Development Process And Characters Of Surface Discharge In Oil-Pressboard Insulation

The safe operation of oil-filed power transformers is vital to the normal operation of the power transmission system,mineral oil-filled transformers,which are widely used in the national grid,apply oil-paper/pressboard insulation system to increase their internal insulating strength,but the problem with the oil-pressboard system is that pressboards always experience insulation malfunctions caused by the poor operation environment like the high electrical field,high temperature,high moisture content,AC combined DC voltage,consequently,partial/surface discharges might be generated on pressboard surface with the degradation of pressboards,especially with the high electrical field concentration.To make things worse,the surface discharge will generate white marks or carbonized marks on pressboard surface layers,these marks will damage the dielectric strength of the oil-pressboard system to such a great extent that the catastrophic flashovers might be generated inside power transformers once the electrical field is high enough.Therefore,the research of surface discharge development process and characters under the operation condition of power transformers is vital to the surface discharge diagnosis and insulation system improvement of transformers.Most researchers focus on the surface discharge characters under the room temperature,they do have some research achievement such as the significant influence of moisture content of pressboards to the surface discharge development speed,however,the oil-pressboard insulation system sustains harsh operating environment like high electrical field,high temperature,negative and positive DC voltages,the influence of these operation conditions on surface discharge characters is still unknown according to the author’s knowledge,so this research aimed to find the surface discharge development process and characters under the operation conditions of power transformers.With the help of the test equipment including high-speed camera,discharge detecting system,scanning electron microscope,this research mainly analyzed the surface discharge characters from three aspects,which were pressboard surface damage,the intensity of surface discharge,pressboard microstructure.The MSc dissertation can be divided into four parts,the first part is the introduction of the international standard test model,circuit,samples,equipment.The second to forth part are test results,the second part is to present the development process of surface discharge from initiation to final flashover,this part also analyzed the mechanisms behind the experimental phenomena,for example,the research found the generation of gases and white marks could stimulate the intensity of surface discharge,but the carbonized marks could desperate the intensity of surface discharge.The third part is to analyze the influence of temperature,electrical field to surface discharge characters,as for the temperature,the research found the surface discharge under high temperature would cause more damage to the pressboard surface layer while the surface discharge under low temperature would cause more damage to the pressboard inner layer.Apart from the influence of temperature,the research also found the damage degree of the pressboard increased first and then decreased with the increase of the electrical field.The fourth part is to find the surface discharge characters under different DC polarities with the help of experimental equipment and simulation software COMSOL.The research found the intensity of surface discharge under negative DC voltage was higher than the figure for the positive DC voltage,after simulation analysis,the research found the space charge on pressboard surface under the negative DC voltage could enhance the intensity of surface discharge,while the figure for the positive DC voltage would desperate the intensity of surface discharge,the conclusion was the negative DC voltage should be avoided during the normal operation of power transformers to protect the oil-pressboard system from damages.