Optical fiber opto-acoustic sensing method and device for simultaneously measuring SF6 purity and decomposition products in gas insulation equipment

Abstract

The invention provides an optical fiber photoacoustic sensing method and device for simultaneously measuring SF6 purity and decomposition products in gas insulation equipment. The method comprises the following steps: obtaining a frequency response curve of an excitation light source; the real-time SF6 purity in the gas insulation equipment is calculated according to the obtained frequency response curve and the linear relation between the resonance frequency and the SF6 purity; adjusting the modulation frequency of an excitation light source according to the calculated real-time SF6 purity; demodulating the F-P dynamic cavity length by using a spectrum demodulation method based on white light interference to obtain a photoacoustic signal; according to the calculated SF6 purity value, the obtained photoacoustic signal value is multiplied by a purity sensitivity coefficient, and SF6 purity compensation is achieved; and calculating the concentration information of the decomposition product according to the obtained compensated photoacoustic signal. The problems that the gas concentration measurement precision is easily influenced by the SF6 purity and the SF6 purity and the decomposition product concentration cannot be monitored at the same time in the application of the existing optical fiber photoacoustic gas sensor are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of on-line monitoring of high-voltage electrical equipment, and specifically to an optical fiber photoacoustic sensing method and device for simultaneously measuring the purity and decomposition products of SF6 in a gas-insulated equipment. Background Art

[0002] The interior of a gas-insulated equipment is usually filled with SF6 gas, which has excellent insulation and arc extinguishing properties. Due to its small floor area, convenient installation, safe and reliable operation, and good breaking performance, the SF6 gas-insulated equipment has been widely used in the high-voltage field. However, the stability and reliability of the gas-insulated equipment depend on the purity of SF6, and the purity of SF6 during equipment operation should not be lower than 97%. In addition, SF6 will decompose under high-temperature discharge or arc action, generating some toxic and corrosive gases, such as H2S. The concentration of H2S is the basis for judging the fault discharge energy. Attention should be paid when the concentration of H2S is greater than 2 ppm, and power should be cut off to find the cause when it exceeds 50 ppm. Once the SF6 purity is too low or the concentration of its decomposition derivative H2S is too high, it may trigger or exacerbate internal faults in the gas-insulated equipment. By simultaneously monitoring the purity of SF6 and its decomposition derivative H2S, the type and degree of defects can be diagnosed in real time, which has important practical significance for the stable and reliable operation of the power grid. Chromatography technology and single-ion mobility spectrometry can be used to detect the purity of SF6. However, off-line detection is not conducive to timely discovering potential faults in gas-insulated equipment. Photoacoustic spectroscopy technology is gradually being widely used in the field of gas-insulated equipment due to its high sensitivity and maintenance-free characteristics. However, the strong electromagnetic field environment near the gas-insulated equipment will affect the stability of traditional photoacoustic spectroscopy devices.

[0003] Based on the fiber-optic cantilever photoacoustic sensing technology, its basic principle is to use a fiber-optic cantilever to detect the acoustic wave signal generated by the absorption of the excitation light by the gas, with many advantages such as high sensitivity, anti-electromagnetic interference, low transmission loss, long-distance measurement, distributed sensing, and applicability to harsh environments. The literature Zhao X, Wang Z, Li C, et al. Ultrahighsensitive trace gas sensing system with dual fiber-optic cantilever multiplexing-based differential photoacoustic detection [J]. Analytical Chemistry, 2024, 96(3): 1046-1053. reported a fiber-optic photoacoustic sensing system for measuring the characteristic decomposition gas H2S in gas-insulated equipment. Based on the dual enhancement of multi-pass absorption and optical differential detection, the detection sensitivity of H2S gas was significantly improved. The photoacoustic pressure wave generated after the H2S gas in the closed photoacoustic cell absorbs the excitation light emitted by the DFB laser is detected by the fiber-optic cantilever. The broadband probe light is transmitted to the cantilever through a fiber coupler, and the reflected interference light is received by a high-speed spectrometer. Dual-channel synchronous photoacoustic detection is achieved through fiber Fabry-Perot interferometric spectroscopy multiplexing. However, when the fiber-optic photoacoustic sensor is applied to the detection of decomposition products in the field of gas-insulated equipment, it is vulnerable to the influence of SF6 purity, seriously affecting the measurement accuracy of gas concentration. Traditional photoacoustic cells usually need to be combined with an off-line SF6 purity sensing device to achieve the measurement of decomposition products and SF6 purity. However, for the application scenario of strong electromagnetic fields near gas-insulated equipment, it is difficult to add a charged device to detect the purity. In addition, adding an additional SF6 purity measurement device in the fiber-optic photoacoustic sensor will also increase the complexity and cost of the system. Therefore, it is of great application significance to invent a fiber-optic photoacoustic sensing method for simultaneous measurement of SF6 purity and decomposition products in gas-insulated equipment. Summary of the Invention

[0004] The object of the present invention is to propose a fiber-optic photoacoustic sensing method and device for simultaneous measurement of SF6 purity and decomposition products in gas-insulated equipment, aiming to solve the problems that the measurement accuracy of gas concentration in the current application of fiber-optic photoacoustic gas sensors is easily affected by SF6 purity and the simultaneous monitoring of SF6 purity and decomposition product concentration cannot be achieved, and to expand a larger space for the application of fiber-optic photoacoustic sensing technology in the on-line monitoring of high-voltage electrical equipment.

[0005] The technical solution of the present invention:

[0006] A fiber-optic photoacoustic sensing method for simultaneous measurement of SF6 purity and decomposition products in gas-insulated equipment, comprising:

[0007] Step S1, obtain the frequency response curve of the excitation light source;

[0008] Step S2, calculate the SF6 purity through the resonance frequency: calculate the real-time SF6 purity inside the gas insulation equipment based on the frequency response curve obtained in Step S1 and the linear relationship between the resonance frequency and the SF6 purity;

[0009] Step S3, adjust the modulation frequency of the excitation light source: adjust the modulation frequency of the excitation light source according to the real-time SF6 purity calculated in Step S2;

[0010] Step S4, obtain the photoacoustic signal through the F-P dynamic cavity length: demodulate the F-P dynamic cavity length using the spectral demodulation method based on white light interference to obtain the photoacoustic signal;

[0011] Step S5, perform SF6 purity compensation on the photoacoustic signal: multiply the photoacoustic signal value obtained in Step S4 by the purity sensitivity coefficient according to the SF6 purity value calculated in Step S2 to achieve SF6 purity compensation;

[0012] Step S6, calculate the decomposition product concentration: calculate the concentration information of the decomposition products based on the compensated photoacoustic signal obtained in Step S5.

[0013] Further, Step S1 specifically includes: scanning the photoacoustic frequency response in the frequency range of 380 Hz - 420 Hz of the excitation light source based on the first harmonic - wavelength modulation spectroscopy technique to obtain the frequency response curve.

[0014] Further, in Step S2, the linear relationship between the resonance frequency and the SF6 purity is: the SF6 purity is obtained through the shift of the resonance frequency, and the change in the SF6 purity will change the peak position of the frequency response, that is, the resonance frequency.

[0015] Further, in Step S4, the F-P dynamic cavity length is calculated by Equation (3), Equation (3)

[0016] is the interference spectrum I(k) of the fiber optic F-P cantilever beam:

[0017] I(k) = 2I0(k)[1 + ηcos(2(l0 + Δl)πk + π)] (3)

[0018] In the formula, I0(k) and k are respectively the intensity and wavenumber of the probe light, η is the contrast of the interference intensity, l0 is the static cavity length, and Δl is the F-P dynamic cavity length.

[0019] Further, in Step S5, the photoacoustic signal value is represented by the photoacoustic amplitude A r (w), and the expression is as follows:

[0020]

[0021] In the formula, Q is the quality factor, C and α(v) are the concentration and absorption coefficient of the gas to be measured respectively, and w r and are the resonance frequency and V r resonator volume respectively, and w, v and are the modulation frequency, wavenumber and intensity of the excitation light respectively, which is the normal mode solution of the wave equation;

[0022] As the purity of SF6 decreases, the photoacoustic signal shows a downward trend.

[0023] An optical fiber photoacoustic sensing device for simultaneously measuring the purity of SF6 and decomposition products in a gas-insulated equipment, comprising:

[0024] A frequency response curve acquisition module, configured to obtain the frequency response curve of the excitation light source;

[0025] An SF6 purity calculation module, configured to calculate the real-time SF6 purity inside the gas-insulated equipment through the frequency response curve obtained by the frequency response curve acquisition module and the linear relationship between the resonance frequency and the SF6 purity;

[0026] A modulation frequency adjustment module, configured to adjust the modulation frequency of the excitation light source according to the real-time SF6

[0027] purity calculated by the SF6 purity calculation module;

[0028] A photoacoustic signal acquisition module, configured to demodulate the F-P dynamic cavity length by using a spectral demodulation method based on white light interference to obtain a photoacoustic signal;

[0029] An SF6 purity compensation module, configured to multiply the photoacoustic signal value obtained by the photoacoustic signal acquisition module by a purity sensitivity coefficient according to the SF6 purity value calculated by the SF6 purity calculation module to achieve SF6 purity compensation;

[0030] A decomposition product concentration calculation module, configured to calculate the concentration information of the decomposition products according to the compensated

[0031] photoacoustic signal obtained by the SF6 purity compensation module.

[0032] Further, the frequency response curve acquisition module is specifically configured to: scan the photoacoustic frequency response in the frequency range of 380 Hz - 420 Hz of the excitation light source based on the first harmonic - wavelength modulation spectroscopy technology to obtain the frequency response curve.

[0033] Further, the linear relationship between the resonance frequency and the SF6 purity is: the SF6 purity is obtained through the shift of the resonance frequency, and the change in the SF6 purity will change the peak position of the frequency response, that is, the resonance frequency.

[0034] Further, the F-P dynamic cavity length is calculated by Equation (3), and Equation (3) is the interference spectrum I(k) of the fiber optic F-P cantilever beam:

[0035] I(k) = 2I0(k)[1 + ηcos(2(l0 + Δl)πk + π)] (3)

[0036] In the formula, I0(k) and k are the intensity and wavenumber of the probe light respectively, η is the contrast of the interference intensity, l0 is the static cavity length, and Δl is the F-P dynamic cavity length.

[0037] Further, the photoacoustic signal value is represented by the photoacoustic amplitude A r (w), and the expression is as follows:

[0038]

[0039] In the formula, Q is the quality factor, C and α(v) are the concentration and absorption coefficient of the gas to be measured respectively, w r and are the resonance frequency and V r resonant cavity volume respectively, w, v and are the modulation frequency, wavenumber and intensity of the excitation light respectively, is the normal mode solution of the wave equation; as the purity of SF6 decreases, the photoacoustic signal shows a downward trend.

[0040] Advantages of the present invention: The present invention detects the purity of SF6 and the concentration of decomposition products through the shift of the resonance frequency and the F-P dynamic cavity length, and can judge the purity of SF6 inside the gas insulated equipment in real time, and make the excited photoacoustic signal hardly affected by the purity. Without additionally adding an electrical SF6 purity detection device, this scheme realizes real-time monitoring of purity. In addition, the SF6 purity value calculated according to the resonance frequency shift is adjusted for the modulation frequency of the excitation light source, and the signal is multiplied by the purity sensitivity coefficient, thereby realizing SF6 purity compensation for the photoacoustic signal and improving the detection accuracy of the fiber optic gas sensor. The proposed fiber optic sensing method has the innate advantages of being non-electrified and anti-electromagnetic interference. The present invention provides a highly competitive technical solution for the simultaneous measurement of high sensitivity and high precision of SF6 purity and decomposition product concentration based on fiber optic photoacoustic sensing. Description of the Drawings

[0041] Figure 1 is the photoacoustic frequency response curve under different SF6 purities;

[0042] Figure 2 is the linear relationship between the resonance frequency and the SF6 purity;

[0043] Figure 3It is a flowchart of a fiber optic photoacoustic sensing method for simultaneously measuring the purity and decomposition products of SF6 in a gas-insulated equipment according to an embodiment of the present invention. Detailed implementation manners

[0044] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0045] The principle of the present invention is as follows: The generation of photoacoustic signals is a complex process of photo-thermal and thermo-acoustic energy conversion. The photoacoustic signals generated after the gas to be measured absorbs the excitation light will change with the change of the SF6 purity, 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 (F-P) interference is adopted, and the gap between the fiber end face and the cantilever beam forms an F-P cavity. The reflectivity of the fiber end face is much less than 1, which makes the intensity of the reflected light approximately cosine-related to the phase difference, and the multi-beam interference is equivalent to the double-beam interference. The demodulation of the dynamic cavity length of the F-P is realized by using a high-speed spectrometer and a demodulation algorithm based on white light interference, that is, the photoacoustic signals are obtained, and then the concentration information of the decomposition products is calculated. As Figure 1 shown, the change of the SF6 purity will change the resonance frequency of the photoacoustic cell. Figure 2 It is a linear relationship between the resonance frequency and the SF6 purity. In the range of 80%-100% of the SF6 purity, as the SF6 purity decreases, the resonance frequency of the photoacoustic cell shifts to the high frequency. The real-time SF6 purity value can be obtained through the measured resonance frequency.

[0046] The present invention adjusts the modulation frequency of the excitation light source through the calculated real-time SF6 purity value to increase the amplitude of the photoacoustic signals; realizes the real-time high-resolution demodulation of the dynamic cavity length of the F-P by using a high-speed spectrometer and a demodulation algorithm based on white light interference; multiplies the demodulated dynamic cavity length of the F-P, that is, the photoacoustic signals, by the purity sensitivity coefficient to realize the SF6 purity compensation of the photoacoustic signals and improve the detection accuracy of the gas; finally, calculates the concentration of the decomposition products by using the compensated photoacoustic signals.

[0047] Specifically, based on Figure 1 the principle that the change of the SF6 purity in Figure 2The linear relationship between the resonance frequency and SF6 purity at different SF6 purities is used to calculate the real-time SF6 purity value inside the gas-insulated equipment; according to the real-time SF6 purity, the modulation frequency of the excitation light source is adjusted; the spectral demodulation method based on a high-speed spectrometer and white-light interference is used to demodulate the F-P dynamic cavity length, that is, the photoacoustic signal; the signal is multiplied by the purity sensitivity coefficient to achieve SF6 purity compensation; the concentration information of the decomposition products is calculated based on the compensated photoacoustic signal.

[0048] As Figure 3 shown, an embodiment of the present invention provides an optical fiber photoacoustic sensing method for simultaneously measuring SF6 purity and decomposition products inside a gas-insulated equipment. The simultaneous measurement of SF6 purity and decomposition product concentration is achieved by detecting the shift of the resonance frequency in the frequency response curve and the F-P dynamic cavity length; by real-time sensing the change in SF6 purity inside the gas-insulated equipment, the modulation frequency of the excitation light source is adjusted to enhance the amplitude of the photoacoustic signal, and through purity compensation correction, the detection accuracy of the decomposition product concentration is improved. The specific steps are as follows:

[0049] Step S1, obtaining the frequency response curve: Based on the first-harmonic - wavelength modulation spectroscopy technology, the photoacoustic frequency response in the frequency range of 380 Hz - 420 Hz of the excitation light source is scanned to obtain the frequency response curve;

[0050] The inside of the photoacoustic cell with good airtightness is filled with the decomposition products inside the gas-insulated equipment to be measured. When the excitation light source is turned on, the intensity of the photoacoustic signal changes with the operating frequency. To obtain the best gas detection performance, the frequency response characteristics of the sensor are measured by scanning the modulation frequency of the excitation light source.

[0051] Step S2, calculating the SF6 purity through the resonance frequency: The real-time SF6 purity inside the gas-insulated equipment is calculated through the frequency response curve obtained in step S1 and Figure 2 the linear relationship between the resonance frequency and SF6 purity in

[0052] The T-shaped photoacoustic cell can achieve a lower resonance frequency and less gas sample consumption without increasing the resonator length. Equation (1) is the expression of the first-order resonance frequency f r of the T-shaped photoacoustic cell:

[0053]

[0054] In the formula, R and T are the gas constant and temperature respectively. v m is the sound speed of the mixed gas. L eff is the resonator correction length considering the boundary effect at the end of the resonator. Under constant temperature and pressure, the resonance frequency of the T-shaped photoacoustic cell is affected by the physical properties of the background gas. γ m and M mare the specific heat ratio and molar mass of the mixed gas, respectively, and are expressed by Equation (2):

[0055]

[0056] where x i represents the proportion of the i-th gas. and represent the specific heat at constant volume and the specific heat at constant pressure, respectively. M i is the molar mass of the i-th gas. Compared with SF6, the specific heat ratio and molar mass of N2 are different, which will cause a change in the sound speed of the mixed gas in the T-type photoacoustic cell, resulting in a shift in the resonance frequency. The SF6 purity is obtained through the shift of the resonance frequency. The change in the SF6 purity will change the peak position of the frequency response, that is, the resonance frequency. According to Figure 2 the resonance frequencies at different SF6 purities in, the real-time SF6 purity is obtained.

[0057] Step S3, adjusting the modulation frequency of the excitation light source: According to the real-time SF6 purity calculated in Step S2, adjust the modulation frequency of the excitation light source;

[0058] Step S4, obtaining the photoacoustic signal through the F-P dynamic cavity length: Demodulate the F-P dynamic cavity length Δl using the spectral demodulation method based on white light interference to obtain the photoacoustic signal;

[0059] The F-P dynamic cavity length Δl is calculated by Equation (3), and Equation (3) is the interference spectrum I(k) of the fiber optic F-P cantilever beam:

[0060] I(k) = 2I0(k)[1 + ηcos(2(l0 + Δl)πk + π)] (3)

[0061] where I0(k) and k are the intensity and wavenumber of the probe light, η is the contrast of the interference intensity, and l0 is the static cavity length.

[0062] Step S5, performing SF6 purity compensation on the photoacoustic signal: According to the SF6

[0063] purity value calculated in Step S2, multiply the photoacoustic signal value obtained in Step S4 by the purity sensitivity coefficient to achieve SF6 purity compensation;

[0064] The intensity of the photoacoustic signal will change with the change of the SF6 purity. For the T-type photoacoustic cell operating in the resonance mode, Equation (4) is the expression of the photoacoustic amplitude A r (w):

[0065]

[0066] Wherein, Q is the quality factor, which takes into account the effects of viscous loss and heat conduction loss of the photoacoustic cell. C and α(v) are the concentration and absorption coefficient of the gas to be measured, respectively. w r and are the resonance frequency and V r resonant cavity volume, respectively. w, v and are the modulation frequency, wavenumber and intensity of the excitation light, respectively. is the normal mode solution of the wave equation. As the purity of SF6 decreases, the photoacoustic signal shows a downward trend. SF6 purity compensation is achieved by multiplying the photoacoustic signal value by the purity sensitivity coefficient.

[0067] Step S6, calculate the decomposition product concentration: Calculate the concentration information of the decomposition product according to the compensated photoacoustic signal obtained in step S5.

[0068] Now, take the example of using this method to test the purity of SF6 and the concentration of decomposition product H2S in a gas-insulated equipment. The H2S-SF6-N2 gas mixture is filled into the fiber optic photoacoustic sensor. The near-infrared DFB laser is used as the excitation light source, and the fiber optic cantilever beam is used as the acoustic wave detector. The detection fiber is connected to a high-speed spectrometer. First, using the first harmonic-wavelength modulation spectroscopy technique, the frequency response curve is obtained by scanning in the range of 380 Hz - 420 Hz. According to Figure 2 the linear relationship between the resonance frequency and the purity of SF6 in, the purity of SF6 inside the gas-insulated equipment is calculated to be 97%. Then, according to the purity value of SF6, the modulation frequency of the excitation light source is adjusted. The modulation frequencies corresponding to 100% and 97% SF6 purity are 193 Hz and 196 Hz, respectively. The higher the SF6 purity, the lower the modulation frequency. According to the calculated SF6 purity value of 97%, the modulation frequency of the excitation light source is adjusted to 193 Hz accordingly. The F-P dynamic cavity length, that is, the photoacoustic signal, is demodulated by using the spectral demodulation method based on white light interference. At 97% SF6 purity, compared with the modulation frequency of 196 Hz, the signal is doubled, that is, the enhancement of the photoacoustic signal is achieved. At different SF6 purities, the intensity of the photoacoustic signal is different. The signal is multiplied by the purity sensitivity coefficient to achieve SF6 purity compensation and improve the detection accuracy of the fiber optic photoacoustic gas sensor. Finally, the concentration of the decomposition product H2S is calculated to be 2 ppm according to the compensated photoacoustic signal.

[0069] The embodiment of the present invention correspondingly provides a fiber optic photoacoustic sensing device for simultaneously measuring the purity of SF6 and decomposition products in a gas-insulated equipment, including:

[0070] A frequency response curve acquisition module, configured to obtain the frequency response curve of the excitation light source;

[0071] The SF6 purity calculation module is used to obtain the frequency response curve obtained by the module through the frequency response curve and calculate the real-time SF6 purity inside the gas insulation equipment according to the linear relationship between the resonance frequency and the SF6 purity;

[0072] The modulation frequency adjustment module is used to adjust the modulation frequency of the excitation light source according to the real-time SF6 purity calculated by the SF6 purity calculation module;

[0073] The photoacoustic signal acquisition module is used to demodulate the F-P dynamic cavity length by using the spectral demodulation method based on white light interference to obtain the photoacoustic signal;

[0074] The SF6 purity compensation module is used to multiply the photoacoustic signal value obtained by the photoacoustic signal acquisition module by the purity sensitivity coefficient according to the SF6 purity value calculated by the SF6 purity calculation module to achieve SF6 purity compensation;

[0075] The decomposition product concentration calculation module is used to calculate the concentration information of the decomposition products according to the compensated photoacoustic signal obtained by the SF6 purity compensation module.

[0076] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. An optical fiber photoacoustic sensing method for simultaneously measuring the purity and decomposition products of SF6 in a gas-insulated equipment, characterized in that Including: Step S1, obtaining the frequency response curve of the excitation light source; Step S2, calculating the SF6 purity through the resonance frequency: calculating the real-time SF6 purity inside the gas insulation equipment based on the frequency response curve obtained in Step S1 and the linear relationship between the resonance frequency and the SF6 purity; Step S3, adjusting the modulation frequency of the excitation light source: adjusting the modulation frequency of the excitation light source according to the real-time SF6 purity calculated in Step S2; Step S4, obtaining the photoacoustic signal through the F-P dynamic cavity length: demodulating the F-P dynamic cavity length using the spectral demodulation method based on white light interference to obtain the photoacoustic signal; Step S5, performing SF6 purity compensation on the photoacoustic signal: multiplying the photoacoustic signal value obtained in Step S4 by the purity sensitivity coefficient according to the SF6 purity value calculated in Step S2 to achieve SF6 purity compensation; Step S6, calculating the decomposition product concentration: calculating the concentration information of the decomposition products based on the compensated photoacoustic signal obtained in Step S5.

2. The fiber optic photoacoustic sensing method for simultaneous measurement of SF6 purity and decomposition products in a gas-insulated equipment according to claim 1, characterized in that: Step S1 specifically includes: scanning the photoacoustic frequency response in the frequency range of 380 Hz - 420 Hz of the excitation light source based on the first harmonic - wavelength modulation spectroscopy technique to obtain the frequency response curve.

3. The fiber optic photoacoustic sensing method for simultaneous measurement of SF6 purity and decomposition products in a gas-insulated equipment according to claim 1, characterized in that: In Step S2, the linear relationship between the resonance frequency and the SF6 purity is: the SF6 purity is obtained through the shift of the resonance frequency, and the change in the SF6 purity will change the peak position of the frequency response, that is, the resonance frequency.

4. The fiber optic photoacoustic sensing method for simultaneous measurement of SF6 purity and decomposition products in a gas-insulated equipment according to claim 1, characterized in that: In Step S4, the F-P dynamic cavity length is calculated by Equation (3), and Equation (3) is the interference spectrum I(k) of the fiber optic F-P cantilever beam: I(k) = 2I0(k)[1 + ηcos(2(l0 + Δl)πk + π)] (3) In the formula, I0(k) and k are the intensity and wavenumber of the probe light respectively, η is the contrast of the interference intensity, l0 is the static cavity length, and Δl is the F-P dynamic cavity length.

5. The fiber optic photoacoustic sensing method for simultaneously measuring the purity and decomposition products of SF6 in a gas-insulated equipment according to claim 1, characterized in that: In step S5, the photoacoustic signal value is represented by the photoacoustic amplitude A r (w), and the expression is as follows: Wherein, Q is the quality factor, C and α(v) are the concentration and absorption coefficient of the gas to be measured, respectively, and w r and are the resonance frequency and V r resonator volume, respectively, and w, v and are the modulation frequency, wavenumber and intensity of the excitation light, respectively, is the normal mode solution of the wave equation; As the SF6 purity decreases, the photoacoustic signal shows a downward trend.

6. An optical fiber photoacoustic sensing device for simultaneously measuring the purity and decomposition products of SF6 in a gas-insulated equipment, characterized in that, Including: A frequency response curve acquisition module, used to obtain the frequency response curve of the excitation light source; An SF6 purity calculation module, used to calculate the real-time SF6 purity inside the gas insulation equipment based on the frequency response curve obtained by the frequency response curve acquisition module and the linear relationship between the resonance frequency and the SF6 purity; A modulation frequency adjustment module, used to adjust the modulation frequency of the excitation light source according to the real-time SF6 purity calculated by the SF6 purity calculation module; A photoacoustic signal acquisition module, used to demodulate the F-P dynamic cavity length using the spectral demodulation method based on white light interference to obtain the photoacoustic signal; An SF6 purity compensation module, used to multiply the photoacoustic signal value obtained by the photoacoustic signal acquisition module by the purity sensitivity coefficient according to the SF6 purity value calculated by the SF6 purity calculation module to achieve SF6 purity compensation; A decomposition product concentration calculation module, used to calculate the concentration information of the decomposition products based on the compensated photoacoustic signal obtained by the SF6 purity compensation module.

7. The fiber optic photoacoustic sensing device for simultaneous measurement of SF6 purity and decomposition products in a gas-insulated equipment according to claim 6, characterized in that: The frequency response curve acquisition module is specifically used for: based on the first harmonic - wavelength modulation spectroscopy technique, scanning the photoacoustic frequency response in the frequency range of 380 Hz - 420 Hz to obtain the frequency response curve.

8. The fiber optic photoacoustic sensing device for simultaneous measurement of SF6 purity and decomposition products in the gas insulated equipment according to claim 6, characterized in that: The linear relationship between the resonance frequency and the SF6 purity is as follows: the SF6 purity is obtained from the shift of the resonance frequency, and the change in the SF6 purity will change the peak position of the frequency response, that is, the resonance frequency.

9. The fiber optic photoacoustic sensing device for simultaneous measurement of SF6 purity and decomposition products in a gas-insulated equipment according to claim 6, characterized in that: The F-P dynamic cavity length is calculated by Equation (3), and Equation (3) is the interference spectrum I(k) of the fiber optic F-P cantilever beam: I(k) = 2I0(k)[1 + ηcos(2(l0 + Δl)πk + π)] (3) Wherein, I0(k) and k are respectively the intensity and the wave number of the probe light, η is the contrast of the interference intensity, l0 is the static cavity length, and Δl is the F-P dynamic cavity length.

10. The fiber optic photoacoustic sensing device for simultaneous measurement of SF6 purity and decomposition products in a gas-insulated equipment according to claim 6, characterized in that: The photoacoustic signal value is represented by the photoacoustic amplitude A r (w), and the expression is as follows: Wherein, Q is the quality factor, C and α(v) are the concentration and absorption coefficient of the gas to be measured, respectively, and w r and are the resonance frequency and V r resonator volume, respectively, and w, v and are the modulation frequency, wavenumber and intensity of the excitation light, respectively, is the normal mode solution of the wave equation; as the purity of SF6 decreases, the photoacoustic signal shows a downward trend.

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