27-03-2012, 12:17 PM
SURGE PROTECTION PRACTICE FOR EQUIPMENT IN SUBSTATIONS
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INTRODUCTION
Surges due to lightning are mostly injected into the power system through long cross-country transmission
lines. Substation apparatus is always well shielded against direct lightning strokes. The protection of transmission
lines against direct strokes requires a shield to prevent lightning from striking the electrical conductors. Adequate
drainage facilities and adequate insulation structures must be provided so that the discharge can drain to ground
without affecting the conductors. This prevents any arc from line conductor to ground. A ground wire placed above
the phase conductors of a transmission line shields the phase conductors from the lightning strokes. A shielding
angle of about 300 gives adequate lightning protection.
PROTECTION OF TRANSMISSION LINES AGAINST LIGHTNING
Protection of transmission lines against natural or lightning over voltages and minimizing the lightning over
voltages are done by suitable line design, providing guard and ground wires, and using surge diverters. Over
voltages due to lightning strokes can be avoided or minimized in practice by
(a) Shielding the overhead lines by using ground wires above the phase wires,
(b) Using ground rods and counter- poise wires,
© Including protective devices like expulsion gaps, protector tubes on the lines, and surge diverters at the
line terminations and substations.
Protection using ground rods and counter- poise wires
When a line is shielded, the lightning strikes either the tower or the ground wire. The path for drainage of the
charge and lightning current is either through the tower frame to ground or through the ground line in opposite
directions from the point of striking. Thus, the ground wire reduces the rise of back flashover in the event of direct
stroke to tower, as the instantaneous potential to which the tower top is raised is reduced by the fact that half the
surge impedance (Zs =Zg/2) of the ground wire appears in parallel to the tower surge impedance (ZT), as the
current path is in three directions.
Protective devices
In spite of the protection of transmission lines described above, sufficiently intense voltage surges can reach
the substation and can damage the apparatus. The apparatus at substation such as switchgear and transformer is
very expensive and outages can be prolonged, and must therefore be provided with almost 100 percent protection
against surges. In regions where lightning strokes are intensive or heavy, the overhead lines with these zones are
fitted with shunt protected devices. On the line itself, two devices known as expulsion gaps and protective tubes
are used. Line terminations, junctions of lines, and substations are usually fitted with surge diverters. A twopronged
approach is followed in the protective scheme for apparatus. Surges before they reach the substation are
modified to reduce the slope of their wave front. Upon reaching the substation, surges above a certain peak value
are diverted into a shunt path to discharge their energies.
CONCLUSION
Surge voltages in power lines and apparatus at substations due to lightning strokes are avoided or
minimized in practice by
• Shielding the overhead lines by using ground wires above the phase wires, and a shielding angle of about
300 gives adequate lightning protection,
• Using ground rods and counterpoise wires, and
• Including protective devices like expulsion gaps, protective tubes on the lines, and surge Diverters at the
line terminations and substations.
For distribution and high voltage power circuits, the primary type of protection is supplied by either sparkgap
or metal oxide surge arrestors. The spark-gap was the first type of surge arrestor. The spark gap surge
arrestor is slower acting and has a higher let-through voltage than the metal oxide surge arrestors. For this reason,
surge protection for power circuits is presently designed with metal oxide arrestors. The only place spark-gap
arrestors are used is on high voltage transmission lines.