17
2006 by Taylor & Francis Group, LLC.
Lightning Protection
William A. Chisholm
Kinectrics/UQAC
17.1 Ground Flash Density ..................................................... 17-1
17.2 Stroke Incidence to Power Lines.................................... 17-2
17.3 Stroke Current Parameters ............................................. 17-3
17.4 Calculation of Lightning Overvoltages on Shielded
Lines ................................................................................. 17-3
17.5
Insulation Strength.......................................................... 17-4
17.6 Mitigation Methods ........................................................ 17-4
17.7 Conclusion ....................................................................... 17-4
The study of lightning predates electric power systems by many centuries. Observations of thunder were
maintained in some areas for more than a millennium. Franklin and others established the electrical
nature of lightning, and introduced the concepts of shielding and grounding to protect structures.
Early power transmission lines used as many as six overhead shield wires, strung above the phase
conductors and grounded at the towers for effective lightning protection. Later in the twentieth century,
repeated strikes to tall towers, buildings, and power lines, contradicting the adage that ‘‘it never strikes
twice,’’ allowed systematic study of stroke current parameters. Improvements in electronics, computers,
telecommunications, rocketry, and satellite technologies have all extended our knowledge about light-
ning, while at the same time exposing us to ever-increasing risks of economic damage from its
consequences.
17.1 Ground Flash Density
The first, negative, downward, cloud-to-ground lightning stroke is the dominant risk element to power
system components. Positive first strokes, negative subsequent strokes, and continuing currents can also
cause specific problems. A traditional indicator of cloud-to-ground lightning activity is given by thunder
observations, coll