Modeling of the dielectric recovery of hot air in insulating nozzles

In standard high voltage gas circuit breakers -important safety elements in today´s power grids-, a small surface layer of polytetrafluoroethylene (PTFE) vaporizes at the high temperatures occurring in the arcing zone during the high current phase of the interruption process. This ablation continues even some hundred microseconds after the arc has been quenched and it actually changes the total gas composition as well as the temperature profile in the arcing zone. Therefore it delays the fast cooling of the arcing zone, which is necessary to prevent dielectric failures. The experimental determination of the dielectric recovery of hot air in insulating nozzles was investigated previously with focus on the substitution of SF₆ (sulfur hexafluoride) in circuit breakers for future power grids. The present investigation gives a physical model for the calculation of the previously measured recovery characteristics. CFD-simulations (computational fluid dynamics) are performed for the determination of the relevant physical properties of the decaying quenching gas (density, temperature etc.). These properties serve as input parameters for the developed model. Based on the gas properties resulting from the CFD-simulations it is possible to calculate the effective ionization coefficients and thus the breakdown voltage applying the streamer criterion. Afterwards the calculated breakdown voltages are compared to the measured recovery characteristics. The comparison shows a good agreement between the measurements and the calculation.