The Principle of Distributed Fiber Temperature Measurement System and the Analysis of the Sensing Process

The Principle of Distributed Fiber Temperature Measurement System and the Analysis of the Sensing Process

Distributed optical fiber temperature measurement systems can be classified into three types based on the backscattering principle: Rayleigh scattering, Raman scattering, and Brillouin scattering. At present, the development is relatively mature, and there is a distributed optical fiber temperature measurement system based on Raman scattering. Its sensing principle is mainly based on the optical time domain reflectometry (OTDR) principle of the fiber and the backward Raman scattering temperature effect of the fiber.

Distributed fiber temperature measurement

I. Introduction

With the development of our country’s economy, the power system is moving in the direction of ultra-high voltage, large power grids, large capacity, and automation. Once an accident occurs, it will cause huge losses to the national economy. How to monitor the running electrical equipment online and carry out safety prediction and temperature change trend analysis? How to scientifically analyze equipment quality, operating environment, operation mode, equipment aging, and load imbalance through real-time data? These are the problems that urgently need to be solved in the power system. Conventional infrared thermometers, infrared imagers, temperature sensing cables, thermal resistance temperature measurement systems, etc. can only measure the temperature of a local part of the power system, and cannot provide scientific basis for safe, economical operation and efficient overhaul. The distributed optical fiber temperature measurement system can realize multi-point, on-line distributed measurement, realize real-time online monitoring of operating equipment, and effectively solve the problem of high temperature, combustion, explosion, fire and other accident emergencies that have occurred on the spot for a long time. . In the power system, this optical fiber temperature measurement technology has a fixed-point temperature sensing application in high-voltage power cables, electrical equipment due to poor contact caused by heating parts, cable interlayers, cable channels, large generator stators, large transformers, boilers and other facilities. Wide application prospects.

Second, the basic principle of distributed optical fiber temperature measurement

Distributed optical fiber temperature measurement systems can be classified into three types based on the backscattering principle: Rayleigh scattering, Raman scattering, and Brillouin scattering. At present, the development is relatively mature, and there is a distributed optical fiber temperature measurement system based on Raman scattering. Its sensing principle is mainly based on the optical time domain reflectometry (OTDR) principle of the fiber and the backward Raman scattering temperature effect of the fiber.

(I) Principle of Optical Time Domain Reflectometry (OTDR)

When laser pulses are transmitted in an optical fiber, scattering occurs due to the microscopic heterogeneity of the refractive index in the optical fiber. In the time domain, the time required for the incident light to return to the incident side of the fiber by backscattering is t. The distance traveled by the laser pulse in the fiber is 2L, where v is the propagation velocity of light in the fiber and C is in vacuum. The speed of light, n is the refractive index of the fiber. At the time t is measured, the distance from the light source L can be determined.

(B) Backward Raman scattering temperature effect of optical fiber

When a laser pulse is injected into the fiber from one end of the fiber, this pulse of light travels along the fiber. Because the light pulse and the internal molecules of the fiber undergo elastic collisions and inelastic collisions, the light pulse will be reflected at every point in the propagation. The reflection has a small part of the reflected light, and the direction is just opposite to the direction of the incident light (also called For backwards). The intensity of this retroreflected light has a certain correlation 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 the intensity of the retroreflected light can be measured, the temperature of the reflection point can be calculated. This is the basic principle of measuring temperature using an optical fiber.

As expressed by the formula: Rayleigh scattering, Brillouin scattering and Raman scattering occur when the laser pulse interacts with the optical fiber molecules while propagating in the optical fiber. Raman scattering is due to the thermal vibration and photon interaction of the optical fiber molecules. Energy exchange occurs. If a part of the light energy is converted into thermal vibration, then a light longer than the wavelength of the light source, called Stokes light, will be emitted; if a part of the thermal vibration is converted into light energy, light with a shorter wavelength than the light source will be emitted, which is called Anti-Stokes light. According to the Raman scattering theory, under the spontaneous Raman scattering conditions, the light intensity of the two reflected lights is related to the temperature, and their ratio R(T) is:

(1) where and are the Stokes intensity and the anti-Stokes intensity, h is the Planck constant, k is the Boltzmann constant, and T is the absolute temperature. From (1), we can see that R(T) is only related to temperature T. Therefore, we can use the ratio of anti-Stokes and Stokes light intensity to achieve temperature measurement.

Third, the distributed fiber temperature measurement system sensing process

As shown in Fig. 1, the sensing process of the distributed optical fiber temperature measurement system is: the computer controls the synchronization pulse generator to generate a pulse with a certain repetition frequency. This pulse modulates the pulse laser on the one hand and generates a series of high-power light pulses. On the other hand, a sync pulse is provided to the high-speed data acquisition card to enter the data acquisition state. The optical pulse enters the sensing fiber through a port of the wavelength division multiplexer and generates backscattered light at various points in the fiber and returns to the wavelength division multiplexer. The backscattered light is filtered out of the Stokes light and the anti-Stokes light through the thin film interference filter in the wavelength division multiplexer, respectively, and output through the other two ports of the wavelength division multiplexer, and enters them respectively. Opto-electronic detectors (APDs) and amplifiers perform opto-electronic conversion and amplification and amplify the signal to the range that the data acquisition card can capture. Finally, it is stored and processed by a data acquisition card for temperature calculation.

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