Microscopic charge provision at interfaces of gas-insulated (HVDC/HVAC) systems

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 (SF ₆ ) as the insulation gas at 0.45 MPa and Al ₂ O ₃ -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.