Development of a Space Charge Measurement Method Applied to HVDC GIS Spacers

The development of HVDC networks leads to an increasing need of Gas Insulating Substations (HVDC-GIS). When an insulator is subjected to high thermoelectric dc DC stress, space charge accumulation can induce local increases of the electric field, which must be known and taken into account for the equipment design when the fields exceed several kV/mm. This work deals with space charge measurement in DC-GIS spacers. To observe the accumulated charges, a non-destructive technique based on a thermal stimulus has been developed for application to GIS insulators. Based on their cylindrical geometry, an experimental bench has been set up following the principle of the thermal step method, which consists of applying a thermal step to the sample and measuring a capacitive signal generated by induced charges under the effect of the electrical disequilibrium due to the temperature diffusion. To study the insulator behavior in its real configuration, samples of alumina-filled epoxy resin have been developed. Their shape allows reproducing the bulk stress endured by the material under the effect of electric field and temperature. The set-up and the experimental validation are presented in this paper.