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Tuesday, 4 November 2014

Shielding method Protection of Transmission Lines against Lightning

Last updated: January 20, 2017

Surges due to lightning are mostly injected into the power system through the long transmission lines. Substation apparatus is always well protected against direct lightning strokes. The commonly adopted and effective method of protecting transmission lines against direct strokes is by the use of overhead ground wires. This method of protection of transmission lines is known as shielding method which does not allow an arc path to form between the line conductor and the ground.

Ground wire is a conductor run parallel to the main conductors of the line. It is supported on the same towers, is placed higher than the main conductors and is adequately grounded at every tower. For horizontal arrangement of conductors, there are two ground wires to provide effective shielding to power conductors from direct lightning strokes whereas in vertical configuration of conductors there is only one ground wire.

Fig 1: A 765 kV transmission line with ground wire

The ground wire is made up of galvanized steel or ACSR conductors. Modern Extra High Voltage (EHV) transmission lines have ground wires of ACSR conductors of the same size as the power conductors. In case of a direct lightning stroke, the ground wire intercepts the stroke and by providing multiple paths for conducting the stroke to the ground they reduce the induced voltage. It also helps to increase the effective capacitance between the conductor and the ground which in turn reduces the voltage induced on the conductors from nearby strokes.

The ratio of the induced voltage on a conductor of a line provided with ground wire protection to the induced voltage which would exist on the conductor in the absence of ground wire is known as protective ratio. Each ground wire has a protective angle which is defined as the-
" Angle between the vertical line passing through the ground wire and the line passing through the outermost power conductor is called the protective angle."
 The protective angle is in the region of 20o to 45o.

The voltage to which a transmission line tower is raised when a lightning strikes the tower is independent of the operating voltage of the system. 

Basic design requirements for protection against direct lightning strokes:

The basic requirements for the design of a line to safeguard it against direct lightning strokes are-

1.      The ground wire used should be mechanically strong and should be so located that they provide sufficient shielding.
2.      There should be sufficient clearance between the power conductors and the tower structure.
3.      There should be an adequate clearance between the line conductors and the ground wires, particularly at the mid-span, so as to avoid flashover to the power conductor upto the protective voltage level used for the line design.
4.      The tower footing resistance should be as low as permissible.