In this article we will discuss about mechanism of lightning and over-voltages caused by it.
Benjamin Franklin performed his famous experiment of flying kite in thunder cloud in 1745. Before his discovery the lightning used to be considered to be “Act of God.” He also proved that the lightning stroke is because of the discharge of electricity. He also invented lightning rods for fixation on tall buildings and grounding for their protection from lightning strokes. Thus Benjamin Franklin is a pioneer scientist in this field.
Lightning is a huge spark, which is due to the electrical discharge taking place between the clouds, within the same cloud and between the cloud and earth. Large number of discharges occurs between or within clouds than to earth, and enough of them terminate on to the earth and result in serious hazards.
Many theories have been put forth to explain the phenomenon where by a thunder cloud becomes electrically charged. However, for our purpose it is quite sufficient to assume that as a result of certain atmospheric processes that take place during thunderstorms, charges are accumulated in clouds (or portions of clouds) and equal and opposite charges are induced in the earth beneath.
According to another theory, the positive as well as negative ions in the air attach themselves to the small dust particles and small drops of water suspended in the air and because of polarization by induction, they get charged to a certain potential under the storm conditions. Thus a cloud gets charged either positively or negatively. Whenever such a cloud passes over the earth, it induces opposite charges in the earth below.
Lightning discharge requires the puncture of air between the cloud and the earth. For breakdown of air at STP condition the electric field required is 30 kV/cm peak. But in a cloud where the moisture content in the air is large and also because of the high altitude (low pressure) it is reduced to 10 kV/cm.
The mechanism of lightning discharge can be well explained with the help of Fig. 9.3. Whenever a local charge concentration near a cloud causes the potential gradient to exceed the critical breakdown value, the air surrounding gets ionized and an electric streamer starts from the cloud towards the earth.
The current in the streamer is of the order of 100 A and the speed of the streamer is 0.15 m/µs. This streamer is known as pilot streamer because it leads to the lightning phenomenon. The process is shown in Fig. 9.3(a). Depending upon the state of ionization of the air surrounding the streamer, it is branched in several paths and it is known as stepped leader. This is illustrated in Fig. 9.3(b).
The leader steps are of the order of 50 m in length and are accomplished in about a microsecond. The charge is brought from the cloud through the already ionized path to these pauses. The air surrounding these pauses is again ionized and the leader in this manner reaches the ground. This is illustrated in Fig. 9.3(c).
Once the stepped leader has made contact with the ground it is believed that a power return stroke [Fig. 9.3(c)] moves very fast up towards the cloud through the already ionized path by the leader. This streamer is very intense where the current varies from 1 k A to 200 kA and the speed is about 10 percent of that of light. It is here where the negative charge of the cloud is being neutralized by the positive induced charge on the earth [Fig. 9.3(d)].
This instant gives rise to lightning flash which is observed by us. There may be another cell of charges in the cloud near the neutralized charged cell. This charged cell will try to neutralize through this ionized path. This streamer is called the dart leader [Fig. 9.3(e)]. The velocity of the dart leader is about 3% of that of light. The effect of the dart leader is much more severe than that of the return stroke.
The discharge current in the return streamer is relatively very large but as it lasts only for a few microseconds the energy contained in it is small and, therefore, this streamer is called the cold lightning stroke while the dart leader is called the hot lightning stroke because even though current in this leader is relatively smaller but it lasts for some milliseconds and so the energy contained in it is relatively larger.
It is found that each thunder cloud may contain as many as 40 charged cells and a heavy lightning stroke may occur. This is known as multiple or repetitive stroke.
Over-Voltages due to Lightning:
If an overhead line is struck by lightning (direct stroke), the voltage rise at the point is given by –
Vd = Id z0 …(9.4)
where Z0 is the surge impedance of the line and ld is the discharge current.
The above equation would have been true if the travelling waves flow only in one direction. But since they travel in both directions, the current is halved and overvoltage is given by the expression
Vd = [1/2 (IdZ0)] …(9.5)
A direct stroke may cause a potential of many million volts. For example if Id is taken 30 k A and Z0 = 500 Ω, then –
Vd = (1/2) x 30 x 1,000 x 500 = 7.5 x 106 V.
Due to direct stroke on a line, the nearby lines are also subjected to over-voltages, but of less magnitude, through electromagnetic coupling. Experiences have shown that voltage rises induced by side strokes may attain a value of 2 million volts but that about 90% of such voltages are below 500 kV. These voltages may also spark over on the insulators.
A direct stroke on the tower or earth conductor causes the voltage rise of the latter to a value given by the expression –
Vd = Id RE + [L (di/dt)] …(9.6)
where Id is the discharge current, RE is the tower footing resistance, L is the inductances of the tower and di/dt is the slope of current wave.
For EHT towers of normal height, L is taken equal to 10 µH. Taking RE = 5 Ω, ld = 30 kA and di/dt = 10 kA/µs, we have –
Vd = [30 x 103 x 5 + 10 x 10-6 x (10 x 1000)/(1 x 10-6)] = 250 kV
If Vd exceeds the spark over voltage of the insulators a backward flashover from tower to conductors will give rise to travelling waves on the conductors in both directions which on reaching the terminal equipment may subject the same to dangerous voltage rises. For all purposes only travelling waves through the lines are considered for protection.