A new simple, effective and inexpensive method for lightning protection of medium voltage overhead distribution line is using long flashover arresters (LFA). A new long flashover arrester model has been developed. It is designated as LFA-M. It offers great number of technical and economical advantages. The important feature of this modular long flashover arrester (LFA-M) is that it can be applied for lightning protection of overhead distribution line against both induced over voltage and direct lightning strokes. The induced over voltages can be counteracted by installing a single arrester on an overhead line support (pole). For the protection of lines against direct lightning strokes, the arresters are connected between the poles and all of the phase conductors in parallel with the insulators.
WHAT IS LIGHTNING ?
Lightning is an electrical discharge between cloud and the earth, between clouds or between the charge centers of the same cloud. Lightning is a huge spark and that take place when clouds are charged to at a high potential with respect to earth object (e.g. overhead lines) or neighboring cloud that the dielectric strength of the neighboring medium(air) is destroyed.
DESIGN OF LFA-M
An LFA-M arrester consists of two cables like pieces. Each cable piece has a semi conductive core of resistance R. The cable pieces are arranged so as to form three flashover modules 1,2,3 as shown in figure1. Semiconductive core of upper piece, whose resistance is R ,applies the high potential U to the surface of the lower piece at its middle. Similarly, the semi conductive core of the lower piece of the same résistance R applies the low potential 0 to the surfaces of the upper piece, also at its center. Therefore the total voltage U is applied to each flashover module at the same moment, and all three modules are assured conditions for simultaneous initiation of creeping discharges developing in to a single long flashover channel.
Tests have been shown that, as the line conductor is stressed by lightning over voltage impulse, flash over channel develop at different rates.Modules1 and 3 flashover first, followed by module 2 ,and thus, forming a rather long flashover channel along the LFA.
Due to long flashover path, a flashover does not give rise to a power arc as the arc extinguishes when the power frequency current crosses zero. This assures uninterrupted power supply of a LFA protected over head line.