Protection of Overhead Transmission line by Ground Wire or Earth Wire

What is Ground Wire or Earth Wire?

    A wire which is provided at the topmost of transmission or distribution tower for the protection of phase conductors from direct lightning strokes is called ground wire or earth wire. A ground wire runs parallel with the phase conductors and has the same length as the phase conductors' length. A ground wire is a connected tower to tower of transmission line and it is earthed at regular intervals for discharging the lightning current. 
Ground wire or Earth wire in Transmission line
    The ground wires are always provided on top of the transmission tower because all direct lightning strokes are firstly interrupted by groud wires and phase conductors of the transmission line are protected.

    A ground wire provides a shield to the phase conductors by attracting direct lightning strokes on it. If the ground wire is not available then lightning strikes directly on phase conductors and damage the transmission line phase conductors. 

    A ground wire only protects transmission lines or distribution lines only for direct lightning strokes during monsoon season and there is no effect for switching surges. A ground wire can not carry current so groud wires are made from galvanised steel. 

    When lightning strikes on the earth wire at any place between the two towers, heavy current waves ( having a current of 10 kA to 50 kA) produced in-ground wire and travelled in the opposite directions along the transmission line. This current wave reached at the adjoining tower and it safely discharge the current wave to earth. 

    The protection provided by the ground wire or earth wire depends upon the tower footing resistance. The tower footing resistance is as low as possible for safely discharging heavy current waves. The ground wire or earth wires are grounded at each transmission tower or pole through as low resistance as possible. 

    For reliable and safe protection of transmission line from direct lightning stroke protective or shielding angle should be from 20 - 30 degrees. The shielding angle or protective angle between 20 to 30 degrees is safe and the shielding angle should not keep more than 40 degrees. 

Shielding angle or protective angle:

Shielding Angle in transmission line
    An angle between the vertical earth wire which is provided at the topmost of the transmission tower and the phase conductor which is to be protected from the direct lightning strokes. The lower the shielding angle, the protection will be greater. The shielding angle is calculated from the outermost top conductor to the earth wire.

Back flashover:

    When the resistance between the tower foot and earth is not sufficiently low and lightning will strikes on the earth wire or tower then lightning cause very high potential across the tower and which will cause flashover between the tower and one more phase conductors is called back flashover.

    The back flashover can be reduced by using the counterpoise earthing and driven rod. The counterpoise and driven rod are used where soil resistivity is very higher. 

Conutrtpoise Earthing:

Counterpoise earthing

    Counter poise is galvanised steel conductor which is buried in the ground. The counterpoise is consists of four-wire which are connected to the one end of the transmission tower, buried these wires in soil up to 600mm depth and kept away from the transmission tower. By increasing the length of the counterpoise wire, we can maintain low earth resistance. This method of earthing is best suited for rocky soil where soil resistivity is very higher.

Driven rod earthing: 

    The driven rod is made from galvanised iron or copper which are buried near the tower and one end of this rod is connected to the tower base. These methods of earthing are best suited for sandy soil. 
Driven Rod Earthing 

    There are two earth wires or ground wires are used in transmission lines where the wider spacing between the conductor is available. Due to wider spacing, the shielding angle increased from 40 degrees which is not safe to protect the transmission lines by single ground wire. 

    For two earth wires or ground wires, surge impedance is low and the coupling factor is increased. 

Coupling factor: 

    The coupling factor is a ratio of voltage induced in the transmission line conductors and the potential of ground wire. 

Advantages of ground wire: 

1. The ground wire provides protection against direct lightning strokes on the transmission lines. The ground wire works as a shield for phase conductors from direct lightning strokes. 
2. The ground wire provides a damping effect on any disturbances which is travelling on the transmission line. The ground wire works as short-circuited secondary. 
3. The ground wire provides the electrostatic shielding against the external field and the ground wire reduce the voltage which is induced on the phase conductors due to discharge of neighbouring clouds. 

Disadvantages of the Ground wire:

1. The ground wire requires additional cost. 
2. The ground wire can break and fall on the phase conductors and cause the short-circuited fault in the transmission line. To eliminated this problem, galvanised make steel conductors are used as a ground wire which provides sufficient strength to the ground wire. 
3. adequate clearance is required between the phase conductors and topmost conductors otherwise short circuit fault occurs between the ground wire and phase conductors. 
4. The low tower footing resistance is required otherwise back flashover occurs between the transmission tower and phases conductors. 

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