Schlieren Images of Negative Streamer and Leader Formations in CO2 and a CF3I-CO2 Electronegative Gas Mixture

Abstract

In electrical networks, SF6 gas is currently used to insulate high-voltage equipment, however, due to its high environmental impact, new alternatives such as CO2 and electronegative gas mixtures such as CF3I-CO2 are being trialled to replace SF6 and create a sustainable energy system. A high-voltage lightning impulse (1.2 μs/50 μs) was used to approximate the disturbance in a high-voltage electrical network caused by a lightning strike and helped to identify the likely streamer and leader formations in different gases insulating a piece of gas insulated switchgear. In this paper, the theoretical and practical aspects of electrical streamer and leader formations in pure CO2 and an electronegative gas mixture of CF3I-CO2 are examined using schlieren videography in small length rod-plane gas gaps between 20 and 50 mm in length. Schlieren allows the examination of gas density in streamer formations and for the differences in weakly attaching gases, such as CO2, and electronegative gas mixtures, such as CF3I-CO2, to be studied. The gas pressure is varied in order to examine the differences in streamer and leader formation as the gas density is varied and hence the probability of electron collision is varied.

Analysis of light emission and Schlieren from short gap high voltage streamers representing lightning impulses

“…The dark image was subtracted from all following images to enhance detail which may not have otherwise been easily visible whilst removing static artefacts present in both images, which were largely found to be a result of the bright LED light source. This technique is the same as given in 13 . The brightness of the images was then increased by 40%, where needed, to emphasise detail for print and display.…”

“…The observation of both light emissions and Schlieren structure have traditionally been used to study streamers and other related phenomena 7 – 9 , with recent examples including using optical cameras to image streamers in air 10 – 12 and Schlieren imaging to analyse streamer and leader structure 13 – 16 . They are sometimes used interchangeably to derive characteristics but are rarely used simultaneously.…”

Analysis of light emission and schlieren from short gap high voltage streamer impulses representing lightning

“…The dark image was subtracted from all following images to enhance detail which may not have otherwise been easily visible whilst removing static artefacts present in both images, which were largely found to be a result of the bright LED light source. This technique is the same as given in [8]. The brightness of the images was then increased by 40%, where needed, to emphasise detail for print and display.…”

“…The observation of both light emissions and Schlieren structure have traditionally been used to study streamers and other related phenomena [4], with recent examples including using optical cameras to image streamers in air [5][6][7] and Schlieren imaging to analyse streamer and leader structure [8][9][10][11]. They are sometimes used interchangeably to derive characteristics but are rarely used simultaneously.…”

Analysis of light emission and schlieren from short gap high voltage streamer impulses representing lightning

“…Flat or rounded edges, such as around a hemispherical electrode, create highly divergent fields resulting in broader structures. The observation of both light emissions and Schlieren structure have traditionally been used to study streamers and other related phenomena [4], with recent examples including using optical cameras to image streamers in air [5][6][7] and Schlieren imaging to analyse streamer and leader structure [8][9][10][11]. They are sometimes used interchangeably to derive characteristics but are rarely used simultaneously.…”