Protection Against Lightning

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.  We shall briefly discuss these methods of protection.

The  Earthing Screen 

The power stations and sub-stations generally house expensive equipment.  These stations can be protected against direct lightning strokes by providing earthing screen.  It consists of a network of copper conductors (generally called shield or screen) mounted all over the electrical equipment in the sub-station or power station.  The shield is properly connected to earth on atleast two points through a low impedance.  On the occurrence of direct stroke on the station, screen provides a low resistance path by which lightning surges are conducted to ground.  In this way, station equipment is protected against damage.  The limitation of this method is that it does not provide protection against the travelling waves which may reach the equipment in the station.

Overhead Ground  Wires

The most effective method of providing protection to  transmission lines against direct lightning strokes is by the use of overhead ground wires as shown in Fig. 24.7.  For simplicity, one ground wire and one line conductor are shown. The ground wires are placed above the line conductors at such positions that practically all lightning strokes are intercepted by them (i.e. ground wires).  The ground wires are grounded at each tower or pole through as low resistance as possible.  Due to their proper location, the *ground wires will take up all the lightning strokes instead of allowing them to line conductors.

When the direct lightning stroke occurs on the transmission line, it will be taken up by the ground wires.  The heavy lightning current (10 kA to 50 kA) from the ground wire flows to the ground, thus protecting the line from the harmful effects of lightning.  It may be mentioned here that the degree of protection provided by the ground wires depends upon the footing resistance of the tower.  Suppose, for example, tower-footing resistance is R1 ohms and that the lightning current from tower to ground is I1 amperes.  Then the tower *rises to a potential Vt given by ; Vt = I1R1 Since Vt (= I1R1) is the approximate voltage between tower and line conductor, this is also the voltage that will appear across the string of insulators.  If the value of Vt is less than that required to cause insulator flashover, no trouble results.  On the other hand, if Vt is excessive, the insulator flashover may occur.  Since the value of Vt depends upon tower-footing resistance R1, the value of this resistance must be kept as low as possible to avoid insulator flashover.

Advantages 

(i) It provides considerable protection against direct lightning strokes on transmission lines. 

(ii) A grounding wire provides damping effect on any disturbance travelling along the line as it acts as a short-circuited secondary. 

(iii) It provides a certain amount of electrostatic shielding against external fields.  Thus it reduces the voltages induced in the line conductors due to the discharge of a neighbouring cloud. 

Disadvantages

(i) It requires additional cost. 

(ii) There is a possibility of its breaking and falling across the line conductors, thereby causing a short-circuit fault.  This objection has been greatly eliminated by using galvanised stranded steel conductors as ground wires.  This provides sufficient strength to the ground wires.

Lightning  Arresters

The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against travelling waves which may reach the terminal apparatus.  The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground.Fig. (i) shows the basic form of a surge diverter.  It consists of a spark gap in series with a non-linear resistor.  One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.  The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap but a dangerously high voltage will break down the air insulation and form an arc.  The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa.  This is clear from the *volt/amp characteristic of the resistor shown in Fig. (ii).

Action.  

The action of the lightning arrester or surge diverter is as under : 

(i) Under normal operation, the lightning arrester is off the line i.e. it conducts **no current to earth or the gap is non-conducting. 

(ii) On the occurrence of overvoltage, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground.  In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line. 

(iii) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to overvoltage, the arc would be a short-circuit on the power system and may cause power-follow current in the arrester.  Since the characteristic of the resistor is to offer high resistance to high voltage (or current), it prevents the effect of a short-circuit.  After the surge is over, the resistor offers high resistance to make the gap non-conducting. 

Two things must be taken care of in the design of a lightning arrester.  Firstly, when the surge is over, the arc in gap should cease.  If the arc does not go out, the current would continue to flow through the resistor and both resistor and gap may be destroyed.  Secondly, I R drop (where I is the surge current) across the arrester when carrying surge current should not exceed the breakdown strength of the insulation of the equipment to be protected.