Study On Characteristics Of Partial Discharge Damage To Oil-impregnated Insulation Paper

Partial discharge (PD) has been considered one of the major causes of the deterioration of insulation materials. The internal insulation of a power transformer mainly consists of oil-paper insulation, in which all kinds of flaws are inevitably formed and generated during manufacture, transportation, assembly, and operation. In a running transformer, these flaws may cause partial discharge. PD may damage insulation and cause potential malfunction or breakdown, seriously threatening the safe operation of the transformer. Furthermore, the higher the voltage level is, the more serious PD damage to insulation materials may be. A thorough investigation of characteristics of PD damage to oil-impregnated insulation paper will encourage further studies on damage and breakdown mechanism, help improve PD resistance of insulation materials, develop an effective PD-based assessment model for insulation aging states, and then will eventually suppress or reduce PD damage to insulation materials. Therefore, the research work is of theoretical and practical importance.In this thesis, according to CIGRE Methodâ…¡, an experimental model was devised to stimulate the flat cavity inside oil-impregnated insulation paper, and PD damage experiments were performed in laboratory for hundreds of hours. According to phase-resolved partial discharge (PRPD) patterns, PD damage process could be divided into several stages. On the one hand, material analysis techniques, such as optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), gas chromatography-mass spectrometry (GC-MS) and Gas-measuring Model, were utilized to analyze surface topography, surface products and surface microstructure of oil-impregnated insulation paper at different damage stages as well as gas within cavity; on the other hand, electrical analysis techniques, such as high resistance meter, broadband dielectric spectra testing system and space charge measurement, were employed to analyze electrical properties of oil-impregnated insulation paper at different damage stages, including polarization, conductance, loss, breakdown and space charge density etc. Meanwhile, combined with material analysis techniques and electrical analysis techniques, the relationship between damage of oil-impregnated insulation paper and its electrical properties was also studied. The creative achievements which this thesis acquired can be summarized as follows: In-depth studies were conducted on the variation laws of gray intensity images, 2-deminsional phase distributions and its features, which has laid the solid foundations for the extraction of new PD features.For the first time, this thesis studied the surface morphology of oil-impregnated insulation paper after PD damage, revealed evolution laws and development tendencies of surface topography, surface roughness and surface conductivity during PD damage process. Meanwhile, after exposure to PD, the difference between the molecular structure variations in crystal regions and the molecular structure variation in amorphous regions was also studied. Furthermore, the distribution of the electric field within the cavity with surface solid was simulated and analyzed.For the first time, this thesis studied the surface products of oil-impregnated insulation paper after PD damage, specified the main constituents of "droplets" and crystalline "solids" generated on insulation surfaces, and revealed the generation mechanism of surface products under the condition of PD. Meanwhile, this thesis obtained the variations on volume and constituents of the gases within the cavity during the PD damage process. The generation mechanism of surface products and gas variation's influence on the transition of discharge types were also analyzed.For the first time, this thesis studied the bulk properties of oil-impregnated insulation paper after PD damage, analyzed the structure features of aggregation state of oil-impregnated insulation paper such as crystallinity, orientation degree and crystal grain size, as well as electrical properties such as polarization, conductance, loss and breakdown etc.; meanwhile, explored the intrinsic relationship between microstructure and electrical property.Based on the analysis of PD signals and PD damage to oil-impregnated insulation paper, this thesis further analyzed the intrinsic relationship between PD and insulation damage, revealed the correlations between gray intensity images,2-dimensional phase distributions and surface damage, gas within the cavity, respectively; revealed the mechanism on how surface conductivity, surface roughness and surface trap density influenced PRPD patterns; provided reference information for assessment of the surface damage of oil-impregnated insulation paper and the evolution of cavity flaws.