Research On Space Charge Characteristics Of Oil-paper Insulation During Several Aging Processes

UHV dc transmission is one of the developing directions of our country to realize long-distance transmission. In the dc high voltage power transmission engineering, UHV converter transformer is the core of the equipment that could transform and transmit power, isolate ac and dc lines. The insulation problems of the UHV dc transmission system directly affect the stability of the grid system, as the core of power transmission and transformation, the converter transformer has higher requirements for insulation. Mixed insulation dielectric consists of cellulose insulation paper and insulation oil is the main insulation material of converter transformer. During the normal operation of transformer, thermal degradation appears in insulation materials under the action of thermal stress, moisture concentration would result in hydrolytic degradation, electricity stress has a great influence on electric field distributions, which would lead to electric field distortions interior the material.All these factors would cause the aging of materials, affect insulation properties of materials, shorten the practical life of materials, result in the breakdown of transformer and even endanger the security of the whole power grid. Aging would lead to the space charge injection, accumulation and migration, meanwhile space charge characteristics would influence the electrical properties of materials, therefore, research of space charge characteristics during aging process, especially under the circumstances of three aging factors is analyzed in the thesis,main work is discussed as follows:Three different experiment schemes are designed based on the actual operating conditions, insulation oil and insulation paper are pretreated and oil-paper samples are prepared.During the thermal aging, oil-paper samples accelerate aging under the circumstances of four kinds of temperature, the PEA method is used to measure the space charge characteristics of aged samples, evolution rules of space charge in the process of thermal aging are analyzed, space charge differences which results from the differences of aging temperature are studied and to explain the differences from the angles of Arrhenius equation and barrier level;Oil-paper samples with different moisture concentrations are studied during the moisture-temperature combined aging experiment, under the conditions of 110℃ and different moisture concentrations, the PEA method is employed to measure the space charge distributions, evolution rules of space charge of the samples with different moisture concentrations in the process of thermal aging are analyzed, differencess of space charge characteristics caused by different moisture concentrations are mainly analyzed;During the electro-thermal aging, oil-paper samples accelerate aging under the conditions of 130℃ and the electric field of 20 kV/mm, the PEA method is employed to measure the space charge distributions, evolution rules of space charge of the samples during the electro-thermal aging are analyzed, differencess of electric field and other characteristic parameters are studied and how electro-thermal aging has a prominent influence on space charge characteristics from perspective of trap energy level.