Compound Electric Field Discharge Characteristics Of Paperboard Overlapping Structure Under Different Moisture Content

With the rapid development of HVDC power transmission,the converter transformer,which is the core equipment of HVDC power transmission system,has always attracted much attention.Its safe and reliable insulation performance has a vital influence on the stable operation of HVDC power transmission system.Converter transformer insulation will be affected by moisture,different electric field forms and other environmental factors during operation,which will greatly affect the electrical performance of oil-paper insulation.Domestic and foreign scholars have carried out a lot of research on the breakdown and creeping discharge characteristics of single-layer paperboard.However,the discharge characteristics of the cardboard overlap insulation structure appearing at the overlap positions such as the enclosure and the corner ring in the converter transformer are not yet clear.In this regard,this dissertation mainly focuses on the following work:Based on the overlapping structure design test model of the cardboard overlapping structure appearing at the connecting position of the insulating parts such as the enclosure and the corner ring of the converter transformer,the electrode structure that can adjust the tangential component of the electric field is designed,and the oil-impregnated cardboard with different moisture content is made,and the interchange is built.The DC superposition circuit,partial discharge and breakdown test platform are pre-processed and parameter tested in accordance with national standards.Experiments are carried out under AC electric field and AC/DC 1:1compound electric field,changing the paperboard overlap length,electrode dislocation distance and paperboard moisture content,and explore the influence of moisture,paperboard overlap length,voltage waveform and electric field tangential component on the paperboard overlap structure The electric field simulation analysis of the test model of the cardboard overlap structure is carried out by using the electric field finite element simulation software for the discharge voltage,the probability of discharge probability and the creeping speed of the cardboard surface,and the characteristics of the surface discharge of the test model are analyzed in combination with the test data.The test results show that the probability of creeping discharge decreases with the increase of the lap length of the paperboard under different moisture content,and the breakdown/ creeping discharge voltage shows an upward trend.The increase of the DC component inhibits the development of creeping discharge of the cardboard overlap structure,reduces the probability of creeping discharge of the cardboard,increases the breakdown/surface discharge voltage,and reduces the creeping time.The tangential component of the electric field promotes the development of creeping discharge,reduces the voltage of creeping discharge,increases the probability of creeping discharge,and speeds up the development of creeping discharge.With the increase of the moisture content of the cardboard,the moisture in the AC electric field has no obvious effect on the breakdown/creeping discharge voltage of the cardboard overlap structure;under the compound electric field,the increase of the moisture content of the cardboard causes the breakdown/creeping discharge voltage to have a downward trend,and the cardboard is more prone to creeping Discharge promotes the development of creeping discharge.The electric field simulation results show that with the increase of the lap length of the paperboard,the maximum field strength modulus under the AC electric field decreases,which is the same as the change of the creeping discharge voltage;the maximum field strength increases under the compound electric field,but the tangential component of the electric field and the maximum field strength The decrease in the ratio results in a decrease in the proportion of creeping discharge in the discharge type,and a decrease in the probability of creeping discharge,which is consistent with the change trend of the experimental conclusion.As the DC component increases,the maximum field strength decreases,resulting in an increase in the breakdown/creeping discharge voltage under the composite electric field,which is consistent with the test results.As the moisture content of the paperboard increases,the maximum field strength of the lap structure increases,and the maximum field strength under the compound electric field changes more,resulting in a decrease in the breakdown/creeping discharge voltage,which is the same as the test results.