Surfing for a corona detection 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.
The DayCor MKI employs bispectral imaging to solve the image overlay problem; that is, it uses a UV beam splitter to create a duplicate of the incoming image. One image is sent through the solar blind filter, which eliminates solar radiation, then through an image intensifier and a charge-coupled device camera. The other image is sent through a standard video camera. The two images are then processed and combined in an image mixer, which produces an image of the corona exactly as it appears on the insulator, conductor, or other line component. This makes it possible for utility personnel to pinpoint the exact location of the corona and take corrective action.
Corona discharges emit UV radiation from 230 nm to 405 nm, but the DayCor camera is designed to detect the radiation in only the 240-280-nm range in order to stay within the UV solar blind band. Although the use of this narrowed range results in a somewhat weaker signal, the DayCor camera is equipped with an image intensifier and is able to provide high-quality, high-contrast images owing to the complete absence of background radiation.
Testing the corona detection technology
Ofil delivered the prototype DayCor camera in the spring of 1999, and EPRI researchers immediately put the unit through a series of indoor and outdoor tests at the Lenox facility. We were all very excited, but as with any new technology, nothing could be taken for granted, says Phillips. We had to invent everything as we went along how to hold the camera, how to keep it steady while it is operating, where to stand in order to get the best image, and so forth.
In the first tests at Lenox, the camera was focused on the end fittings of a 500-kV NCI that was installed without grading rings. Corona in such situations is common and not only results in customer complaints about audible noise and radio interference but also degrades the polymeric rubber material of the NCI, which in turn can cause premature failure of the insulation. In the first test, the corona activity was audible but not visible to the naked eye, says Phillips. Yet, as we had hoped, the DayCor camera was able to produce a good image of the corona, something that two nondaytime corona cameras could not do. This showed that we were on the right track.
The next series of tests involved a defective NCI that had been removed from the field after four years of service. To simulate in-service conditions and viewing positions, the researchers installed the NCI outdoors on a simulated tower at a height of 48 feet (15 m). After the tests were completed, the researchers compared results from the DayCor camera with those from two other inspection devices: a nondaytime corona camera and an infrared camera.
As before, the DayCor camera was able to observe significant discharge activity and hence identify the defective insulator. The nondaytime corona camera was unable to observe any discharge activity, even though the inspection was conducted on a heavily overcast day. This confirmed previous EPRI work indicating that sunlight renders nondaytime corona cameras unsuitable for daytime inspections. A small amount of heating was observed in the image from the infrared camera, but if it had not been an overcast day and the sun had been shin-ing brightly, this increase in temperature would not have been discernible from background solar radiation, says Phillips. The DayCor camera clearly was the best method for identifying an NCI with this type of defect.
After the successful tests at Lenox, Phillips and his team took the DayCor camera on the road to test its capabilities on structures belonging to several utilities that were participating in the development project. Here, too, the camera identified numerous corona sources in broad daylight. During an in-service inspection of one utility's 500-kV steel lattice tower, the DayCor camera identified two principal sources of corona: broken wire strands on conductors about one-quarter span from the tower, and a possibly defective porcelain bell near the energized end of the vee-string configuration that supports the center phase. The utility's line workers had reported a high level of audible noise coming from the tower, but they had not been able to identify the cause of the noise or its location, says Phillips.
In another test, EPRI researchers used the DayCor unit to evaluate transmission line components on a number of 115-, 161-, 230-, 500-, and 765-kV line structures. Some of the NCIs had been in service for more than 20 years, while others had been installed recently, notes Phillips. In one case, the DayCor camera plainly showed that the grading ring attached to the insulator was inappropriate for the application. In other tests, the camera captured images of corona activity from a 765-kV substation bushing and from broken conductor strands on 230-, 500-, and 765-kV transmission lines.
While the field tests strongly validated the DayCor's potential value to utilities, they also revealed an important shortcoming of the camera prototype. Insulators on the 765-kV structures were a considerable distance about 90 feet [27 m] from the ground, says Phillips. This fact, along with the DayCor camera's relatively large field of view, made observations of corona activity difficult. We will be reducing the field of view of the final version of the DayCor, allowing closer, more effective inspections of such structures.
Helicopter inspections for tranmission lines
In July 1999, EPRI and TVA staff conducted the first aerial inspections for corona activity with the DayCor camera. Until then, the use of a helicopter to perform visual inspections of long sections of transmission line for corona activity had been all but impossible: airborne inspections are practical only during the day, but corona could be viewed only at night. We were confident that the DayCor camera could be used successfully from an airborne platform, and these tests confirmed it, says Phillips. Inspections by helicopter will revolutionize the maintenance of transmission lines.
During such an inspection, the operator observes power lines or components in real time through the DayCor's viewfinder, and the images can be saved simultaneously on a videotape recorder. This has two benefits, Phillips notes. It enables the operator to conduct an inspection quickly, which helps to reduce costs a critical factor with airborne inspections and it allows a detailed review of the results at a more convenient time and place. In the not-too-distant future, he goes on, inspectors will be able to re-fly a transmission line and look for corona with a cup of coffee in one hand and a computer joystick in the other.
For the TVA helicopter inspections, the DayCor MKI prototype was mounted on the lap of an operator who sat in the rear seat and controlled the camera's viewing direction and settings. The lap mount for the camera consisted of a board strapped to the operator's legs. A three-axis tripod head was mounted on the board, allowing the operator to easily adjust the camera's view-ing angle. A single-axis gyrostabilizer was installed between the tripod head and the camera to prevent unwanted vibrations from affecting the images an enhancement that the researchers found unnecessary because of the camera's wide field of view. The need for a gyrostabilizer will be reassessed for later versions of the DayCor camera, which will have a narrower field of view.
The entire TVA inspection was recorded on 8-mm videotape, which clearly shows corona activity on 500-kV conductor bundles at several points on the TVA system. Subsequent helicopter inspections of NCIs on the Alabama Power and Georgia Power systems also identified problem insulators. We're not yet sure what remediation might be required in these cases, but the utilities are happy that we were able to pinpoint potential problems before they created serious difficulties for them and their customers, says Phillips.
Real-world problem solver for corona incidents
The six months of laboratory and field testing have strongly convinced investigators of the corona camera's capabilities. But the DayCor's success is not limited to tests. One utility has already used the camera to resolve a potentially serious problem. When a local radio station in upstate New York began to experience radio interference, the problem was thought to be corona from a nearby New York Power Authority 765-kV transmission line. To resolve the issue quickly, NYPA asked Phillips to use the prototype DayCor camera to identify the source of the problem.
The device found a number of sources of corona activity on the 20-year-old transmission line, but none of these sources were in the beam pattern of the radio station's antenna. Says NYPA's Pete Muench, The technology for daytime corona viewing developed by EPRI allowed us to prove that our line was not the source of radio interference. The utility avoided a lengthy investigation of its transmission line, which could have resulted in additional expenditures or line repair.
Other utilities involved in the development of DayCor technology are just as enthusiastic. This device works, says Rick Stearns, a project manager at the Bonneville Power Administration. I'm very excited about the promise it holds for use as a routine maintenance inspection tool. The early detection of corona discharge on high-voltage apparatus will allow us to replace defective components before catastrophic failure. In turn, Stearns continues, this will help to improve system reliability and greatly reduce the financial risks that are associated with costly line dropping, tower and hardware damage, and unplanned outages.
Paul A. Dolloff, an engineer with East Kentucky Power Cooperative, shares these sentiments. The DayCor camera will allow us to routinely check the integrity of NCIs in the field, thereby avoiding a systemwide change-out program.
Future corona camera developments
Even though the technology has met every goal so far, Phillips and his utility partners in the DayCor project are already working to improve the camera. The unit's weight and size will be reduced, and the ergonomics improved, for easier handling in the field. Technical refinements will include increasing the optical magnification and sensitivity, reducing spurious noise, and developing the capability to capture individual image frames for record-keeping purposes.
Next year EPRI will begin work on a DayCor camera application guide in effect, a user's manual for utilities. Publication is slated for 2001. The guide will not only discuss what you can see with the camera but also explain how to interpret the results, notes Phillips. It will present detailed instructions on how to hold, mount, and use the camera, as well as specify appropriate viewing distances and other parameters for obtaining maximum benefit from this new technology.
Preproduction prototypes of the improved DayCor MKII will be supplied to some of the utilities that have partnered in the camera's development, including TVA, NYPA, Alabama Power, Allegheny Power, East Kentucky Power Cooperative, and Central Hudson Gas & Electric. And there are still opportunities for funding development and for investigating further applications. DayCor units are expected to be available on the open market by the end of the year 2000.
The DayCor technology has the ability to fundamentally change the way utilities deal with corona problems, says Phillips. Laboratory and field test results have already exceeded our expectations. With further improvements, which are under way, we believe this device will prove indispensable for the inspection of transmission line and substation components. Not a bad result from a little Net surfing.
Background information for this article was provided by Andrew Phillips, EPRI Energy Delivery and Utilization Center, Lenox, Massachusetts.
Reproduced from the EPRI Journal with permission from the Electric Power Research Institute (EPRI)