Method for simultaneously detecting micro water and air pressure in gas insulation equipment based on optical fiber photoacoustic sensing

Abstract

The invention discloses a method for simultaneously detecting micro-water and air pressure in gas insulation equipment based on optical fiber photoacoustic sensing. Double-parameter measurement of the micro-water content and the air pressure is realized by simultaneously detecting F-P dynamic cavity length and peak offset of second harmonics. The modulation depth and the bias current of the excitation light source are adjusted by sensing the air pressure change in the gas insulation equipment in real time so as to enhance the amplitude of the photoacoustic signal; and through air pressure compensation correction, the detection precision of the micro-water content is improved. According to the invention, the real-time monitoring of the air pressure is realized under the condition of not additionally adding an electrical air pressure sensor. In addition, the modulation depth and the bias current of the excitation light source are adjusted according to the air pressure value obtained through peak offset calculation of the second harmonic, the signal value is multiplied by the air pressure sensitive coefficient, and then air pressure compensation is achieved. The provided optical fiber sensing method has the innate advantages of being uncharged and resisting electromagnetic interference. The invention provides a new idea for simultaneous measurement of high-precision micro-water content and air pressure based on optical 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 micro-water and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing. Background Art

[0002] SF6 gas has extremely excellent insulation and arc extinguishing properties and is widely used in high-voltage switchgear, transformers and urban pipe corridors. Compared with traditional air-insulated electrical equipment, electrical equipment using high-pressure (about 4 atmospheres) SF6 gas as the insulating medium has many advantages such as small size, high insulation strength and safe and reliable operation. During the long-term operation of gas-insulated equipment, SF6 gas leakage may occur, causing moisture in the atmosphere to penetrate into the equipment or wet the surface of the solid medium. The standard for micro-water content during the operation of gas-insulated equipment is less than 300ppm. When the micro-water content in SF6 gas exceeds the standard, the excess water vapor may condense into ice at low temperatures and adhere to the insulation surface, which may cause insulation accidents. In addition, SF6 gas reacts with a large amount of H2O to generate decomposition derivatives, causing corrosion to the equipment. Therefore, real-time monitoring of the micro-water content in gas-insulated equipment helps to promptly detect the problem of micro-water exceeding the standard, thereby arranging equipment maintenance more scientifically and reasonably. In addition, the leakage of SF6 gas will cause the internal air pressure of gas-insulated equipment to decrease, and the change in air pressure will further affect the detection accuracy of micro-water. 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 the microphone, and the concentration information of the gas to be tested 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 in the application, which seriously affects the measurement accuracy of the micro-water content. For the application scenarios with strong electromagnetic fields near the 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, aiming to solve the problems that the measurement accuracy of trace water content of current photoacoustic sensors is easily affected by air pressure and the simultaneous monitoring of air pressure cannot be achieved, thereby expanding the space for the application of optical fiber sensing technology in 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 tested 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. A high-speed spectrometer and a demodulation algorithm based on white light interference are used to realize the demodulation of the FP dynamic cavity length, that is, to obtain the photoacoustic signal, and then calculate the micro-water content information. Figure 1 As shown, changes in gas pressure will change the shape of the gas absorption spectrum. 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] The collision broadening of the absorption spectrum is caused by the mutual collision between molecules. When the gas pressure gradually increases, the intensified collision between gas molecules constantly changes the molecular state, making the molecular excited state energy level further shortened than the average lifetime of spontaneous radiation. According to the uncertainty principle, shortening the lifetime of molecules in a certain energy state can lead to increased uncertainty in molecular energy. The more frequent the molecular collisions, the shorter the energy state lifetime, and the wider the spectrum line. The width of the spectrum line increases with the increase of gas pressure, which will cause the modulation depth and bias current of the excitation light source to change accordingly. The modulation depth and bias current of the excitation light source are adjusted by the calculated real-time gas pressure value to increase the amplitude of the photoacoustic signal. The real-time high-resolution demodulation of the FP dynamic cavity length is realized by a high-speed spectrometer and a demodulation algorithm based on white light interferometry. The demodulated FP dynamic cavity length, that is, the photoacoustic signal, is multiplied by the pressure sensitivity coefficient to realize the pressure compensation of the photoacoustic signal and improve the detection accuracy of the gas. Finally, the compensated photoacoustic signal is used to calculate the micro-water content.

[0008] The technical solution of the present invention:

[0009] A method for simultaneously detecting micro-water content and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing realizes dual-parameter simultaneous measurement of micro-water content and air pressure by detecting FP dynamic cavity length and second harmonic peak offset; by real-time sensing of air pressure changes inside the gas-insulated equipment, the modulation depth and bias current of the excitation light source are adjusted to enhance the amplitude of the photoacoustic signal, and the detection accuracy of micro-water content is improved by air pressure compensation correction; the specific steps are as follows:

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

[0011] Step S2, calculating 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 analyzing 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 air 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, by 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 beam:

[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 detection 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: according to 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 will change with the change of air pressure. The higher the air pressure, the higher the collision frequency between gas molecules, resulting in a faster heat conduction rate. In addition, the effect of air pressure on the response of the cantilever beam is mainly reflected in the damping, which increases with the increase of air pressure. Therefore, the intensity of the photoacoustic signal will change with the rise and fall of air pressure. Air 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 according to the compensated photoacoustic signal obtained in step S5.

[0024] Beneficial effects of the present invention: The present invention can detect the air pressure and micro-water content through the peak offset of the second harmonic and the FP dynamic cavity length, so as to judge the air pressure inside the gas-insulated equipment in real time, and make the excited photoacoustic signal almost unaffected by the air pressure. This scheme realizes real-time monitoring of the 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 by the peak offset of the second harmonic, 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 under different gas pressures;

[0026] Figure 2 It is the second harmonic diagram 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 micro-water and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing is as follows:

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

[0031] Let's take the use of this method to test the trace 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 beam is used as the acoustic wave detector, and the detection fiber is connected to a high-speed spectrometer. First, the second harmonic is scanned 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 the air pressure is calculated to obtain that the air pressure inside the gas-insulated device is 4atm. Then, the modulation depth and bias current of the excitation light source are adjusted according to the air pressure value. The modulation depths corresponding to 1atm and 4atm are 9mA and 30mA, respectively. The higher the air pressure, the greater the modulation depth. The bias currents corresponding to 1atm and 4atm are 94mA and 96.5mA, respectively. The higher the air pressure, the drift of the central current of the second harmonic toward the large current, which is caused by the asymmetry of the spectrum line. According to the calculated air pressure of 4atm, the modulation depth and bias current of the excitation light source are adjusted to 30mA and 96.5mA, respectively. The spectral demodulation method based on white light interference is used to demodulate the FP dynamic cavity length, that is, the photoacoustic signal. Under the air pressure of 4atm, the signal is doubled compared with the modulation depth of 9mA, that is, the enhancement of the photoacoustic signal is achieved. Under different air pressures, the intensity of the photoacoustic signal is different. The signal is multiplied by the air pressure sensitivity coefficient to achieve air pressure compensation, which improves the detection accuracy of the fiber 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 simultaneously detecting micro-water and air pressure in gas-insulated equipment based on optical fiber photoacoustic sensing, characterized in that: The dual parameters of micro-water content and gas pressure are measured simultaneously by detecting the FP dynamic cavity length and the peak shift of the second harmonic. The modulation depth and bias current of the excitation light source are adjusted to enhance the amplitude of the photoacoustic signal by sensing the gas pressure change inside the gas-insulated equipment in real time. The detection accuracy of micro-water content is improved by gas pressure compensation correction. The specific steps are as follows: Step S1, obtaining the second harmonic of the micro-water absorption signal: using the second harmonic-wavelength modulation spectroscopy technology to detect the micro-water absorption signal in the gas-insulated equipment, scanning in the current range of 60mA to 125mA to obtain the second harmonic of the micro-water absorption signal; Step S2, calculating the gas pressure by the peak shift of the second harmonic of the micro-water absorption signal: calculating the real-time gas pressure inside the gas-insulated device, the real-time gas pressure is obtained by the peak shift of the second harmonic of the micro-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 air 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, by 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 beam: 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 detection 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: according to 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 micro-water content: calculating the micro-water content information according to the pressure-compensated photoacoustic signal obtained in step S5.

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