(245c) To the Determination of Thermodynamic Properties of g3 “C3F7cn - CO2 - O2” Insulating Gas for Electrical Circuit Breaker

In the medium and high voltage electrical circuit breaker manufacturing industry, and particularly at General Electric, the use of a substance like SF₆ for the insulation of the electric arc produced during breaking is essential. To reduce the dependence on SF₆ technology in worldwide electrical grid and to reduce the bulk electric system’s carbon footprint and greenhouse gas emissions, GE Grid Solutions has developed a ternary gas mixture which is the g³, to replace SF₆ (which has a global warming power 23,000 times greater than that of CO₂ [1]) used as an electric arc insulator. The molar composition of g³ ternary mixture is between 82 and 83.5% of carbon dioxide (CO₂), oxygen (O₂) at 13% and Novec 4710^® (C₃F₇CN) between 3.5 and 5%. C₃F₇CN is a fluorinated compound with a dielectric strength similar to or even better than that of SF₆ but its liquefaction temperature is high (-5 °C), hence the advantage of adding CO₂ in order to reduce it, and the presence of O₂ makes it possible to improve the recombination capacities of the arched g³ mixture. The g³ mixture requires to determine its thermodynamic behaviour under storage and operating conditions and to determine its thermodynamic properties.

For the determination of the thermodynamic behaviour of the mixture g³, we need to determine a phase diagram describing the phase changes and particularly the changes from the gaseous state to the liquid state and in particular the bubble and dew points curve. Information on isothermal Pxy diagram for each binary mixture and the PT envelope for the g³ mixture are also require. Concerning the determination of the PT envelop it is essential to develop a thermodynamic model adapted to the g³ system. To do this work we chose the Peng Robinson [2] cubic equation of state (PR EoS). Predictions are validated by comparison with experimental data. The PR EoS used in the case of ternary mixture with classical mixing rules should be able to correctly describe the intermolecular interactions between each molecule by adjustment of the binary interaction parameter (BIP) on experimental vapor liquid equilibrium (VLE) data.

Experimental data concerning the CO₂-O₂ binary mixture are available in the public literature [3,4], and by using these data we were able to determine the BIP parameter of this binary system. Concerning the two others binary mixtures involving C₃F₇CN and given the confidential nature and the novelty of the g³ project, the VLE experimental data do not exist in the public literature. For this reason, we have chosen to measure our own data. New VLE data concerning the C₃F₇CN - CO₂ binary system were measured from 0 to 60 ° C and pressures up to 70 bar (figure 1). VLE measurements are performed using a home-made equipment whom experimental technic is based on static analytic method with a fixed volume cell and samples analyses by using a gas chromatography device.

Concerning the determination of BIP of the binary system C₃F₇CN - O₂ and for reasons related to the risk of flammability due to the presence of oxygen we have chosen to do the Pressure Volume Temperature (PVT) measurements, by using a synthetic method measurement device with variable volume cell. Using the experimental bubble point, we well determine the last BIP and calculate the PT envelope. Predicted dew point pressure, will allow us to define the liquefaction conditions of the mixture g³ and will be compared to the data provided by General Electric. Figure 2 presents the PT phase envelope calculated with BIP = 0 for each binary system, PT envelop calculated with BIP (C₃F₇CN - CO₂) = -0.0065, BIP (CO₂-O₂) = 0.15, and Antoine’s law equation provided by GE.

The results obtained show that the values of BIPshave a great influence on the prediction of bubble pressure but not of the prediction of dew point pressure. This confirms the importance of measuring the bubble pressure of the g3 mixtures.

The adjustment of the BIPs will help us to develop an accurate thermodynamic model able to predict the phase behavior of g³ under all conditions of temperature and pressure. This work will give to GE grid solutions and its partners (who use electrical circuit breakers operating with g³ as insulating gas) a solution to fully understand the phase behavior of g³ mixtures under storage and use conditions. With this experimental and modeling works GE Grid Solutions has an experimental database, and an adapted thermodynamic model which can be also used to determine the other thermodynamic properties for the g³ mixtures.

References :

[1] General Electric, Development plan for g³ products by 2025, 2017, 12, p.3.

[2] Peng, D,-Y, and D,B, Robinson, A new two-constant equation of state, Ind, Eng, Chem, Fundam, 1976, 7, p.3

[3] Fredenslund A.A., Sather G.A., J.Chem.Eng.Data, 1970, 15(1), p.17-22

[4] Kaminishi G.-I., Toriumi T., Kogyo Kagaku Zasshi, 1966, 69, p.175