The Story in Brief

The presence of corona activity is a good indication of trouble with nonceramic insulators and other transmission line equipment. But since corona radiates in the ultraviolet region of the light spectrum, it is invisible to the naked eye, and conventional corona cameras are ineffective in daylight because sunshine drowns out the corona’s image. Responding to the need for better technology, EPRI researchers and engineers from Ofil Ltd. developed the DayCor daytime corona camera, which features bispectral imaging for effective use in daylight. This breakthrough camera, which has already been successfully field-tested on utility transmission systems, will enable utilities to perform comprehensive airborne inspections safely and economically.

Surfing for a solution

In 1997 Phillips, who had come to EPRI the previous year from the University of the Witwatersrand in South Africa, began a study of techniques for evaluating NCIs in service—work that culminated in the publication of the Application Guide for Transmission Line Nonceramic Insulators (TR-111566). During that study, Phillips determined that the presence of corona activity is one of the best indicators of defective NCIs; unfortunately, the equipment to detect corona in daylight did not exist.

“We had evaluated all current corona inspection equipment, and nothing worked as well as we had hoped,” he says, “so I was surfing the Internet to see what else might be available. This led me to Ofil Ltd., an Israeli company that makes something called solar blind UV filters for use by the Israeli military and for fire detection. I immediately knew this would change everything.”

Located in the UV, or ultraviolet, range of the light spectrum, the solar blind band is of great interest, notes Phillips, because all the sun’s radiation in this frequency band is absorbed by the ozone layer in the upper atmosphere; none actually reaches the earth. “Therefore, this spectral band is as ‘dark’ at noontime as it is at midnight,” he says. “Because corona emits radiation in this band, it would be possible to detect corona even in bright sunlight—if you had the right equipment.”

To develop that equipment, Phillips immediately contacted Ofil. “I told them I was interested in their technology but needed proof it could be used to view corona. They forwarded a measurement they had made—not even an image, just a spectral graph. While the graph supported the concept we were pursuing, it wasn’t what we were looking for. We knew we should be able to observe corona discharges in daylight by viewing them through a solar blind bypass filter, but without the ability to overlay the corona image on an image of the structure under scrutiny, it would be impossible to determine the exact location of the activity. We needed to see an actual image of the corona.

“After some discussion, Ofil conducted tests with cameras hooked up in parallel and was able to capture an image of corona on a transmission line, and that really got me excited. We were finally able to view corona on actual structures during the daytime. We then turned to the task of developing a practical device that utilities could use; the DayCor MKI prototype was the first step in this process.” Phillips confesses to being amazed at the speed of the camera’s development from concept to prototype. “All our communication was via e-mail,” he says. “The Internet made it all possible.”