Distributed fiber temperature measurement system based on the principle of backscattering can be divided into three types: based on Rayleigh scattering, based on Raman scattering and based on Brillouin scattering. Currently developed more mature, and products are applied to the project is based on Raman scattering distributed optical fiber temperature measurement system. Its sensing principle is mainly based on the OTDR principle of optical fiber and the temperature effect of backward Raman scattering of optical fiber.
Distributed optical fiber temperature measurement
I. Introduction
With the economic development of our country, the power system is moving in the direction of ultra-high voltage, large power grid, large capacity and automation. In the event of an accident, it will cause huge losses to the national economy. How to conduct on-line monitoring of running electrical equipment and conduct safety prediction and temperature trend analysis? How to conduct scientific analysis on equipment quality, operating environment, operation mode, equipment aging and load imbalance through real-time data? These are the urgent problems to be solved in the power system. Traditional infrared thermometer, infrared imager, temperature cable, RTD temperature measurement system can only measure the local temperature of the power system and can not provide scientific basis for safe, economical operation and efficient overhaul. The distributed optical fiber temperature measurement system can realize multi-point and on-line distributed measurement, real-time on-line monitoring of running equipment and effectively solve the problem of unforeseen emergency such as high temperature, burning, explosion and fire on site . In the power system, this kind of optical fiber temperature measurement technology has the advantages of temperature fixed-point sensing in high-voltage power cables, heating parts caused by bad contact of electrical equipments, cable interlayer, cable channels, large generator stator, large transformers , boilers and other facilities A wide range of applications.
Second, the basic principle of distributed optical temperature measurement
Distributed optical fiber temperature measurement system based on the principle of backscattering can be divided into three types: based on Rayleigh scattering, based on Raman scattering and based on Brillouin scattering. Currently developed more mature, and products are applied to the project is based on Raman scattering distributed optical fiber temperature measurement system. Its sensing principle is mainly based on the OTDR principle of optical fiber and the temperature effect of backward Raman scattering of optical fiber.
(A) Optical Time Domain Reflectometry (OTDR) principle
When a laser pulse is transmitted in an optical fiber, scattering occurs due to microscopic non-uniformities of the refractive index in the optical fiber. In the time domain, the time required for incident light to backscatter back to the fiber entrance end is t, and the distance traveled by the laser pulse in the fiber is 2L, where v is the propagation speed of light in the fiber and C is the vacuum Of the speed of light, n is the refractive index of the fiber. At the measured time t, the distance from the light source L can be obtained.
(B) fiber Raman scattering temperature effect
When a laser pulse is incident on an optical fiber from one end of the optical fiber, the optical pulse propagates forward along the optical fiber. Since the light pulse collides with the internal elements of the fiber elastically and inelastically, the light pulse is reflected at every point in the transmission. A small part of the reflected light is reflected in the opposite direction to that of the incident light For backward). The intensity of this retroreflected light correlates with the temperature of the reflection point in the light. The higher the temperature of the reflection point (the ambient temperature at which the fiber is located), the greater the intensity of the reflected light. Using this phenomenon, if you can measure the intensity of retroreflected light, you can calculate the temperature of the reflection point, which is the basic principle of using fiber temperature measurement.
As expressed by the formula: Rayleigh scattering, Brillouin scattering and Raman scattering occur when the laser pulse interacts with the optical fiber when it propagates in the optical fiber. Raman scattering is caused by thermal vibration and photon interaction of the optical fiber molecule Generated by the exchange of energy. If part of the light energy is converted to thermal vibration, then a light longer than the wavelength of the light source is called Stokes; if part of the thermal vibration is converted to light energy, a light shorter than the wavelength of the light source will be emitted, called Anti-Stokes light. According to the Raman scattering theory, under the conditions of spontaneous Raman scattering, the light intensities of the two reflected light beams are related to the temperature, and their ratio R (T) is:
(1) where, and are respectively the Stokes light intensities and the anti-Stokes light intensities, h is the Planck's constant, k is the Boltzmann's constant, and T is the absolute temperature. It can be seen from (1) that R (T) is related to temperature T only. Therefore, we can use the anti-Stokes and Stokes light intensity ratio to achieve the temperature measurement.
Third, the distributed optical fiber temperature sensing system sensing process
As shown in Figure 1, the sensing process of the distributed optical fiber temperature measurement system is as follows: The computer-controlled synchronous pulse generator generates a pulse with a certain repetition frequency. On the one hand, this pulse modulates the pulse laser to generate a series of high-power optical pulses, On the other hand to provide high-speed data acquisition card synchronization pulse, into the data acquisition state. The optical pulse enters a sensing fiber through one port of the wavelength division multiplexer and produces backscattered light at each point in the fiber, returning it to the wavelength division multiplexer. The backscattered light filters the Stokes light and the anti-Stokes light respectively through the thin film interference filter in the wavelength division multiplexer and is outputted to the other two ports of the wavelength division multiplexer and respectively enters into Photodetector (APD) and amplifier optoelectronic conversion and amplification, the signal is amplified to the data acquisition card can capture the range. Finally, the data acquisition card for storage and processing, for temperature calculation.
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