or equal to Il,cr
The efficiency of LFA lightning protection against direct lightning strokes can be expressed as the ratio of the number of lightning outages n0 for unprotected line to
n|0 for lines protected by LFA arresters .
K
= 
=
Where k is the outage reduction factor of lightning outages caused by direct lightning strokes.
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