Electricity + Control October 2018

ANALYTICAL INSTRUMENTATION

ing the subsequent de-ionisation phase, photons are emitted with emis- sion lines related to the spectral properties of these gases. Air is made up of about 80% nitrogen, which has its dominant spectral lines in the UVA and UVB spectrum and minor lines in the UVC spectrum. With regards to heat radiation, any material emits at a wavelength that depends on body temperature, its so-called kinetic surface energy. Ac- cording to Wien’s displacement law, peak wavelength, λ max, is at 2898/T, where λ is expressed in micrometres (1.10-6) and T is temperature in de- grees Kelvin. Simple calculation reveals that a body at 27°C emits peak radiation at 10.55 μm such that, for example, a clamp at 100°C will have maximum radiation at 7.76 μm. Calculation shows that, for any line inspection application where there are only low temperature sources, a heat detector is required that con- verts radiation at 8-10 μm wavelengths into the visible spectrum. There are a number of heat detectors available on the market and these are classified according to spectral sensitivity as determined by material com- position of the detector. The mercury cadmium telluride (MCT) and Quantum Well (QWIP) de- tectors, for example, are relatively expensive since Sterling engines are required to cool them to -70°C. The micro-bolometer detector, by contrast, is used by many camera manufacturers for industrial IR inspections. This type of detector does not need cooling and works at room temperature. It is also compact with high pixel resolution, low cost and easy availability. Conclusions UV and IR inspection are not simply two alternative ways to look at the same problem. Instead, each inspection technology records a particular defect or abnormality within the component or equipment being inspect- ed. In general, it can be said that UV recordings indicate the presence of corona activity while IR recordings highlight heat phenomena. Moreover, UV corona recordings have to do with surface discharges and indicate the presence of high electric field. In contrast, infrared measurements high- light the presence of leakage current. The first phenomenon depends on surface condition while the second depends on an internally generated heat source. The latest multi-spectral camera will enable the power industry to si- multaneously inspect electrical equipment for corona discharges and IR hotspots. A specialised software program will assist the camera user to record, process, store and retrieve these recordings.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Electricity + Control

OCTOBER 2018

5