| Biaxially oriented polypropylene（BOPP）,a typical linear dielectric material,has ultra-low dielectric loss factor,high dielectric strength,low carbon-hydrogen ratio,and suitable self-healing property,low cost,and it is convenient for large-scale preparation,so it is widely used in metalized film capacitors.In the flexible DC transmission converter valve,the BOPP film dielectric of the DC link capacitor is subjected to high temperature and high field,and the complex,strong electric field and high-temperature thermal field will accelerate the aging of the film dielectric,deteriorating the electrical properties of the film.To explore the evolution law of the microstructure and electrical properties of BOPP film after aging under DC link,this thesis carried out the research on the DC electrothermal aging characteristics of capacitor BOPP film.In this thesis,an electrothermal combined aging test platform for capacitor films under DC link was built.Under the aging field of 220 kV/mm,the electrothermal aging test was carried out on the BOPP film with aging temperatures of 85℃,95℃ and 105℃.The research results show that the insulation and energy storage properties of the aged samples aged at 85℃ are slightly improved for BOPP films.This is attributed to the strong effect of recrystallization at this aging temperature,but weak degradation effect,tighter molecular arrangement,and reduced defects in the sample.However,with the increase of the aging temperature,the performance degradation of the aging samples aged at 95℃ and 105℃ is more obvious,mainly manifested as the decline of insulation performance,including the increase of leakage current,the decrease of breakdown strength,the dielectric constant and the increase of the dielectric loss will affect the energy storage characteristics of the film,resulting in a significant decrease in the discharge efficiency.Under the field of 200kV/mm,the leakage current density increased to 3.50×10-4 A/m2 after aging for 21days,the breakdown strength decreased to 602.4 kV/mm,and the dielectric constant increased from 2.13 increased to 2.19,the loss factor increased from 2.82×10-4 to6.68×10-4,the discharge efficiency decreased from 93.5%to 89.8%,the energy loss was larger during the discharge process,and the discharge energy density did not change significantly.This is attributed to the destruction of the crystal region structure of the thin film by the action of high temperature and high field,resulting in the reduction of the crystal region,the expansion of the amorphous region,the increase of the free volume in the amorphous region,and the increase of internal defects in the sample.Using magnetron sputtering technology to grow SiO2 insulating layer on the surface of BOPP film can increase the barrier height of the electrode-dielectric interface under high temperature and high field,thereby effectively inhibiting the injection of electrode charges and improving the insulation and energy storage performance of the composite dielectric.In addition,an electrothermal aging test with an aging temperature of 105℃ was carried out on the SiO2/BOPP/SiO2composite dielectric after sputtering.The research results show that the aging of the composite dielectric shows almost the same change trend of insulation and energy storage performance as that of the BOPP film after aging.However,compared with the BOPP film dielectric aged under the same conditions,the insulation and energy storage performance of the sputtered composite dielectric after aging is better than that of the aged BOPP film,even better than the unaged BOPP film.Under the field of 200 kV/mm,the leakage current density of the composite dielectric aged at 105℃for 21 days is 4.45×10-5 A/m2,the dielectric constant is 2.36,the loss factor is3.69×10-4,and the discharge efficiency The discharge energy density is 96.3%.The discharge energy density is 0.4148 J/cm3.The above properties are better than the BOPP film aged under the same conditions,which shows that the sputtered composite dielectric has good anti-aging characteristics of insulation and energy storage properties.This thesis clarifies the evolution law of the microstructure and electrical properties of BOPP film dielectrics during the aging process,analyzes the DC electrothermal aging mechanism of BOPP film dielectrics,and provides an essential reference for the insulation and energy storage performance optimization of capacitor BOPP films. |
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