Sensitivity Study and Operational PD Monitoring Experiences of SF₆-free GIS

Sulfur hexafluoride (SF₆) has been widely used as an insulating gas in compressed gas-insulated equipment across the power industry. However, SF₆ has the highest global warming potential (GWP), 24,300 times greater than CO₂, which necessitates the search for viable substitution gas candidates. In recent years, there are strong legislative incentives for transitioning away from SF₆ with environmentally friendlier gas alternatives. Condition monitoring of gas-insulated switchgear (GIS) filled with a new alternative gas mixture is key for assessing the integrity and reliability of such asset. The use of partial discharge (PD) monitoring has already been well proven in SF₆ insulation applications as a useful technique for identifying various insulation defects including protrusions, voids in insulation spacers and floating metal particles. However, there is limited evidence that the current established knowledge for SF₆ can be easily transferable to new SF₆ alternatives. Assessing the efficacy of current SF₆ PD commissioning tests or condition monitoring setups in new SF₆-free GIS installations is an important step to identify key differences that may exist in the PD behaviour between SF₆ and its alternatives.

The first use case of Novec™ 4710 based gas mixture (4% C₃F₇CN and 96% CO₂) at the National Grid Sellindge substation in the UK with only 6-7 years of user experience, which is a relatively short time when compared to the experience the industry has with SF₆ since 1960. The reliability and behaviour of SF₆ based GIS are therefore better reported in the literature and the wider power industry. Limitations in the current knowledge involved with the implementation of transferable SF₆ GIS test procedures and recommendations to SF₆-free solutions are presented in this paper. There is little reported literature on SF₆ and let alone SF₆ alternatives on PD sensitivity testing of GIS. Performing a sensitivity study will help outline the key attenuation and frequency response characteristics that exists inside a GIS specific to the insulating gas medium used. In this paper, a sensitivity study based on the current industry recommendations for SF₆ was performed on a GIS filled with a C₃F₇CN gas mixture. The existing PD monitoring setup was examined to underline aspects that can directly affect the signal amplitude and phase angle. A modified approach to perform the sensitivity study which utilises existing PD monitoring equipment is presented. Finally, the modified method of performing the sensitivity study highlighted that statistical measurement might be easier to comprehend and quantify than S-parameter. This study shows technical viability of applying the same procedure for sensitivity study as adopted for SF₆ GIS.