There are two main ways in which lightning affects a line: 1. Direct Lightning Strokes 2. Surge Due to Electrostatic Induction.
Way # 1. Direct Lightning Strokes:
A direct stroke can take place in several ways. In one way a cloud attains a large amount of charge, it induces an opposite charge on the tall objects, such as temples, churches or mosques. In between these two charges an electrostatic field is set up and when the intensity of electrostatic field becomes sufficiently great to ionize the neighbouring air, the air breaks down and discharge takes place between the cloud and the object. Such a discharge is known as A stroke, and is characterised by the comparatively long time taken to produce it and the fact that it strikes the highest and most sharply pointed object in the neigbourhood.
Another way results in a much more sudden stroke, which is produced in the manner shown in Fig. 9.6. Three clouds are involved. Clouds 1 and 3 are positively charged and cloud 2 is negatively charged. The potential of cloud 3 is reduced due to presence of the charged cloud 2. On flashover from cloud 1 to cloud 2, both these clouds are discharged rapidly and cloud 3 assumes a much higher potential and flashes to earth very rapidly.
This is the B stroke, and is characterized by its rapidity and the fact that it ignores tall objects and reaches ground in a random manner. Thus the stroke B is the result of stroke A between clouds 1 and 2.
It is most dangerous stroke. No protection can be designed to avoid a stroke B whereas protection can be provided against stroke A by channelizing the charge through a lightning conductor placed at the highest point on the building and earthling properly at the other end. The stroke B is sometimes called the induced stroke.
For a transmission line, there is practically no protection against the direct lightning strokes. In case of a lightning stroke landing on a conductor, tremendous amount of overvoltage waves will travel on both the sides of the transmission line from the striking point and shall shatter the transmission line towers and insulators.
In case of a stroke quite away from the generating station, the energy will be dissipated in the impedance of the line and the generating plant will not get damaged significantly. But when the stroke is quite near, the plant will get certainly damaged severely, if it is not protected against such over-voltages.
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Luckily direct strokes are very rare and if such strokes occur, it may be difficult to save the electrical equipment from such strokes completely. The most that can be done from protective devices is that they will limit the damage and prevent the resulting travelling waves affecting plant as far as possible.
A lightning discharge may have currents in the range of 10 kA to 90 kA and the duration of the flash may be of the order of one-millionth of a second. The real danger of the stroke consists in its short duration i.e., the over-voltages will have travelling waves of very steep wave front which is actually responsible for the damage.
Way # 2. Surge Due to Electrostatic Induction:
The most of the surges in a transmission system are, due to lightning, and are caused by electrostatic induction in the manner illustrated in Fig. 9.7. A cloud electrostatically charged, and lying above a transmission line, will induce in the adjacent section of the line a corresponding charge of opposite sign known as a bound charge. This charge has the maximum value below the cloud and then gradually tails away.
At the distant end of the line, charges of like sign to that in the cloud are located. This charge on the line will not flow since it is a bound charge. The positive charge on the far ends of the line will however leak to earth slowly through insulators, metallic parts etc., thus leaving only the negative charge on the line.
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So long as the cloud remains stationary the static distribution on the line persists. In this static condition though no oscillations are set up but the potential of the line immediately below the cloud may rise to such a value that ‘spill-over’ at the insulators is caused and thus a static over-potential is a dangerous phenomenon.
When the cloud discharges to earth or to another cloud, the negative charge on the line is isolated as it cannot passes quickly to earth over the insulators. The line thus acquires a high negative potential, which is maximum at the place nearest the cloud and drops slowly to a small value at a distance.
The charge will flow from a higher to a lower potential and the result is travelling waves in both directions. The two waves will be equal and thus each wave will have half the potential of the charge at the time of discharge of the cloud; they will also have the space-voltage distribution of the original charge, as illustrated in Fig. 9.8.
The waves travel in exactly the same way as the waves due to switching, so that current at any point of the line is the voltage divided by the surge impedance. In a line without resistance or leakage the waves travel without change in shape, but the effect of resistance and leakage is to attenuate the wave and to flatten the wave front.
The above travelling waves will be of quite large amplitude (10 to 15 kV) and shall have very steep wave fronts which can damage the unprotected equipment connected to the line and hence these must be passed to the earth.
The charge induced on the conductor is given as –
Q = CV …(9.2)
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where C is the capacitance between conductor and ground –
and V = E h …(9.3)
where E is the field gradient and h is the height of the line from ground.