Detection Of Acetylene Gas Dissolved In Transformer Oil Based On Hollow Core Photonic Crystal Fiber

The concentration of dissolved acetylene in oil and the rate of gas production are often used to monitor early failures of power transformers.Traditional gas chromatography has the disadvantages of poor anti-electromagnetic interference ability,easy contamination of the chromatographic column,consumption of carrier gas,and need for calibration.However,optical detection methods can effectively solve the above problems,so it has been widely studied.The traditional gas cell is large in volume,complicated in structure,expensive and consumes a lot of characteristic gases.This paper uses hollow core photonic crystal fiber(HC-PCF)as the sensing gas cell,combined with the fiber loop ring-down spectroscopy technique(FLRDS),to carry out the gas phase detection research of acetylene dissolved in transformer oil.Firstly,this paper studies the processing method of the microchannels on the side of the fiber with small aperture,number of holes,and depth to the core based on the focused ion beam technology.The ion beam processing parameters required for the fiber microchannel with an outer hole size of 3×3 μm,an inner hole size of 1×1 μm,and a depth to the fiber core have been mastered.The hollow core photonic crystal fiber sensing gas cell with low loss and fast response is developed.After processing 4 micro-holes on the side of the HC-PCF with a length of 0.85 m,it takes about 581 s for the acetylene gas to diffuse into the hollow core region to the saturated state,which is 93%shorter than the time required for diffusion from both ends of the fiber.The average transmission loss of the single hole processed in this study is about 0.13 dB,which has little effect on the transmission loss of the system and is suitable for acetylene gas sensing.Then,the effects of three different topological forms and optical path parameters of the fiber cavity ring-down system of the FLRDS system are simulated and studied.The optimal topological form of the detection system is determined.The optimal optical path parameter selection scheme such as the coupler splitting ratio,the length of the sensing gas chamber,and the length of the delay fiber is formed,thereby improving the sensitivity,the lower detection limit and the concentration resolution of the detection system.The simulation results show that the overall performance of the acetylene detection system is optimal when the ring fiber cavity(2×2 coupler,splitting ratio is 90:10)topology is used,the length of the sensing gas cell is 1 m,and the length of the delay fiber is 2.74 km.The lower detection limit is 0.14 ppm.When the gas concentration is 10 ppm,the sensitivity is 2.88 ns/ppm,and the concentration resolution is 0.14 ppm.Finally,according to the optimally designed ring fiber cavity topology and optimal optical path parameters,a FLRDS acetylene detection system with HC-PCF as the sensing gas cell is built.This paper combines the dual-wavelength differential detection method to detect low-concentration acetylene gas.The quantitative relationship between the acetylene concentration and the ring-down time at dual wavelengths is established,and the acetylene concentration can be back calculated using the measured ring-down time.Experiments show that the lower detection limit of the FLRDS acetylene detection system developed is 0.64 ppm.When the acetylene concentration is 9.97 ppm,the system sensitivity is 1.03 ns/ppm and the concentration resolution is 0.39 ppm.The maximum absolute error of the system is 5.82 ppm,the single-point repeat error is as low as 1.45%.The developed system achieves ppm-level acetylene detection.This method is also suitable for the detection of other hydrocarbon gases dissolved in transformer oil.A new method for online monitoring of dissolved gas in transformer oil is proposed.