Study On Insulation Characteristics Of C-C₄F₈ And Its Gas Mixtures Substituting SF₆

Because of its excellent dielectric and arc extinguishing properties, SF6 has been widely used in high voltage apparatus as dielectric medium. However, Global Warming Potential (GWP) of SF6 is high about 23900 as CO2, and has been listed as one of the restricted gas for its high GWP in the ordination of Kyoto protocol signed in 1997. It has been urgent to find environment friendly alternative gases with low GWP and high dielectric strength (E/N)lim to SF6.(E/N)lim, GWP and liquefaction temperature (Tb) should be considered when choosing the substitutes to SF6. c-C₄F₈ gas mixtures have been ranked as the most potential gases for being used as gaseous dielectric in NIST Technical Note 1425 in 1997. Though few scholars home and abroad have studied the insulation characteristcs of c-C₄F₈ gas mixtures, they only analyzed the GWP and Tb qualitatively or even ignored these two indexes when analyzing the insulation characteristics of c-C₄F₈ gas mixtures. In this thesis, (E/N)lim, GWP and Tb these three indexes of pure c-C₄F₈ and its mixtures were analyzed quantitatively synchronously for the first time, studying the insulation characteristics of c-C₄F₈ and its mixtures substituting SF6, besides, discharge parameters in c-C₄F₈/N2O and 10%c-C₄F₈ ternary gases were also measured and computed for the first time. The methods adopted in this thesis are Steady State Townsend (SST) experiment method and Boltzmann equation analysis method when studing the discharge characteristics in c-C₄F₈ and its mixtures. In this thesis, the SST mathematical model was modified, and density-reduced ionization coefficientα/N (α:ionization coefficient;N:number density) and density-reduced attachment coefficientη/N(η:attachment coefficient)were fit directly, so the accuracy was improved while the compution time was reduced. By measuring the ionization current in SF6, the stability of initial electrons was determined. By measuring the discharge parameters in neutral gas N2, weaK electronegative gas CO2 and strong electronegative gas SF6, the feasibility of the designed SST apparatus and the modified mathematical model were proved. Two methods evaluating (E/N)lim were analyzed and compared, and one method was choosen to determine (E/N)lim in c-C₄F₈ and its mixtures. Based on the modified SST model, over the range of 1804F₈/CO2,c-C₄F₈/CF4,c-C₄F₈/N2 and c-C₄F₈/N2O were measured by SST experiment and (E/N)lim were also determined. By comparing the (E/N)lim of c-C₄F₈ binary mixtures with the one of pure c-C₄F₈, the effect of buffer gases CO2, CF4, N2 and N2O on the insulation characteristics of pure c-C₄F₈ were analyzed, and such conclusions are obtained that there is synergetic effect in c-C₄F₈/CO2, slightly synergetic effect in c-C₄F₈/CF4, and positive synergetic effect in c-C₄F₈/N2 and c-C₄F₈/N2O. So, adding CO2, CF4, N2 and N2O to c-C₄F₈ can reduce the use of c-C₄F₈, and can ensure that these mixtures have high (E/N)lim.The feasibility of c-C₄F₈ binary mixtures substituting SF6 were analyzed by comparing the (E/N)lim of c-C₄F₈ binary mixtures with the one of SF6.Discharge parameters calculated by Boltzmann equation analysis method can include not onlyα/N andη/N, but also transport coefficient (drift velocity Ve and diffusion coefficient D ), so in this thesis Boltzmann equation analysis method was also used to calculate the discharge parameters in c-C₄F₈ binary mixtures over the wide range of 1804F₈ were explained from the view of molecular motion, which extended the studied results by SST experiment. By analying the relationship between GWP and the ratios of c-C₄F₈ in its binary mixtures, it is can be found that the GWP of these four c-C₄F₈ binary mixtures studied in this thesis are much lower than SF6 when their comparing to that SF6. So, these four c-C₄F₈ binary mixtures can redece the effect on atmosphere greatly when they are used as insulation gases.The liquefaction temperature (Tb) of pure is high. Considering the possible use of c-C₄F₈ mixtures in environment with low temperature, this thesis studied the insulation charateristcs of 10%c-C₄F₈/CO2/N2,10% c-C₄F₈/N2O/N2 and10% c-C₄F₈/CO2/N2O by Boltzmann equation analysis.In this thesis, GWP and Tb were calculated in pure c-C₄F₈, c-C₄F₈ binary mixtures and 10% c-C₄F₈ ternary mixtures when their (E/N)lim were comparable to SF6. Calculated results show that, it is not feasible for pure c-C₄F₈ substituting SF6 because of its high Tb, when the environment temperature is over 0℃, c-C₄F₈/ N2 and c-C₄F₈/ N2O in c-C₄F₈ binary mixtures are better substitutes than c-C₄F₈/CO2 and c-C₄F₈/CF4 to SF6 for the ratios of c-C₄F₈ in these two mixtures are low, and GWP are also low, and 10%c-C₄F₈/N2O/N2 are better substitutes than 10%c-C₄F₈/CO2/N2 and 10% c-C₄F₈/CO2/N2O to SF6 in 10% c-C₄F₈ ternary mixtures for the required gas pressure is low when (E/N)lim are comparable to SF6 under the environment temperature above -20℃. (E/N)lim,GWP and Tb in c-C₄F₈ binary mixtures and 10% c-C₄F₈ ternary mixtures obtaind in this thesis can provide reference for practical engineering use.