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一部分气体分子在吸收了激光的能量后被激发到高能级。一般来说,自发辐射和碰撞弛豫是吸收能量释放的两种主要方式。然而,振动能级辐射弛豫时间太长抑制了能量的释放。因此,激发态气体分子主要通过碰撞弛豫释放能量,转化为气体分子的平动能,引起气体的周期性振动,产生光声压力波。在非共振圆柱形光声管中,光声声波处处均等,光声(photoacoustic, PA)压力波pPA的振幅可表示为:
$ p_{\mathrm{PA}}(f)=\frac{(\gamma-1) P_0 \alpha(\lambda) l}{V} \frac{\tau}{\sqrt{1+(2 {\rm{ \mathsf{ π} }} f \tau)^2}} $
(1) 式中,pPA是频率f的函数,用pPA(f)表示; γ为气体的质量热容比; P0为激光功率; α(λ)为气体吸收系数, 是吸收光波长λ的函数; l为光声管的长度; V为光声管的体积; τ表示气体的热阻尼,是一个随温度变化的函数,τ越大,光声压力波越大。光声管产生的光声压力波通过光纤FPI声波传感器检测,通过检测F-P腔变化量获得光声信号大小。
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为实现2f-WMS检测,对激光器提供一个频率为500Hz的正弦信号进行调制,根据波长调制光谱技术原理,二次谐波信号的幅度随调制深度变化,通过优化激光器的调制深度,可以提高信噪比。首先对体积分数为250×10-6的CO气体进行测试,控制激光器在中心波长2331.9nm处扫描,该波长对应CO气体一个吸收峰。改变正弦激光调制电流的有效值从1mA~6mA,记录光声信号的输出值,图 6所示为信号幅度随调制电流的变化关系。从实验结果看出,调制电流为3.25mA时,光声信号的二次谐波幅度最大,即最佳调制电流为3.25mA。
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为了测试所设计的光纤光声气体传感器的一氧化碳体积分数的响应特性,将不同气体体积分数的一氧化碳/高纯氮气混合气体依次通入测试气室,并且记录二次谐波信号。使用质量流量控制器(S48-300,HORIBA)控制气体流速,对气体体积分数进行稀释。将高纯氮气与气体体积分数为1000×10-6的一氧化碳标准气体安装一定比例混合,得到气体体积分数分别为1000×10-6,500×10-6,250×10-6,100×10-6,50×10-6和25×10-6的测试气体。设定锁相积分时间为1s,将DFB激光器的偏置电流调制范围为90mA~100mA,实现在2331.9nm附近的波长扫描。为进一步提信噪比,采用小波去噪法对2f-WMS进行去噪。测量的2f-WMS光谱光声信号如图 7所示。提取测得的波长调制光谱信号的峰值,分析信号峰值随气体体积分数的变化,所得结果如图 8所示。通过线性拟合,估算了设计的光纤光声气体传感器对一氧化碳气体体积分数的检测灵敏度为0.345pm/10-6, 计算线性拟合的R2≈0.996。这表明该传感器对气体体积分数小于1000×10-6的一氧化碳气体具有良好的线性响应。
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为进一步确定设计的光纤光声传感器对油中溶解一氧化碳气体的测量精度,分别配置了油中溶解气体体积分数为5×10-6, 10×10-6, 50×10-6和100×10-6的油样各3份进行测试。将溶解不同气体体积分数CO的油样充入油室,控制锁相积分时间为5s,脱气温度为50℃,为保证脱气充分平衡,等待2.5h后对CO进行检测。结合CO气体响应度和气室油室的气体体积分数比,对测得的光声信号测量值进行标定获得油中溶解气体体积分数,测试结果如表 1所示。当油中溶解一氧化碳气体体积分数大于10×10-6时,测量值误差均在±15%以内,并且最低检出限达到5×10-6,完全满足《变压器油中溶解气体在线监测装置技术规范》中对一氧化碳的A级误差要求[20]。
Table 1. Measurement results of dissolved carbon monoxide gas in oil with different gas volume fraction
oil sample
volume
fractionstest result 1 test result 2 test result 3 absolute error 100×10-6 101.9×10-6 100.8×10-6 105.6×10-6 5.6×10-6 50×10-6 53.7×10-6 48.8×10-6 53.9×10-6 3.9×10-6 10×10-6 9.3×10-6 11.3×10-6 10.1×10-6 1.3×10-6 5×10-6 6.5×10-6 4.6×10-6 5.5×10-6 1.5×10-6
变压器油中溶解一氧化碳气体的光纤传感技术
Optical fiber sensing technology for detection of dissolved carbon monoxide gas in transformer oil
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摘要: 为了满足变压器中绝缘纸板因过热或者放电故障产生的一氧化碳气体的在线监测需求, 提出了一种基于光纤光声传感的油中溶解一氧化碳气体检测技术。采用光声光谱气体检测技术、并结合光纤传感和膜分离技术, 设计了集成油气分离和气体检测功能于一体的光纤光声传感探头, 油中溶解的一氧化碳气体通过油气分离膜进入到光纤探头中的微型气腔; 采用两根光纤将探头连接到解调仪器, 分别传输近红外激发光和探测光; 气体吸收光能产生的光声信号被光纤法布里-珀罗传感器探测, 并被设计的光纤光声解调模块进行信号处理, 获得系统对一氧化碳气体体积分数的检测灵敏度为0.345pm/10-6。结果表明, 所设计的光纤传感系统对油中溶解一氧化碳气体体积分数检出限达到5×10-6。该研究具有精度高、抗电磁干扰、脱气简单的优势, 为变压器油中溶解一氧化碳气体的检测提供了新方法。Abstract: In order to realize the online monitoring of carbon monoxide gas generated by overheating or discharge failure of insulating paperboard in transformers, a detection technology of carbon monoxide gas dissolved in oil based on optical fiber photoacoustic sensing was proposed. Combining photoacoustic spectroscopy, optical fiber sensing, and membrane separation technology, a fiber optic photoacoustic sensing probe that integrates oil and gas separation and gas detection functions was designed. The carbon monoxide gas dissolved in the oil enters the miniature air cavity in the optical fiber probe through the oil and gas separation membrane. Two optical fibers were used to connect the probe to the demodulation instrument, and transmit near-infrared excitation light and probe light respectively. The photoacoustic signal generated by the absorption of light energy by the gas was detected by the optical fiber Fabry-Perot sensor and processed by the designed optical fiber photoacoustic demodulator. The detection sensitivity of the system for volume fraction of carbon monoxide gas was 0.345pm/10-6. The experimental results show that the designed optical fiber sensing system has a detection limit of 5×10-6 for the volume fraction of dissolved carbon monoxide gas in oil. This research has the advantages of high accuracy, anti-electromagnetic interference, and simple degassing, and provides a new method for detecting carbon monoxide gas dissolved in transformer oil.
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Table 1. Measurement results of dissolved carbon monoxide gas in oil with different gas volume fraction
oil sample
volume
fractionstest result 1 test result 2 test result 3 absolute error 100×10-6 101.9×10-6 100.8×10-6 105.6×10-6 5.6×10-6 50×10-6 53.7×10-6 48.8×10-6 53.9×10-6 3.9×10-6 10×10-6 9.3×10-6 11.3×10-6 10.1×10-6 1.3×10-6 5×10-6 6.5×10-6 4.6×10-6 5.5×10-6 1.5×10-6 -
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