Recommendations for surface conductivity characterisation under high voltage direct current (HVDC)

Phenomena of charge accumulation on the insulator surface are of major interest in Gas Insulated Substations (GIS) under Direct Current (DC). These phenomena mainly depend on electric properties of the insulating parts. The accumulated charge can then modify the electric field distribution in GIS. In order to better understand / simulate the electric field distribution in a DC GIS, surface resistivity characterization must be performed. The latter is usually realized following the standard recommendations from IEC-60093. It consists in measuring the current onto the insulator surface, induced by a tangential electric field applied on the material. However, the appearance of leakage current through gas and the presence of gas water content may influence the measured current, and lead to a wrong estimation of attributed surface resistivity. The aim of this work is to identify the impact of currents passing through gas and their influence on the characterization of surface resistivity.
A comparative simulation study of two experimental setups for surface resistivity characterization has been performed. An estimation of the measurement error introduced on the measurement of surface conductivity is presented for both configurations. Then, experimental measurements in both dry and “wet” environment in the new configuration, for electric field up to 10kV/mm and temperature up to 100°C are presented. This geometry allows decreasing the field intensification at the edges of the HV electrode, and shift to a more uniform electrical field.

This paper presents a comparative simulation study of two experimental setups for surface resistivity characterization. Currents measured using the uniform geometry allows determining the influence of current through gas. It evidences that depending on the environmental conditions, the electric field and the geometry considered, the surface resistivity attributed to the insulator will vary. From the results obtained, recommendations for surface resistivity characterization are presented.