| As a novel type of intelligent substation equipment for future power transmission and distribution especially for flexible DC applications, high frequency power transformer (HFPT) can be utilized to realize flexible voltage and energy control as to meet various requirements of the power grids. Normally running at high frequency sinusoidal-like mode, the temperature rise of HFPT is much higher than that of the power frequency equipment. The insulation medium polyimide (PI) in HFPT will experience frequency-dependent electric-thermal stresses, which may result in insulation failure or even severe threat to normal operation of HFPT. However so far, it still remains unclear regarding the deterioration mechanism and discharge physics caused by high frequency and temperature changes, consequently, there lacks of mature guidelines for insulation structure optimization and fault diagnosis methodology, which to some extent hinders the development of HFPT towards high voltage and large capacity. Therefore, it is of urgent need and significance to study the mechanisms of partial discharge (PD), surface discharge and molecular deterioration of PI insulation according to operational conditions of HPFT.In order to realize comprehensive simulation of the frequency-dependent electric and thermal effect, a multi-functional test platform incorporating thermal-electric stresses was established, with test frequency up to 50kHz and test temperature up to 220℃. The discharge data was picked up by a high frequency pulsed-current sensor. Further, signal processing techniques were adopted including de-noising, reconstruction and statistical analysis as to extract the discharge pulses, combined with the Sym8 wavelet analysis plus windowing processing and subsection statistics. Application of the established test system has demonstrated its accuracy and efficiency in processing high frequency discharges.Based on the established platform mentioned above, experimental studies were carried out regarding the PI partial discharges of the gas-solid insulation under different temperatures and frequencies. An "inflection point" was discovered for PD parameters corresponding to the frequency effect as well as the temperature variations. There existed a tendency that the pulse numbers and amplitudes of the partial discharges raised in the first stance and decreased thereafter with increase of the excitation frequency, in which 10 kHz rendered to be the turning point. As the temperature increased, the maximum and average PD amplitudes as well as the PD pulse number showed a rising tendency. To account for the phenomenon described above, field-induced electron emission characteristics were studied. Based on the ion mobility equation and the conductivity tests of PI, space charge accumulation and/or dissipation behavior were elucidated. With frequency- and temperature-dependent dielectric spectra, the dielectric-charge interaction mechanism as well as frequency-thermal synergism was investigated. The impact mechanism of the frequency dependent electric-thermal stresses on partial discharge characteristics was revealed from the viewpoints of space charge and dielectric properties.Needle-rod type electrodes were adopted to study the progressing behavior of surface discharge under frequency-dependent electric-thermal stresses. The discharge scenario including discharge patterns and surface deterioration conditions was observed to acquire the surface discharge characteristics from the viewpoint of discharge statistics and physical phenomenon. Referring to the traditional discharge parameters, several dimensionless parameters including Kurtosis, Skewness and Pulse index as well as their specific variation percentages were adopted for diagnosing the severity of surface discharge under frequency-dependent effects. Based on the variation law of discharge parameters, and combined with the analysis of space charge accumulation/dissipation, charge collision/photoionization, charge trapping/de-trapping and initial electronic excitation, the impact mechanism of frequency-dependent electric-thermal effects on surface discharge development was clarified and fully elucidated.Explorative research from the microscopic physics and chemistry was further carried out with Reactive Force Field (ReaxFF) molecular dynamics simulation in the Kapton-type polyimide pyrolysis process as to study the impact of thermal stresses caused by high frequency effect. A micro PI model composed of 10 single chains with degree of polymerization 4 was constructed in the pyrolysis simulation software ADF. The whole pyrolysis procedure of polyimide could be divided into three stages based on the decomposition rate of PI, hence, the initial cleavage, microscopic dynamic reaction path in the pyrolysis process and the formation process of the main products were analyzed accordingly. The distribution curves of predominant products (CO2, CN and CO) versus temperature and time were obtained. Pair electron theory was employed to analyze the formation path of the principal products (CO2 and CN), and the formation mechanism were investigated in details based on the simulation trajectories obtained. Furthermore, based on the Scanning Electron Microscope (SEM) technology and high frequency PD tests, the interaction mechanism between electrical properties and micro-cleavage reaction was analyzed. With the above discussions, the pyrolysis and failure mechanism of PI under frequency-dependent thermal effect as well as the influential mechanism of temperature on the pyrolysis products were summarized in details at the atomic level. |
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