Micro discharges in HVDC gas insulated systems

In the present contribution, we analyze and quantify micro-discharge currents originating from rough electrode surfaces in HVDC gas insulated switchgear (GIS) applications. These currents can be significantly larger than those in the gas phase caused by natural ionization or those from bulk conduction in the solid insulators. They may thus dominate the charging of the insulator surface and are thus a key dimensioning criterion. Five different electrode systems with different, but clearly defined, roughness were designed. To quantify the amplitude of the micro discharge current as a function of the electrical field strength on the electrode surface, the consequence of these discharges in the form of accumulated charges on the surface of a PTFE sample was measured. The measurements were compared to simulation results from a model that uses the Townsend approximation for unipolar saturation ionic current, which is a widely used approximation in many areas of applications. As a main finding of our investigations, to avoid micro discharges, electrical field strengths of 7.1 kV/mm should not be exceeded in HVDC GIS design using typical industrial electrode roughness around 2 μm and 0.5 MPa SF6.