Abstract
This paper investigates the effect of ageing due to free-burning arcs on the dielectric performance and the chemical composition of technical air (79% N2, 21% O2) at 1.3 bar absolute pressure. The ageing is performed by exposing the gas to a series of arcs dissipating an accumulated energy of more than 300 kJ, a level corresponding to relevant arcing energies for medium voltage load break switches. To evaluate the long-term dielectric performance, lightning impulse (LI) and power frequency AC breakdown tests are conducted in a 10 mm rod-plane gap before and after ageing. Gas decomposition is also investigated with gas chromatography coupled with mass-spectrometry. Results indicate that the LI breakdown voltage of air may enhance temporarily after arcing. This is probably due to the formation of short-lived air by-products (e.g., NO, NO2, NO3 and O3) which may reduce initial electron generation. However, changes in AC breakdown voltage might not be that significant as the presence of primary electrons is presumably guaranteed in the latter case.