Effect of Forced Gas Flow During Lightning Impulse Testing of CO2 in a Weakly Inhomogeneous Field

A memory effect from previous impulses might be observed during up and down lightning impulse (LI) testing of CO2 in a weakly inhomogeneous field. This could be due to residual space charges or formation of by products. To address the issue, this paper investigates the effect of forced gas flow when the enclosed gas volume is subjected to LI tests. At first, reference up and down tests were performed as per IEC 60060-1 in $\mathbf{1. 3}$ bar (absolute pressure) CO2 under standard LI voltages in a 10 mm rod-plane gap. Afterwards, the effect of forced gas flow was investigated by repeating the tests with a fan placed near the gap. The consecutive impulses appeared to be statistically dependent in the reference series and an increasing trend was observed in the LI breakdown voltage, whereas breakdowns occurred within a stable scattering range in the test series with forced gas flow. The results indicate that the standard for LI testing of insulation gases might require further review to ensure statistically independent test results. Ensuring gas flow by a fan near the insulation gap was experimentally proven to be effective in this regard.