| SF6 gas was used widely in high voltage switchgear because of its excellent insulation performance. When moisture content in SF6 swtichgear reaches to a certain extent, it could be compounded with product of SF6's decompse under arc action and then produces poison which not only results in chemic corrupt but also arouses the drop of insulated performance. The operation of SF6 switchgear is seriously influenced by moisture content, so it is very improtant to detect moisture content of SF6 swtichgear on-line.By means of comprehension and analysis of former research achievements, the paper presents the flaws and limitations of traditional methods and advances spectrum absorption technology. Based on molecule spectrum theory and experimental, the spectrum absorption characteristic of water in near infrared region is studied. Each kind of gases has inherent absorptive spectrum. When a light source's emitted wavelength is corresponding to the gas's absorptive wavelength, resonating absorption will happen. The absorptive intensity is related to the gas's concentration, so the gas's concentration can be known by the detection of absorptive intensity.The designs of the circuit, light path and gas cell are based on the theoretical analysis in the paper. For the signals are very weak, the lock-in amplifier is employed to extract the signals in the circuit design. It can inhabit the noise in the most and maximize the signal-to-noise.The experiment results show that the on-line monitoring system design can detect the moisture content in SF6 gas and provide a good technique for on-line monitoring system.
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