Study On The Characteristics Of Low Temperature Creeping Discharge Of Oil-paper Insulation Under Composite Electric Field

UHV transmission network plays an important role in reconciling China's power resources and power load imbalance.DC transmission is widely used in China's UHV long-distance transmission because of its large transmission capacity,small floor space and low operating loss.The converter transformer is the core equipment of DC transmission.When working,the valve-side winding insulation of the converter transformer will withstand the AC-DC superimposed composite electric field,so that the breakdown characteristics of the insulation and the flashover characteristics of the surface are different from those of the common AC transformer.Regarding the creeping discharge characteristics of oil-paper insulation of converter transformer under the action of AC-DC composite electric field,domestic scholars have carried out a lot of work,but most of them are test results obtained at normal temperature and high temperature.Considering the low temperature operating environment that may occur in converter transformers,it is necessary to study the surface discharge characteristics of oil-paper insulation at low temperatures.In this paper,the processing technology of transformer oil and insulating paperboard is given.The test electrode system is prepared.The low-temperature creeping discharge test platform,resistivity test platform and surface potential measurement platform of oil-paper insulation under composite electric field are built,and the temperature,water content and external.The DC component of the composite electric field is taken as the research variable,and the discharge characteristics of the oil-paper insulation along the tangential electric field and the normal electric field are studied.Under the pure tangential electric field,the discharge voltage along the surface of the oil-paper insulation decreases with the increase of water content in a certain temperature range.Under the low water content,the discharge voltage along the surface of the oil-paper insulation increases first and then decreases with the increase of temperature,while the oil paper with high water content increases.The discharge voltage of the insulation creeping surface decreases with the increase of temperature.Under the low water content,the creeping discharge voltage of the oil-paper insulation increases first and then decreases with the increase of the DC component of the applied composite voltage.Under the tangential and normal electric field superposition,the discharge voltage of the oil-paper insulation creeping surface increases with the increase of water content in the temperature range of-40°C~20°C,and the change trend is obvious.Under the tangential and normal superimposed electric fields,the discharge voltage along the surface of the oil-paper insulation decreases first and then increases with the increase of temperature.The creeping discharge voltage of the oil-paper insulation under the external pure voltage is always lower than the creeping discharge voltage of the oil-paper insulation under the external pure DC voltage.By comparing the characteristics of the surface discharge between the pure tangential electric field and the tangential normal electric field,it is found that the normal electric field has a significant effect on the creeping discharge under AC voltage,and has little effect on the creeping discharge under pure DC voltage.The increase in the thickness of the oil-impregnated paperboard causes the oil-paper insulation creeping discharge voltage to rise.The conductivity of oil-impregnated paperboard and transformer oil increases with the increase of temperature,which increases the water content of the oil-impregnated paperboard,which reduces the accumulation of space charge and increases the dissipation rate.