Electrical breakdown in pure gases with different field enhancement factor

Sulphur hexafluoride (SF₆) is a gas commonly used in high voltage devices such as circuit breakers, gas-insulated switchgear etc., for insulation and switching applications due to its excellent dielectric characteristics. However, it is an extremely potent greenhouse gas with a GWP (global warming potential) of 23,500 times that of CO₂. Research is ongoing to find a suitable replacement gas with a lower ecological footprint, and able to provide satisfactory dielectric properties.

In this context, we study the physical characteristics of discharges in pure CO₂, Air, N₂, CO for different electric field geometries obtained with different electrode types, under positive and negative voltage impulse and DC. The point-plane geometry with sharp radius of curvature (Rc=0.2mm at 50mm distance from the plane) allows obtaining a strongly inhomogeneous field. It is not directly representative of a real application, but strongly facilitates the study of pre-breakdown phenomena (i.e. streamer or leader) by “forcing” their inception. The sphere-plane geometry (Rc=5mm, 50 mm distance) is more representative of a real application, while the plane-plane geometry provides an uniform field and can be compared with the classical Paschen’s law.

In inhomogeneous field breakdown is controlled by pre-discharge propagation conditions. In homogeneous field the breakdown becomes controlled by pre-discharge initiation: when a pre-discharge is initiated, it always leads to breakdown. Pressure influences the initiation voltage of these pre-discharges. The results obtained allows comparing breakdown voltages in several pure gases over a wide range of conditions, and points to the complex and sometimes erratic behavior of CO₂.