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Humidity has been considered as one of the main influencing factors that determine the conduction processes and electric strength of gas-insulated systems. Whereas in the past, various studies focused on the change in the partial discharge inception voltages, breakdown strength of homogeneous and inhomogeneous field arrangements, and insulator flashover voltage, recent studies have investigated the changes in ion currents measured through different gas gaps. In the framework of this contribution, a highly precise humidity control circuit has been developed to analyse the significance of humidity in the range from −25 to −5°C frost-point, which is fully applicable to operating gas-insulated devices. Using sulphur hexafluoride (SF6) as the insulation gas at 0.45 MPa, Al2O3-filled epoxy resin insulators, and technically rough electrodes, the humidity was found to significantly influence the intensity of microdischarges at interfaces. Charge generation from microdischarges at the interfaces substantially increased with increasing humidity. For an electric field of 5 kV/mm that were applicable to the dimensioning of gas-insulated devices, humidity strongly influences the charge provision from technically rough interfaces and potentially contributes to the surface-charge accumulation at insulator surfaces. On the other hand, for low-field conduction phenomena, no increase in the ion currents from natural ionisation or electrophoretic conduction was observed. For the investigated range of parameters, humidity is expected to be highly relevant for the design of gas-insulated devices., High Voltage, 5 (4), ISSN:2397-7264

A detailed understanding of the charge generation and transport in gaseous insulation forms the basis for modeling numerous problems in high-voltage engineering. Some examples for processes relevant to the dimensioning of gas-insulated devices are the surface-charging phenomena at insulator surfaces, prediction of initiatory electron provisions for partial discharges in small voids, estimation of statistical time lags for breakdown inception in gaseous insulation, etc. Here, the intensity and continuity of the charge provision strongly influence the operating states of the devices and, owing to their impact on the mean energy input from the discharges, the long-term performance and aging behavior of the insulation. Over the years, researchers have made enormous progress using continuous ion-current measurements or surface-charging experiments with model spacers downscaled from gas-insulated components. Whereas the low sensitivity of the former method limits its measurement range, the latter method is time consuming and demands comprehensive modeling. To overcome the limitations of the classical current-measurement techniques, this thesis introduces a new measurement approach that enables studying spatially resolved currents in the sub-femtoampere range. Low-conductive polymeric interfaces function as information storage, which accumulate the conduction and charge provision processes from solid or gaseous domains and interfaces. Using Al2O3-filled epoxy resin for solids and sulfur hexafluoride (SF6) for gaseous insulation, new fundamental insights on the processes are presented to determine the dimensioning of gas-insulated devices. Low-field charge provision and transport processes serve as the basis for the simulation of surface-charge decay under no-load conditions, modeling of initial electron provisions for partial discharges in small volumes, and explanation of statistical time lags for breakdown inception in gaseous insulations. The established theories assume that charge generation from natural ionization is the main source of charge carriers, which defines the current flowing through the gaseous insulation in DC devices. In this thesis, the ion-pair generation from natural ionization in a gas was demonstrated to be significantly influenced by the surrounding solid materials and therefore scales with the insulation volume. This charge generation was assumed to be independent of the applied electric field in cases where the electric field is sufficiently below the onset of microdischarges at the interfaces. Enhanced currents that were measured in a limited range of low electric fields revealed further conduction phenomena, which could be explained by electrophoretic conduction. Charge generation at the interfaces of gas-insulated devices is mostly studied from a macroscopic point of view. Integral current measurements are among the common methods of determining the average inception field strengths and intensities of microdischarges. However, because of the lack of spatial resolution and measurement sensitivity, these experiments do not provide deeper insights on the microscopic processes. Spatially resolved analyses of charge-generation processes in technically rough aluminum electrodes and insulating interfaces reveal a highly inhomogeneous charge generation from microdischarges. The analysis of the charge generation from a macroscopic perspective has been demonstrated to possibly underestimate the local charge generation by up to three orders of magnitude. From the results, microdischarges are expected to be highly relevant for the dimensioning of gas-insulated devices. The influence of humidity on the electric strength and conduction phenomena in gas-insulated devices has been discussed for many decades. In the framework of this thesis, a highly precise humidity control circuit has been developed to analyze the significance of humidity in the range applicable to operating gas-insulated devices. Whereas humidity is determined to have no influence on low-field ion currents through gaseous insulation, an increase in humidity significantly enhances the intensity of microdischarges at the interfaces and potentially influences the dimensioning of gas-insulated devices.