Abstract
This paper explores how ageing by free-burning arcs affects the dielectric properties and decomposition of a mixture of 7.5% C5-fluoroketone (C5-FK) with 92.5% technical air (79% N2, 21% O2) at a pressure of 1.3 bar absolute. The gas is aged by exposure to consecutive arcs with a cumulative energy ∼315 kJ, a level relevant to medium voltage load break switches (LBS). To assess the long-term insulation performance, lightning impulse (LI) and AC breakdown tests are performed in a rod-plane electrode configuration before and after the ageing process. Additionally, gas decomposition is examined using gas chromatography and mass spectrometry. The results suggest that the LI breakdown voltage of the C5-FK/air mixture may decrease after ageing, likely due to C5-FK decomposition. Calculation of electron transport coefficients shows reduced attachment with C5-FK dissociation, which may decrease the discharge time delay. However, the discharge delay effects might not be observed in case of a slowly changing field, explaining why the AC breakdown voltage appears to be unaffected by ageing.