Charge generation in gas-insulated high-voltage direct current (HVDC) equipment has been investigated since surface charge accumulation was detected as the main challenge to insulation coordination. 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 timeconsuming and demands comprehensive modeling. In this paper, more than 3,000 hours of surface charge measurements are presented, covering the full range of charge generation processes from ion currents owing to natural ionization to the inception of micro-discharges. In accordance with gas-insulated equipment, Al2O3filled epoxy resin was used for the solids, and sulfur hexafluoride (SF6) was used for the gaseous insulation. It has been demonstrated that the ion pair generation from natural ionization in the gas is significantly influenced by the surrounding solid materials, and therefore, scales with the insulation volume. From the onset of micro-discharges, sharp and locally limited charge patterns are formed, as surface conductivity does not appear to influence the investigated charge distributions. Whereas the primary charge dissipation from surface areas centered at the distribution could be explained in terms of the gas volume size, the discharge currents could not be reproduced with common ion-current models. At low electric fields, the observed charge decay indicates the presence of low-field conduction processes that cannot be explained by charge generation from natural ionization in the gas.

Charge generation at the interfaces of gas-insulated switchgear or gas-insulated lines is mostly studied from a macroscopic point of view. Integral current measurements are a common method to determine the average inception field strengths and intensities of micro discharges. However, because of the lack of spatial resolution and measurement sensitivity, these experiments do not provide deeper insights on the microscopic processes. By applying surface potential measurement techniques, studying the charge generation of single interface protrusions and drawing conclusions relative to their spatial distribution have become possible. This paper presents a spatially resolved analysis of charge-generation processes at rough aluminum electrodes and insulating interfaces. The results reveal a highly inhomogeneous charge generation at the interfaces even for electric fields below 6 kV/mm. Analyzing the charge generation from a macroscopic perspective has been demonstrated to possibly underestimate the local charge generation by up to three orders of magnitude. Using sulfur hexafluoride (SF6) as the insulation gas at 0.45 MPa and Al2O3-filled epoxy resin insulators, the inception field strength for charge generation at the insulating interface is measured to be 3 kV/mm. For a technical insulation system that includes both a rough aluminum electrode and an insulating interface, significant discharge intensities are observed at a 5-kV/mm electric field. From these results, micro discharges are expected to be highly relevant for dimensioning of gas-insulated devices.

The established theories behind low-field ion-drift currents assume that the charge generation from natural ionization is the main source of the charge-carriers, and that it defines the current flowing through the insulation gas in gas-insulated direct current (DC) devices. This charge generation is assumed to be independent of the applied electric field in the cases where the electric field is sufficiently below the onset of microdischarges at the interfaces. However, the results of a few previous studies have suggested the presence of a field-dependent current contribution from some conduction processes, which might significantly accelerate the decay of the surface charges distributed on the insulator surfaces. In this paper, an extensive study on low-field charge transport is presented, and the influences of the insulation volume, electric field, and gas pressure on low-field charge transport are discussed. In accordance with the common structure employed in gas-insulated devices, Al2O3-filled epoxy resin is used in the solid parts of the device and sulfur hexafluoride (SF6) is used for gaseous insulation. At electric fields of approximately 1800 V/m, a gas current that is up to 30 times higher, when compared to the conduction currents caused by the charge generation from natural ionization, is observed. This enhanced current reveals a further conduction process acting in a limited range of low electric fields and is characterized to be independent of the applied voltage polarity. With an increase in the gas pressure, the current amplitude is found to increase linearly and higher electric field is required to reach its maximum. At electric fields below 200 V/m, the conduction current reverses its pressure dependency and increases with decreasing gas pressure. The reported findings strongly indicate that the enhanced low-field charge transport in gasinsulated devices may be owing to electrophoretic conduction.

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Abstract-This paper presents a current measurement setup to determine polarization and conduction processes of solid insulation materials under HVDC stress. The setup enables simultaneous polarization, depolarization, and conduction current measurements of up to four material samples. The volume or surface current measurements are performed in a pressure vessel, adjustable to the whole range of possible longterm operating conditions present in gas-insulated equipment. Besides discussing the electrical and mechanical construction, this contribution addresses restrictions of the measurement range caused by an insufficient signal-to-noise ratio.