New innovative methods for non-intrusive optical diagnostics of high-voltage insulator materials
Abstract: The purpose of this PhD thesis work has been to develop optical techniques that can help utility engineers for assessing the state of insulators in service and in a laboratory. The possibilities to use remote laser induced fluorescence for detection and imaging of biological growth such as algae and fungi have been investigated. Reflectance spectroscopy has also been investigated as a technique for discrimination between clean and polluted surfaces. Remote laser-induced breakdown spectroscopy has been tested for the detection and removal of salt on metal and insulator surfaces. Optical second harmonic generation has been studied for the purpose of measuring surface charge, and electric field calculations have been performed by using the finite element method. It has been shown that remote laser- induced fluorescence can be used to detect and measure the distribution of algal contamination on polymeric insulators. Remote fluorescence imaging of fungal growth on polymeric high-voltage insulators has been performed using an Nd:YAG laser equipped mobile Lidar system using a laser wavelength of 355 nm. Insulator areas contaminated by fungal growth could be discriminated from clean surfaces and imaged. It has been shown that the fluorescence from pure fungal growth is shifted to longer wavelengths compared to the fluorescence from clean material. Detailed spectral studies have been performed in the laboratory using a fibre-optic fluorosensor incorporating an optical multi-channel analyser system (OMA) and a nitrogen laser emitting radiation at 337 nm. The detection of contamination such as salt in outdoor high-voltage insulator systems and its subsequent removal are vital for a reliable transmission of electric power. Remote detection of salt on a copper metal surface was carried out by using a mobile LIBS Lidar system with a laser wavelength of 355 nm. Detection of salt on a polymeric high-voltage insulator was obtained when a lens was inserted into a collimated laser beam and focused the laser light onto the target. The detection sensitivity was estimated from the number of photons that was detected using a calibrated white light source at the target point. Ablative cleaning could readily be carried out with LIBS and was verified by observing the disappearance of the sodium D-line emission. The second harmonic generation experiments, where the aim was to detect surface charge on metal and polymeric surfaces, did not show any correlation between the signal and the surface charge. Oxide layers and other surface contaminants contributed strongly to the signal, making the technique difficult to implement.
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