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Lasers Help Power Companies Find Good Vibrations

Photonics Spectra
Sep 2000
Dan Drollette

Electric power companies may soon have a faster, safer and more effi-cient means for checking the wooden crossarms that support many high-voltage transmission power lines: a helicopter-mounted HeNe laser beam. Power companies now conduct yearly checks for weaknesses in their transmission lines by employing a team of helicopter-borne observers to fly alongside their rights-of-way for hundreds of miles. To see damaged insulator strings, bad splices or other problems, these aircraft routinely hover 20 to 30 feet away from 46,000-V transmission lines.

But to inspect the condition of the wood crossarms that hold up the lines, someone must climb each 70-foot "H-frame" transmission tower to bang on the crossarms with a hammer and judge their sturdiness: Hard, solid wood emits a different sound than softer, rotting wood. This method is thorough but imprecise, time-consuming and sometimes hazardous.

Just reaching an H-frame can be an ordeal, because transmission lines often march up and down the sides of mountains, across ravines and through swamps. Because of these obstacles, comprehensive crossarm checks are done only about once every five years.

But with a remote detection system developed by researchers at Georgia Institute of Technology's National Electrical Energy Testing Research and Application Center, power companies could make cross-arm inspections more easily and at the same time as their line surveys.

Paul Springer, principal investigator, explained that the laser sensing system works by using the vibrations generated by the noise of the helicopter's own engine and rotor blades. These "beat up the air pretty severely," he said, and cause the crossarm to shake in a pattern that is picked up by an onboard model OFV 353 laser Doppler vibrometer manufactured by Polytec PI of Auburn, Mass.

Like the sounding board of a musical instrument, a wood crossarm gives off complex patterns, which a neural net system interprets with algorithms to tell the operator whether the crossarm is emitting the vibration pattern of a sturdy piece of wood or a rotten one.

Springer said that his team demonstrated proof of principle by testing the condition of 100 H-frames in four hours. However, he noted that, to make the shaking of the crossarms readable from 30 feet, highly reflective material had to be attached to each beam.

By this fall, he expects to have completed a working vibrometer capable of operating at this range without any targeting aids.

The system will undergo more extensive tests late this summer, checking the condition of 10,000 crossarms maintained by Entergy Services Corp. of New Orleans. Springer said it would cost as little as 10 percent of today's manual inspections -- which currently average $50 per crossarm.

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