Space Charge Measurements for HVDC GIS Spacer using the Thermal Step Method

High voltage direct current (HVDC) technologies are currently emerging to develop new energy transmission networks able to integrate renewable energy sources with remote locations from consumers. Gas Insulated Substations (GIS) have been widely used in alternating current (AC) transmission due to their low footprint and high reliability. While this technology is mature under AC, it requires investigations under direct current (DC). DC GIS also include high voltage conductors maintained by dielectric spacers, which allow insulating the high voltage parts from the ground. These embodiments are enclosed in a gas pressurized metallic vessel. Indeed, the governing physical phenomena in DC differ from AC: the electric field distribution in the GIS depends much more on volume and surface conductivities of the insulating materials and on the associated charge phenomena, than on their permittivity, as in AC. To develop and optimize the design of a DC GIS, the DC properties of the insulating materials must be well known. Various investigations have been conducted the last decade, on the dependency of volume and surface resistivities on temperature and electric field. More recently, investigations have been performed on the surface charge accumulation along the spacer surface. Another issue is the effect of space charge accumulation in the material bulk, which can occur under high DC thermoelectric stress, leading to local electric field reinforcements that favor ageing. Less information is available on the space charge behavior in filled epoxy resin in DC GIS, and particularly for configurations close to GIS application. This paper aims to present a non-destructive technique for characterizing alumina-filled epoxy resin insulators, using a thermal stimulus (Thermal Step Method). The first part of the paper deals with an experimental set up developed for this purpose in a coaxial geometry. This configuration allows reproducing the bulk stress applied to a GIS spacer under the effect of high voltage and temperature. Space charge characterizations of samples submitted to different DC constraints, stress durations and temperature conditionings, are then presented