Mid-infrared photoacoustic spectrum analysis of SF6 gas-decomposition system

Until now, the photoacoustic spectrum (PAS) analysis technology cannot face the need for SF6 gas-decomposition products due to the high noise level, big shock and low accuracy. In this manuscript, we present a portable sulfur hexafluoride (SF6) gas-decomposition PAS gas analysis system based on mid-infrared quantum cascade laser (MI-QCL). Because the narrow linewidth, high speed tunable and stability wavelength of MI-QCL, our PAS gas analysis system has a good performance. A platform for MI-QCL PAS system is set up in our work. The accuracy of quantitative detection for sulfur dioxide (SO2), hydrogen sulfide (H2S) and carbon monoxide (CO) in SF6 gas background mixture gas is 0.5, 0.1 and 0.1 ppm, respectively. Experiment results demonstrate the MI-QCL PAS system not only has a high detection accuracy, but also has a small system volume. This work gives a novel solution method for PAS system miniaturization in the future.


Introduction
Nowadays, sulfur hexafluoride (SF 6 ) gas is extensively utilized in circuit breakers, busbars, transformers and gas insulated switchgear.High-voltage electrical equipment filled with SF 6 gas has several advantages, including a smaller footprint, improved safety and non-flammability.These advantages significantly enhance the reliability and safety of electrical equipment operations, contributing to its widespread application [1,2].Pure SF 6 gas is colorless, odorless, nontoxic, and is a good insulating medium.When an insulation fault occurs in electrical equipment, the high-temperature arc generated by the discharge causes the decomposition of SF 6 gas, producing toxic low-fluorine sulfides [2,4].In the absence of impurities, the decomposition products of pure SF 6 gas will quickly recombine and revert back to SF 6 as the temperature decreases.However, in actual use, SF 6 gas is contaminated with air and water.Usually when a fault occurs, sulfur tetrafluoride (SF 4 ) is decomposed first, and then, water (H 2 O), carbon dioxide (CO 2 ) and oxygen (O 2 ) react to generate sulfur dioxide (SO 2 ), hydrogen sulfide (H 2 S) and carbon monoxide (CO).Therefore, by monitoring the concentration changes of one or more of SO 2 , H 2 S and CO in the SF 6 decomposition products, fault warning analysis and diagnosis of SF 6 -filled electrical equipment can be performed.
Recently, various methods have been developed to measure the concentrations of the three fault characteristic gases CO, SO 2 , and H 2 S.These methods include gas chromatography, mass spectrometry, Fourier transform infrared absorption spectroscopy and electrochemical methods.However, the electrochemical method has significant limitations, such as poor detection accuracy, weak resistance to interference, and a short lifespan.As a result, it can only qualitatively detect equipment failures, but cannot determine their development and severity.On the other hand, while gas chromatography, mass spectrometry and Fourier transform infrared absorption spectroscopy offer high precision, they require specialized laboratory conditions for use.To address these challenges, it is necessary to develop a device that can quickly detect SF 6 decomposition products at substation sites.Such a device would allow for early detection, early warning and accurate assessment of the potential dangers posed by SF 6 electrical equipment [3][4][5].In earlier researches, scientists have discovered that photoacoustic spectroscopy has exceptional sensitivity and rapid response times in detecting characteristic gases, making it a widely researched technology for ultra-sensitive gas concentration analysis [5][6][7][8][9][10][11].The advent of various high-output, narrowlinewidth lasers in the 1960s led to a rapid advancement of photoacoustic spectroscopy technology.In 1968, Kerr and Atwood applied the technology for gas concentration detection using CO 2 and ruby lasers [12].By the year 2000, scientists at Zurich University of Technology in Switzerland used a CO 2 laser to measure the concentrations of gases such as C 2 H 2 and CO 2 [13].In 2003, Chongqing University's research group, led by Chen Weigen, expanded the types of gases that could be detected using photoacoustic spectroscopy with the use of a 2-µm near-infrared semiconductor laser as the light source [14].In 2018, a photoacoustic spectroscopy technology was proposed by Shuo from Wuhan University of Technology that utilized an active gas chamber structure.The technology combined a distributed feedback laser and a photoacoustic cell and eliminated the need for a chopper, allowing for the detection of the gases C 2 H 2 , CH 4 , and CO with a limit of 0.5 ppm [15].
At present, photoacoustic spectroscopy based on distributed feedback lasers has demonstrated successful highprecision detection of certain SF 6 gas decomposition products [16,17].However, the limited wavelength range of semiconductor lasers presents a challenge in detecting gas concentrations with absorption peaks exceeding 1.6 µm.This results in significant cross-interference between different gas photoacoustic signals during multi-component gas detection, seriously affecting the system detection accuracy.In order to solve these problems, this study used a mid-infrared quantum cascade laser (MI-QCL) combined with an integrated photoacoustic cell to construct a photoacoustic spectroscopy measurement system [18].Due to the application of MI-QCL, it is possible to detect the absorption peak signal above 3.0 µm, which significantly reduce the overlap and crosstalk of photoacoustic signals from different gases.Furthermore, the use of programmable modulation modules in the laser allows for direct modulation and the generation of optical pulses with desired frequency.The integrated combination of the laser and the photoacoustic cell greatly reduces the volume of the photoacoustic spectroscopy system, laying the foundation for the miniaturization of the system.

Method of photoacoustic spectroscopy detection
After the gas to be measured is introduced into the photoacoustic cell, the optical signal corresponding to the wavelength of the gas's absorption peak is directed into the cell.The gas molecules that absorb the signal are first excited to a high-energy state and then return to a low-energy state through either a spontaneous emission transition or a nonradiative transition.In the non-radiative transition, the energy absorbed by the gas molecules will be converted into the translational and rotational kinetic energy of the molecules, manifested as an increase in the temperature of the gas and an increase in the pressure of the gas chamber.If the incident light intensity changes at a certain frequency, the temperature and pressure inside the photoacoustic cell also change at the same frequency, that is, a photoacoustic signal is generated.
According to this working principle, the photoacoustic signal can be expressed as: where P( − → r , ω) is the heat source, S m is the detection sensitivity of the acoustic sensor,α λ is the gas absorption coefficient at the corresponding wavelength,c is the concentration of the gas to be measured, C cell is the cell constant of the photoacoustic gas chamber, P 0 is related to the incident laser intensity.When other factors remain unchanged, the photoacoustic signal's amplitude is linearly proportional to the gas concentration.Hence, it is essential to design the structure of the photoacoustic cell in a rational manner, ensuring that it has an optimal cell constant.In the design phase, we start by considering the three key factors-resonance frequency, quality factor and cell constant.
We then use the one-dimensional longitudinal photoacoustic cell network method to design the multi-cell structure and determine the optimal microphone placement.To streamline post-processing, we aim to incorporate a regular geometric structure into our design.In terms of circuit board design and fabrication, we adopt a multilayer circuit board stacking technique to ensure that the system is both miniaturized and portable.Figure 2 depicts the finite element calculation results of the sound pressure signal in the first-order longitudinal mode photoacoustic cell used in our system.In the calculation process, the mathematical equation of thermoacoustic wave in the photoacoustic cell can be expressed as: The calculation only considers the transmission of sound waves along the radial and longitudinal directions.Equation 2can use the hyperbolic second-order partial differential equation to determine its corresponding parameters: where u, c, e n and f are the career, diffusion coefficient, mass coefficient and active term in the variable analog thermoacoustic wave equation, respectively.In the calculation process, the optical windows at both ends are designed at the node positions, meaning that the photoacoustic signal at the ports is zero.The side walls of the acoustic gas chamber are designed to not absorb sound waves.As shown in Fig. 1, the photoacoustic signal only propagates in the longitudinal direction, with the largest amplitude of the sound pressure signal being in the middle of the resonant cavity.This makes the middle of the resonant cavity the optimal location to measure the sound pressure signal.However, due to the limitations of the boundary conditions in the calculation, the sound pressure signal at the ports is approximately equal to zero.In reality, the sound pressure at the ports is expected to be higher than the sound pressure in the buffer cavity as the photoacoustic effect generates and radiates sound waves.To eliminate the thermal signal from the optical windows at both ends during the photoacoustic cell assembly, we implement a technique that involves passing polarized light through the Brust window at a specific angle.and CO is 0.01, 0.02 and 0.02 μ V/ppm, respectively.The signal noise ration of our photoacoustic spectrum system is about 60 dB, which mainly decided by the performance of microphone.The acquired photoacoustic signal is transmitted to the computer acquisition card through a microphone.
The computer then employs phase-lock amplification on the short time domain signal (the integration time of photoacoustic signal is about 10 s) to extract the photoacoustic signal.In our measurement, gas flow is an important reason to induce acoustic noise.Thus, we need to close all gas valves when we measured.Moreover, the photoacoustic cell was put in a noise enclosure to avoid the influence of ambient noise on the results.The use of this type of photoacoustic cell significantly decreases the volume of the photoacoustic spectroscopy detection system and provides technical support for the rapid on-site detection of trace gases.In the experimental measurement, for SO 2 and H 2 S gas content not more than 10 ppm and CO gas content not more than 50 ppm, calculate the measurement error and instrument repeatability of the detector according to formulas ( 4) and ( 5), respectively: In the formula, e , J and C, respectively, represent the measurement error, the instrument indication value and the actual concentration value of the injected gas; C g , C and C i present the repeatability of the instrument, the arithmetic mean value of each instrument indication and the second instrument indication.For SO 2 and H 2 S gas content greater than 10 ppm, CO gas content greater than 50 ppm, calculate the measurement error and instrument repeatability of the detector according to formulas ( 6) and (7): where ε e and n represent the measurement error and the number of detections, respectively, and C v represents the relative standard deviation.Generally speaking, a trace gas sensor for detecting the decomposition components of SF 6 must exhibit superior stability and a lower detection limit for specific concentrations.To assess the performance of the SF 6 decomposition component trace gas sensor, this paper conducts quantitative detection experiments on multi-component standard gases with varying concentrations of SO 2 , H 2 S, and CO.According to the DL/T 1876.1-2018"Technical Conditions for Sulfur Hexafluoride Detector-Decomposition Product Detector" standard and the analysis of sulfur hexafluoride on-site detection data, the high, medium and low (1, 2, 5, 10 ppm), and above 10 ppm (20, 50 ppm); high, medium, low (1, 2, 5, 10 ppm) below 10 ppm in the CO range, and above 10 ppm (20, 50 ppm).These concentrations are used for measurement.Figure 3 shows the detection results of different concentrations of SO 2 gas by the mid-infrared photoacoustic spectrometer.The concentration of each SO 2 gas is 0.50, 1.96, 5.10, 10.0, 20.0 and 49.6 ppm, respectively.It can be seen from the test results that when the SO 2 gas concentration is less than 10 ppm, the accuracy of this equipment fluctuates within the range of ± 0.10 ppm, and the repeatability is about ± 0.1 ppm.When the concentration of SO 2 gas is greater than or equal to 10 ppm, the accuracy of this equipment fluctuates in the range of − 1.0 to − 4.5%, and the repeatability is about 0.2%.The results indicate that as the concentration of SO 2 gas increases, the amplitude of the photoacoustic signal grows accordingly, but so does the amplitude of the background gas signal.As a result, the sensitivity of the measurement of SO 2 gas concentration does not vary greatly between low and high concentration regions.
Figure 4 shows the detection results of H 2 S gas with different concentrations by the mid-infrared photoacoustic spectrometer.The concentrations of each H 2 S gas were 1.00, 2.00, 5.00, 10.0, 20.0 and 50.3 ppm.It can be seen from the test results that when the H 2 S gas content is less than 10 ppm, the accuracy of the equipment fluctuates within the range of ± 0.05 ppm, and the repeatability is about ± 0.1 ppm.When the H 2 S gas concentration is greater than 10 ppm, the accuracy of the device fluctuates within ± 1.5%, and the repeatability is about 0.2-0.3%.Compared with the previous SO 2 gas, the accuracy of the mid-infrared quantum cascade photoacoustic spectroscopy measurement system for low and high concentrations of H 2 S gas has been improved.From the results of measuring different concentrations, it is noticeable that the measured values of most H 2 S gas samples are slightly higher than the standard value.This is because the light absorption coefficient of H 2 S gas at its corresponding absorption peak is lower than that of SO 2 and CO, causing molecular vibrations generated by photoexcitation to be more active at this wavelength.As a result, the amplitude of the actual photoacoustic signal increases, leading to higher measurements of H 2 S gas concentration.
Figure 5 shows the detection results of different concentrations of CO gas by the mid-infrared photoacoustic spectrometer.The concentrations of each CO gas were 1.00, 2.00, 5.00, 10.0, 20.0 and 50.0 ppm, respectively.It can be seen from the test results that when the CO gas concentration is less than or equal to 10.0 ppm, the accuracy of the device fluctuates within ± 1.5 ppm, and the repeatability is about ± 0.2 ppm.When the CO gas concentration is greater than 10.0 ppm, the accuracy of the device fluctuates in the Fig. 3 SO 2 testing with different concentration levels range of ± 0.45%, and the repeatability is about 0.1-0.2%.Compared with the measurement results of H 2 S gas at the same concentration, the measurement error of CO gas at low concentration is smaller and the accuracy is higher.The thermal noise can be effectively reduced during the interaction with the CO gas, resulting in a more accurate detection of the photoacoustic signal by the microphone.
In the experiment, the measured results of SO 2 , H 2 S and CO concentrations fluctuate slightly above and below the standard value; the main reasons are: (1) The flow of gas in the resonance tube and the collisions between gas molecules and the inner wall of the resonance tube generate mechanical noise, thus affecting the acoustic signal acquisition of the microphone.The mixed gases measurement results are shown in Table 1.The accuracy requirement for low concentrations of SO 2 and H 2 S is ± 0.5 ppm, while for high concentrations, it must Fig. 5 CO testing with different concentration levels be ± 5% of the measured value.For low CO concentration, the accuracy requirement is ± 2 ppm, and for high concentration, it must be ± 4% of the measured value.The mixed gas of the three measured gases was tested at different combined concentrations of high, medium and low, and the results are presented in Table 1.The statistical analysis of the results in the table reveals that all test indicators meet the standard requirements.

Conclusion
This paper proposes a gas concentration detection system for SO 2 , H 2 S and CO based on MI-QCL and an integrated photoacoustic cell structure.The experimental results indicate that the detection limits of SO 2 , H 2 S and CO can reach 0.5, 1.0 and 1.0 ppm, respectively, in a mixed gas environment, all of which meet the detection requirements of SF 6 equipment in power systems.Furthermore, the adoption of laser direct modulation technology eliminates the requirement for a mechanical chopper, resulting in a reduction of the volume of the SF 6 gas comprehensive tester to a mere 0.05 m 3 .The results of the experiments suggest that utilizing a MI-QCL in conjunction with an integrated photoacoustic cell structure can significantly reduce the volume of the detection system while ensuring the accuracy of gas detection.This solution not only reduces the amount of gas required for detection but also eliminates the need for degassing and photoacoustic gas chamber cleaning, making it an ideal solution for on-site rapid detection in photoacoustic spectroscopy.

Fig. 2 a
Fig. 2 a Experiment scheme of quantum cascade laser photoacoustic spectrum system; b schematic diagram of photoacoustic cell

Fig. 4
Fig. 4 H 2 S testing with different concentration levels