Daytime exposure for corona16 October 2000
Daytime aerial inspections are set to revolutionise the maintenance of transmission line equipment. Suzanne Pritchard explains how the development of a corona camera which utilises solar blind filters has made this possible
Walking on the moon became a reality in 1969 thanks to revolutionary technological advancements. But until 1999 effective daytime inspections of overhead transmission lines were almost impossible, and a mere dream for hydro utilities who wished to make safe inspections in remote, inaccessible and rough terrain.
One of the most effective indications of problematic transmission lines or substation equipment is the presence of corona activity. Corona is a luminous discharge caused by high voltage electrical breakdown, and results in emissions mainly in the ultraviolet (UV) region of the light spectrum. It produces light as discrete lines across a broad band of ultraviolet but the signal is weak in comparison to the sun and cannot be seen during daylight, although it is audible.
Corona activity may indicate the presence of faults such as shorting, cracks or excessive pollution on ceramic insulators, or broken strands on conductors strung between transmission towers. On non ceramic insulators corona discharge may indicate a severely degraded unit that may in time fail electrically or mechanically. AM radio and television interference is another problem.
The ability to detect corona can be of immense value, possibly saving power transmission companies millions of dollars by rectifying faults at an early stage. Conventional corona cameras have not been successful in detecting corona activity during daylight hours, and for safety reasons airborne inspections of long sections of transmission lines are only practical during the day. Corona faults can be surveyed using infrared imagers but they can only detect heat when severe damage to lines or equipment has already occurred. Although UV cameras can work at night when solar radiation does not blanket the weak corona effect, such inspections are known to be difficult, inefficient and expensive. However, the successful partnering of Ofil, an Israeli company, and epri, the energy research organisation, is set to revolutionise the maintenance of transmission lines and high voltage power equipment.
Solar blind filter
The evolution of the DayCor corona camera can be traced back to a fruitful search on the internet. In 1997 Andrew Phillips, senior research engineer at the EPRIsolutions Engineering and Test Centre in Massachusetts, US, was trying to locate equipment which could detect corona in the daylight. He discovered that Malka and Pinhas Lindner, chemical engineers who founded Ofil, had developed a filter that would only transmit the ultraviolet C (UVc) band of wavelengths.
The filter has been designed to detect radiation in the 240-280nm UVc range — a band of wavelengths called the solar blind range which is inactive except for primarily man-made energy sources. In the solar blind spectral range the ozone in the atmosphere absorbs all of the sun’s radiation, so it is as dark at noon as it is at midnight. Phillips says that he knew straight away that solar blind UV filters would help give corona activity some well deserved daylight exposure.
EPRI and Ofil agreed to work together on the development of the DayCor camera using the two organisations’ respective power industry and optics expertise. Several EPRI member US electric utilities, including New York Power Authority, Tennessee Valley Authority, Alabama Power, City Public Service of San Antonio, Allegheny Power, East Kentucky Power and Bonneville Power Administration, have also been involved in the camera’s development.
Phillips says that in order to locate corona on equipment it was necessary to overlay an image of the corona onto an image of the structure being inspected. The result is the DayCor bi-spectral camera which consists of two video imaging channels.
The camera works by splitting the incoming image in two by using a UV beam splitter. It sends the first image through the solar blind filter, to block out the sunlight, then onto an image intensifier. As corona discharges emit UV radiation from 230-405nm, and the solar blind filter only operates from 240-280nm, this narrower range results in a weaker signal. As the corona signal consists of only a few photons, the intensifier is necessary to make it visible. High contrast images are obtained due to the absence of background radiation.
The intensifier then sends the UV image to one charge-coupled device camera, while the image of the object under investigation is sent to a second standard video camera. A special imaging process super-imposes the two images, producing a final image of the corona exactly as it appears on the insulator, conductor or other line component.
In June 1999 Ofil sent a prototype of the camera to EPRI. ‘Laboratory and field tests have already exceeded our expectations,’ Phillips says. ‘With further improvements, which are under way, we believe this device will prove indispensable for the inspection of transmission lines and substation components.’ And there is already evidence of this.
New York Power Authority (NYPA), which has 4270MW of hydro power, has already used the camera to resolve a potentially serious problem. To detect corona on a transmission line, which was thought to be creating interference for a local radio station, the power authority used the prototype of the DayCor camera. Corona activity was indeed found in several places on the 20-year-old transmission line but was in the wrong location to affect the radio station. This saved the company from spending over US$14,000 on lengthy investigations which could have resulted in additional expenditure or line repair work. Instead the DayCor inspection lasted for one day, costing only US$3600, and proved that the corona was not actually the source of radio interference.
According to Phillips, research has found that the device can be used to identify a wide range of defective components, such as broken strands and defective porcelain insulators. The early detection of potential problems could ultimately lead to a reduction in dropping lines, tower and hardware damage and unplanned outages — all of which carry a substantial financial burden.
Another NYPA inspection was carried out in response to complaints of reported interference with police radio signals. Corona on a 345kV transmission line was suspected but could not be located with the DayCor camera. However, after five hours, the camera found the likely source of corona to be loose strands damaged by a gunshot. Even though the radio interference was still reported after the line was repaired, NYPA was able to rectify a potentially serious problem on a critical transmission line.
Testing has also proved the camera’s effectiveness both on the ground and in the air. In July 1999 EPRI and Tennessee Valley Authority (TVA) staff carried out the first aerial tests using the DayCor camera. Corona activity on 500kV conductor bundles was discovered at several points. ‘This is a breakthrough technology,’ says Fisher Campbell, a TVA project manager. ‘It allows daytime corona inspections that were not previously possible. Ultimately we would like to employ the camera in our airborne inspections. That would allow us to inspect over 27,000km of transmission lines for corona and to work the camera into our routine line inspection programme.’
Phillips says that EPRI is doing a considerable amount of research into the application of the camera for overhead transmission and substation aerial inspections and distribution. ‘This can be a valuable maintenance tool,’ he adds, ‘as long as it is used properly. That is why we are doing application research. If you do not know how to use the tool correctly you can get misleading results. Corona occurs for numerous reasons. Some are bad and some are benign; such as corona from a small broken strand which needs to be fixed and corona from bird droppings which will just wash off with time. One needs to distinguish between these.’ Consequently EPRI is developing a comprehensive guide for using the DayCor camera on overhead transmission lines. This is scheduled for publication in 2001 and will explain how to use the device, what you can see and how to interpret results. On and off-site training for utility personnel using the camera, and full DayCor inspections of transmission lines and substations, are also available from EPRI.
The first DayCor model is available for sale worldwide and even though this has exceeded performance expectations, improvements are taking place. A lighter, smaller and ergonomically improved DayCor II has been developed and a number have already been supplied to participating EPRI members together with training. The results from the DayCor I prototype and from the commercialised DayCor II camera have shown the immense promise of this new technology, Phillips says.
‘The aims in developing DayCor II were to enable the detection of corona at greater ranges, to incorporate more flexible controls and to exploit the capabilities of modern image processing to enhance the visibility of the phenomena,’ says Jeremy Topaz, electro-optics consultant to Ofil.
‘DayCor I was built around an off-the-shelf lens of which there is a limited choice. To improve on this we went to a custom-designed objective lens with a longer focal point,’ he adds. ‘This both increased magnification and gave more sensitivity by having a larger collecting area. To keep the volume and weight down a mirror-type lens was selected. The new model also has a built-in LCD monitor to display the image.’ The electronics component of DayCor II enables functions such as automatic focus, a selection of display options and an on-screen status display. The digital signal processor can filter out random noise, mark suspected corona spots with a graphic overlay and count the rate of photons to give a quantitative evaluation of the corona source.
It is anticipated that DayCor II should be available on the open market during the early part of 2001.
Those involved with the development of this new technology believe it has the ability to fundamentally change the way utilities deal with corona problems. Inspections can now take place during the day, including airborne inspections, while more structures will be inspected each day.
Dan Ninedorf from Ox Creek Energy, the North American distributor of DayCor I, says that another application for the corona camera is generator winding inspections — open frame hydro generators over 4160V can be inspected while running for corona. All of these applications ultimately mean reduced maintenance costs and increased service reliability for utilities.
It may not exactly be space travel, but this new camera can have just as many far reaching implications.