Research On Optical Fiber Detection Of Dissolved Acetylene In Transformer Oil Based On Phase Demodulation Method With Dual Phase Detectors

The concentration of oil-dissolved gas in transformer is an important indicator of early transformer defects,among which acetylene gas is one of the seven characteristic gases most commonly used to evaluate the operating state of transformers.Optical fiber sensing is widely used in the status monitoring of power equipments such as transformers due to its good insulation and resistance to electromagnetic interference.However,the existing detection technology for oil-dissolved gas in transformer is difficult to achieve high-sensitivity when detecting gas in oil phase directly.The operation of oil extraction and oil-gas separation always cause oil sample contamination,resulting in large errors in measurement results,high false alarm rate and underreporting rate.In optical sensing,heterodyne interference can effectively overcome the phase fading problem,suppress the influence of environmental disturbance,and has the advantages of high sensitivity and low detection limit.However,the traditional phase demodulation method of heterodyne interference signal has problems such as high sampling rate and large amount of data,so it is difficult to realize real-time demodulation.Therefore,the research on optical fiber detection of dissolved gas in transformer oil based on dual phase detectors phase demodulation method is conducted in this paper.First,this paper analyzes the reason why it is difficult to achieve a low detection limit for gas detection in open-path oil phase,clarifies the problem that the laser beam is easy to diverge in open-path oil phase,and analyzes the feasiblity of gas detection in hollow-core photonic crystal fiber with oil-filled core.The light field simulation model of the oil-core photonic crystal fiber(OC-PCF)was established.The light-guiding mechanism of OC-PCF was clarified.It was veriified that the laser beam can efficiently transmit in the oil core of OC-PCF,and the laser beam radius can reach to several micrometers,which solves the problem of laser beam divergence in open-path oil phase.Moreover,a simulation model of the temperature field of OC-PCF is established,and the simulation results show that the frequency of the temperature field of the OC-PCF under the action of photothermal effect can reach to 20kHz,and the amplitude of the temperature field change is in the order of mK.The mechanism and main influencing factors of photothermal phase modulation are clarified,which provides a good theoretical basis for improving the sensitivity and detection limit of the sensing system.Secondly,this paper starts with the analysis of the phase signal attenuation and distortion caused by the existence of the nonlinear region of the single-chip phase detector,and proposes the dual phase detector phase demodulation method,and the sampling accuracy and lower limit of detection of the method is improved.The dual phase detectors phase demodulation method has a good linear relationship between the output signal and the phase signal,and the linear fitting degree can reach 0.999.the dual phase detectors phase demodulation method has a smaller minimum detectable limit than differential-cross multiplication(DCM)demodualtion method,which meets the detection requirements of micro phase signal.The dual phase detectors phase demodulation method uses phase detectors to realize the frequency reduction of 100MHz interference signals,which greatly reduces the sampling rate and data volume,which is 1125 times faster than the DCM software demodulation method.A phase demodulation method with high signal-to-noise ratio,low detection limit and real-time detection is provided for the detection and sensing of dissolved gas in transformer oil.Finally,in order to verify the actual demodulation effect of the dual phase detector phase demodulation method on the heterodyne interference signal,a Mach-Zehnder heterodyne interference detection platform for dissolved acetylene gas in transformer oil based on photothermal spectroscopy was built.The results show that the signal-to-noise ratio of the dual phase detectors phase demodulation method is 15dB higher than that of the DCM demodulation method,and the lower detection limit is 58%lower.A detection platform for dissolved acetylene gas in transformer oil was built.The test results show that the system has a good linear response to the concentration of acetylene gas,and the linearity can reach 0.991.The pump light modulation method has been improved,the output signal can increase by 102%under the same concentration,and the detection limit of the system reaches 0.45 ppm.The relative standard errors of the 10 sets of measurement results within 100 minutes are less than 10%,which has good long-term stability.