Study On Surface Discharge And Breakdown Characteristics Of Paperboard Overlap Structure Under Composite Electric Field

Considering the fact that China’s energy resources and load centers are far apart,long-distance and high-capacity transmission projects across provinces,cities,and regions are imperative.High voltage direct current transmission projects have rapidly developed and achieved a series of significant results due to their low line losses and strong transmission capacity.As the hub connecting the AC system and the DC system,the safe and stable operation of the converter transformer is crucial for the entire power system.Scholars have done a lot of research work on the failure of its internal oil paper insulation system,mainly focusing on the single-layer paperboard.There is little research on the surface discharge and breakdown characteristics of the paperboard bonding structure at the junction of the converter inner screen and the corner ring.Especially when the paperboard with different moisture content suddenly contacts the high-temperature transformer oil at room temperature,the influence of the moisture content in the paperboard on the discharge characteristics of the paperboard during the rapid heating process is still unclear.According to the lap structure at the junction of insulation components such as the converter transformer’s internal enclosure and corner ring,the paper board lap structure model is designed.Based on the actual electric field distribution in the converter transformer,appropriate electrodes are selected and the appropriate electrode support is designed.The AC/DC superposition circuit without partial discharge and partial discharge test circuit are built.The oiled paper board with different aging degrees and different moisture content is prepared and processed and tested according to national standards.Experiments were conducted on low moisture content cardboard lap models with different degrees of aging in AC and AC/DC 1:1composite electric field environments at room temperature and high temperature,respectively,to explore the surface discharge and breakdown characteristics of the cardboard lap model.To investigate the influence of moisture in cardboard on the discharge characteristics during the rapid heating process of cardboard when there is a temperature difference between oil and paper,experiments were conducted on high moisture content cardboard with different degrees of aging at high oil temperature.The discharge characteristics were compared with the lap model of low moisture content cardboard under high temperature conditions,and the surface discharge and breakdown characteristics of oil-paper composite insulation of high-temperature oil and high moisture content cardboard at room temperature were explored.Through experiments,it was found that compared to the AC/DC superimposed electric field,the voltage withstand value of the cardboard lap model under the AC electric field is lower and the probability of creep is higher.Under the combined electric field of AC and AC/DC,the withstand voltage value of the cardboard lap model is positively correlated with the lap distance,and negatively correlated with aging time.At the same overlap distance,the deepening of aging degree will cause a decrease in the initial voltage of the overlap model,an increase in the probability of creepage,and a decrease in the discharge time;At the same aging level,the increase in overlap distance leads to a decrease in the creep probability of the overlap model and an increase in discharge time.Under high temperature conditions,the pressure resistance value of the low moisture content cardboard lap model has been improved,the creep probability has decreased compared to room temperature conditions,and the discharge time has slightly decreased;High moisture content cardboard,due to its high moisture content and the temperature difference between oil and paper in high-temperature oil,quickly releases a large amount of water and accumulates at the junction of oil and paper,resulting in a decrease in withstand voltage,a significant increase in creepage probability,and a significant reduction in discharge time.Based on actual working conditions,when the moisture content of cardboard is high,it is necessary to undergo a long period of preheating and oil filtration treatment before starting the converter to avoid abnormal discharge caused by the rapid temperature rise of the winding during startup,which may cause temperature difference between the oil paper;When the moisture content of the inner cardboard is low,the grid connection operation can be directly started,eliminating the steps of preheating and oil filtration,and simplifying the process.