Overhead Ground Wires :
☛Overhead transmission lines have the thousands of kilometer age 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 conductor 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 is the 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.
☛It may be mentioned here that the tower footing resistance of the tower decides the degree of protection provided by the ground wires.
☛Suppose V, is the approximate voltage between tower and line conductor
☛And V, 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 V, is less than insulator flashover voltage then no trouble occurs.
☛Since the value of V, depends upon the tower footing resistance R, the value of this resistance must be kept as low as possible to avoid insulator flashover.
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Overhead Ground Wires |
Advantages of Ground Wire
(1) Ground wire provides considerable protection against direct lightening strokes on transmission line.
(2) It provides a certain amount of electrostatic shielding against external
fields. Thus it reduces the voltage induced in the line conductor due to the discharge of a neighboring cloud.
(3) Since the grounding wire acts as a short-circuited secondary it provides damping effect on any disturbance travelling along the line.
Disadvantages of Ground Wire
(1) Since the ground wire has to run along the length of transmission line, it requires additional cost.
(2) There is a possibility of breaking and falling of ground wire across the line conductors, further it causes a short-circuit Fault.
☛Using galvanised stranded steel conductors as ground wires can eliminate this difficulty. Galvanized stranded steel provides sufficient strength to the ground wires.
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