Protection Against Lightning Overvoltages 21d43

Lightning Overvoltage Protection

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Another point to be considered is that when the traveling wave reaches a point where there is a change in value of surge impedance of the line, a reflection of the wave will occur, the reflection being whole or partial depending on the amount of change in the surge impedance. The intensity of voltage, at the point of reflection will therefore be more, even double, depending upon the nature of reflection. The oscillograms of the lightning current indicate that the lightning current is very high initially and has short front times in the range of 10 ms. The low current portion follows next and lasts for a longer duration in the range of milliseconds. Although this last portion is low in magnitude, it can cause thermal damage to insulation owing to its longer duration. Time taken to reach the peak value and the rate of rise are also important. Based on practical data, it can be concluded that 50% of lightning stroke currents have a rate of rise higher than 7.5 kA/ ms. Only for 10% of lightning strokes, it is more than 25 kA/ms. Actual measurements of lightning overvoltages reveal that a maximum voltage of 5000 kV occurs on the transmission lines. But, in general the lightning strokes give rise to overvoltages of not more than 1000 kV on the transmission lines. The wave-front time ranges from 0.8 to 10 ms and the tail times are generally between 20 to 100 ms. The typical value of rate of rise of voltage is about 1 MV/ms.

17.2 PROTECTION AGAINST LIGHTNING OVERVOLTAGES Protection of transmission lines against lightning overvoltages is done by suitable line design, providing ground wires and using surge diverters. Overvoltages due to lightning strokes can be avoided or minimised in practice by 1. Shielding the overhead lines by using ground wires above the phase wires 2. Using ground rods 3. Using counter-poise wires 4. Use of lightning arresters or surge diverters

17.2.1

Lightning Protection using Shielding Wires or Ground Wires

Referring to Fig. 17.5, a ground wire is a conductor run parallel p p : Phase wires to the main conductor of the transmission line, ed on G : Ground wire G the same tower and earthed at every equally and regularly qs : Shielding angle spaced towers. It is run above the main conductor of the line. qs The ground wire shields the transmission-line conductor from p p induced charges, from clouds as well as from a lightning discharge. The effective protection given by the ground wire depends on the height of the ground wire above the ground and the shielding angle, qs, as shown in Fig. 17.5. The shielding angle of 30° is considered adequate for a tower height of 30 Fig. 17.5 Shielding of line conductor metres or less. If a positively charged cloud is assumed to be above the line, it indicates a negative charge on the portion below it, i.e., the transmission line. With the ground wire present, both the ground wire and the line conductor get the induced charge. But the ground wire is earthed at regular intervals, and as such, the induced charge is drained off. Moreover, the potential difference between the ground wire and the cloud and that between the ground wire and the transmission line wire will be in the inverse ratio of their respective capacitances. This is discussed as follows.

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Power System Protection and Switchgear

Referring to Fig. 17.6, i V1 = ____, the voltage to which the ground wire is stressed w C1 i V2 = ____, the voltage to which the phase wire is stressed w C2

and

As the phase wire is insulated by porcelain insulators and as the Cloud distance between the phase wire and ground is very small compared to the distance between the ground wire and cloud, C2 >> C1, considering air as a dielectric medium between the cloud C1 and the line V1 \ V1 >> V2 i Thus, the electric stress due to lightning is largely taken care off G i by the ground wire, reducing the electric stress on the phase wire with respect to ground to a very small value. Thus, the overvoltage on the V2 phase wire is minimised. Stress concentration never occurs on ground C2 wires as the electric charges leak to the ground. When the lightning strikes, it falls either on the tower or on the p ground wire. The lightning current and charge can flow to the ground Fig. 17.6 Reduction of electric stress through three possible paths. The first path is through the tower metal frame to ground. The second and third paths are through the ground wire in both the directions. As all three paths are leading to the ground, the current will be divided in these paths. Hence, the instantaneous magnitude of voltage to which the tower top can rise, is reduced considerably. Let us have a closer look at this tower-top instantaneous voltage with the help of Fig. 17.7. Lightning

IG I0

ZG

ZG

@ I0

IT

IT ZT

Fig. 17.7

Significance of tower footing resistance

VT = IT ZT where, VT ZT ZG I0 IG IT

IG

= instantaneous magnitude of voltage at tower top = surge impedance of the tower = surge impedance of the ground wire = lightning current = lightning current through the ground wire path = lightning current through tower path

ZT

Lightning Overvoltage Protection

453

The equation given above can be further simplified by using division of current in parallel paths as shown below. I0 ◊ ZG VT = IT ZT = _______ ◊ ZT ZT + ZG \

I0 ZT VT = ______ ZT 1 + ___ ZG

Thus, it can be concluded that VT is greatly dependent on the surge impedance of the tower which is nothing but the effective tower footing resistance. So, the final conclusion is that the use of ground wire must be ed by the reduction in the effective tower-footing resistance. This can be achieved by the use of driving ground rods and counter-poise wires connected to the tower-bottom foundation.

17.2.2

Use of Ground Rods

The ground rods are made up of galvanised iron or copper-bearing steel to prevent corrosion. The diameter of a rod is 15 mm. There are a number of rods each of 2.5 to 3 metre length driven into the ground to form a mesh. The purpose of the ground rods connected to the legs of the tower is to reduce the tower footing resistance. The type of soil also affects the configuration of ground rods. For soils that are hard, the rods have to be driven down to a greater depth. The design of the ground rods depends on the requirement of the value of the effective tower-footing resistance. By using 10 rods of 4 m length and a spacing of 5 m, a value of 10 W may be achieved for the effective tower-footing resistance.

17.2.3

Use of Counter-poise Wires

This is an alternative method for reducing the effective tower footing resistance. In this method, the wires are placed inside the ground at a depth of 0.5 to 1.0 m, running parallel to the transmission line conductors. Of course, these wires are connected to the tower legs. The length of these wires may be from 50 to 100 m. In this method, depth does not affect the value of the tower-footing resistance. The wires are laid at a sufficient depth so that the theft of wires can be prevented. To reduce the effective tower-footing resistance, it is necessary to use a large number of parallel wires than a single wire. With this method of counter-poise wires, the effective tower-footing resistance may be reduced to as low as 25 W. The only problem is the difficulty in laying counter-poise wires as compared to ground or driven rods. However, with the modern development in erection techniques, it may be easier to lay counter-poise wires.

17.2.4

Use of Lightning Arresters or Surge Diverters

To be really effective, a surge diverter should reduce the crest of the surge voltage and at the same time absorb the transient energy to an extent sufficient to prevent reflection. Also, it should be noted that the surge diverter should provide a path of low impedance, only when a traveling wave of surge reaches it, neither before nor after it. The lightning arrester should have the following characteristics: 1. The lightning arrester should not absorb any current during the normal operation, but during overvoltages it must provide an easy path to earth. 2. The impulse spark-over voltage of the arrester must safeguard the insulation of the terminal apparatus.

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Power System Protection and Switchgear

3. The diverter must be capable of carrying the discharge current for a short duration without being damaged. 4. The arrester must, after discharge, cease to carry any current, i.e., it must seal-in itself. 5. After operation, the arrester must be in a condition to accept and deal with ensuing surges i.e., there must be no failure of the arrester itself. Figure 17.8 shows the functioning of a simple lightning arrester. The successive stages of operation are shown diagrammatically. In Fig. (a), the front of the wave approaches the diverter which is protecting the terminal equipment (not shown), connected to the line to the left of the diverter. By (b), the surge has reached the diverter and in about 0.25 ms, the voltage has reached a value sufficient (V ) to break down the spark gaps. During (c), the surge current flows to earth. As the voltage applied increases, and just as rapidly, the resistance of the element decreases, thus permitting further surge energy to discharge, and so limiting the voltage impressed on the terminal apparatus to a safe value. At (d), the front of the wave is shown approaching and during (e) the tail of the wave ing the arrester, and in consequence the current through the arrester decreases while the resistance increases, reaching a stage when the current flow is interrupted by the spark gaps, thus sealing the diverter as shown by (f ). This entire operation takes place in a matter of microseconds, typically 30 ms. Surge

Line

V

LA

Fig. 17.8

(a)

(b)

(c)

(d)

(e)

(f)

Functioning of a lightning arrester

17.3 DIFFERENT TYPES OF LIGHTNING ARRESTERS USED IN PRACTICE The overhead transmission lines and connected apparatus, viz., transformers, switchgears and other apparatus are subjected to overvoltages due to lightning discharges caused by atmospheric disturbances and switching overvoltages due to switching operations. These overvoltages act across the insulation and as the time taken by these voltages to rise to their peak values is very small, very severe voltage stresses are imposed on insulation of transformer windings, circuit breakers, bushings and other equipment. The lightning arrester discharges these overvoltages to earth. Different types of lightning arresters (LA) used in practice are 1. Rod-gap LA 2. Sphere-gap LA