A method for simultaneously detecting micro-water and gas pressure in a gas insulated device based on fiber-optic photoacoustic sensing

Abstract

A kind of gas insulated equipment micro water and gas pressure simultaneous detection method based on optical fiber photoacoustic sensing, by simultaneously detecting F-P dynamic cavity length and the peak shift of second harmonic, the dual parameter measurement of micro water content and gas pressure is realized;By real-time sensing the gas pressure change inside gas insulated equipment, the modulation depth and bias current of excitation light source are adjusted to enhance the amplitude of photoacoustic signal;Through gas pressure compensation correction, the detection accuracy of micro water content is improved.The present application realizes real-time monitoring of gas pressure without additional electrical pressure sensor.In addition, the modulation depth and bias current of excitation light source are adjusted according to the gas pressure value calculated by the peak shift of second harmonic, and the signal value is multiplied by the gas pressure sensitivity coefficient, and then the gas pressure compensation is realized.The proposed fiber sensing method has the innate advantages of no electricity and anti-electromagnetic interference.The present application provides a new idea for high-precision micro water content and gas pressure simultaneous measurement based on fiber sensing.

Description

Technical Field

[0001] The invention belongs to the technical field of online monitoring of high-voltage electrical equipment, and relates to a method for simultaneously detecting trace water and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing. Background Art

[0002] SF6 gas has excellent insulation and arc-extinguishing properties and is widely used in high-voltage switchgear, transformers, and urban utility tunnels. Compared to traditional air-insulated electrical equipment, electrical equipment using high-pressure (approximately 4 atmospheres) SF6 gas as an insulating medium offers numerous advantages, including compact size, high dielectric strength, and safe and reliable operation. During long-term operation, gas-insulated equipment may experience SF6 gas leakage, causing atmospheric moisture to penetrate the equipment or wet the surfaces of solid dielectrics. The standard for trace moisture content in gas-insulated equipment during operation is less than 300 ppm. When the trace moisture content in SF6 gas exceeds this standard, the excess water vapor may condense into ice at low temperatures and adhere to the insulation surface, potentially causing insulation failure. Furthermore, SF6 gas reacts with large amounts of H2O to produce decomposition derivatives, which can corrode the equipment. Therefore, real-time monitoring of the trace moisture content in gas-insulated equipment helps promptly detect excessive trace moisture levels, enabling more efficient and effective maintenance planning. Furthermore, SF6 gas leakage can cause a decrease in the internal pressure of gas-insulated equipment, and this pressure change can further affect the accuracy of trace moisture detection. Therefore, it is necessary to simultaneously monitor the water content and gas pressure inside the gas-insulated equipment to determine the SF6 gas leakage fault and to carry out timely maintenance.

[0003] At present, the main methods for detecting trace gases in gas-insulated equipment include gas chromatography, electrochemical sensing, ultraviolet absorption spectroscopy, non-dispersive infrared spectroscopy, and photoacoustic spectroscopy. Photoacoustic spectroscopy gas detection is an online gas monitoring solution with high sensitivity, good gas selectivity, and fast response speed. The periodic acoustic wave signal generated by the target gas after absorbing the modulated excitation light is detected by a microphone, and the concentration information of the gas to be measured can be obtained based on the amplitude of the photoacoustic signal measured by the microphone. et al.Diode laser based photoacoustichumidity sensors[J].Sensors and Actuators B:Chemical,2003,91(1-3):219-226. Using a resonant photoacoustic cell, a photoacoustic system for micro-water measurement was proposed, achieving a sub-ppm detection limit and a linear detection range of micro-water of 4 orders of magnitude. However, there is a strong electromagnetic field near the gas-insulated equipment, which will cause electromagnetic interference to the traditional electrical acoustic wave sensor. In addition, the photoacoustic sensor is easily affected by air pressure during application, which seriously affects the measurement accuracy of micro-water content. For strong electromagnetic field applications near gas-insulated equipment, it is difficult to add an electrical air pressure sensor to monitor the air pressure. In addition, adding an additional air pressure sensor to the photoacoustic detection system will also increase the complexity and cost of the system. The basic principle of cantilever-based fiber optic photoacoustic sensing technology is to use the fiber optic cantilever to detect the acoustic wave signal generated by the gas absorbing the excitation light. It has many advantages such as high sensitivity, anti-electromagnetic interference, low transmission loss, long-distance measurement, distributed sensing, and can be used in harsh environments.

[0004] Therefore, inventing a method for simultaneously detecting micro-water and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing has important application significance. Summary of the Invention

[0005] The purpose of the present invention is to propose a method for simultaneously detecting trace water content and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing. The method aims to solve the problems existing in the current application of photoacoustic sensors, namely that the accuracy of trace water content measurement is easily affected by air pressure and that simultaneous air pressure monitoring cannot be achieved. The method expands the space for the application of optical fiber sensing technology in the online monitoring of high-voltage electrical equipment.

[0006] The principle of the present invention is as follows: the generation of photoacoustic signals is a complex photothermal and thermoacoustic energy conversion process. The second harmonic-wavelength modulation spectroscopy technology is used to detect the photoacoustic signal to improve the detection sensitivity. The photoacoustic signal generated after the gas to be measured absorbs the excitation light will change with the change of air pressure, which will affect the accuracy and reliability of the test results of the photoacoustic sensor. The fiber optic photoacoustic sensing technology based on Fabry-Perot (FP) interference is adopted, and the gap between the fiber end face and the cantilever beam constitutes the FP cavity. The reflectivity of the fiber end face is much less than 1, which makes the intensity of the reflected light approximately cosine-related with the phase difference, and the multi-beam interference is equivalent to double-beam interference. The demodulation of the FP dynamic cavity length is realized by using a high-speed spectrometer and a demodulation algorithm based on white light interference, that is, the photoacoustic signal is obtained, and then the micro-water content information is calculated. Such as Figure 1 As shown, changes in gas pressure will change the shape of the gas absorption line. Figure 2is the second harmonic of the micro-water absorption signal under different air pressures. In the range of 1atm to 4atm, as the air pressure increases, the central current of the second harmonic will drift toward the large current. This is because Figure 1 The asymmetry of the absorption spectrum of H2O in the atmosphere causes the real-time pressure value to be obtained by measuring the second harmonic.

[0007] Collision broadening of absorption lines is caused by molecular collisions. As gas pressure increases, the intensified collisions between gas molecules continuously alter the molecular state, further shortening the lifetime of the excited state relative to the average lifetime of spontaneous emission. According to the uncertainty principle, shortening the lifetime of a molecule in a particular energy state increases the uncertainty of the molecular energy. More frequent molecular collisions shorten the energy state lifetime, resulting in wider spectral lines. The line width increases with increasing gas pressure, which in turn causes corresponding changes in the modulation depth and bias current of the excitation light source. The modulation depth and bias current of the excitation light source are adjusted based on the calculated real-time gas pressure, thereby increasing the amplitude of the photoacoustic signal. A high-speed spectrometer and a demodulation algorithm based on white-light interferometry are used to achieve real-time, high-resolution demodulation of the FP dynamic cavity length. The demodulated FP dynamic cavity length, i.e., the photoacoustic signal, is multiplied by the pressure sensitivity coefficient to achieve pressure compensation of the photoacoustic signal, improving gas detection accuracy. Finally, the compensated photoacoustic signal is used to calculate the trace water content.

[0008] The technical solution of the present invention:

[0009] A method for simultaneously detecting trace water content and air pressure in gas-insulated equipment based on fiber-optic photoacoustic sensing achieves dual-parameter simultaneous measurement of trace water content and air pressure by detecting the FP dynamic cavity length and the peak shift of the second harmonic. By sensing the air pressure changes inside the gas-insulated equipment in real time, the modulation depth and bias current of the excitation light source are adjusted to enhance the amplitude of the photoacoustic signal. Air pressure compensation is then used to improve the detection accuracy of trace water content. The specific steps are as follows:

[0010] Step S1, obtaining the second harmonic of the trace water absorption signal: using the second harmonic-wavelength modulation spectroscopy technique to detect the trace water absorption signal, scanning in the current range of 60mA to 125mA to obtain the second harmonic;

[0011] Step S2, calculate the gas pressure by the peak shift of the second harmonic: the second harmonic obtained in step S1 and Figure 2 Calculate the real-time gas pressure inside gas-insulated equipment;

[0012] The air pressure is obtained through the peak shift of the second harmonic. The change of air pressure will change the shape of the absorption spectrum, and then change the shape of the second harmonic and the position of the central current. Figure 2 The real-time air pressure is obtained by measuring the second harmonic of the micro-water absorption signal under different air pressures.

[0013] Step S3, adjusting the modulation depth and bias current of the excitation light source: adjusting the modulation depth and bias current of the excitation light source according to the real-time gas pressure calculated in step S2;

[0014] The increase in air pressure will broaden the width of the H2O absorption spectrum. When measuring the water absorption signal, the modulation depth and bias current of the excitation light source should be adjusted accordingly according to the air pressure value to enhance the amplitude of the photoacoustic signal. The modulation depth Δv is related to the line width Δv. L The relationship between is expressed as:

[0015] Δv=m·Δv L (1)

[0016] Where m is the dimensionless modulation coefficient.

[0017] Step S4, obtaining a photoacoustic signal through the FP dynamic cavity length: demodulating the FP dynamic cavity length, i.e., the photoacoustic signal, using a spectral demodulation method based on white light interferometry;

[0018] The FP dynamic cavity length Δl is calculated by formula (2), which is the interference spectrum I(k) of the fiber FP cantilever:

[0019] I(k)=2I0(k)[1+ηcos(2(l0+Δl)πk+π)] (2)

[0020] Where I0(k) and k are the intensity and wave number of the probe light, respectively, η is the contrast of the interference intensity, and l0 is the static cavity length.

[0021] Step S5, performing air pressure compensation on the photoacoustic signal: based on the air pressure calculated in step S2, the photoacoustic signal value obtained in step S4 is multiplied by the air pressure sensitivity coefficient to achieve air pressure compensation;

[0022] The intensity of the photoacoustic signal changes with changes in air pressure. Higher air pressure increases the frequency of collisions between gas molecules, leading to faster heat transfer. Furthermore, the effect of air pressure on the cantilever beam's response is primarily reflected in damping, which increases with increasing air pressure. Therefore, the intensity of the photoacoustic signal changes with rising and falling air pressure. Pressure compensation is achieved by multiplying the photoacoustic signal value by the air pressure sensitivity coefficient.

[0023] Step S6, calculating the trace water content: calculating the trace water content information based on the compensated photoacoustic signal obtained in step S5.

[0024] Beneficial effects of the present invention: The present invention detects air pressure and micro-water content through the peak offset of the second harmonic and the FP dynamic cavity length, thereby being able to judge the air pressure inside the gas-insulated equipment in real time, and making the excited photoacoustic signal almost unaffected by the air pressure. This solution realizes real-time monitoring of air pressure without adding an additional electrical air pressure sensor. In addition, the modulation depth and bias current of the excitation light source are adjusted according to the air pressure value calculated according to the second harmonic peak offset, and the signal is multiplied by the air pressure sensitivity coefficient, thereby realizing air pressure compensation of the photoacoustic signal and improving the detection accuracy of the optical fiber gas sensor. The proposed optical fiber sensing method has the inherent advantages of being non-electrical and resistant to electromagnetic interference. The present invention provides a highly competitive technical solution for the simultaneous measurement of high-precision micro-water content and air pressure based on optical fiber sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the absorption spectrum of H2O at different pressures;

[0026] Figure 2 is the second harmonic graph of the micro-water absorption signal under different air pressures;

[0027] Figure 3 It is a step diagram of the method of the present invention. DETAILED DESCRIPTION

[0028] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0029] A method for simultaneously detecting trace water and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing is as follows:

[0030] based on Figure 1 The change of atmospheric pressure will lead to the broadening of gas absorption spectrum. The second harmonic of water absorption signal will be affected by atmospheric pressure. The absorption signal of gas is detected by second harmonic-wavelength modulation spectroscopy. The second harmonic is obtained by scanning within a certain current range. Figure 2 The second harmonic graph of the micro-water absorption signal under different gas pressures is used to calculate the real-time gas pressure inside the gas-insulated equipment; the modulation depth and bias current of the excitation light source are adjusted according to the real-time gas pressure; the FP dynamic cavity length, i.e., the photoacoustic signal, is demodulated using a spectral demodulation method based on a high-speed spectrometer and white light interferometry; the signal is multiplied by the gas pressure sensitivity coefficient to achieve gas pressure compensation; and the micro-water content information is calculated based on the compensated photoacoustic signal.

[0031] Let's take the method of testing the water content in gas-insulated equipment as an example. SF6 gas is filled into the fiber photoacoustic sensor, the near-infrared DFB laser is used as the excitation light source, the fiber cantilever is used as the acoustic wave detector, and the detection fiber is connected to a high-speed spectrometer. First, the second harmonic is obtained by scanning in the range of 60mA-125mA using the second harmonic-wavelength modulation spectroscopy technology, and the central current is 96.5mA. Figure 2 The relationship between the second harmonic and gas pressure was calculated, and the gas pressure inside the gas-insulated device was 4 atm. The modulation depth and bias current of the excitation light source were then adjusted based on the gas pressure. The modulation depths corresponding to 1 atm and 4 atm were 9 mA and 30 mA, respectively. Higher gas pressures increase the modulation depth. The bias currents corresponding to 1 atm and 4 atm were 94 mA and 96.5 mA, respectively. With higher gas pressure, the center current of the second harmonic shifts toward higher currents due to spectral asymmetry. Based on the calculated gas pressure of 4 atm, the modulation depth and bias current of the excitation light source were adjusted to 30 mA and 96.5 mA, respectively. The FP dynamic cavity length, or photoacoustic signal, was demodulated using a spectral demodulation method based on white light interferometry. At a gas pressure of 4 atm, the signal was doubled compared to the modulation depth of 9 mA, achieving photoacoustic signal enhancement. The photoacoustic signal intensity varies under different gas pressures. By multiplying the signal by the pressure sensitivity coefficient, pressure compensation is achieved, improving the detection accuracy of the fiber-optic photoacoustic gas sensor. Finally, the water content of SF6 gas was calculated to be 260 ppm based on the compensated photoacoustic signal.

Claims

1. A method for simultaneous detection of trace water and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing, characterized in that: By detecting the FP dynamic cavity length and the peak shift of the second harmonic, the dual parameters of trace water content and gas pressure are simultaneously measured. By sensing the pressure changes inside the gas-insulated device in real time, the modulation depth and bias current of the excitation light source are adjusted to enhance the amplitude of the photoacoustic signal. Through pressure compensation correction, the detection accuracy of trace water content is improved. The specific steps are as follows: Step S1, obtaining the second harmonic of the trace water absorption signal: using the second harmonic-wavelength modulation spectroscopy technique to detect the trace water absorption signal in the gas-insulated equipment, scanning in the current range of 60mA to 125mA to obtain the second harmonic of the trace water absorption signal; Step S2, calculating the gas pressure by the peak shift of the second harmonic of the trace water absorption signal: calculating the real-time gas pressure inside the gas-insulated equipment, the real-time gas pressure being obtained by the peak shift of the second harmonic of the trace water absorption signal; Step S3, adjusting the modulation depth and bias current of the excitation light source: adjusting the modulation depth and bias current of the excitation light source according to the real-time gas pressure calculated in step S2; The increase in air pressure will broaden the width of the H2O absorption spectrum. When measuring the micro-water photoacoustic signal, the modulation depth and bias current of the excitation light source should be adjusted accordingly according to the air pressure value to enhance the amplitude of the micro-water photoacoustic signal. The modulation depth Δv of the excitation light source is related to the line width Δv. L The relationship between is expressed as: Δv=m·Δv L (1) Where m is the dimensionless modulation coefficient; Step S4, obtaining a photoacoustic signal through the FP dynamic cavity length: demodulating the FP dynamic cavity length, i.e., the photoacoustic signal, using a spectral demodulation method based on white light interferometry; The FP dynamic cavity length Δl is calculated by formula (2), which is the interference spectrum I(k) of the fiber FP cantilever: I(k)=2I0(k)[1+ηcos(2(l0+Δl)πk+π)] (2) Where I0(k) and k are the intensity and wave number of the probe light, respectively, η is the contrast of the interference intensity, and l0 is the static cavity length; Step S5, performing air pressure compensation on the photoacoustic signal: based on the air pressure calculated in step S2, the photoacoustic signal value obtained in step S4 is multiplied by the air pressure sensitivity coefficient to achieve air pressure compensation; Step S6, calculating the trace water content: calculating the trace water content information based on the pressure-compensated photoacoustic signal obtained in step S5.

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