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We demonstrate the utility of cavity-enhanced Raman spectroscopy (CERS) as a unique multigas analysis tool for power transformer diagnosis. For this purpose, improvements have been added to our recently introduced CERS apparatus. Based on optical feedback frequency-locking, laser radiation is coupled into a high-finesse optical cavity, thus resulting in huge intracavity laser power. With 20 s exposure time, ppm-level gas sensing at 1 bar total pressure is achieved, including carbon dioxide (CO 2 ), carbon monoxide (CO), hydrogen (H 2 ), methane (CH 4 ), ethane (C 2 H 6 ), ethylene (C 2 H 4 ), acetylene (C 2 H 2 ), nitrogen (N 2 ), and oxygen (O 2 ). By using the internal standard gas (sulfur hexafluoride, SF 6 ), the quantification of multigas with high accuracy is also realized, which is confirmed by the measurement of calibration gases. For fault diagnosis, transformer oil is sampled from a 110 kV power transformer in service. Dissolved gases are extracted and analyzed by the CERS apparatus. Then the transformer is diagnosed according to the measurement results. CERS has the ability to analyze multigas with high selectivity, sensitivity, and accuracy, it has great potential in gas sensing fields.

Raman spectroscopy is a powerful method for gas sensing but is limited by the inherently weak Raman effect. Due to the increase in intracavity laser power or improvement in spontaneous emission, cavity-enhanced technology is a useful enhancement method for the improvement of the limits of detection, which permits Raman spectroscopy to get better applications in gas sensing. In this article, a brief review of several cavity-enhanced technologies used in gas sensing by Raman spectroscopy is presented, the enhanced-cavities including multiple-pass cavities, Fabry-Perot cavities, laser cavities, and microcavities. Finally, the advantages and limitations of these different technologies are discussed.

Optical fibre sensing technology is a powerful method for long-term reliable sensing in harsh environments, which means it is particularly suitable for the detection of electrical equipment characteristic state parameters, including characteristic gases, abnormal vibration, ultrasonic produced by partial discharge, and abnormal temperature rise. This paper reviews several individual optical fibre sensors for different state parameters detection, discusses the advantages, limitations and possible improvement methods, and finally presents the most promising optical fibre sensor for each state parameter.

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