The compact design and the independence from environmental conditions of gas-insulated systems leads to an ideal suitability of this high-voltage equipment for the use in a modern power supply system. The operation of the assets under DC voltage stress is unavoidable due to the increasing distance between the areas of power generation and consumption. The reliable operation during the estimated lifetime of several decades is only feasible with a precise partial discharge diagnosis. Hence, a sensitive measurement and a doubtless interpretation of the results is necessary. Nevertheless, it is necessary to take into account, that under alternating voltage stress established physical mechanisms of the discharge processes and the influence of dielectric interfaces cannot be adopted directly, due to the changed voltage stress with a constant electric field and the related surface and volume charge accumulation.
Aim of this thesis is the analysis of defects in gas-insulated systems with and without dielectric interfaces under DC voltage stress and thereby to contribute to a reliable interpretation of partial discharge measurements. In addition, known electrical and novel optical measurement methods are investigated with respect to their capabilities and limitations when used under DC voltage stress. The experimental investigations are carried out in model electrode arrangements. The weakly inhomogeneous electrical field of the gas-insulated systems is replicated in three configurations, one for each defect investigated. The detailed analysis of the discharge processes is enabled by a direct measurement of the partial discharge currents. A distinction between impulse currents and pulseless currents is made.
Due to assembly faults or insufficient material quality fixed, metallic protrusions can be created within the insulation system. The experimentally observed dependencies of the discharge processes on the polarity of the defect, the insulating gas pressure and the voltage stress permit a classification of four different types of discharge. In addition to the investigations in the most commonly used insulating gas sulphur-hexafluoride a comparison of the results with measurements in the climate-friendly alternative synthetic air are made. Derived from this, commonalities and differences in the discharge behaviour are discussed. Free moving, metallic particles can adhere to the gas-solid interface. The accumulation of surface charges at the solid insulator influences the partial discharge inception significantly. Due to the steady-state discharge behaviour, which is comparable to the fixed, metallic protrusion without contact to a dielectric interface, distinguishing between the two defects based on pulse repetition rates and amplitudes is challenging.
A unique aspect under DC voltage stress are discharges at the orthogonal interface between electrode coating and insulating gas. The analysis of the causes of the occurrence of these discharges, their optical and electrical characteristics and strategies for the prevention are investigated. Derived from the results, recommendations for partial discharge diagnosis of gas-insulated DC systems are discussed. These recommendations are an essential component for the future use of this asset in a high-performance electric power system with high reliability of the power supply.