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DAS是一种利用普通通信光纤作为传感介质,光纤自身的反向瑞利散射信号,能够精确有效测量光纤所受应力变化的全分布式光纤传感系统[4-6]。图 1是DAS系统反向瑞利散射信号示意图。
当光脉冲信号入射一段光纤后,激光在传播的光纤路径上会产生瑞利散射光,部分散射光方向与光纤前进方向相反,反向瑞利光沿光纤反向继续传播,抵达激光发射端。类似于相机闪光灯拍摄原理,在拍摄瞬间发射光脉冲,胶卷对返回的光曝光形成胶片底片。区别在于两者光线不同,相机曝光底片的光是反射光,DAS系统利用的是散射光。
利用散射光原理,当外界声音信号震动引起所在位置光纤应力发生变化,即对光纤的折射率产生影响,最终都会导致反向瑞利散射信号发生相应的变化。DAS利用这个反向瑞利散射信号的变化,提取到该位置在不同时刻的应力信号,从而实现对该位置外界信号的定量测量。
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已知两束光强为A、角频率为ω的激光,干涉后的光强信号I为:
$I=2 A+2 A \cos (\omega t+\psi)$
(1) 式中,t是时间, ψ是光纤收到外界物理信号影响导致的相位变化信号。
通过高通滤波器,即可得到光强信号I的交流部分IAC:
$I_{\mathrm{AC}}=2 A \cos (\omega t+\psi)$
(2) 式中, 角频率ω是系统通过声光调制器调制上去的,属于已知信号,所以可以利用正交调制/解调算法解调出未知的待测信号ψ。
由于ω是已知信号,可以得到本地信号S1、S2:
$S_1=\cos (\omega t)$
(3) $S_2=-\sin (\omega t)$
(4) 让IAC和S1混频得:
$\begin{gathered}I_{\mathrm{AC}} \times S_1=2 A \cos (\omega t+\psi) \times \cos (\omega t)= \\ A \cos (2 \omega t+\psi)+A \cos \psi\end{gathered}$
(5) 过滤其中高频部分cos(2ωt+ψ), 得到信号S1:
$S_1=A \cos \psi$
(6) 让IAC和S2混频得:
$\begin{aligned} I_{\mathrm{AC}} \times S_2 & =2 A \cos (\omega t+\psi) \times[-\sin (\omega t)]= \\ & -A \sin (2 \omega t+\psi)+A \sin \psi\end{aligned}$
(7) 过滤其中高频部分sin(2ωt+ψ), 得到信号S2:
$S_2=A \sin \psi$
(8) 用S2/S1,得到:
$S_2 / S_1=\tan \psi$
(9) 再用反正切运算,即可得到待测信号ψ。
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为了从反向瑞利散射信号的变化解调出不同位置的外界信号,DAS系统可以利用激光调制解调结构对发出的激光脉冲进行调制,在此基础上对返回的反向瑞利散射信号做解调,得到不同位置的外界信号。图 2是DAS系统激光调制解调示意图。
激光器采用窄线宽的单波长激光器,工作波长1550 nm。系统通过声光调制器(acoustooptic modulator, AOM)调制激光器发出的激光,从而产生不同频率及不同时间间隔(角频率相差ω)的两束激光脉冲A和B,这两个激光脉冲经过掺铒光纤放大器(erbium doped fiber application amplifier, EDFA)放大,通过环形器后进入待测量光纤,从而形成对应的两束反向瑞利散射信号A和B,这两束反向瑞利散射信号经环形器输入到耦合器后,A路经过延迟,B路不延迟,这样在光纤内同一位置的两束不同频率的反向瑞利散射信号就会产生干涉效果,干涉后的光强信号通过光电探测器(photodetector,PD)变为电信号,再通过模数转换器(analog to digital converter,ADC)转换为数字量,并利用中央处理器(central processing unit,CPU)的解调算法,从而解调出对应位置的相位信号(即待测信号)。
光纤分布式声传感系统在GIS耐压测试中的应用
Application of optical fiber distributed acoustic sensing system in GIS insulation withstand voltage test
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摘要: 气体绝缘开关设备(GIS)在国家电网变电站建设领域应用广泛,通常在GIS设备投产之前需要做绝缘耐压试验,以对现场故障设备进行定位。为了进行更快速有效的定位, 针对实际应用环境,利用分布式声传感系统(DAS)和相位敏感光时域反射仪,通过光纤自身的反向瑞利散射信号,进行多波长多路径调制解调,由光纤实时监测GIS管道的不同位置的振动信号,减少测试盲点。结果表明,所研制的DAS系统能够实现对GIS故障设备进行实时快速准确的定位,定位精度可达到3 m内。该研究对于GIS现场故障的及时排除和维修处理,保障电力设备安全运行具有较好的参考价值。
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关键词:
- 传感器技术 /
- 光纤分布式声传感系统 /
- 气体绝缘开关设备 /
- 耐压测试
Abstract: Gas insulated switchgear (GIS) is widely used in the construction of State Grid Substations, and it is usually necessary to do insulation withstand voltage tests before GIS is put into production. To locate faulty equipment on site, according to the practical application environment, a fast and effective method was presented to locate the fault equipment in GIS insulation withstand voltage test by using distributed acoustic sensing (DAS) system. Through the back Rayleigh scattering signal of the optical fiber itself, the phase heterodyne was adopted phase-sensitive optical time-domain reflectometer (φ-OTDR) multi wavelength multi path modulation and demodulation method, which can visually and real-time monitor the optical fiber vibration signals at different positions of the metal shell of the GIS system to reduce the test blind spots. The experimental test results show that, fast and accurate positioning of fault equipment in GIS can be realized with the developed DAS system, and the positioning accuracy can reach within 3 m. It has a good reference value for timely troubleshooting and maintenance of GIS field faults and ensuring the safe operation of power equipment. -
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