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Distributed fiber optic temperature measurement system is a new technology gradually developed in recent years for real-time measurement of spatial temperature field. It can continuously monitor the temperature of various points within a range of more than ten kilometers along the fiber optic cable, with tens of thousands of temperature collection points, positioning accuracy of up to 1m, temperature measurement accuracy of ± 1 °C, and temperature measurement range of -20 ℃ to+150 ℃. It is very suitable for large-scale, harsh environments, and multiple temperature monitoring points in coal mine underground power supply systems. The commonly used temperature monitoring methods for coal mine electrical equipment include: artificial infrared induction gun inspection, thermistor temperature measurement system, and thermocouple temperature measurement system. Traditional temperature measurement methods have drawbacks such as inability to monitor in real time, poor stability, poor insulation, hazardous on-site environment, high electromagnetic interference, high transmission loss using analog signals, and low temperature measurement accuracy. In response to the main causes and characteristics of electrical fire accidents in coal mines, independently developed distributed fiber optic temperature measurement technology to comprehensively monitor and warn the temperature of the underground power supply system. Even for some non Class I important loads, a feedback control scheme of power-off locking can be provided before the heating amplitude reaches the ignition point.
Principle of Distributed Fiber Optic Temperature Measurement System
Distributed fiber optic temperature measurement refers to a temperature monitoring system that comprehensively utilizes the Raman scattering effect (Raman) of optical fibers and optical time-domain reflection measurement technology (OTDR) to obtain spatial temperature distribution information. Some physical quantities in daily life, such as temperature, pressure, and tension, can affect glass fibers and locally alter the optical transmission characteristics in optical fibers. Due to the attenuation of light in quartz glass fibers through scattering, the location of external physical effects can be determined, making optical fibers suitable for linear sensors. Thermal effects cause lattice oscillations in fiber optic quartz solids. When light falls on these thermally excited molecular oscillations, light scattering occurs when there is interaction between the electrons of light particles and crystal molecules, also known as Raman scattering. Unlike incident light, the spectral shift of this scattered light is equivalent to the resonance frequency of lattice oscillations. The light scattered back from the optical fiber contains three different spectra: Rayleigh scattering light, Stokes light, and anti Stokes light. The anti Stokes light band has a strong temperature dependence, while Stokes light is almost temperature independent. The intensity of anti Stokes light in the optical fiber changes due to external temperature effects. The ratio of anti Stokes light intensity to Stokes light intensity can be used to calibrate temperature, and this principle can be used to achieve distributed measurement of temperature fields at various points along the optical fiber.
The principle of optical time-domain reflection measurement technology (OTDR) is to emit light pulses to the tested fiber, generate Raman scattering effect, and the backscattered light formed is transmitted back to the starting end of the fiber (i.e. the injection end of the light pulse). Since each backscattered light corresponds to a scattering point on the fiber, the location of the scattering point on the fiber can be determined based on its propagation time.
Coal mine temperature measurement
In recent years, the number of mechanical and electrical equipment used in coal mines in China has gradually increased, and the power supply system has become increasingly complex, with long lines, multiple branches, high voltage levels, multiple equipment units, and high power. The reality of multiple nodes such as switches and junction boxes along the line has brought greater fire hazards to the coal mine power supply system. In coal mine production practice, many fixed laid cables have long service life, aging insulation, poor contact of cable joints, and lack of effective inspection and timely maintenance, which can easily cause electric leakage and fire; بالإضافة, poor contact of the contacts inside the high-voltage switch or junction box chamber can also easily cause arc ignition, overheating of internal components of the equipment, and electrical fires. The above are the main causes of coal mine electrical fires. The major fire accidents caused by the ignition of underground cables or high and low voltage switchgear in coal mines seriously threaten the safety of coal mines.
The distributed fiber optic temperature measurement system has designed corresponding fiber optic cable laying schemes for several different temperature measurement objects underground.
Cable joints for laying optical cables underground in coal mines are weak points in cable connections and are prone to short circuits, leakage, and overheating. In response to the relatively coarse manual production of cable joints, a temperature sensing optical cable double ring winding method is used to fix it at the cable terminals and joints, which can make it fully and tightly contact, and monitor the temperature of the entire cable joint more densely and sensitively.
There are corresponding solutions for the layout of temperature measuring optical fibers in cable joints, the installation of detection optical fibers in cable trays, and the layout of temperature measuring optical fibers in cable trays.
In the actual laying process of various underground cables in coal mines, in order to achieve aesthetic and standardized requirements, cable trays are often used for enclosed installation. This causes great inconvenience to the heat dissipation of cables and manual temperature measurement. Based on the characteristics of such monitoring objects, the installation of optical cables adopts an S-shaped curve laying method, and the temperature of the cables in the bridge can be controlled in real time.
Temperature monitoring method for cables suspended by cable hooks
Various types of cables in underground coal mines are often hung in rows through cable hooks. At this time, each cable is tightly attached to a fiber optic cable, and the temperature measurement fiber optic cable layout method for hanging cables in rows underground coal mines is adopted
Online monitoring of static contact and busbar temperature in high-voltage switchgear
High voltage switchgear, underground explosion-proof high opening, mobile transformers, combination switches, frequency converters, and other box type electrical equipment in various coal mine surface substations can be installed using fiber optic winding and fixing methods, with a focus on monitoring specific hazardous points,
After the application of fiber optic temperature measurement system in coal mines, a total of 6314 temperature monitoring points were arranged along various types of high and low voltage cables with a length of about 57260m within a 15.7km underground tunnel range. The wide distribution of measurement points, the large amount of monitoring information, and the long transmission distance are far beyond the reach of traditional temperature measurement methods. بالإضافة, the fiber optic temperature measurement system has excellent intrinsic safety, مقاومة التآكل, مقاومة الجهد العالي, and electromagnetic interference resistance, and can automatically detect the precise position of fiber optic cable breakpoints, providing convenience for rapid system repair.
Layout method of temperature measuring optical fibers inside high-voltage switchgear
In terms of safety and economic benefits, the fiber optic temperature measurement system effectively solves the huge hidden dangers caused by traditional coal mine electricians due to factors such as heavy workload, negligence, and insufficient sense of responsibility, such as inadequate manual inspections and untimely detection of fire hazards (often only discovered when smoke is emitted), greatly saving labor costs for coal mines and improving the safety and reliability of underground power supply systems.
