Overvoltage Lightning

 Lightning

An electric discharge between cloud and earth, between clouds or between the charge centres of the same cloud is known as lightning. 

Lightning is a huge spark and takes place when clouds are charged to such a high potential (+ve or −ve) with respect to earth or a neighbouring cloud that the dielectric strength of neighbouring medium (air) is destroyed.  There are several theories which exist to explain how the clouds acquire charge.  The most accepted one is that during the uprush of warm moist air from earth, the friction between the air and the tiny particles of water causes the building up of charges.  When drops of water are formed, the larger drops become positively charged and the smaller drops become negatively charged.  When the drops of water accumulate, they form clouds, and hence cloud may possess either a positive or a negative charge, depending upon the charge of drops of water they contain.  The charge on a cloud may become so great that it may discharge to another cloud or to earth and we call this discharge as lightning.  The thunder which accompanies lightning is due to the fact that lightning suddenly heats up the air, thereby causing it to expand.  The surrounding air pushes the expanded air back and forth causing the wave motion of air which we recognise as thunder.

Mechanism of  Lightning Discharge 

Let us now discuss the manner in which a lightning discharge occurs.  When a charged cloud passes over the earth, it induces equal and opposite charge on the earth below. Fig. shows a negatively charged cloud inducing a positive charge on the earth below it.  As the charge acquired by the cloud increases, the potential between cloud and earth increases and, therefore, gradient in the air increases. When the potential gradient is sufficient (5 kV*/cm to 10 kV/cm) to break down the surrounding air, the lightning stroke starts.  The stroke mechanism is as under : 

(i) As soon as the air near the cloud breaks down, a streamer called leader streamer or pilot streamer starts from the cloud towards the earth and carries charge with it.  The leader streamer will continue its journey towards earth as long as the cloud, from which it originates feeds enough charge to it to maintain gradient at the tip of leader streamer above the strength of air.  If this gradient is not maintained, the leader streamer stops and the charge is dissipated without the formation of a complete stroke.  In other words, the leader streamer will not reach the earth.  Fig. shows the leader streamer being unable to reach the earth as gradient at its end cloud not be maintained above the strength of air.  It may be noted that current in the leader streamer is low (<100 A) and its velocity of propagation is about 0·05% that of velocity of light.  Moreover, the luminosity of leader is also very low.

(ii) In many cases, the leader streamer continues its journey towards earth [See Fig.] until it makes contact with earth or some object on the earth.  As the leader streamer moves towards earth, it is accompanied by points of luminescence which travel in jumps givingrise to stepped leaders.  The velocity of stepped leader exceeds one-sixth of that of light and distance travelled in one step is about 50 m.  It may be noted that stepped leaders have sufficient luminosity and give rise to first visual phenomenon of discharge. 

(iii) The path of leader streamer is a path of ionisation and, therefore, of complete breakdown of insulation.  As the leader streamer reaches near the earth, a return streamer shoots up from the earth [See Fig.] to the cloud, following the same path as the main channel of the downward leader.  The action can be compared with the closing of a switch between the positive and negative terminals; the downward leader having negative charge and return streamer the positive charge.  This phenomenon causes a sudden spark which we call lightning.  With the resulting neutralisation of much of the negative charge on the cloud, any further discharge from the cloud may have to originate from some other portion of it. The following points may be noted about lightning discharge : 

(a) A lightning discharge which usually appears to the eye as a single flash is in reality made up of a number of separate strokes that travel down the same path.  The interval between them varies from 0·0005 to 0·5 second.  Each separate stroke starts as a downward leader from the cloud. 

(b) It has been found that 87% of all lightning strokes result from negatively charged clouds and only 13% originate from positively charged clouds. 

(c) It has been estimated that throughout the world, there occur about 100 lightning strokes per second. 

(d) Lightning discharge may have currents in the range of 10 kA to 90 kA.

Types  of  Lightning Strokes 

There are two main ways in which a lightning may strike the power system (e.g. overhead lines, towers, sub-stations etc.), namely; 

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 overvoltages 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 overhead line in this case as shown in Fig. 24.5 (i).  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 Fig. 24.5 (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. Two points are worth noting about direct strokes.  Firstly, direct strokes on the power system are very rare.  Secondly, stroke A will always occur on tall objects and hence protection can be provided against it.  However, stroke B completely ignores the height of the object and can even strike the ground.  Therefore, it is not possible to provide protection against stroke B. 

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. 24.6.  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 Fig.  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 1 µs and decay to half the peak value in about 5µs.  Such a steep-fronted voltage wave will initiate travelling waves along the line in both directions with the velocity dependent upon the L and C parameters of the line. 
(i) The travelling waves produced due to lightning surges will shatter the insulators and may even wreck poles. 
(ii) If the travelling 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 “piles up” against the transformer (or generator).  This induces such an excessive pressure between the windings that insulation may breakdown, resulting in the production of arc.  While the normal voltage between the turns is never enough to start an arc, once the insulation has

* The degree of protection by ground wires depends upon the shielding angle (i.e. the angle subtended by the outermost line conductors at the ground wire).  The lower this angle, the greater the protection.

broken down and an arc has been started by a momentary overvoltage, 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.

Protection  Against  Lightning 

Transients or surges on the power system may originate from switching and from other causes but the most important and dangerous surges are those caused by lightning.  The lightning surges may cause serious damage to the expensive equipment in the power system (e.g. generators, transformers etc.) either by direct strokes on the equipment or by strokes on the transmission lines that reach the equipment as travelling waves.  It is necessary to provide protection against both kinds of surges.  The most commonly used devices for protection against lightning surges are : 
(i) Earthing screen 
(ii) Overhead ground wires 
(iii) Lightning arresters or surge diverters Earthing screen provides protection to power stations and sub-stations against direct strokes whereas overhead ground wires protect the transmission lines against direct lightning strokes.  However, lightning arresters or surge diverters protect the station apparatus against both direct strokes and the strokes that come into the apparatus as travelling waves.