Liquid spray injection in the expansion volume of a CO2 high voltage circuit breaker
Liquid spray injection
in the expansion volume of a CO2 high voltage circuit breaker
Most high voltage gas circuit breakers (HVCB) 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 to this gas. The performance of CO2 as an interruption medium in HVCB has been previously examined in the literature and it was established the CO2 intrinsic performance was clearly worse than that of SF6. Optimization of circuit breaker design to account for the specific properties of CO2 is likely to bring some improvement, although without matching the performance and small size of designs optimized for SF6. To further improve the interruption properties of a CO2 circuit breaker, a spray of a selected liquid, typically a mineral oil, is injected in the circuit breaker expansion volume so as to: i) favour the pressure increase mechanism leading to a more efficient arc expansion process; ii) generate a mixture likely to ensure a better heat release at current-zero and improve the dielectric performance. A spray injection device has been experimentally developed and is complemented with a numerical simulation of the interaction between the spray and the hot gas flow within the expansion volume. The numerical model relies on an Eulerian-Lagrangian description of the flow which combines a RANS modelling of the gaseous phase and a Lagrangian description of the discrete phase, including a turbulent dispersion model and an evaporation model of Abramzon-Sirignano type for the spray droplets. Both descriptions are coupled through a two-way approach which accounts for the effects of the continuous phase on the droplets and the retro-action of the droplets on the carrier phase. The computational domain includes the expansion volume and part of the nozzle connecting the arc region to the expansion volume with inlet / outlet conditions applied on the nozzle boundary obtained from a full computation of the HVCB, including the plasma region, performed with a dedicated software. The thermodynamic description of the mixture between CO2, Polytetrafluoroethylene (PTFE) vapour resulting from the ablation of the PTFE nozzle walls and oil vapour from the evaporating spray droplets relies on look-up tables.