Testing the 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.

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