Design and optimization of photoacoustic CO gas sensor for fault diagnosis of SF<sub>6</sub> gas insulated equipment

Abstract

Trace gas analysis for SF<sub>6</sub> decomposition is a powerful diagnostic method to identify partial discharge problem occurring in electrical equipment. In particular, it is recognized that the SF<sub>6</sub> decomposition gases (such as CO, H<sub>2</sub>S, SO<sub>2</sub> and CF<sub>4</sub>) can effectively determine the inner insulation condition of the electrical equipment. Currently, most of researches of diagnostic methods cannot meet the online high-precision detection of gas derivatives in SF<sub>6</sub> electrical insulation equipment. Therefore, there is a need of developing a sensitive, selective and cost-effective sensor system for CO detection in an SF<sub>6</sub> buffer gas environment due to the fact that the power system is filled with pure SF<sub>6</sub> as the dielectric gas, which means that the concentration of SF<sub>6</sub> is usually > 99.8%. The traditional photoacoustic CO gas sensors cannot be directly used in power system, since several SF<sub>6</sub> physical constants strongly differ from those of N<sub>2</sub> or air. In addition, SF<sub>6</sub> molecule reveals uninterrupted and strong absorption lines in the mid-infrared spectral region. In this work, a CO gas sensor working in high concentration SF<sub>6</sub> background gas is designed by using a distributed feedback (DFB) laser as an excitation source with a center wavelength of 2.3 μm. The absorption line strength of 2.3 μm is ~ two orders of magnitude higher than the absorption line strength around 1.56 μm, which can improve the sensor detection performance. A background-gas-induced high-<i>Q</i> differential photoacoustic cell is simulated numerically and tested experimentally. The quality factor of the designed photoacoustic cell in pure SF<sub>6</sub> gas is 85, which is ~ 4 times higher than that in N<sub>2</sub> carrier gas. The experimental results show that the maximum gas flow rate of the differential structure photoacoustic cell is ~ 6 times higher than that of the single resonant cavity photoacoustic cell. After optimizing the resonance frequency, gas velocity and working pressure of the sensor system, the detection sensitivity of the volume fraction of 1.85 × 10<sup>–6</sup> is achieved. In the case of the volume fraction of 50 × 10<sup>–6</sup> CO/SF<sub>6</sub> gas mixture, the maximum photoacoustic signal amplitude of 19.6 μV is obtained, the corresponding normalized noise equivalent concentration (1σ) is 3.68 × 10<sup>–8</sup> cm<sup>–1</sup>·W·Hz<sup>1/2</sup> in 1 s integration time. A linear fitting is implemented to evaluate the response of the sensor from 50 × 10<sup>–6</sup> to 1000 × 10<sup>–6</sup>, resulting in an <i>R</i> square value of 0.9997. The CO photoacoustic gas sensor has high sensitivity, good selectivity and strong noise immunity, which can provide an on-line detection technology for potential insulation fault diagnosis in the power system. The capability of CO gas sensor can be improved by using a higher excitation optical output power and/or reducing the PAC resonator volume to increase the cell constant.