| During the formation of early faults or new faults inside the oil-immersed power transformer,the insulating material will decompose and generate small molecular characteristic gases.The new generated small molecular gases will continuously dissolve in the oil.Acetylene is one of the most frequently used key characteristic gases in the evaluation of transformer operation status among many characteristic gases.Therefore,oildissolved acetylene detection is one of the most important methods for condition monitoring of oil-immersed power transformers.Optical sensing has gradually become a research hotspot due to its advantages of good insulation performance,strong antielectromagnetic interference ability,and excellent detection performance.However,for the existing optical techniques of oil-dissolved acetylene detection,there are still demerits as the usage of large gas cells,complex structures,cross-sensitivity of various characteristic gases,insufficient sensing sensitivity,susceptible to vibration or noise,over-reliance on oil-gas separation and other difficult problems.In order to solve the above problems,this paper adopts a new optical technique to carry out the direct monitoring of oil-dissolved acetylene.The main research work and conclusions obtained are as follows:1)Aiming to solve the problem that oil and gas must be separated before detection and the detection period is long,an oil-core photonic crystal fiber(OC-PCF)optical probe is proposed for the first time in this paper.In order to achieve low-loss transmission of laser in transformer oil,a low-loss OC-PCF preparation scheme is designed.The air holes on the end face of the photonic crystal fiber are sealed via the fusion process,and the micro-sized microchannels are processed along the axial direction of the fiber based on the focused ion beam technique.Transformer oil can only flow in and out the central core and a small number of air holes in the cladding through the microchannels.Through the simulation calculation of the mode field distribution in the OC-PCF and the experimental test of the optical transmission performance,it is proved that the laser can always be confined in the oil core to propagate,in turn the beam radius is effectively bound to the order of μm.The laser can efficiently contact the oil sample in the central core region of OC-PCF.There is no problem of beam divergence,and successfully overcome the problem that the laser cannot transmit in the transformer oil with low loss.Moreover,due to the good insulation properties and excellent resistance to strong electromagnetic interference,OC-PCF can be directly placed into the transformer oil tank,which lays the foundation for the direct detection of oil-dissolved acetylene.2)In order to improve the direct detection accuracy of dissolved acetylene in transformer oil,the similarities and differences of photothermal effect in OC-PCF oil core and in free gas phase space are analyzed,and the temporal and spatial distribution of temperature rise in OC-PCF is deduced.A scheme for the direct detection of oil-dissolved acetylene based on photothermal spectroscopy is proposed.According to the distribution of the mode field inside the oil core of an OC-PCF,the temperature rise distribution in the OC-PCF is mastered,and the key parameters affecting the efficiency of the photothermal effect are clarified.In order to realize the accurate measurement of the macroscopic characterization of the photothermal effect,a detection system based on the Mach-Zehnder Interferometer is established,which converts the detection of the small temperature rise into the detection of the phase modulation information of the probe light.To improve the sensitivity of the detection system,the influence of environmental noise and shot noise on monitoring accuracy is analyzed,and a noise reduction method based on heterodyne interference is introduced.In order to achieve high-sensitivity detection of dissolved trace acetylene in transformer oil,a sensitizing structure based on a fiber loop cavity is proposed.The feasibility of the sensitization scheme is verified by the detection of trace oil-dissolved acetylene.The results shows that the detection signal-to-noise ratio is greatly improved by 190%after the introduction of the fiber loop cavity.3)For the purpose of solving the serious problem of environmental interference to the detection system utilized in-situ,this paper proposes a dual pump-wavelength differential noise reduction method.Using the dual pump-wavelength differential noise reduction method,the demodulation results can reflect the phase modulation induced only by the photothermal effect.The negative influence of low-frequency interference such as transformer body vibration,environmental noise,and possible high-frequency interference such as partial discharge ultrasound on the detection results can be greatly suppressed.After the introduction of the dual pump-wavelength differential noise reduction method,the detection results when external interference is applied are only 2.66%deviated to the results when single pump wavelength is used and no interference is applied.Transformer oil samples with traces of acetylene dissolved in the laboratory are prepared,and the detection performance of the high-sensitivity all-fiber photothermal interferometry monitoring system for oil-dissolved gas is tested.The results show that the system response time is about 70 minutes,and the response time has nothing to do with the gas concentration.The detection limit of acetylene is as low as 1.40 ppm.The maximum deviation between the optical system detection results and the gas chromatograph detection results do not exceed ±4 ppm.Deteriorated oil samples are prepared in the laboratory,results have verified that the developed optical sensing system has the advantages of high real-time performance,high sensitivity and low dispersion,which can meet the needs of oil-dissolved acetylene sensing and has a good application prospect. |
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