Production of Graphite During the Extinguishing Arc with New SF6 Alternative Gases

Most high voltage circuit breakers in operation use SF₆ as the arc interruption medium because of its high dielectric strength and good arc interruption properties. However SF₆ also displays a high global warming potential which motivates the investigation of possible alternatives such as CO₂–Novec?4710 and N₂–Novec?5110 mixtures proposed by different HVCB manufacturers. CB power test comparison between pure CO₂ and CO₂–Novec?4710 mixture shows that the latter created a large amount of graphite and reduced CB performances after several current breaking shots. Using Gibbs free energy method, a theoretical study has been conducted on SF₆ replacement candidates listed above. Aiming to understand graphite formation, several parameters impact were discussed: carrier gas such as CO₂ and N₂, pressure and O₂ initial mixture proportions. The study focuses on graphite condensation temperature and graphite production amount evolutions in respect to the study parameters as well as in respect to CB inter-contacts space condition in terms of pressure and temperature at current-zero. It has been shown that a high percentage of CO₂ in the mixture reduces graphite condensation temperature by 2600 K while adding N₂ do not make such an impact. Pressure increases graphite condensation temperature and O₂ reduces it. The amount of graphite produced is pressure independent but decreases with the increase of O₂ proportion in the initial mixture. It has been shown that adding a sufficient O₂ amount can prevent graphite production at all.