CSIR (www.corocam.com) has for several years been interested in a way of observing corona discharge, the internal electric gas discharge phenomenon associated with high voltages.
In the electrical power industry corona can typically occur:
* Around line conductors.
* On spacers and dampers.
* On damaged insulators.
* On polluted insulators.
* At the live ends of insulator assemblies and bushings.
* At any point on electrical field equipment where the electric field strength exceeds 3 kV/m.
* Where there is a gap discharge of spark, mini-spark or surface type.
The measurement of corona discharge benefits utilities such as Eskom in terms of the location of faults. The need for a corona camera was identified some years ago and research was initiated to develop such a camera and to overcome the many foreseen problems. The objective was eventually achieved through the use of four technologies: first to use a solar blind UV intensified detector; use appropriate filters to reduce daylight contamination; then control the aperture of the lens; and finally, appropriate image enhancement.
Over the years CSIR (Division of Manufacturing and Materials Technology) has developed six different cameras, all of which are available on order at prices that range from $10 000 to $60 000 - the user can make a choice based on the available budget.
The first, very basic, camera developed was the CoroSMART which only works well at night or indoors (without lights). The CoroSMART was a simple 'point-and-shoot' device and a simple eyepiece was used to view the image of the phosphor of the image intensifier. The lessons learned from the CoroSMART were applied in the development of the next device, the CoroCAM I. Once again a point-and-shoot device with good operational characteristics, also mainly for use at night or indoors. A major change was the addition of a CCD detector which allowed the output to be in video format (PAL or NTSC). Both the CoroSMART and the CoroCAM I used a wide bandpass ultraviolet filter.
The UV filter is critical to operation of UV cameras and such a device had been developed in South Africa by Grinaker Avitronics for use in its UV-based missile attack warning system (MAWS). The CoroCAM II was the first commercial device to offer such a filter (it is not known whether this is the current version of the Avitronics filter) and was the first camera developed that operated in both daylight and at night. The CoroCAM II offered three selectable filters. The first of these was a broadband filter (300 to 800 nm), and then a UV bandpass filter (300 to 410 nm) and finally the narrow bandpass filter (340 ±20 nm). A video output was again provided, the image being displayed on a monochrome video monitor (480 TV lines). The CoroCAM II was also the first camera to offer a computer-based control station, running Windows NT with a Pentium III computer chip. Amongst other things, this offered complete remote control and the leakage current and voltage were measured in 16 consecutive cycles with a 16 video field image storage capacity.
The next version developed was the CoroCAM III which was specifically designed for on-site inspections under certain daylight conditions and at night. The main new features of this camera were that it was fully automated to set its own parameters according to the conditions. It also has a portable video recorder option again for on-site use. The camera also had a self-protection feature and shuts itself off if the illumination is too bright or too dark. Once again, this camera was fitted with the standard UV wide bandpass filter and a narrow bandpass filter.
The subsequent development to this was the CoroCAM Mark IV+ (circa 2002) which is capable of full daylight operation. The benefits of using the IV+ is that it is a powerful diagnostic tool and can locate areas of high stress, pollution build up on non-ceramic insulators, and can determine whether corona stress release rings have been properly mounted. The IV+ has a focus range of 4 m to infinity and possesses a 480 line colour or 560 line monochrome video displays. Image processing and enhancement features are standard and the operator can add site data and a date and time stamp to the video recording. For the first time, the daylight channel was fitted with a zoom lens. An integration mode (up to 64 frames) removes photon noise from the image.
The company has become more innovative in allocating names to its products and the very latest system is the CoroCAM 504 (the video output is CC504N for NTSC and CC504P for PAL). Again this product was developed in partnership with Eskom, and this has ensured that there are both customer and developer-wanted features.
The 504 is a full daylight camera (with a low light option) and has a fully automatic focus controlled by a PC through a USB port. The colour video system makes use of zoom lenses and there is also a digital zoom capability. The camera has a powerful onboard digital signal processor which has such features as visible/UV overlay, image enhancement, image integration, zooming and stabilisation, together with image compression and storage. GPS is an optional extra. In addition to the ability to store data on the video, a microphone input is available for audio notes.
Together with the 504, a new software package called CoroSOFT has been developed to complement the appropriate cameras. The software is capable of managing corona images, generating reports and assists in predicting and locating potential faults. CoroSOFT operates on either Windows 98 or NT. Overall, the 504 plus software represents a significant upgrade on the previous daylight system. The group is now working on a multispectral camera where it is hoped that UV, visible and far infrared (8 to 12 microns) images can be viewed simultaneously.
The CoroCAM product range is sold locally and globally through a distributor network. It also has had the distinction of being evaluated by the US Navy.
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