1. Direct stroke
2. Indirect stroke
1. Direct stroke: In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the subject equipment e.g. an overhead line. From the line, the current path may be over the insulators down the pole to the ground.
The over voltages set up due to the stroke may be large enough to flashover this path directly to the ground. The direct strokes can be of two types viz. (i) Stroke A and (ii) stroke B.
(i) In stroke A, the lightning discharge is from the cloud to the subject equipment i.e. an over head line in this case as shown in Figure.
- The cloud will induce a charge of opposite sign on the tall object (e.g. an overhead line in this case). When the potential between the cloud and line exceeds the breakdown value of air, the lightning discharge occurs between the cloud and the line.
(ii) In stroke B, the lightning discharge occurs on the overhead line as a result of stroke A between the clouds as shown in Figure. There are three clouds P, Q and R having positive, negative and positive charges respectively.
- The charge on the cloud Q is bound by the cloud R. If the cloud P shifts too near the cloud Q, then lightning discharge will occur between them and charges on both these clouds disappear quickly.
- The result is that charge on cloud R suddenly becomes free and it then discharges rapidly to earth, ignoring tall objects.
2. Indirect stroke: Indirect strokes result from the electro statically induced charges on the conductors due to the presence of charged clouds. This is illustrated in Figure.
- A positively charged cloud is above the line and induces a negative charge on the line by electrostatic induction. This negative charge, however, will be only on that portion of the line right under the cloud and the portions of the line away from it will be positively charged as shown in Figure.
- The induced positive charge leaks slowly to earth via the insulators. When the cloud discharges to earth or to another cloud, the negative charge on the wire is isolated as it cannot flow quickly to earth over the insulators.
- The result is that negative charge rushes along the line is both directions in the form of travelling waves. It may be worthwhile to mention here that majority of the surges in a transmission line are caused by indirect lightning strokes.
HARMFUL EFFECTS OF LIGHTNING:
- A direct or indirect lightning stroke on a transmission line produces a steep-fronted voltage wave on the line.
- The voltage of this wave may rise from zero to peak value (perhaps 2000 kV) in about I is and decay to half the peak value in about Sits. Such a steep-fronted voltage wave will initiate travel ling waves along the line in both directions with the velocity dependent upon the Land C parameters of the line.
(i) The traveling waves produced due to lightning surges will shatter the insulators and may even wreck poles.
(ii) If the traveling waves produced due to lightning hit the windings of a transformer or generator, it may cause considerable damage. The inductance of the windings opposes any sudden passage of electric charge through it.
Therefore, the electric charges “up” against the transformer (or generator). This induces such an excessive pressure between the windings that insulation may breakdown, resulting in the prod of arc. While the normal voltage between the turns is never enough to start an arc; once the insulation has broken down and an arc has been started by a momentary over voltage, the line voltage is usually sufficient to maintain the arc long enough to severely damage the machine.
(iii) If the arc is initiated in any part of the power system by the lightning stroke, this arc will set up very disturbing oscillations in the line. This may damage other equipment connected to the line.