Basic Research Of The Environmentally Friendly Insulating Gas CF₃I For Its Application In Electric Power Apparatus

Recently,trifluoroiodomethane?CF₃I?has been broadly proposed as a potential replace-ment for SF₆owing to its outstanding dielectric strength and physico-chemical properties.In view of that there were few studies on the insulation characteristics of CF₃I gas mixtures and lack of data for practical application in power equipment,we aim to adopt the Boltzmann equation method as well as gas breakdown tests to comprehensively investigate the insulation properties of binary gas mixtures of CF₃I with N₂and CO₂.Firstly,this thesis reviews the physicochemical and environmental parameters for CF₃I,and expounds on the concerned issues for practical application,including the greenhouse ef-fect,liquefaction temperature,toxicity,and by-products after breakdown.From the view point of environment,CF₃I presents an extremely low GWP,and its overall atmospheric lifetime is estimated to be less than 2 days,thereby ensuring that CF₃I does not contribute significantly to ozone depletion.However,in spite of its environmental advantages,CF₃I has a high boiling point.As a result,the application of CF₃I for insulation inevitably needs it to be mixed with some ordinary gases such as N₂,CO₂and air,in which the buffer gas composing the greater por-tion can efficiently decrease the liquefaction temperature.In this work,the ionization coefficient?,attachment coefficient?,electron drift velocity V_e,transverse diffusion coefficient ND_T,and electron mean energy?for the gas mixtures of CF₃I with N₂and CO₂are calculated by solving the electron Boltzmann equation in the condition of steady-state Townsend discharge.These electron swarm parameters are essential for the study of discharge mechanism and development process in CF₃I gas mixtures.The calculation results indicate that the insulation strength of pure CF₃I is 1.19 times greater than that of SF₆,meanwhile,the values of limiting field strength are larger in CF₃I-N₂than in other gas mixtures of CF₃I with CO₂,Ar,He,or Xe.It is also found that CF₃I-N₂gas mixture with 70%CF₃I presents a very similar dielectric strength to that of SF₆.In addition,CF₃I gas mixtures have advantages on the electric field sensitivity compared with SF₆,however,disadvantages on the synergistic effect.After the investigations of electron swarm parameters in CF₃I gas mixtures,a new test rig has been designed and constructed specifically to be used for the insulation research of CF₃I gas mixtures.Extensive AC and standard lightning impulse breakdown tests on CF₃I gas mix-tures have been completed in the condition of quasi-homogeneous and extremely non-uniform electric field,focusing on the CF₃I-N₂and CF₃I-CO₂gas mixtures with CF₃I content from 5%to 30%,gas pressure from 0.1 MPa to 0.3 MPa,and gap distancefrom 5 mm to 60 mm.The present investigations provide scientific basis for the design and optimization of the environ-mentally friendly gas insulated switchgear in the coming future.The AC breakdown voltages of CF₃I-N₂gas mixtures are slightly higher than those of CF₃I-CO₂under quasi-homogeneous field,even though both of the gas mixtures linearly increase with the gas pressure and gap dis-tance.In the condition of non-uniform field,the AC breakdown voltages of CF₃I-N₂gas mix-tures approach to saturation with the increasing of gas pressure.However,for CF₃I-CO₂gas mixtures,the breakdown voltages present the well-known N shape characteristics as a result of the strong electronegativity of CF₃I.The pressure interval of abnormal breakdown would transfer with CF₃I gas content and gap distance.Regarding to the lightning impulse breakdown characteristics,the negative impact coefficient of lightning is approximately equal to 1 under quasi-homogeneous field.While in the condition of non-uniform field and positive polarity,the lightning impulse breakdown voltage falls far below the AC breakdown voltage,and the impact coefficient is as low as 0.6.Based on the foregoing analysis of breakdown characteristics,this thesis reveals the effect laws of CF₃I gas content,gap distance,gas pressure,polarity and electric field non-uniform coefficient on the insulation performance of CF₃I gas mixtures.In general,CF₃I-N₂gas mixture presents a superior dielectric strength than CF₃I-CO₂under different electric field sets.The experimental results indicate that 20%and 30%content CF₃I-N₂gas mixtures can achieve nearly50%and 55%insulation strength of pure SF₆.After a systematic consideration of the insulation performance,environmental properties,and liquefaction temperature,this thesis does propose to apply CF₃I-N₂gas mixtures with 20%³0%CF₃I as insulation medium to replace SF₆in the electric power apparatus.