Influence Of Environmental Factors And Thermal Aging On Dielectric Properties Of Oil-paper Insulation

HVDC transmission is widely used in transmission networks due to its advantages such as low cost,large capacity,and low loss.Converter transformer as one of the important equipment of HVDC,the stability of the power system is directly affected by its safe and reliable operation.The oil-paper insulation structure inside the converter transformer will be affected by the surrounding environment such as temperature,moisture,AC and DC superimposed electric fields,and impurities.At the same time,the converter transformer will also be affected by factors such as electricity and heat during long-term operation.Therefore,it is necessary to study the insulation performance of oil paper insulation in different environments.At present,the research on oil-paper insulation at home and abroad mainly focuses on the insulation performance at room temperature and low field strength.Considering that the converter transformer will operate in some alpine regions.It is necessary to carry out research on aging and dielectric properties of oil-paper insulation under wide temperature.In this paper,a dielectric parameter measurement system for transformer oil and oil-impregnated cardboard,a breakdown field strength measurement system,and an isothermal surface potential measurement system are established.Making transformer oil and oil-impregnated cardboard with different moisture contents and aging levels,and refer to relevant national standards for processing and testing.The temperature,moisture content,aging time and electric field strength were used as variables to study the dielectric properties of oil-paper insulation.Through experimental research,the dielectric parameters of transformer oil and oil-impregnated cardboard will increase to varying degrees with the increase of temperature and moisture,the relative dielectric constant and dielectric loss factor of transformer oil are less affected by temperature and moisture.At low temperatures,the electrical conductivity of oil-impregnated cardboard increases first and then decreases with increasing electric field strength.The electric conductivity of oilimpregnated cardboard increases with the increase of electric field strength at high temperature.With the increase of the aging degree,the dielectric parameters of the oil-paper insulation have risen to a certain extent,increased relaxation strength of oilimpregnated cardboard.Its internal polarization can be more fully established.The degree of oil-paper insulation aging has almost no effect on breakdown field strength.But the deepening of aging will reduce oil-impregnated cardboard trap level,the rate of space charge accumulation and dissipation increases.