Electricity + Control October 2018

ANALYTICAL INSTRUMENTATION

UV vs IR in diagnostic inspection of lines and substations

Roel Stolper, CSIR

One of the issues in maintenance of overhead lines and substations relates to better understanding as to what is being revealed during ultraviolet or infrared inspections of equipment and hardware.

Take Note!

1. In simple physical terms, corona is plasma discharge whenever ambient gases are ionised. 2. During the subsequent de-ionisation phase, pho- tons are emitted with emission lines related to the spectral properties of these gases. 1 2

Typical problems on lines Electric utilities and research organisations world- wide have launched programmes to investigate and classify all the typical faults that occur on over- head lines. A past working group within CIGRE, for example, has been dedicated to better under- standing the behaviour of polymeric insulators un- der different environmental conditions. Current inspection technologies Years of experience in line inspection have indi- cated that there is still no single best technology that meets every need, i.e. there is no diagnostic sensor that will detect and locate all the possible different faults that may appear on a line. Given this fact, it is probably correct to state that the ide- al inspection tool is one that integrates different types of sensors into a single instrument. Generally speaking, modern inspection tech- nology can be classified into two main groups: ul- trasound radio telescopes and camera detectors. Both make use of the basic phenomenon that every defect emits radiation (energy in the electro- magnetic spectrum) that can then be detected and recorded by an inspection device. Basic principle of ultrasound detectors A round dish focuses ultrasound radio waves onto an RF detector (microphone) that amplifies and presents any weak signal detected to the operator by means of sound produced by an earphone. Basic principle of camera detectors Light from a source is collected by a lens and pro- jected through a filter onto a detector that converts

the light energy into electric signals. The signals are electronically manipulated into a raster image and displayed to the operator. The images shown in Figures 1 to 6 are from various UV and IR camera recordings of electro- magnetic radiation related to unique defects oc- curring in line components. For example, a simple structural defect (e.g. a cut in a silicone insulator housing or a damaged ceramic disc) generates corona activity due to distortion of as well as in- crease in electric field. Similarly, an internal defect in a composite insulator can result in leakage cur- rent along the FRP core rod, causing heat dissipa- tion. Viewed this way, it is clear that corona and thermal cameras are essentially complementary and that no single inspection technology is inher- ently superior. One can therefore also conclude that, ideally, IR and UV inspection should be con- ducted simultaneously. To demonstrate: Figure 1 shows ultraviolet and infrared recordings of the same object. There is only corona activity present due in this case to sharp edges on the clamps. But there is no heat dissipation, suggesting that corona does not nec- essarily generate heat. UV/IR inspection with single device Corona discharges are detected using a UV de- tector that converts UV radiation to wavelengths that are visible to the naked eye. This same prin- ciple applies to IR heat radiation detected by the IR uncooled micro-bolometre and subsequently converted to wavelengths in the visible spectrum. In simple physical terms, corona is plasma dis- charge whenever ambient gases are ionised. Dur-

There is no diagnostic sensor that will detect and locate all the possible different faults that may appear on a line.

Author: Roel Stolper is an expert researcher in diagnostics with the CSIR.

4 Electricity + Control

OCTOBER 2018