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.

This thesis investigated the AC insulation performance of a Fluoronitrile gas mixture with a varying amount of humidity in the gas. Also the AC insulation performance of a humid Fluoronitrile gas mixture at different operating pressures is investigated. Furthermore, the partial discharge (PD) behaviour of corona in a Fluoronitrile gas mixture is investigated. This is achieved by comparing the Partial Discharge Inception Voltage (PDIV), the Phase Resolved Partial Discharge (PRPD) pattern and some of the PD characteristics (e.g. rise time, fall time) of corona in a dry and humid Fluoronitrile gas mixture.

The fresults from this research confirm that humidity influences the electrical breakdown behaviour of a Fluoronitrile gas mixture. An increase in the amount of humidity in the gas results in a decrease of the AC insulation performance. This also applies to higher operating pressures, although the influence of humidity on the AC insulation performance decreases as the operating pressure increases. The PD behaviour of corona in a Fluoronitrile gas mixture is also affected by the presence of humidity in the gas. The PRPD pattern and PD characteristics of corona in a Fluoronitrile gas mixture differ with the presence of much humidity in the gas, as the charge of the corona discharges in a humid Fluoronitrile gas mixture are larger. The PDIV however did not change with the presence of much humidity in the gas.