Protection against Lightning

Lightning surges may cause serious damage to the equipments in the power system, either by direct or by strokes on the transmission lines that reach the equipment as travelling wave. It is necessary to provide against both kinds of surges.The most commonly used devices for protection against lightning surges are :

1. Earthing screen
2. Overhead ground wires
3. Lightning arresters (or surge diverters)
4. Surge absorber

1. 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 direct strokes.

2. Overhead ground wires

The principle consists in creating one or more preferred impact points for a lightning strike using low impedance, conductor elements. These then conduct and dissipate the lightning current into the ground.The ground wires are placed over the transmission tower above the line conductors. When direct lightning strokes occur, ground wires will take up all the lightning strokes instead of allowing them to line conductors. The ground wires are grounded at each tower through a low resistance.In this way, station equipment is protected against direct strokes.

3.Lightning arresters (or surge diverters)

Lightning arrestor is connected between the line and earth. It is a protective device which conducts the high voltage surges (due to both direct and indirect stroke) on the power system to the ground and protects the equipments connected to it.

Basic operation of Lightning Arrestor

It consists of a spark gap in series with a non linear resistor. One end is connected to the protected terminal and other end connected to ground.The spark gap length is design to protect normal line voltage not enough to produce an arc. Non linear resistance,at normal frequency power system voltages the resistance is high and resistance decreases with the surge voltage increases.

• Under normal working: It is an off stage ie, it conducts no current to earth.
• Over votlage condition : The air in the spark gap breaks down and an arc is formed, it provide a low resistance path for the surge to ground and protected the equipments.
• After the surge is over, the resistor offers high resistance to make the gap is non conducting.

Types of lightning arresters :

• Rod gap arrester
• Horn gap arrester
• Thyrite type lightning arrester / Valve type

4.Surge absorber

A surge absorber is a protective device which reduces the steepness of wave front of a surge by absorbing surge energy. Although both surge diverter and surge absorber eliminate the surge, the manner in which it is done is different in the two devices. The surge diverter diverts the surge to earth but the surge absorber absorbs the surge energy.