Nozzle ablation model: Calculation of nozzle ablation intensity and its influence on state of SF6 gas in thermal chamber

Energy released by electric arc during short circuit switching is mostly absorbed by the surrounding cold SF₆ gas. However, a considerable part of this energy is also transferred and absorbed by other elements of the circuit breaker interrupter which are located near the electric arc. The most important parts are the transfer of energy to the arcing contacts and to the nozzles, absorption of the energy by these elements and the resulting effects. The absorption of the energy causes heating, melting and finally the vaporization of structural material and it is the main cause of wearing of arcing contacts and nozzles, where the latter is commonly referred to as the nozzle ablation. The nozzle ablation causes an increase in the nozzle throat diameter which generally has a negative effect on the circuit breakers breaking performance. The other significant effect is the mixing of SF₆ gas and the nozzle vaporized material in the nozzle space and in the surrounding chambers. It is obvious that the ablation process has a considerable influence on the state of SF₆ gas in the contact gap but also in the adjacent interrupting chambers, in particular on the state of gas in the thermal chamber in case of self-blast interrupting units. In this paper, a method of calculation of intensity of nozzle ablation is presented as well as a variety of calculation results. The calculated nozzle ablation intensity is verified by comparing the calculated results of the nozzle diameter increase and mass losses, with experimentally obtained data. In addition to the nozzle ablation intensity, the influence of the ablated nozzle material on the state of SF₆ gas in the thermal chamber is also analyzed and discussed. The model is incorporated into a computer application for high voltage circuit breaker interruption simulation.