Electric Field Distribution And Discharge Characteristics Of Oil-pressboard Insulation Of Converter Transformer With Dynamic Parameters

Converter transformer is the core equipment in direct current transmission power system,and its insulation reliability directly affects the safety of power grid.Oil paper insulation is the main form of transformer insulation.The dielectric parameters in the operation process will be affected by temperature,moisture,aging and electric field strength etc.and change with time(that is,dielectric parameters are dynamic parameters that change with the external environment,non-constant).The variation of dielectric parameters will further affect the electric field distribution.In the past,constant dielectric parameters were used to calculate the electric field,which obviously caused errors.In addition,there are many lap structures in transformer insulation,which are seldom studied in the past.Therefore,it is necessary to study the electric field distribution and discharge characteristics of the main insulation structure of converter transformer under dynamic working conditions in order to improve the reliability of the main insulation of converter transformer.Oil-impregnated pressboard and transformer oil with different moisture and thermal aging degree were prepared.The changes of conductivity and permittivity of oil-impregnated pressboard and transformer oil with temperature,moisture content,thermal aging degree and electric field strength were obtained through experimental measurement.The results show that the electric field strength has different effects on the conductivity of transformer oil at different temperatures.When the temperature is low,the conductivity of transformer oil increases monotonically with the increase of electric field strength.When the temperature is higher,the conductivity of transformer oil decreases first and then rises with the field strength,which is close to saturation.Electric field strength has different effects on conductivity of oil-impregnated pressboard at different temperatures,too,and the variation trend of conductivity of oil-impregnated pressboard is obviously different from that of transformer oil with the increase of electric field strength.When the temperature is low,the conductivity of oil-impregnated pressboard decreases first and then rises with the increase of electric field,and this phenomenon occurs at all water content.When the temperature is high,the conductivity of oil-impregnated pressboard increases with the increase of temperature,but the increase rate decreases gradually and tends to saturation,and the saturation trend becomes more obvious at the lower the temperature.The conductivity of oil-impregnated pressboard changes with the electric field strength due to thermal aging.With the increase of electric field strength,the conductivity of unaged oil-impregnated pressboard increases gradually and tends to saturation.The conductivity of aged oil-impregnated pressboard decreases with the increase of electric field strength and tends to saturation.And the more serious the aging degree,the more obviously the conductivity decreases with the increase of field strength.The relative permittivity of transformer oil and oil-impregnated pressboard varies inversely with increasing temperature.With the increase of temperature,the relative permittivity of transformer oil with different water content decreases linearly.The relative permittivity of oil-impregnated paperboard increases with the increase of temperature and presenting a nonlinear trend.Moisture has an obvious effect on the relative permittivity of oil-impregnated pressboard.When moisture is low,the growth rate of the relative permittivity of oil-impregnated pressboard decreases gradually with the increase of temperature and tends to saturation.When the moisture is higher,the relative permittivity increases with the increase of temperature.Based on molecular dynamics simulation and quantum calculation,the local density of state(DOS)of transformer oil and oil-immersed pressboard under the influence of water and aging acid impurities was obtained,and the influence mechanism of conductivity of oil-paper insulation by the environment was analyzed.Water has little effect on the HOMO(Highest Occupied Molecular Orbital)-LUMO(Lowest Unoccupied Molecular Orbital)energy level of transformer oil.Acid introduces low density isolated bound states between HOMO-LUMO levels,and the coexistence of acid and water results in a greater reduction of HOMO-LUMO levels.Impurities affect the density of local states around them.The HOMO-LUMO levels decreases in the area affected by impurities,which increases the electron conductivity at high field intensity.As a result,the electron conductivity shows a u-shaped curve with the electric field strength.In order to study the influence of dynamic dielectric parameters of oil-paper insulation on electric field distribution of main insulation in converter transformer,the finite element simulation model was established based on the real structure of the end of the winding in converter transformer.It is found that the calculation results of dynamic nonlinear parameters are different from those of constant parameters.Temperature has a significant effect on the electric field distribution of valve side main insulation.When the temperature is low,the transition process is very slow,and the electric field is very concentrated,and the maximum field intensity can reach more than 2 times of the operating temperature.The thermal gradient aggravates the uneven distribution of parameters in the medium,causing space charges generated in the interface of oil paper and insulating paper,and then distort the electric field.The location of the maximum field strength varies greatly with the temperature gradient.There will be a large electric field in the pressboard lap part.The larger the temperature difference is,the more serious the field distortion is,and the larger the maximum electric field intensity is.Thermal aging does not affect the position of the maximum field strength,but does affect its value.The maximum value of the field strength increases first and then decreases with the deepening of the aging degree.The higher the DC component is,the greater the field intensity is.In order to study the discharge characteristics of pressboard overlapped structure under AC+DC voltage,the breakdown/creeping discharge platform was designed and established,and the tests of double layer lapped pressboard structure were carried out under the influence of DC component of electric field,pressboard lap length,water content,temperature,temperature difference between oil paper and other factors.The results show that the discharge voltage increases with the increase of the lap length.Creepage is the main form of discharge with short lap length,and the creepage probability decreases with the increase of lap length.Compared with AC voltage,surface discharge is less likely to occur under AC+DC voltage,and the breakdown strength is higher than that under AC voltage,but its development speed is faster.At constant temperature,the creepage probability at high temperature is lower than that at room temperature,so the discharge voltage is higher than that at room temperature.When the water content of pressboard is low and the oil temperature is high,the discharge is mainly breakdown.The creeping discharge with high probability occurs when the standing time is short,thus reducing the average discharge voltage.With the increase of standing time,the creepage probability decreases,and the discharge voltage increases.The discharge voltage at high oil temperature is higher than that at low oil temperature.When the moisture content of pressboard is high,the discharge form is creeping discharge regardless of oil temperature.The discharge voltage of high oil temperature is lower than that of low oil temperature.Thermal aging will increase the probability of creeping discharge and reduce the discharge voltage,and the development rate of discharge is faster after aging.It is found that compared with the dynamic dielectric parameters of oil-paper insulation,the calculation results of electric field are larger when the dielectric parameters of oil-paper insulation constant are used,and the design size may be conservative.It is pointed out that the key moment is in the early stage of operation,especially in the low-temperature operation,and the insulation assessment is more stringent.The operating temperature of converter transformer is suggested.The experimental model of pressboard lap structure in converter transformer is presented.The discharge characteristics of lap structure under AC+DC are obtained.It provides the foundation for improving the reliability and economy of converter transformer.