Study On Dielectric And Thermal Conductivity Properties Of H-BN/RIP For Dry HVDC Bushing

Dry DC high-voltage bushing is one of the most important devices for HVDC transmission,and its main insulation is epoxy resin impregnated paper composite insulation(RIP insulation).A large amount of heat will be generated when a large current flows through the center conductor during operation.Due to the poor thermal conductivity of RIP insulation,there will be a large temperature gradient between the inner and outer insulation layers of the bushing.Under DC voltage the temperature gradient will make the uneven distribution of electric field and accelerate the thermal aging of the insulation near the center conductor.At the same time,space charge will accumulate in the bushing under DC voltage,which will cause electric field distortion and seriously threaten the safe operation of dry DC high voltage bushing.In order to solve this problem,h-BN(hexagonal boron nitride)was used to modify RIP insulation to improve its thermal conductivity and space charge accumulation characteristics.The specific research contents are as follows:In this paper,h-BN modified insulation paper is prepared by sheet making in the laboratory,and cellulose nanofiber(CNFs)is used to wrap the surface of h-BN to obtain the CNFs@h-BN modified insulation paper.The modified insulating paper was impregnated with unmodified epoxy resin in vacuum,and then cured by step heating to obtain h-BN/RIP and CNFs@h-BN/RIP sample.SEM and XRD results showed that h-BN was successfully filled into the insulation paper,and CNFs formed a cage structure on the surface of h-BN,CNFs are closely connected with pulp cellulose.The test platform for dielectric properties testing is built.The influence of h-BN modification on the thermal conductivity of RIP insulation is studied.The results show that h-BN can effectively improve the thermal conductivity of RIP insulation,and the thermal conductivity rises with filler content.When the filler content is 30wt%,the thermal conductivity of CNFs@h-BN/RIP is 0.376W/(m.K),and the thermal conductivity increases by 85.2%.It indicates that the material has high potential application value in the main insulation of dry DC high-voltage bushing.In order to further verify the feasibility of its application in the main insulation of dry DC high-voltage bushings,this paper explores the effects of h-BN and CNFs@h-BN modified RIP on the relative permittivity,dielectric loss factor,DC conductivity,AC and DC breakdown characteristics.The results show that compared with unmodified RIP,the relative permittivity of h-BN/RIP decreases,the dielectric loss factor increases slightly;at the same time,the DC conductivity increases,the dependence of DC conductivity on temperature decreases;the DC breakdown strength decreases slightly,the AC breakdown strength first increases and then decreases with the increase of h-BN content.The maximum AC breakdown field strength of 15wt% h-BN/RIP is8.10% higher than that of unmodified RIP;compared with h-BN/RIP,the relative permittivity of CNFs@h-BN/RIP increases slightly,and the dielectric loss factor in the low frequency band decreases.When the filler content is the same,the DC conductivity becomes smaller and the DC breakdown strength increases,and10wt% CNFs@h-BN/RIP has the maximum DC breakdown field strength,which is 14.11% higher than that of the unmodified RIP.However,the improvement effect of the AC breakdown field strength is slightly reduced.Based on the above test results,this paper uses finite element simulation methods to conduct electro-thermal coupling analysis and calculation on the typical bushing structure insulated with h-BN/RIP and CNFs@h-BN/RIP,and calculate the electric field and temperature field in the bushing respectively.The simulation results show that the use of modified RIP as the main insulation of the dry DC high-voltage bushing can effectively reduce the temperature gradient between the inner and outer insulating layers,and make the internal electric field distribution uniform.The effect of h-BN content and temperature on the space charge characteristics of RIP is studied.The results show that at 25 ?,5wt% and 10wt%h-BN/RIP and CNFs@h-BN/RIP inhibit the space charge accumulation,reduce the electric field distortion rate in the sample.But when the temperature rises to50? and 75?,5wt% and 10wt% h-BN/RIP samples accumulate a large amount of heteropolar charges,which aggravates the electric field distortion;compared with h-BN/RIP,5wt% and 10wt% CNFs@h-BN/RIP samples have significantly reduced heteropolar charge accumulation.10wt% CNFs@h-BN/RIP suppresses space charge injection at various temperatures and reduces the electric field distortion rate;In addition,this paper uses the TSC method to test the trap parameters of the nano-modified RIP,and analyzes the mechanism of nano-h-BN and CNFs@h-BN on the dielectric properties of the RIP combined with the trap parameters and the interface model.The analysis results show that the trap characteristics have an important influence on the dielectric properties of RIP.The deeper the trap energy level,the greater the trap density,the more beneficial it is to inhibit the accumulation of space charge and increase the DC dielectric strength.