Cu/W Electrode Ablation and Its Influence on Free-Burning Arcs in SF₆ Alternatives

Abstract

A free-burning arc experiment was performed in a gas mixture composed of CO2 / O2 / C5F10O (85 % / 10 % / 5 %). An optical emission spectroscopy diagnostic was developed to obtain spatially resolved arc spectra at a high frame-rate. Spectral measurements are compared with simulated spectra to estimate arc parameters including temperature and composition. These techniques are used to characterize the arc and its associated electrode jets over a range of conditions, in order to study the influence of ablated Cu/W on arc characteristics. The observed spectra indicate that the ablation of the contact tip primarily occurs through vaporization rather than the expulsion of droplets. The metal vapor content of the arc is investigated as the arc driving current is varied, and a transition threshold for increased ablation at electrode current densities above 50 A mm −2 is determined. The expansion angle of the electrode jet is estimated from the concentration of metal vapor in the arcing region at different axial positions, using a basic conical expansion model. The dependence of the measured voltage on the arc composition is examined, and a positive correlation between Cu content and arc voltage is identified. This trend is attributed to the higher emissivity of the metal vapor, which suppresses the central arc temperature and increases its resistivity.

Voltage–Current Characteristic of Free Burning Arcs in SF₆ Alternative Gas Mixtures

“…Voltage-current characteristics at 5 bar show that the minimum arc voltage measured in mixtures with C 4 F 7 N is always lower than that of the base gas, which is consistent with the results in the literature [5], however at lower pressures this is not always the case. This is to be expected considering the gas decomposition leading to a lower fluorinated additive level and assuming a higher fractional metal vapor content of the arc at lower pressures [13]. Characteristics for C 5 F 10 O mixtures show results of both higher and lower minimum arc voltage across all varied parameters.…”

“…The clear difference between the arc voltage in SF 6 and other gases for 20 mm shown in Fig. 12 is even more significant considering that for this condition the arc voltage is most likely highly influenced by the copper vapor [13].…”

“…The unblown arc experiment was also used to investigate the free burning arc with optical emission spectroscopy [13]. Results of the investigation showed an increase of the arc voltage with the rise of copper vapor content in the arc.…”

“…Each arc event erodes the surface of the arcing contacts changing its geometry which leads to different field uniformity factor. A more detailed description of this nonlinearity, as well as information about arc temperature and composition affecting the arc voltage, based on the unblown arc experiment measurements, can be found in the article by Engelbrecht et al [13].…”