This master’s thesis investigates the feasibility of retrofilling existing L-SEP Gas Insulated Switchgear (GIS). This is motivated by the European F-Gas regulations phasing out the usage of Sulphur Hexafluoride (𝑆𝐹6) due to its large Global Warming Potential (GWP). Retrofilling the L-SEP GIS with a dielectrically weaker insulation gas is a two-pronged problem. One side is on a component level, where the physical required alterations are investigated to obtain a new withstand voltage based upon alternative insulation gas. The other side is a system based study, where the grid of Stedin is examined on what transient overvoltages can occur. The latter part is the focus of this thesis and is important as some locations might stress the L-SEP to its rated maximum withstand voltage, not allow for retrofill. The research methodology involves devolvement and investigation of the key transient origin applicable to the Stedin grid by usage of the ATP-EMTP software. An analysis of transient origins identified fault clearing as the highest priority for investigation. Key components (such as cables and transformers) were identified and represented using frequency dependent models such as the ULM and BCTRAN Grey box model. Both of which were tuned and validated using theory, measurement data and datasheets. A sensitivity analysis was then performed for differing fault locations and component parameters to identify the primary drivers of the maximum transient overvoltages. The results show that the magnitude of the overvoltages are governed by grid earthing configuration and total shunt capacitance on the source side of the breaker. Non effectively earthed systems consistently produce higher overvoltages due to a large pole clearing factor. Furthermore, larger parallel capacitance reduces circuit damping which increases the contribution of the oscillatory part to the overvoltage. The highest simulated transient overvoltage was simulated to be a phase to phase overvoltage. In the case of a non effectively earthed systems with very low damping, where the oscillations of different phases align in opposition. This study concludes that the worst case transient overvoltages remain well below the L-SEPs (rated at 72.5kV) rated lightning withstand voltage of 325kVp for 𝑆𝐹6. This implies that the possibility of retrofilling the GIS with weaker insulation gas (which lowers the withstand voltage) might be possible. In order for this to be confirmed, tests must be done on the altered GIS to find the new withstand voltage for the alternative gas. To improve the grid overvoltages remedial actions can be taken such as improving grid earthing and strategic installation of surge arresters.

High-voltage circuit breakers protect and control the electrical power grid, making their reliability of the essence. Robustly assessing the technical `health' of every circuit breaker in operation is key for an optimal maintenance strategy and safety measures, but is traditionally not performed in a data-driven, statistical fashion. The goal is to design an indexation method for the health of the high-voltage SF6 circuit breakers of TenneT TSO NL using its available (monitoring) data. This indexation is performed through modelling the hazard rate of major and minor component failures with the Cox proportional hazards model. Failure modes and their causes are investigated using large-scale surveys on historical failure data and covariates for the model are chosen accordingly. The hazard rate for minor failures is found to be increasing with the asset's operating voltage level, its average switching frequency, certain manufacturers, and its relative SF6 leakage rate, of which the last relation has not been modelled before to our knowledge. Using the model output, the circuit breakers' probabilities of future failure are estimated, shown to have some predictive power, and incorporated into a color-coded scoring system. Using this quantitative measure of their condition, the TSO should be able to better manage its asset portfolio and risks. However, data quality, especially for major failures, is found to be low and poses a limitation, as well as a possibility for future research.