Influence Of Temperature On The Breakdown And Creeping Characteristics Of Oil-paper Insulation Under Compound Electric Field

With the increasing demand for long-distance and large-capacity power transmission in our country and the development of power electronics technology,HVDC transmission has achieved rapid development in recent years and has achieved many results due to its large transmission capacity and low line loss.In the high-voltage direct current transmission system,the converter transformer plays an extremely important role,and it is particularly necessary to study the performance and mechanism of the insulation structure of the converter transformer.At present,there are many researches on the creepage discharge characteristics of cardboard and the oil-paper composite insulation structure in converter transformers,but the breakdown/creepage discharge characteristics of the insulation structure at the overlap between the inner enclosure of the converter transformer and the corner ring are still insufficient.In this paper,the paperboard lap structure model is established,the tangential component adjustable electrode structure is made,and the AC/DC superposition test circuit without partial discharge and the dielectric parameter measurement system are built.The paperboard lap model is studied with temperature,voltage type,electric field tangential component size,and paperboard lap length as variables.A simulation model of the electric field of the paperboard lap structure was built,and the electric field distribution at room temperature and high temperature was simulated and analyzed under AC and AC/DC superimposed voltages.The influence of the paperboard lap length on the amplitude and tangential component of the electric field was studied.Then the breakdown/creepage discharge characteristics are explained by the simulation results.Through research,it is found that temperature has a more obvious impact on the model withstand voltage.In AC and AC /DC superimposed electric fields,the model’s withstand voltage will be increased to a certain extent after the temperature rises;at the same time,the increase in temperature will reduce the probability of creepage.Regardless of whether it is at room temperature or high temperature,the model is always more prone to creepage in the AC electric field,and so the withstand voltage value is lower.With the increase of the paperboard lap length,the creepage initial voltage of the model increases.When the lap length increases to a certain critical value,the model’s withstand voltage value tends to stabilize,and the discharge is dominated by breakdown,and the creepage is rare.With the increase of the electric field tangential component,the model is more prone to creepage,and the withstand voltage value decreases;but the relationship between the direction of the electric field tangential component and the direction of the paperboard overlap has little effect on the withstand voltage value.The simulation results show that the electric field strength is the highest at the edge of the paperboard overlap,and the maximum value of the field strength increases with the rise of temperature,which is opposite to the trend of the discharge voltage;mainly because the temperature rise increases the solubility of gas and water in the oil,which improves the breakdown voltage,the positive effect of temperature increase on the improvement of insulation oil breakdown performance is higher than the negative effect of the increase in field strength caused by temperature increase,which ultimately leads to an increase in breakdown voltage.The electric field strength under AC voltage is higher than the value under AC and DC superimposed voltage.This rule exists both at room temperature and high temperature,resulting in the breakdown/creepage voltage under AC voltage being lower than the value under AC and DC superimposed voltage.With the increase of the paperboard lap length,the maximum value of the field strength under AC voltage decreases,which is the same as the change trend of the breakdown voltage;the maximum value of the field strength increases under the AC and DC superimposed voltage,but the ratio of the tangential component to the normal component of the electric field increases with the decreasing of the lap length,which leads to an increase in the proportion of breakdown in the discharge type and a decrease in the probability of creeping discharge,which is consistent with the test results.The lap direction of the paperboard has little effect on the field strength,which is the same as the change trend of the breakdown voltage.