Ageing of Environmentally Friendly Insulation Gases by Partial Discharges

Due to the high global warming potential (GWP) of SF₆, the current trend in gas-insulated switchgear (GIS) is to replace SF₆ with more ecologically friendly alternative gases. Among different alternatives, the natural origin gases, e.g., technical air (79% N2, 21% O₂) and CO₂ offer low environmental footprint at a lower cost. These gases can also be used with small amounts of fluorinated admixtures like C5-fluoroketone (C5-FK) or C4-fluoronitrile (C4-FN) to enhance dielectric performance while maintaining a significantly lower GWP. The insulation gas in a GIS can be subjected to various kinds of electrical discharges, e.g., partial discharges (PD), spark, and arc discharges. Imperfections or contaminations in practical GIS may cause PD due to localized field enhancement which may lead to gas decomposition in the highly stressed zone. Unlike SF₆, the alternative gases may not fully recombine upon decomposition and the dielectric properties may deteriorate eventually, leading to ageing. This article presents a specially designed laboratory setup to evaluate the effect of ageing on the dielectric performance and chemical composition of the alternative gases upon their exposure to PD. To age the gas to a level relevant to practical applications, the net charge transfer during PD activity is calculated for a standard GIS compartment. An equivalent amount of charge is then transported through the gas in a 130 L pressure vessel by exposing it to corona discharges initiated in a needle-plane electrode arrangement. The insulation performance and PD characteristics of the aged gas can subsequently be evaluated, while gas samples can be routinely extracted to analyze gas decomposition. The paper also presents some sample results from an experiment in technical air at an absolute pressure of 1.3 bar, where power frequency AC breakdown tests were performed in a 10mm rod-plane gap. The results indicate that the AC breakdown performance of air remains unaffected due to high voltage DC corona discharges with a net charge transport of around 40 C, a level exceeding the estimated lifetime exposure of a typical GIS compartment.