Abstract：Sulfur hexafluoride (SF6) is an excellent insulating medium, which is widely used in gas insulated equipment. When insulation faults such as partial discharge (PD) or overheating occur, SF6 will decompose and react with micro-water, micro-oxygen, or other solid insulating media. Detection of SF6 decomposition products has become an effective means of operation status monitoring, fault diagnosis and early warning for gas insulation equipment. H2S is a special product related to serious insulation faults such as high energy PD or high temperature local overheating. CO and CO2 are products of overheating or severe breakdown involving organic materials. However, current detection technologies cannot take into account both detection accuracy and practicability. To address the problem, this paper designs a high precision gas detection system based on the cavity ring-down spectroscopy (CRDS) with high sensitivity, low limit of detection and fast response speed, which can detect H2S, CO and CO2 gases in SF6 background. This paper first presents a prototype of the detection device with integrated optical components and embedded microprocessor units, builds an optical resonator with length of 260 mm and free spectral range of 576.9 MHz, proposes a nonlinear fitting strategy based on Levenberg-Marquardt (LM) algorithm, and designs a DFB laser driving circuit. The laser central wavelength is linearly correlated with the control temperature and current in a small range of 5~10 nm. The proposed temperature control module is monitored a current variation of ±0.02 mA and a temperature variation of ±0.001℃ within 24 hours. Then, this paper verifies the linear responses to H2S concentration in the SF6 and N2 background, measures the absorption coefficient of H2S over the concentration range from 0 to 20×10-6, and explores the response time and detection accuracy of the CRDS system. It is found that the output data at H2S concentrations of 1×10-6, 2×10-6, 5×10-6, and 10×10-6 show normal distribution characteristics with standard deviations of 22×10-9, 21×10-9, 26×10-9, and 43×10-9, respectively, and the measurement errors increase with increasing concentration. The Gaussian noise can be reduced by means of moving average. Allan variance analysis shows that the limit of detection (LoD) of H2S in SF6 is 4.25×10-9 in the CRDS system. In addition, by modulating the laser wavelength, CO and CO2 are measured over the concentration range from 0 to 200×10-6, and the LoD can reach 10.4×10-9 and 24.8×10-9. Finally, considering that the gas reactions in insulation faults are complex and generate multiple products simultaneously, this paper conducts the cross-interference experiments on H2S, CO, and CO2. The spectra of mixed gases show that the CO spectrum is relatively independent, while the H2S spectrum at 6 336.617 cm-1 and the CO2 spectrum at 6 336.242 cm-1 overlap significantly. Multiple linear regression is used to correct the cross- interference between H2S and CO2. Before correction, every 120×10-6 of CO2 resulted in a 1×10-6 over- indication of H2S, and every 5×10-6 of H2S resulted in a 12.8×10-6 over-indication of CO2. After correction, the detection error is less than 5% of the gas concentration.
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