Study On The Dielectric Properties Of Insulating Pressboard Modified With SiC And MMT

Converter transformer lies in the core position of alternating current(AC)and direct current(DC)mutual transformation,and its good internal insulation is the prerequisite to ensure the safe and reliable operation.Converter transformer withstands AC/DC and polarity reversal voltages in operation,the key problems of which are the nonuniform electric field distribution under DC voltage and the highly concentrated field strength in transformer oil under polarity reversal voltage.So the method of pressboard modification doped with nanometer SiC and MMT is adopted to improve the problems of electric field distribution nonuniformity in oil/pressboard insulation and space charge accumulation at the interface from the aspect of electrical conductivity self-regulation.Referring to the preparation technology of industrial insulation pressboard,the modified pressboard samples were prepared in laboratory,and the samples were oiled impregnation according to the standard;the distribution pattern of the fiber and nanoparticles in pressboard were observed by scanning electron microscope,verifing the feasibility of pressboard modification;the dielectric parameters of modified insulating pressboard were measured,and the modification effect was verified by simulation with the experiment data.The study results on the dielectric properties of SiC modified pressboard show that when the SiC doping ratio is greater than 2.5%,the pressboard conductivity with electric field strength shows obvious nonlinear characteristics;the relative permittivity increases then turns to stable and then rapidly increases with the SiC doping ratio increasing;the dielectric loss factor increases with SiC doping ratio increasing;the breakdown field strength decreases with the nano SiC doping ratio increasing under AC and DC voltages.The electric field distributions in oil－pressboard insulation system and the space charge accumulation at the interface with non-modified and modified pressboard were simulated and analysed by Comsol software.The simulation results show that by using SiC modified pressboard,the electric field distribution in oil-pressboard system can be more homogeneous and the space charge accumulation characteristics at the interface can be improved under DC and polarity reversal voltages.The study results on the dielectric properties of MMT modified pressboard show that the pressboard conductivity shows certain nonlinear characteristics with field strength increasing;the relative permittivity decreases then increases and then turns to stable with MMT doping ratio increasing,and the dielectric loss factor increases with MMT doping ratio increasing;the breakdown field strength reaches a peak when the MMT doping ratio is 1%,and then decreases with the MMT doping ratio increasing under AC and DC voltages.The dielectric properties of SiC一MMT modified pressboard were studied with the optimal MMT doping ratio changeless.The results show that the noulinear characteristics of pressboard modified with composite materials is between that modified with SiC and MMT alone;the relative permittivity increases then decreases with the SiC doping ratio increasing;the dielectric loss factor decreases then increases with SiC doping ratio increasing;the breakdown field strength decreases rapidly with the nano SiC doping ratio increasing under AC and DC voltages.