| Serious faults of electric breakdown due to the static electrification owing to oil flow in extreme high voltage(EHV) power transfonners have been reported both inland and abroad. The static electrification problem for large power transformers with voltage higher than 500 kV has become one of the key factors threatening the transformer safe performance, and it has caused great interest and concern of the related academic organizations and researchers all over the world. The static electrification owing to oil flow is under the influence of oil velocity, oil temperature, applied AC field and characteristics of insulating materials involved. To deal with this problem, it is required a combination of the knowledge of electrostatics, hydromechanics, electro-chemistry and insulating materials. Besides, the problem itself is of great diversity. Hence it is of great theoretical interest and industrial significance to study this problem. Up to now, for the complicated oil-path structure of EHV transformers, there is no mathematical model which can be applied to deal with the effects of both the flow- induced charge in the oil-path and AC field-induced charge, no laboratory model which can be used to investigate the static electrification phenomena at the bottom of the transformers. The purpose of this dissertation work is aimed at solving these problems. The main work and achievements in the dissertation are as follows: 1. By taking both the flow-induced and the AC field-induced charge effects into consideration, two groups of constraint conditions are obtained for solving the electrical charge density equation under the condition of turbulent flow state in the oil. Thus the difficulty of obtaining the unknown charge density at the oil/paper interface is avoided. By this model, the influences of oil velocity, oil conductivity, oil temperature, applied AC field, oil viscosity and the effects of ions in the oil such as ion mobility and diffusion coefficients can be determined. 2. Owing to the feature of existing moving medium in the related problem, the finite difference algorithm with combined scheme and the finite-element algorithm with up-wind scheme are applied for solving the charge density equation to avoid the irregular oscillating phenomena in the numerical solution obtained by the finite difference algorithm with central scheme or the conventional finite-element algorithm. 3. The fault caused by static electrification usually occurs at the bottom of transformers where high oil velocity and strong AC field appear. To take this phenomenon into consideration, a simplified model with uniformly flat rectangular section is analyzed theoretically. Then a 3-layer laboratory model with uniformly flat rectangular section for experimental study is designed carefully, and the layers of the model can be put in series or in parallel. High AC voltage can be applied for studying the effect of AC field with the model. The low-voltage electrode is made into an electrically insulated 7-section plate, hence the leakage current at different parts along the oil path can be measured. 4. Based on the theoretical analysis, the experiment with the laboratory model are well organized and designed, so that the tests are carried out smoothly and successfully, and the results in the experiment are highly repeatable. According to the way of leakage current in the small collecting tank, a simple and effective method is proposed to study the influence of strong AC field upon the st...
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