Study On The Dielectric Properties Of Nano SiC And Al₂O₃ Modified Insulation Pressboard

Converter transformer plays an important role in HVDC transmission system.During its operation,the main insulation on its valve side will be subjected to the AC,DC,AC superimposed DC voltage as well as the polarity reversal voltage.Under DC voltage,the electric field distribution in oil-pressboard main insulation mainly depends on the conductivities of oil and pressboard,which are closely related to temperature,so the electric field distribution may be concentrated,leading to the occurrence of abnormal discharge or even breakdown in the oil-pressboard insulation structures.Moreover,the space charge may be accumulated in oil and pressboard under DC voltage,resulting in electric field distortion and threatening the insulation performance of convertor transformer.In aim to homogenize the distribution of electric field in main insulation and outlet device of converter transformer,meanwhile to promote the breakdown strength and to inhibit the space charge in pressboard,the research on dielectric properties and mechanism of nano-Si C and nano-Al2O3 modified pressboard has been carried out in this paper.The main topics of this paper are as bellow.By referring to the production flow of industrial insulation pressboard,the nano-modified pressboard is prepared by using minitype devices in laboratory.The polyethylene glycol(PEG)is used as dispersant in the surface modification process to avoid aggregation and to promote the remains of nanoparticles.The measurement results indicate that the nano-modified pressboards have uniform physical properties.Also,the space charge,the dielectric parameter and the TSC testing platforms are constructed to satisfy the experimental requirement,respectively.The effects of nano-doping on the dielectric properties are investigated.The results show that the conductivity and breakdown strength of nano-Si C modified pressboard present the increasing and decreasing trends as the nano-particle content increasing,respectively.And its conductivity exhibits nonlinear characteristic with electric field strength at the high nano-particle content.While,the resistivity and breakdown strength of nano-Al2O3 modified pressboard rise firstly and fall afterwards with the increase of nano-particle content.The conductivities of both nano-modified pressboards increase as the temperature increasing.And the ?r and tan? of them express a trend of decline and then incline as nano-particle content increasing.In addition,the simulation result shows that the electric field distribution in oil-pressboard insulation of converter transformer barrier system can be well homogenized by using the nonlinear characteristic of nano-modified pressboard conductivity.The test of space charge is carried on in laboratory to study effect of nano-doping on space charge behavior of modified pressboard.The experimental results show that nano-Al2O3 modified pressboard at low content can inhibit the accumulation of space charge and the distortion of electric field strength in pressboard.In comparison,the space charge accumulation of nano-Si C pressboard is intensified,and its accumulation and dissipation rate are higher than that of non-modified pressboard.Moreover,the space charge accumulation rate and the electric field distortion rate increase but the space charge dissipation indeed increases with the increase of electric field strength and temperature.In order to analysis the effect of nano-doping on dielectric properties,molecular dynamics simulation is used to calculate the forbidden bandwidth of cellulose microcrystal,nano-Si C and nano-Al2O3.Furthermore,the nanoparticle distribution and energy band model in modified pressboard system are built,and then verified by TSC test.Theoretical and experimental results show that the change of trap parameters caused by nano-doping contributes to the different dielectric properties of nano-modified pressboard.In details,the depth and density of trap in pressboard can be increased by doping nano-Al2O3,and these are contrary to the nano-Si C doping.Both of the depth and density of trap increase firstly and then decrease with the increase of the nano-particle content,which cause the above changing in dielectric and space charge characteristics of nano-modified pressboard.