21-09-2008, 10:09 AM
Introduction
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 overvoltages 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.
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.
TYPES OF LIGHTNING STROKES
There are two main ways in which the lightning may strike the power system . They are
1. Direct stroke
2. Indirect stroke
Direct Stroke
In direct stroke, the lightning discharge is directly from the cloud to the an overhead line. From the line, current path may be over the insulators down to the pole to the ground. The over voltage set up due to the stroke may be large enough to flashover this path directly to the ground. The direct stroke can be of two types
1. stroke A
2. stroke B
In stroke A, the lightning discharge is from the cloud to the subject equipment(e.g. overhead lines). The cloud will induce a charge of opposite sign on the tall object. When the potential between the cloud and line exceed the breakdown value of air, the lightning discharge occurs between the cloud and the line.
In stroke B the lightning discharge occurs on the overhead line as the result of stroke A between the clouds. There are three clouds P,Q and R having positive, negative and positive charge respectively. Charge on the cloud Q is bound by cloud R.If the cloud P shift too nearer to cloud Q,Then lightning discharge will occur between them and charges on both these cloud disappear quickly. The result is that charge on cloud R suddenly become free and it then discharges rapidly to earth, ignoring tall object.
