Voltage-current characteristics of free burning arcs in SF6 and air have been known for decades. As the demand for an SF6-free solution is increasing, there is an accompanying need to determine arc parameters in the alternative gases. An unblown arc experiment has been established to determine the voltagecurrent characteristics of SF6 alternative gases, which have not yet been thoroughly studied. In this experiment free burning arc measurements were performed in a number of gases under consideration of SF6 alternatives, including CO2 and mixtures of CO2 / O2 with and without C4F7N or C5F10O additives at concentrations of up to 10 %. Measurements were also performed in air and SF6 for comparison. Arc voltage was measured in each gas at pressures ranging from 1 bar to 5 bar absolute, and electrode separations ranging from 20 mm to 95 mm. Voltagecurrent characteristic measurements for air and SF6 show good agreement with previously published results. A linear relationship of the arc voltage to the arc length is shown, as well as fourth root dependence of the arc voltage on the gas pressure. It was shown that neither the O2 nor the fluorinated additives to CO2 have any significant influence on the voltage-current characteristic. The minimum arc voltage in all measured gases was slightly higher than in SF6, but the arc in SF6 was the least stable and had the highest elongations resulting in high voltage peaks. The arc voltage in air had a similar minimum value to the CO2 based gases, but the arc was much more stable, resulting in lower effective voltage, especially at low currents.

In recent years, significant achievements have been made with respect to the development of SF6-free gas insulated substations (GIS). In parallel, the interest in installing SF6-free GIS by utilities increased steadily and tenders for new substations or upgrades, which regularly also include alternative technologies. The excellent performance of SF6 was unequivocally accepted by all vendors and users so that the community became used to single technology solutions. This is no longer the case with alternative gas mixtures, and multiple technological solutions are available. However, from the present body of literature it is not possible to make a full and comparative evaluation of different alternative gas switchgear, i.e. circuit-breakers and disconnectors. Thus, the High-Voltage Laboratory of ETH Zürich started investigations and measurements of basic experiments that allow an unbiased comparison of properties of alternative gas mixtures relevant for switching. The two main purposes of these investigations are to define a set of measurements that allow an estimation and comparison of switching performance with different gas mixtures, independent of a specific interruption nozzle geometry, drive system, electrostatic design, and other design specific features, and to perform (some of) these measurements comparing pure SF6, with air, pure CO2, CO2/O2 mixture, and further specific gas mixtures that are currently proposed by manufacturers for SF6 replacement. The basic analysis behind the definition of measurements will be given in detail and the design principles of the chosen test devices and the derived test currents and diagnostics will be introduced. Test results themselves will not be given, rather they will be the subject of separate future publications.

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.

In gas insulated substations, the main trend is to move from SF6 to more environmentally friendly alternatives. The main SF6 alternatives are mixtures based on CO2, with additives of O2 and the fluorinated gases C4F7N or C5F10O. Mixtures with fluorinated additives of up to 10 % were investigated in a test setup with a free burning arc. Gas composition was measured for each gas after mixing, and in-between some arcing sequences. Gas mixtures were investigated by three mobile gas analyzers and by three laboratories. The results showed that for clean gas mixtures, all methods showed comparable results. Determination of the heavily arced gas mixture decomposition is challenging and requires detailed laboratory analysis. A range of decomposition rates were shown for both C4F7N and C5F10O. In addition, the influence of the gas used for arcing on the arcing contacts made of Cu/W (20/80wt%) was investigated. A higher mass loss on the anode than on the cathode was found for all contacts. There is an indication of a higher erosion rate for arcing contacts used in SF6 alternatives than for those used with SF6, but further investigation is required. No major differences were found during visual inspection of the pins.