Earthing Screen
- It Consist of a network of copper or aluminium conductors mounted above all the equipment's installed in the sub-station or generating station.
- This conductor is also known as shield and is connected to earth at least two time through low impedance.
- This type of protection is used for substations and power stations because they contain very expensive equipment in their premises.
- As the earth screen is located above all the equipments , direct lighting stroke likely to occurs on it.
- Thus it provides the protection against direct lighting stroke.
- But the main disadvantage of it is that it is unable to provide protection against travelling waves.
Overhead Ground Wires
- Overhead Transmission lines have the thousands of kilometer range run also they are exposed to atmosphere hence, the chances of lightening stroke on overhead lines are more than any other equipments of power system.
- The ground wire provides the perfect protection against the lightening stroke for overhead lines.
- The ground wire is placed above the line conductors i.e at the top of the tower.
- The arrangement is as shown in fig.
- When the direct lightening stroke occurs on the transmission line, it is subjected on the ground wire instead of transmission line since the ground wire tallest object in this case.
- Since the ground wire is earthed at each pole/tower through low impedance, the heavy lightening current from the ground wire flows to the ground , thus protect the overhead transmission line from the harmful effects of lightening.
- The tower footing resistance of the tower decides the degree of protection provided by the ground wires.
- Suppose Vt is the approximate voltage between tower and line conductor and Vt is also the Voltage that will appear the string of insulator, which is given by I1R1.
Where I1 = Lightening current flowing through the tower to ground
R1 = Tower footing resistance
If the value of Vt is less than insulator flashover voltage then no trouble occurs.
- Since the value of Vt depends upon the tower footing resistance R1 the value of this resistance must be kept as low as possible to avoid insulator flashover.
Advantages :
- Ground wire Provides considerable protection against direct lightening strokes on transmission lines.
- It provides a certain amount of electrostatic shielding against external fields. thus it reduce the voltage induced in the line conductor due to the discharge of neighboring clouds.
- Since the Grounding wire acts as a short circuited secondary it provides damping effect on any disturbance travelling along the line.
Disadvantage :
- Since the Ground wire has to run along the length of transmission line , it requires additional cost.
- There is a possibility of breaking and falling of ground wire across the line conductors, further its causes a short circuit fault.
- Using Galvanized stranded steel conductors as ground wire can eliminate this difficulty. Galvanized stranded steel provides sufficient strength to the ground wires.