TYPES OF LIGHTNING STROKES

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 flash over this path directly to the ground. The direct strokes can be of two types:

(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 Fig 4 (i).The cloud will induce a charge of opposite sign on the tall object (eg. an overhead line in this case). When the potential between the clouds 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 stroIj4 between the clouds as shown in Fig. 4 (ii). 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 electrostatically induced charges on the conductors due to the presence of charged clouds. This is illustrated in fig 5. A positively cloud is above the line and induces a negative charge on the line by electrostatic induction. This negative charge 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 Fig 5. 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 a negative charges rush along the line is both directions in the form of travelling waves. Majority of surges in transmission lines are caused by indirect lightning strokes.

Written by arjun on March 20th, 2009 with 2 comments.
Read more articles on Lightning and Power System and Surge Over Voltages and Switchgear&Protection.

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