Dissolved Gases Detection In Power Transformer Oil Based On Optical Sensing Techniques

Online dissolved gases analysis(DGA)in transformer oil is of great importance for condition monitoring of oil-immersed transformers.When it comes to DGA,a variety of methods can be used,including gas chromatography,physical and chemical sensing and so on.Among them,optical detection has advantages of non-contact measurement,anti-electromagnetic interference,calibration-free,etc.However,as to existed optical detection for dissolved gas analysis,there are still three problems to be solved:multiple cross-sensitivity,less sensitivity or subject to vibration noise,excessive dependence on oil and gas separation.In order to overcome the above shortcomings,novel optical technologies for oil dissolved gas monitoring and analysis are proposed in this thesis.The main research work carried out and conclusions obtained are as follows:Dissolved hydrogen sensor in transformer oil with high sensitivity is prepared,based on fiber Bragg grating(FBG).Aiming to measure hydrogen concentration in oil,the volume expansion of hydrogen sensing material on the sensor caused by ambient hydrogen is transferred into Bragg wavelength shifts.Due to high hydrogen permeability rate,palladium-silver alloy is introduced into hydrogen-sensitive membrane.This membrane increases the amount of hydrogen per unit and enhances the mechanical strength as well.Moreover,side polished fiber Bragg grating(SP-FBG)is put forward to apply in dissolved hydrogen measurement.In SP-FBG,the cladding is processed by grinding wheel at micrometer level.Then hydrogen-sensitive material is sputtered on the SP-FBG laterally.Owing to bending deformation of SP-FBG after absorbing hydrogen,the sensitivity of the hydrogen sensor is obviously increased.Using palladium-silver alloy hydrogen sensitive composite film and SP-FBG substrate,in,the sensitivity of the sensor has reached 2.1(?L/L)/pm,when the temperature of transformer oil is 60 ?.In addition,influence factors of oil temperature and other gases are tested in the laboratory.Interference of other gases can be eliminated and the developed hydrogen FBG sensor is proved to be available in full working condition monitoring.Based on wavelength modulation spectroscopy(WMS)and tunable diode laser absorption spectrum(TDLAS)technique,detection of hydrocarbon gases dissolved in transformer oil is proposed in this thesis.In the proposal,quantitative relationship between the second harmonic signal and the hydrocarbon gas concentration to-be-measured is established.Wherein,the selection of central wavelength for four hydrocarbon gases is critical.The hydrocarbon gases detection system is built in the laboratory with features of short oil-gas separation time,simple operating procedures,no carrier gas.In the system,multi-gas cross-sensitivity is overcome,and sensitivity of symbol gases(methane and acetylene)at ?L/L level is achieved.Degradation oil samples are prepared and detected(methane as the example)in the laboratory.Results show that the test cycle is less than 5 minutes,the lower limit of detection of hydrocarbon gases reach 0.28 ?L/L.Direct acetylene detection based on Fourier transform infrared spectrometry is put forward and explored.Transmittance distribution of transformer oil in near and middle infrared wavelength range is tested for the first time.Moreover,optimal detection wavelength of acetylene dissolved in transformer oil is 1534 nm,correspondingly,full width at half maximum(FWHM)is 9.5 nm.We also analyze the difference of acetylene absorption position and FWHM between the gas phase and oil condition.Sensitivity of dissolved acetylene at hundred ?L/L level is achieved.What's more,this scheme lays foundation for free oil-gas separation.Online dissolved gas analysis based on proposed optical sensing technique is carried out in China Southern Power Grid(CSG),Zhaoqing city.The system is installed in#1 main transformer,Duanzhou 220kV substation,integrated with wavelength demodulation based hydrogen sensors and light intensity demodulation based hydrocarbon gas sensing system.A high-reliability cable is used to connect to the communication module.A specialized software is written and added into equipment,and online monitoring and warning system is formed with combination of early warning information.A comparison test between developed online DGA device and existed DGA equipment is carried out and results of two devices agree with each other.The similar results give evidence that the developed DGA device based on optical technique can be applied to monitor symbol gases concentration of transformer oil in the field.