Stipendiatstilling 7 SINTEF (2021-2023)

Gas insulated switchgear (GIS) are essential parts of a safe, robust, and flexible energy system. They are responsible for interrupting short-circuit currents when faults occur, isolating parts of the grid during maintenance or faults, and for redirecting power and changing the grid infrastructure when needed. The most common gas used as an insulation medium in GIS today is sulfur hexafluoride, SF6. However, SF6 is the most potent greenhouse gas in existence with a global-warming potential approximately 25 000 times that of CO2. Replacing this gas would reduce the national and global use of SF6, making GIS-technology more sustainable. The project will obtain fundamental knowledge on discharge behaviour of the alternative gases and gas mixtures through experimental and computational research. A PhD candidate started at NTNU in the fall of 2021 and is working on experimental characterization and computational modelling of electrical discharges in SF6-alternatives. A setup consisting of a 400 L, 1.5 bar pressure vessel with custom made electrodes has been established and connected to a lightning impulse generator. Advanced high-speed cameras capture the discharge development in the tank with up to 1 billion frames per second. A photo-multiplier tube detects the discharge light emission. The initial tests have been performed in both air and C5-FK gas mixture, and a conference paper on these initial tests has been presented at Nordic Insulation Symposium in Trondheim (Norway) in 2022. The focus has then been changed to streamer inception probability, and we have performed both experiments in the lab and big simulations using a supercomputer to verify a probability-based model for discharges in different gas mixtures. These tests were performed in both air and in a Fluoroketone gas mixture, and resulted in a conference paper that was presented at the International Symposium on High Voltage Engineering in Glasgow (UK) in 2023. The comparison of the simulation results and the experimental results for air (and thus verification of the simulation model) resulted in a journal paper that was submitted to the journal Plasma Sources Science and Technology in the summer of 2024. This paper is currently in the review process. An interesting polarity dependency for streamer inception probability in air was discovered, and experiments were consequently conducted with a slightly different impulse voltage in collaboration with a master student. These experiments, in combination with the previous experiments in air, resulted in a conference paper which was presented at the IEEE International Conference on High Voltage Engineering and Application in Berlin (Germany) in 2024. The PhD candidate spent the spring of 2024 (5 months, January-May) on a research stay (exchange) at ETH Zürich. The polarity dependency for streamer inception probability in air was still the main focus, but this time from a different perspective: here the candidate studied the time lags for streamer inception when the electrode gap was stressed by a DC voltage. The laboratory setup at ETH Zürich that the candidate used was specifically designed to study time lags, and is something that she would not have had the opportunity to do at NTNU. A PhD candidate from ETH Zürich was at an exchange stay at NTNU during the same period in order to use laboratory equipment that they don’t have at ETH. The results from the time lag experiments are planned to result in a journal publication, potentially in combination with simulations run on a supercomputer. Going forward, there will also be conducted experiments with AC voltage to further study the aforementioned polarity dependence in collaboration with a master student.