Dissolved Gases On-Line Analysis Technology Of Insulation Oil Based On Membrane Extraction And Photoacoustic Spectroscopy

On-line DGA (dissolved gas analysis) is an important means by which not only the running condition of oil-filled electrical equipment, such as transformers, can be monitored promptly, but also the state maintenance of the electrical quipment can be achieved. The condition and fault of transformers are diagnosed by the separation and quantification of fault gases(CH₄,C₂H₂,C₂H₄,C₂H₆,CO,CO₂,H₂) dissolved in insulation oil of transformers. The fault gases can be separated from insulation oil by membrane continuously and quantifid by PA (photoacoustic spectroscopy) with high resolution. Combining the requirement of on-line condition monitoring and state maintenance, this dissertation studied the issues existed in the on-line dissolved gas analysis systems which adopt membrane extraction technology and PA gas quantification technology.Through the analysis of the extraction process of gases dissolved in insulation oil through membranes; the factors that influence the gas-in-oil extraction processes and results are also studied. Aiming at the defection of original calculation method, a new algorithm - dynamic algorithm is proposed based on those analyses. The algorithm utilizes the concentrations of gases in the gas separation cell at the same time interval to calculate the concentrations of gases dissolved in insulation oil, when the equilibrium state of the extraction process is not reached; also the algorithm consists of a temperature modified coefficient, which can remove the temperature affection of gas concentrations calculation theoretically.After the expetiment for comparision of extraction abilities of membranes in the laboratory is done, an on-line dissolved gas analysis system which amounts membrane as gas extractor is constructed, detecting the concentrations of H₂ and CO in the insulation oil. The off-line GC results are set as reference values, and the dynamic algorithm has been used to calculate the concentration of H₂ and CO dissolved in transformer insulation oil. Compared with the original calculation metrod, the accuracy and the immediacy of the calculation of concentrations of H₂ and CO are both improved. The validity of the dynamic aolgorithm with modified temperature coefficient for on-line DGA has been approved. Based on the research report and survey results of IEC and CIGRE, accuracy, repeatability and reproducibility are set as the quality indicator of on-line DGA results. Typical value, caution value and pre-failure value and the approaches to the setting of those values are proposed to evaluate the on-line DGA resuls. Base on the relationship between the concentrations of gases dissolved in oil and the probability of failure-related events in service, a sampling interval calculation method utilizing the on-line DGA results for calculating the next on-line DGA sampling interval is proposed.Based on the study of the model of molecule's structure and the molecule's vibration and rotational spectrum, the charaticristic of CH4, C₂H₂, C₂H4, C₂H₆, CO and CO2 structure and spectrum are analyzed. The photoacoustic spectroscopy theory for gas quantification and the inspiration process of PA signal is analyzed. The difference between resonant PA signal and non-resonant PA signal is studied particularly. The reflection of the sound pressure and the transmission of the sound pressure are both studied. The releationship between photoacoustic signal and the output singal of microphone is deduced. Those researches can provide the theoretical foundation for the design of the non-resonant PA cell. A non-resonant PA gas analysis model is constructed, the influence of the crossover interference is investigated, and the crossover of spectrum of fault gases is disquisitived. The dependence of photoacoustic signal on the concentration of each kind of gas in a mixture is deduced, which can provide the foundation for the application of non-resonat PA in the transformer insulation oil on-line DGA.A non-resonant photoacoustic spectroscopy experimental system applied for on-line analysis of gases dissolved in insulation oil was established. The dependence of photoacoustic signal on the modulation frequency is analysied, and an infrasonic photoacoustic way, which is decreasing the modulation frenquency of light to produce a higher amplitude photoacoustic signal, is proposed. Then the noises in the system have been researched, and high SNR gas cell is designed and constructed. The measurements of the concentrations of the CO, CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆ in the fault gases are carried out, and the limited sensitivity of each kind of gas is calculated. The accuracy, repeatability and reproducibility of the detection results are evaluated. Comepared with the GC results, the experiment results demonstrate that the probability of the non-resonant photoacoustic spectroscopy experiment system and the experiment system's feasibility of application in field test of the analysis of gases dissolved in transformer oil is approved.