All the electrical equipment must be protected from severe damage due to the lightning strokes. The problem of protection of power systems against lightning can be studied under the following heads:- 1. Protection of power stations and substations from direct lightning strokes 2. Protection of overhead transmission lines from direct lightning strokes 3. Protection of electrical equipment from travelling waves.
1. Protection of Power Stations and Substations from Direct Lightning Strokes:
Power stations are usually indoor while substations may be indoor or outdoor. For protection of a structure from direct strokes there are three requirements which are to be fulfilled. These requirements are interception, conduction and dissipation.
These requirements involve:
(i) An object in good electrical connection with the earth so that the leader stroke may get attracted,
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(ii) A low impedance path joining this object to earth so that the discharge follows it in preference to any other path,
(iii) A low resistance connection with the earth body.
For 1, the upper portion of a metal structure may be employed. Alternatively a separate metallic system, often called the shield, either mounted on the structure or near to and above it may be provided. A particular shield configuration in the form of masts or overhead ground wires is considered to provide good shielding.
For 2, the requirements are:
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(i) Low resistance (i.e., adequate conductivity and cross- section, properly bounded joints, free from possible corrosion),
(ii) Low reactance (i.e., absence of sharp bends, or loops and short conductors),
(iii) And sufficient clearance from any other conducting objects that might provide separate uncontrolled path to ground.
Outdoor substations have much of equipment carried on metal gantries and the interconnection of the upper portion of these will screen the apparatus. Usually, there is suitable grounding provided.
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Shielding of the station and the incoming lines (about 0.8 km out from the station) to restrict the severity of the waves that can enter the station through the lines is a desirable supplement, particularly in the case of hv lines (66 kV and above) to the lightning arrester located in the station [Fig. 9.10(b)].
Where overhead ground wires cannot be provided on the incoming lines due to existing structure/construction, additional protection of the station equipment against direct lightning strokes can be provided by equipping each line with protector tubes at the entrance to the structure of the station and at each tower for a distance of about 0.8 km out from the station, as illustrated in Fig. 9.10(c). However, shielding of the power station/substation is the only way of eliminating direct strokes to the station itself.
2. Protection of Overhead Transmission Lines from Direct Lightning Strokes:
The two methods of protecting overhead transmission lines against lightning strokes are:
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(i) Overhead ground wires and
(ii) Expulsion protector tubes.
(i) Protection of Overhead Transmission Lines from Direct Lightning Strokes by Ground Wires:
A ground wire is a form of lightning protection employing a conductor or conductors, well-grounded at regular intervals, preferably at each support (pole or tower), and attached from support to support above the transmission line conductors.
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No doubt the ground wire can be run below the line but running of ground wire above the line is considered better as it provides more effective shield. The ground wire shields the phase or line conductors by attracting itself the lightning strokes which, in the absence of ground wire, would strike the line conductors.
The elevation at which the ground wire is strong is likewise determined by calculation and selected so that the line conductors come within the zone of protection of the respective ground wire. For reliable protection the protective angle α is taken equal to 20-30 degrees. This is the angle between the vertical line through the ground wire axis and the line passing from the ground wire axis to the outermost line or phase conductor.
Coupling Factor:
The entire ground wire voltage does not show up across the line insulators as a voltage of the same polarity as the ground wire is also induced in the line conductors. The ratio of the induced voltage on the line to the potential of ground wire is called the coupling factor, i.e.,
The voltage across the line conductor and earth is the product of coupling factor and ground wire potential.
Consider an arrangement of line conductors and ground wire, as illustrated in Fig. 9.12. When the ground wire is struck by a lightning stroke at point A, currents flow as indicated in the figure. Currents I1 and I2 flow in opposite directions from point A. Again at point B, the current subdivides and let I4 flow, through earth connecting wire, to the ground.
Suppose the ground resistance RE is high. The currents in the down lead and into the earth (I4) causes a voltage drop in the ground connection, thereby raising the potential of ground (or shield) wire above true earth potential.
Let this be Vg –
Potential between the line conductor and earth,
Where k is the coupling factor and is given as –
where h is the distance from phase conductor to ground wire, h1 is the distance between conductor image and ground, H is the height of ground wire above ground and r is the radius of ground wire.
In the absence of corona the electrostatic and electromagnetic coupling factors are equal.
In case of corona, electrostatic coupling increases while the electromagnetic coupling remains unaffected.
The electromagnetic coupling factor is determined as above using the actual diameter of the ground wire while the electrostatic coupling factor is determined by using the increased diameter of the ground wire due to corona as follows –
where r1 is the increased radius of ground wire and Kfs is the electrostatic coupling factor,
The resultant coupling factor,
where kfm is the electromagnetic coupling factor.
Figure 9.13 represents the equivalent electrical circuit corresponding to arrangement shown in Fig. 9.12. C1 and C2 are the capacitances between conductor and earth and between ground wire and conductor respectively. Let V1 and V2 be the potential drops across these capacitances.
Also from the equivalent electrical circuit diagram shown in Fig. 9.13, we have –
V2 = Vg – V1 …(9.13)
From Eqs. (9.13), (9.9) and (9.8), we have –
V2 = Vg – V1 = Vg – K Vg = Vg (1 – K) = I4 RE (1 – K) …(9.14)
i.e., the potential difference between the ground wire and the line, V2 may be high. If the clearance between the down lead and the line conductor is not sufficient or if the support is metallic and the insulator is small, a flashover will be caused from the ground wire to the line conductor.
This is just as bad as a direct hit on the line conductor with flashover from it to ground because either may cause a short circuit. Sometimes, special measures such as counterpoise rods (horizontal rods buried in ground) may be necessary to obtain reasonable ground resistance (about 5 – 10 Ω) and sometimes it may be necessary to offset the down leads so that adequate clearances are obtained.
Besides taking the brunt of a direct stroke, the ground wire reduces the voltage electrostatically or electromagnetically induced in the conductors by the discharge of a neighbouring cloud. For example, referring to Fig. 9.14, if C1 is the capacitance of cloud to the line and C2 the capacitance of the line to ground.
The induced voltage on the line is 
times the cloud voltage. The presence of the ground wire above the line causes a considerable increase in C2 and therefore, a reduction of the induced voltage on the line.
The induced voltage could be very much reduced by an array of ground wires but this is too expensive to install in practice. Peak has made a laboratory study of the protective effect of ground wires and has found that a single ground wire reduces the induced lightning voltage to one-half of that without ground wire; for two ground wires the reduction is one-third, while for three ground wires it is to one- fourth. These results were obtained under the favourable conditions of good earths and low impedance for the earth connections.
The ground wire also affords an additional protective effect by causing an attenuation of any travelling waves that are set up in the lines by acting as a short-circuited secondary of the line conductors. For this reason its resistance should not be too large. Ground wire is usually made of steel, which has a high permeability and thus possesses a resistance which increases with frequency.
The objections to the ground wire are the additional cost, and the possibility of the wire breaking and falling across the line conductors, thus causing a direct short circuit. Failure due to the latter cause, however, is rare, as substantial galvanized stranded steel conductors are usually employed. Indeed, such a steel conductor joining the tops of the towers generally adds greatly to the mechanical strength and stability of the line, particularly if the line is of the flexible type.
Ground wires are extensively used for direct-stroke protection of transmission lines for voltages of 110 kV and upward (up to 500 kV). Ground wires are usually strung on all vital transmission lines and on all sections of transmission lines running through regions subject to frequent lightning storms.
The selection of size, number and arrangement of ground wires is of great importance in line design. The selection of size of ground wire is based on the consideration of mechanical strength rather than electrical considerations.
In practice, one ground wire is generally used, which allows the intensity of over-voltages to be considerably reduced, and lessens the work demanded from the lightning arresters connected to the system. In exposed situations, or lines subject to severe lightning disturbances, it would probably be good practice to install one or two additional wires. In case of horizontal spacing of conductors on the tower, two ground wires are run on the top portion of the tower.
On wood-pole transmission lines within the working voltage range of 33 to 110 kV, ground wires are provided only at the approaches to the power station and system substations. No special means of lightning protection is provided on the remaining sections of such lines as advantage can be taken of the insulation provided by the wood-pole supports. By installing the ground wires on the substation approaches of the lines the latter will be guarded against the possibility of lightning strokes close to the substations.
(ii) Protection of Overhead Transmission Lines from Direct Lightning Strokes by Protector Tube:
Even after reduction in the induced voltage by using a ground wire, there still exist over-voltages in the system which must be removed by using additional protective devices such as lightning arrester that bypasses the surges to the ground. Another device that is quite common in use is the protector tube.
Expulsion protector tube consists of a backlisted fibre tube containing two built-in electrodes between which an internal gap is provided. An outer electrode or arcing horn, made from steel wire 5 or 6 mm in dia is attached to the bolt at the upper end of the protector tube (Fig 9 15) to provide an external spark gap between it and a line conductor.
The internal gap can be adjusted by turning the tube about the bolt which secures the tube to the bracket. The external gap is adjusted to templates made from wire 3 or 4 mm in diameter.
On a transmission tower one tube is often mounted below each line so that the upper electrode is connected to an arc-shaped horn located at the proper distance below the line, thus forming a series gap G2 with it [Fig. 9.16(a)], The lower electrode is solidly grounded.
When a surge of sufficient voltage travels along the phase conductor and reaches the point where the expulsion tube is mounted, both of the series air gaps (internal air gap G1 and external air gap G2) breakdown and drain the surge current to ground through the tube and its earthing conductor; thereby reducing the crest value of surge voltage.
On breakdown of the tube gaps on two or three phases, or on one phase of solidly-grounded neutral circuits, the operating voltage simultaneously initiates a flow of short circuit or power current. This current must flow through the tubes and set up arcs between their spark gaps.
The high temperature of the arc across the gap in the tubes then produces a large amount of gases due to decomposition of some of the tube material. These gases flash out of the tube under pressures reaching from 100 to 500 atm and intensely deionize the arc. The latter is thus extinguished and the circuit insulation returned to its normal value with respect to earth.
Arc extinction duration will be only one or two half-periods. This interval is too short for the protective relays of the line to come into action, the circuit breaker remains closed and the line remains in operation. Immediately after the gases have been expelled and the arc suppressed, every tube is ready for a new operation.
The purpose of external air gap G2 is to isolate the expulsion tube from the line conductor. Failure to provide the external gap would otherwise place the tube at the operating potential of the conductor and cause flow of leakage currents over the tube surface and to eventual carbonizing of the tube material and final destruction of the tube.
3. Protection of Electrical Equipment from Travelling Waves:
The ground wire or earthling screen used for the protection of overhead lines and power stations and substations not only provides an adequate protection against lightning but also reduces the over-voltages induced electrostatically or electromagnetically, but such shielding is inadequate in providing protection against travelling waves which may reach the terminal equipment and cause damage to it.
The damages that may be caused by travelling waves are:
i. The high peak or crest voltage of the surge may cause flashover in the internal winding thereby spoil the winding insulation.
ii. The steep wave front of the surge may cause internal flashover between inter-turns of the transformer.
iii. The high peak voltage of the surge may cause external flashover, between the terminals of the electrical equipment which may result in damage to insulators.
iv. The steep wave front resulting into resonance and high voltages may cause internal or external flashover of an un-predicable nature causing building up of the oscillation in the electrical apparatus.
Thus it is absolutely necessary to provide some protective device at the power stations or substations to prevent transformers and other equipment from being subjected to travelling surges reaching there. The most common devices used for protection of equipment at the substations against travelling waves are lightning arresters or surge diverters.
A surge diverter is a device that is connected between line and earth, i.e., in parallel with the equipment to be protected at the substation.
The action of the surge diverter can be studied in reference to Fig. 9.17:
When a travelling wave reaches the diverter, it sparks-over at a certain prefixed voltage as illustrated by point A in the figure, and provides a conducting path of relatively low impedance between the line and ground.
The surge impedance of the line restricts the amplitude of current flowing to ground. This is necessary in order to protect the insulation of the equipment. Fig. 9.17 shows the shape of voltage and of current at the diverter terminals.
It should, however, be noted that the surge diverter should provide a path of low impedance only when the travelling surge reaches the surge diverter, neither before it nor after it.
An ideal surge diverter should have the following characteristics:
i. It should not draw any current during normal operating conditions, i.e., its spark over voltage must be above the normal or abnormal power frequency that may occur in the system.
ii. Any abnormal transient voltage above the breakdown value must cause it to breakdown as quickly as possible so that it may provide a conducting path to ground.
iii. When the breakdown have taken place, it should be capable of carrying the resulting discharge current without getting damaged itself and without the voltage across it exceeding the breakdown value.
iv. The power frequency current following the breakdown must be interrupted as soon as the transient voltage has fallen below the breakdown value.
There are many types of surge diverters which are used to protect the power system.
The choice of lightning arrester depends upon the following factors:
(i) Voltage of the line.
(ii) Frequency of the lightning.
(iii) Cost.
(iv) Weather conditions.
(v) Reliability.