Electrical And Thermal Aging Characteristics Of Cellulose Insulating Pressboard Modified By Nano-TiO₂

A converter transformer is one of the key equipment of the HVDC transmission system,and its insulation status is directly related to the safe operation of power systems.The oil-paper insulation inside it suffers from complex voltage and high operating temperature.Oil-paper insulation is subjected to heat,electromagnetic,mechanical,chemical and other external factors,which will lead to its accelerated aging and breakdown.Therefore,improving the main insulation performance and anti-aging level has become an important direction in inernal insulation research field of converter transformers.This paper aims to improve the electrical and thermal aging properties of the main insulation from the converter transformer.Nano-modification technology is used to improve the performance of oil-immersed insulation paperboard.Combined with finite element simulation,the influence of thermal aging on the AC and DC fileds distribution of oil-paper insulation is analyzed.All in all,the performance of nanocomposite oil-immersed paperboard is evaluated synthetically.The main research contents and results of this paper are as follows:（1）After beating,filter,dissociation and papermaking,ordinary insulating pressboard（named P0）was prepared.TiO₂ nanoparticles were selected as the additive modified material.Composite pressboard with different mass fraction and particle diameters nano-TiO₂ were prepared in the laboratory.The two-parameter Weibull distribution model was used to calculate the insulation failure possibility of nanocomposite pressboard,and the relationship between breakdown voltage and doping amount of TiO₂ was analyzed.The insulating pressboards modified by 10 nm nano-TiO₂ with 5 wt%were corfirmed as optimal modified samples,and they were named as B5.The samples of P0 and B5 went through the accelerated thermal aging test.（2）The simulation model of oil-paper insulation based on finite element analysis was established.According to the variation of relative permittivity and volume resistivity of insulating oil and pressboard,the AC and DC electric field distribution of oil-paper insulation under different thermal aging stages were obtained.The influence of aging on AC and DC electric field distribution of oil-paper insulation was analyzed.The simulation results show that the oil-paper insulation has a capacitive characterisitic under AC voltage and a resistive characteristic under DC voltage.Compared with the unmodified insulating paperboard P0,the pressboard undertakes a higher proportion of AC and DC field strength in oil-pressboard nanocomposite insulation B5.（3）In different aging stages,the electrical and physical properties of P0 and B5cardboard were measured.The experimental results show that the electrical performances of nano-TiO₂ modified insulating pressboard are always better than that of unmodified one,and the aging products are less and the aging rate is slow.During the thermal aging process,the breakdown field strength of P0 and B5 decreases,and the decreasing ratio of B5 is less than that of P0.Combined all this with the simulation results,it is concluded that the proportion of the AC and DC field strength inside the B5 pressboard is always higher than P0.Also,the AC and DC breakdown voltage of the B5 is higher than P0.In addition,the hydroxyl groups generated by the surface hydrolysis of nano-TiO₂ in B5 unite with the hydroxyl groups on the cellulose surface to form hydrogen bonds.This structure improves the stability of the cellulose structure and delays effectively the thermal aging of oil-paper insulation.