Influence of thermal treatment on space charge in alumina-filled epoxy resin for HVDC GIS

A Gas Insulated Substation (GIS) consists of a high-voltage conductor maintained by spacers placed in a metal enclosure filled with a dielectric gas. The development of high voltage direct current (HVDC) networks leads to an increasing need of DC-GIS. A problem to be dealt with when an insulator is subjected to thermoelectric stress is the accumulation of space charges, which can induce a local increase of electric field that must be known and taken into account for the equipment design when the fields exceed several kV/mm.
The proposed work concerns GIS insulators made of alumina-filled epoxy resin. It has been observed that the electrical properties of this material are substantially dependent on the moisture content. Thus, the present study deals with the effect of heat treatment on the accumulation of space charges in cylindrical samples. In order to observe these phenomena, we have used a non-destructive technique allowing measuring space charge and electric field in solid insulating materials: the thermal step method (TSM). This technique consists in applying a thermal step to an insulating sample and measuring a signal generated by the influence charges at electrodes under the effect of the electrical disequilibrium due to the diffusion of the temperature. To perform the study, cylindrical samples have been designed to reproduce the electrical behavior of real insulators near the conductor. An experimental bench following the principle of the TSM has been designed and implemented.

A comparative study between “raw” and heat-treated samples is presented in the paper. After the application of dc electric fields up to 10 kV/mm, the charge accumulation phenomena are studied. The first part of the paper presents the sample geometry, the experimental bench and its operation. The experimental procedure is then detailed. Finally, experimental results and their interpretations are presented.