Study On Accumulative Characteristics Of Partial Discharge In Gas-gap Model Of Oil-paper Insulation Under AC And DC Combined Voltage

Converter transformer is the core equipment of ultra-high voltage direct current(UHVDC)systems,converter transformer' stable running is key to the reliable,safe running of the entire power transmission system.The hidden trouble of converter transformer mainly comes from the deterioration of insulation,and the main factor causing insulation deterioration is partial discharge.Oil-paper insulation is the main insulation material of converter transformer.During operation of converter transformer,the oil paper insulation material is stimulated by the change of electromagnetic environment for a long time,inevitably produces the deterioration of insulation performance,resulting in the production of partial discharge,which in turn leads to the further damage,to a certain extent,causing a breakdown accident,serious threat to transformer safety and reliable operation.The deterioration of partial discharge on oil paper insulation is a cumulative process.With the increase of the voltage level of UHVDC system,the cumulative effect will be further aggravated,and the cumulative time will be further shortened.Therefore,it is of great significance and application prospect to study the cumulative effect of partial discharge on the oil paper insulation material of converter transformer,to improve the insulation monitoring and fault diagnosis ability and to study the theory and method of restraining or slowing down the accumulative deterioration of insulation materials by partial discharge.The main content of this thesis are as follows:(1)In this paper,the electric field environment of the converter transformer is simulated,the development process of partial discharge under AC and DC combined voltage is carried out.The characteristic parameters of the partial discharge are extracted and the variation of the characteristic quantity is studied.(2)This paper concluded how the damage and development process of the insulating paper surface were measured and analyzed by scanning electron microscope,and the change and development trend of the surface morphology and surface coarseness of oil-immersed insulation paper is studied by the test of the long time partial discharge of Gas-gap model under AC and DC combined voltage.At the same time,the accumulative effect of air-gap discharge on the microstructure of insulation paper under AC and DC combined voltage is also studied.(3)The formation process and microcomponent of the surface products of air-gap model insulating paper are analyzed by X ray electron spectroscopy,and the effect of surface products on the air-gap discharge and the deterioration of insulation paper performance is studied.(4)In this paper,the frequency spectrum method based on frequency medium response is used to study the change of dielectric parameters and its influence on the deterioration of insulation performance during air-gap discharge at AC and DC combined voltage.(5)By using the PEA method,the space charge density in the insulating paper which in the air-gap discharge process under AC and DC combined voltage is measured.The regularity of space charge distribution of the sample is analyzed.The change of the electric field intensity caused by the charge change and its effect on the deterioration of the insulation performance are studied.The above research work is based on the test to obtain data,and uses relevant methods,techniques and means for analysis and research.The research results can provide experimental basis and theoretical support for the insulation state assessment,fault diagnosis and maintenance strategy formulation of the converter transformer,and can also provide some exploration for the internal insulation design of the converter transformer.