Evaluating Resistance to Corona and Ozone in Polymeric Materials for High Voltage Outdoor Applications

Licentiatavhandling, 2009

Resistance to corona discharges and ozone, the well known ageing factors for polymeric materials used in outdoor insulation, has been listed as being of great importance among twelve properties identified by CIGRE WG D1.14 – Material Properties for Non-ceramic Outdoor Insulation. Numerous earlier experiences indicated that the influences of corona discharges on properties of polymeric insulators may yield among different effects material discoloration, surface erosion, and sheath/shed cutting, each of them affecting practically important material properties. The effects may vary depending on type of corona source, its spatial distribution and on discharge intensity. Therefore, for the purpose of investigating the corona ageing mechanisms, various types of artificial corona ageing test arrangements have been used. In this thesis a multi-needle corona electrode arrangements was selected and used for testing the corona and ozone resistance in mechanically pre-stressed specimens of materials for outdoor applications. The main goal of this work concentrated first on designing the necessary equipment and then on defining test conditions. Thereafter, by strictly following the test procedures and specifications elaborated, changes imposed by corona and ozone treatment on electrical properties of five different grades of commercially available polymeric materials were investigated, including epoxy composite (EP), two types of silicone rubbers (LSR and HTV) ethylene-vinyl-acetate (EVA) and ethylene-propylene-diene (EPDM) elastomers, all containing fillers. It has been found that the 100 h long treatment had rather strong influences on surface resistivity of all the materials concerned, except for EVA, for which surface resistivity remained practically unchanged. The treatment had also an influence on mechanical properties of the tested materials. The observed changes could mainly be attributed to oxidation of the material surfaces, as revealed by additionally performed physical-chemical analysis. However, the changes incurred seemed to still lie far away from the level that could be considered as dangerous for material performance in outdoor environments. The effects of the treatment on volume resistivity, dielectric permittivity and dissipation factor (dielectric losses), all being the properties of the material bulk, did not exhibit high sensitivity to the treatment. In addition, based on the elaborated procedure, the resistance to corona and ozone was investigated for epoxy based composition containing micro- and nano-fillers at different proportions. Micro- and nano-fillers were found playing a significant role in the observed changes imposed by corona exposure. A clear effect of the nano-filler addition on activation energy of the dielectric response could be seen. The elaborated testing procedure has been proposed to CIGRE for further evaluation through performing an international round-robin test.