基于微型共振光声池的高灵敏度油中溶解气体检测技术

Ultra-high Sensitivity Detection Technology of Dissolved Gas in Oil Based on Miniature Resonant Photoacoustic Cell

针对真空脱气法脱气效率高但气量少的特点,提出了基于微型共振光声池的高灵敏度油中溶解气体检测技术。使用真空脱气法对变压器油中溶解的乙炔气体进行脱气,结合气室体积为12.4 mL的小体积H型共振式光声池,实现对油中溶解乙炔气体浓度的高灵敏度测量。利用掺铒光纤放大器对激光功率进行放大,并分析在近红外波段乙炔、水蒸气和二氧化碳的气体吸收光谱,选择较合适的气体吸收谱线进行光声信号激发。通过实验对激光调制电流和调制频率等参数进行优化。配置不同浓度的油样对系统的性能进行测试,得到系统对乙炔油样的最低检测限为0.2μL/L。

The detection of dissolved acetylene(C2H2)in transformer oil is an important means to detect the operating state of transformers.In order to realize the low concentration of dissolved acetylene gas in oil,a detection system of dissolved C2H2gas in oil based on a miniature resonant Photoacoustic(PA)cell is designed.The oil sample was put in a vacuum oil tank.Then the dissolved gas in oil diffused into the vacuum part of the oil tank.Because of the diffusion of the dissolved gas,the vacuum of the oil tank decreased.The gas was pumped into a gas cell after a set time.The gas in the gas cell was pumped into the detection system by a gas pump to obtain the concentration of C2H2.The degassing time was about 40 min.A Distributed Feedback(DFB)laser was selected as the excitation source of the PA signal.According to the spectrum data of C2H2obtained from the HITRAN database,the central wavelength of the DFB laser was chosen to be 1532.83 nm with an emission spectrum width of 0.3 pm after considering the cross interference of H2O and CO2.The output power of the DFB laser was 13.2 mW.To keep the laser power stable,the temperature of the DFB laser was kept to be 27℃.The output of the DFB laser was modulated by a sine signal which was produced by a signal processing circuit of a Field Programmable Gate Array(FPGA).The power of the laser was amplified to 200 mW by an erbium doped fiber amplifier before the laser was emitted into the PA cell through a fiber collimator.The volume of the PA cell was 12.4 mL.In the PA cell,the laser was reflected by a gold-plated mirror with 98%reflectance to double the absorption length.According to the principle of the PA,the PA signal was increased with the increase of the absorption length.A constant temperature device was used to stabilize the temperature of the PA cell at 30℃to prevent the resonant frequency of the PA cell from drifting due to temperature changes.An electric microphone was used to detect the PA signal.The responsibility of the electric microphone was good between 10 Hz and 10 kHz.A lock-in amplifier based on FPGA was used to extract the second harmonic signal from the electric signal of the microphone.The response frequency range of FPGA lock-in amplifier was 1 Hz~10 kHz.Finally,the electric signal was processed by a LabVIEW program to obtain the concentration of dissolved C2H2in oil.The signals of 50μL/L standard C2H2gas was recorded during the frequency range from 1000 Hz to 3000 Hz.The maximum value of the signals was obtained at the frequency of 2565 Hz.Because of the use of second harmonic technology,the resonance frequency of the PA cell was 5130 Hz.Modulation current and bias current were determined to be 5 mA and 102.8 mA respectively at the resonance frequency.Dissolve different volumes of C2H2standard gas into transformer blank oil to prepare oil samples with concentrations of 2μL/L,10μL/L,20μL/L,40μL/L and50μL/L.Under the condition mentioned above,the responsibility of dissolved C2H2in oil was1.475μV/(μL·L^(-1)).The performance of the system was verified by repeated experiments on dissolved C2H2in oil samples of various concentrations.And the system had good repeatability with the absolute error value of each test within±0.2μL/L.The minimum detection limit of the system for dissolved C2H2-in-oil samples is 0.2μL/L.The detection system realizes an ultra-high sensitivity measurement of dissolved acetylene gas in oil,which can provide a new technical solution for on-line monitoring of dissolved gas in transformer oil.