SF6 Gas Leakage Monitoring Technology Based on Diffuse Photoacoustic Sensor
Leakage monitoring of Gas Insulated Switchgear(GIS)is of great significance for the stable operation of power systems.SF6 is an artificially synthesized inert gas that is colorless,odorless,non-toxic,and non-flammable,with excellent stability.80%of SF6 worldwide is used in the power industry.GIS is a closed electrical equipment,and SF6 is stored in a pressurized state within the equipment.During long-term use,problems such as decreased airtightness may occur,leading to SF6 gas leakage,affecting the insulation performance of the equipment,and causing great safety hazards.SF6 is also one of the six greenhouse gases mentioned in the Kyoto Protocol,and its greenhouse effect is tens of thousands of times that of the same volume of carbon dioxide.Leakage of SF6 gas into the external environment will exacerbate the deterioration of the greenhouse effect.The traditional SF6 gas detection methods have some problems,such as low measurement accuracy,complex operation,and short service life.To effectively meet the challenges posed by the emerging field of intelligent power systems,there is an urgent need to enhance the accuracy,reliability,and convenience of detection technologies.The application of photoacoustic spectroscopy to obtain gas concentration information is a non-radiative relaxation process and belongs to the indirect detection method of gas concentration.This paper designs a gas sensor with high sensitivity,miniaturization,and low-cost characteristics for SF6 gas leakage monitoring.The SF6 gas photoacoustic spectroscopy detection technology mainly utilizes the mid infrared band.According to the Hitran database,the absorption spectra of SF6,H2O,and CO2 in the wavelength range of 10 µm～11 µm at room temperature and atmospheric pressure were plotted.When the wavelength is 10.5 µm,SF6 has a very high absorption coefficient and there is almost no cross interference from interfering gases.The wide spectrum infrared thermal radiation light source EMIRS200 with a wavelength range of 2～14 µm was selected as the excitation light source,combined with a filter with a central wavelength of 10.5 µm and a bandwidth of 0.5 µm,effectively avoiding the absorption interference of H2O and CO2 in the air.The non-resonant photoacoustic cell adopts a diffusion inlet structure,and SF6 enters the photoacoustic cell in a free diffusion form through the micropores on the outer wall of the photoacoustic cell.The gas diffusion structure uses a breathable hole with a radius of 0.65 mm,covering a hydrophobic breathable film with a pore size of 0.2～0.5 µm.The breathable film is made of expanded polytetrafluoroethylene,which has good chemical stability,an opening rate of 80%,and a thickness of 0.3 mm.The support layer is made of polypropylene material.The breathable film can be used normally in environments ranging from-40℃to 120℃.This structure reduces the volume of the sensor and lowers the cost.The overall dimensions of the sensor are 1.4 cm×1.5 cm×1.65 cm,and the volume of the photoacoustic cell is 0.13 mL.The sound pressure inside the non-resonant photoacoustic cell is uniformly distributed,and the microphone is embedded in the cell to receive and convert the sound wave signal into an electrical signal.The relationship between the photoacoustic cell radius and sound pressure was simulated using COMSOL Multiphysics finite element analysis software.It was found that there is a higher photoacoustic signal when the modulation frequency is low,and the photoacoustic signal increases with the decrease of the photoacoustic cell radius.However,the size of the photoacoustic signal is proportional to the incident light power,and a balance should be made between the radius and the incident light power.Therefore,the photoacoustic cell radius is designed to be 2 mm.The response time of the sensor is the time required for the photoacoustic signal value to increase from 10%to 90%.The experimental results show that the sensor has a response time of 178 seconds for SF6 detection.To test the photoacoustic response of the sensor,SF6/N2 gas mixtures of different concentrations were configured with a response degree of 14.47 µV/ppm within the concentration range of 0～100 ppm.The goodness of fit of the response curve is R2=0.997 6,photoacoustic signal response has a good linear relationship with SF6 concentration.According to the analysis of variance,when the integration time is 10 s,the detection limit of SF6 gas reaches 0.3 ppm.
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