Kyle King, EPRIsolutions-Lenox
What causes lightning?
Thunderclouds form when the upper atmosphere contains cold, dense air and the lower atmosphere contains warm, moist air. As the warm air rises, the moisture condenses to form clouds, and the cold air descends. As the cycle repeats, friction between the liquid and frozen water particles builds electrical charges in the cloud. When the electrical charge is large enough, the air between the base of the cloud and ground will break down, allowing some of the charge to reach the earth below in what we know as a lightning stroke.
Is it possible to determine if lightning caused a power line outage?
Yes, by using data from the National Lightning Detection Network (NLDN), which was developed by EPRI, various universities, and government agencies over the past 20 years and is operated by Visala-GAI Inc. (formerly Global Atmospherics). The network records the location, precise time, and magnitude of the 20 million lightning strikes that hit the ground in the U.S. every year. By comparing historical lightning data from the NLDN with the time and location of particular disturbances, a utility can determine whether lightning was responsible.
In general, how does lightning affect power lines?
When lightning hits a power line, the surge of electricity can cause a flashover, and the appropriate corrective action depends on how flashover occurred.
A backflashover occurs when a lightning stroke hits a shield wire or a tower structure. When this happens, current flows in both directions and down the tower into the ground, developing a voltage on the crossarm sufficient to flash over the insulator string. A backflash is typically caused by large stroke currents, high tower surge impedances (tall towers) and/or high footing resistance.
A shielding failure can also cause a flashover. A much lower current lightning stroke may cause a line to flash over if the stroke hits the phase wire, bypassing the overhead shield wire. On high-voltage transmission lines, low-current lightning strokes generally cause shielding failures while the high-current strokes result in backflashovers of the tower to the phase conductors. On lower-voltage lines this distinction is not clear.
An induced flashover commonly occurs on sub-transmission and distribution lines when the impulse insulation level is below approximately 400 kV. In such cases a lightning stroke hits the ground near the line, or some other object near the right-of-way, without striking the line directly. The electromagnetic field resulting from the rapidly changing currents in the stroke can induce severe voltages on the phase conductors and cause flashovers on one or more of the insulators.
How can utilities protect power lines from flashovers?
There are four principal lightning mitigation measures:
“- Improving the shielding by adding or moving shield wires will reduce the number of shielding failures, and resulting flashovers, on a transmission line. A poorly placed shield wire can allow an excessive number of lightning strokes to hit the phase conductors and cause flashovers.
“- Improving tower grounding by reducing the ground impedance of a tower will lower the voltage developed on the structure when a lightning stroke hits the structure or shield wire. This lowers the crossarm voltage, which, in turn, reduces the insulation stress during a lightning event along with the number of backflashovers on the line. When soil resistivities are high, counterpoise, either continuous or radial, can be used to obtain acceptable footing impedances.
“- Increasing insulation on an existing transmission line can be difficult because there is not much room to significantly increase insulator length. Small increases in length will have little effect on shielding failure flashovers, but the improved insulation may have a significant effect on induced flashovers if the original insulator impulse insulation level is below approximately 400 kV.
“- Installing transmission line surge arresters (TLSA) to limit voltages between phase conductors and the tower structure can prevent lightning-related flashovers in both high-footing-resistance areas (backflash prevention) and poorly shielded designs (shielding failure prevention).
Utility engineers should be aware that taking corrective action to improve transmission line lightning performance without first determining the types of flashovers they are experiencing can result in a large expense with little improvement in performance.
How do you know which mitigation method is right for a particular line?
Absolute precision and accuracy are not possible in calculating power system lightning performance. Lightning ground stroke densities, and resulting flashovers, are statistical in nature and can vary significantly from year to year. The equations developed to calculate power system lightning performance only approximate complex physics and depend on accurate data to provide dependable results.
EPRI’s TFlash power line lightning calculation program, for instance, uses a traveling wave simulation to calculate voltage and current distributions on power systems caused by lightning. These results can be used to analyze relative performance of different line configurations and to identify problem areas where excessive lightning flashovers may occur along a line.
King is director of operations for EPRIsolutions Transmission and Distribution Engineering Center in Lenox, Mass. He may be contacted at firstname.lastname@example.org or at 413-448-2459. The Lenox Center is a 35-acre high-voltage/ high-current research and test facility for electrical transmission and distribution systems.