380 kV transmission overhead lines seem to be vulnerable to lightning discharges since they are, mostly, the things that have largest height in field. Lightning discharges most likely prefer to breakdown the air through overhead lines in plain fields. This is a very bad condition for power system since it may result temporary overvoltages, insulation failures or partial energy cuts. That's why overhead lines should be protected against lightning discharges. Most of the time, the protection is satisfied using lightning rods, especially in substations. Actually, sometimes it is inevitable for HV transmission lines to be exposed to lightning discharges. Therefore, the excessive energy should be directed to a point which causes much less damage to power system. Shielding is a kind of protection against the harmful effects of lightning discharges, which is always used in high voltage transmission lines unless it is protected by lightning rods or any other conductor that has higher altitude. Shielding requires to direct the lightning strokes to ground wire instead of phase wires. This aim is satisfied by simply placing the ground wire at a higher place than phase wires so that the nearly all of the strokes are directed towards ground wire. The shielding position of wires can be determined using shielding angle, theta, which is defined as the angle between tower direction and the line passing through ground wire and outermost phase wire. Figure 1 is showing an effective shielding.
Figure 1: Effective Shielding
The most important thing to take into consideration for effective shielding is determining the shielding angle. While high shielding angle may result to missing some strokes to phase wires, low shielding angle may result to extra unnecessary tower cost and construction handicaps. Let me try to explain the method for choosing the best procedure for determining shielding angle.
Lightning Stroke History in Field
Provide the lightning stroke historical data for determining the characteristics of the strokes to be protected. Then determine the maximum protection level ,in other words, determine the stroke energies that are most likely to happen and eliminate the extreme (rarely happened) events. If we try to protect the line for even extreme cases, the construction cost will increase dramatically. By using the historical data for lightning strokes we can determine the equipotential lines. Please note that the higher the electrical charges collected in a stroke the the higher the shielding angle is. In other words, if the charge enclosed in a lightning stroke is high, the stroke starts at a longer height. Thus, high shielding angle is enough. However if the charges in a lightning stroke is less which means the lightning stroke starts discharging at the distances closer to earth, then the low shielding angle is needed which can be possible only with high tower length. Please take a look at the graphs shown in figure 2, 3 and 4, they give the relations between shielding angle and lightning strokes.
Figure 2: Percentage of Strokes vs Lightning (kA)
Figure 3: Protective Angle vs Lightning (kA)
Figure 4: Percent Lightning Strokes vs Angle
It had better to represent a noneffective shielding, figure 5 represents a non effective shielding.
Figure 5: Noneffective Shielding
Figure 6 shows a practical application of shielding in a thermal plant in Turkey.
Figure 6: Practical Shielding in Kemerköy Thermal Plant, Muğla / TURKEY