Technology Study On Heat Transfer And Discharge Characteristics Of Perfluorohexanone-Paper Insulation System In Phase Change

As one of the important symbols of rail transit modernization,high-speed railway plays an important role in the development of social economy,culture and science.China Railway High-speed Train and Fuxing Bullet Train have become diplomatic business card of China.As one of the core equipment of the train,the transformer’s service performance determines the safe and stable operation of the train.Years of operating experience of the current EMU traction transformers show that the heat dissipation capacity of the insulating oil convection heat dissipation method is difficult to meet the operating requirements of the train in the face of high load conditions.Due to the limitation of the installation location,the body structure of the transformer is relatively compact,resulting in its internal thermal field is more concentrated.In order to ensure timely heat dissipation during the operation of the transformer,special heat dissipation equipment is usually installed on the bottom of the vehicle,which makes it difficult to achieve miniaturization and light weight of the transformer.Two-phase immersion cooling removes heat by vaporizing the liquid medium after reaching boiling point.It has a heat dissipation efficiency far exceeding that of convection heat dissipation and has been applied in important fields such as nuclear energy engineering,energy chemical industry and electronic industry.Two-phase immersion cooling can be considered in transformer miniaturization and weight reduction.In this paper,perfluorohexanone(C₆F₁₂O)is used as a two-phase immersion cooling medium.Through heat dissipation and discharge experiments,combined with molecular simulation technology,the discharge characteristics of C₆F₁₂O-paper insulation system under phase change heat dissipation are studied The discharge decomposition mechanism of gaseous C₆F₁₂O and the damage mechanism of gaseous C₆F₁₂O discharge decomposition to insulating paper are proved,which provides certain reference and guidance for the design and operation of the traction transformer of the EMU adopting the two-phase immersion cooling method.Based on the boiling heat transfer experiment,the boiling heat transfer characteristics of C₆F₁₂O on the surface of cellulose insulating paper,Nomex insulating paper,polyester film and polyimide film are investigated.It is found that when cellulose insulating paper and polyester film are used,the best and worst heat transfer abilities can be obtained respectively.Through the analysis of the surface roughness of the four insulating papers,it is found that the insulating paper with higher roughness will make the heat transfer ability of C₆F₁₂O stronger.Molecular dynamics simulation is used to prove the insulating paper.The molecular structure of C₆F₁₂O will affect the movement ability of C₆F₁₂O,and when its movement ability is stronger,it will obtain better heat transfer ability.Through the partial discharge experiment of C₆F₁₂O in the boiling state,its discharge characteristics in the nucleate boiling range under the uniform electric field and the non-uniform electric field are proved.The discharge decomposition characteristics of gaseous C₆F₁₂O are studied by molecular simulation and experimental methods.The simulation results show that the C-C bond breaks first.There are exothermic and endothermic reactions in the decomposition process of C₆F₁₂O,and a large number of free radicals and small molecules are generated.At the same time,the distribution of these products varies in the pressure range of 0.1 MPa to 0.5 MPa.By comparing and analyzing the simulation and experimental results,the corresponding C₆F₁₂O aging state assessment suggestions and actual transformer operation and maintenance suggestions are put forward.Through the damage test and molecular simulation technology of cellulose insulating paper under the gaseous C₆F₁₂O discharge environment,the structural properties of the new cellulose insulating paper and the cellulose insulating paper after discharge damage are compared and analyzed,and the damage mechanism of the cellulose insulating paper by discharge is proved.Among them,the thermal damage to cellulose insulating paper caused by partial discharge will cause its molecule to be cleaved from the C-O bond,which will eventually reduce the number of original chemical groups in cellulose and introduce aldehyde groups（-CHO）.The chemical damage of partial discharge to cellulose insulating paper is mainly reflected in the introduction of C-F groups,and the reactivity of different F-containing radicals is different.