Study On The Effect Of Electric Field Types And Defects On The Breakdown Characteristics Of Oil-Paper Insulation

Converter transformer is the core equipment of HVDC which influence the reliability of transmission system.As the major solid insulating medium,pressboard is mixed with air gap and metal impurities defects when it is produced which will affect the partial discharge and breakdown characteristics of pressboard in converter transformer under AC/DC superimposed electric field.Therefore it is necessary to study partial discharge and breakdown characteristics of pressboard with different defects to ensure that converter transformer could working stably.Insulation pressboards are made with industrial manufacturing method which including beating papermaking and calendaring.There are two kinds of defects including air gap and metallic impurities.Subsequently,breakdown,partial discharge and space charge test platform is built to study the characteristics of breakdown,partial discharge and space charge distribution of insulated cardboard with different defect types.In the breakdown test,the point-dynamic boost method is used to boost the pressure until the sample breaks down.In the partial discharge test,the step-up method is used to boost the voltage based on the principle of pulse current method,which ensures the accuracy of the measurement results and provides the test efficiency.In the space charge test,the distribution of space charge in the sample is measured by using the principle of electroacoustic pulse method.The results show that,the defects reduce the breakdown voltage and initial discharge voltage under the electric field with DC component and the degree of decline is different with different defects.Under the AC and AC/DC superimposed electric field,The amplitude and frequency of air gap defect develop with the trend of increasing and decreasing then with the voltage increasing and extension of applied voltage time.The frequency is higher.The amplitude and frequency of metallic impurities samples develops with the trend of increasing.The frequency is lower.Space charges make electric field distortion in the pressboard which is an important reason of effect the partial discharge parameters and its developing process.