Research On Low Thermal Resistance Backfill Materials Of Cables

With the rapid development of society and economy,the demand for electric energy is increasing,and the application of high capacity and large load power cables becomes more and more widely.The safe and economical operation of cable is the key to ensure the transmission of power cable system.The underground cable system as an important part of modern distribution and transmission network system,the temperature of the cable directly affects its thermal resistance and carrying capacity,and backfill materials heat transfer performance directly affect the heat transfer between underground cables and surrounding media,thus affecting the temperature rise of cable.Therefore,the research on backfill materials and its thermal transfer and process is helpful to improve the operating value of cable lines.In this paper,by adjusting the thermal resistance environment of cables,a bentonite-based filling material suitable for underground cable engineering is designed and the composite formalized phase change materials(CPCMs)are developed for heavy load and high load cable laying conditions,thereby forming the new cable backfill material system which can be applied to different cooling conditions.Based on the finite element analysis technology,the heat transfer characteristics and the lifting effect of underground cable system are explored and analyzed to provide theoretical guidance and material support.Specific work includes:(1)The bentonite-based backfill materials which suitable for underground cable system is designed.The influence of bentonite on the temperature rise of cable was studied by selecting bentonite as the basic backfill system.Secondly,in order to meet the working performance of engineering applications,the ratio parameters of bentonite-based system were adjusted to optimize the design of the system,and a kind of backfill materials suitable for cable laying engineering has been developed,which has good thermophysical and working properties and stable physical and chemical properties.(2)Two kinds of CPCMs have been developed.The paraffin/porous ceramsite and paraffin/expanded graphite composite phase change materials were prepared by physical adsorption method and applied to the underground cable system as the functional components of the cable backfill materials,so as to alleviate the excessive "heat accumulation" of the underground cable system.Among them,the porous carrier has excellent adsorption capacity,which can carry a large amount of paraffin,and the chemical compatibility between the paraffin and porous carrier was good.These two kinds of composite phase change materials have suitable phase change temperature and excellent latent heat.The corresponding latent heats were 63.38 J/g and 156.4 J/g,respectively.They can maintain thermal stability below 200℃,and their thermal reliability was improved after packaging and setting.(3)The dissipation effect of the new cable backfill materials system was verified.Firstly,according to the typical cable line laying mode,the thermal resistance environment is classified according to the severity,corresponding to different cable backfill materials: backfill type-A,backfill type-B and backfill type-C,to provide an optimal choice for underground cable engineering with different heat dissipation requirements.Secondly,the simulation test platform of cable heat dissipation effect was built.It was found that the backfill materials could effectively reduce the temperature fluctuation around the cable and improve the heat dissipation conditions.The addition of composite phase change material has a better dissipation effect,suitable for heavy load cable environment requiring high heat dissipation.(4)The heat transfer characteristics of the new type of cable backfill materials in underground cable system are expounded.For the new cable filling material system,a heat transfer model based on solid heat transfer and equivalent specific heat was established for numerical calculation,to explore the heat transfer characteristics of the underground cable system and the lifting effect of carrying capacity.The new type of cable backfill materials system can effectively reduce the temperature fluctuation and improve the cable current carrying capacity.Under the same conditions,the carrying capacity of type-A,type-B and typeC are increased by 15.5%,23.6% and 29.1%,respectively.In addition,when PCM backfilling conditions with different mass fractions(10%,20%,30%)were used,the overall heat dissipation is improved.However,excessive doping ratio will affect the overall thermal conductivity,and the advantages of PCM backfilling will continue to decrease.