Effects of Corona and Ozone Exposure on Properties of Polymeric Materials for High Voltage Outdoor Applications

Doctoral thesis, 2011

Despite the past four decades of service experiences with polymeric insulators in power systems all over the world, an adequate standard allowing for proper selection of polymeric materials for applications in high voltage outdoor environments has still been lacking. Therefore CIGRE Working Group D1.14 concentrated during recent years numerous activities aiming at defining all the physical parameters important for the use of polymeric materials in outdoor insulation and on checking if relevant test methods for their evaluation are available. Among the twelve parameters identified, the resistance to corona/ozone belongs to the group of remaining four parameters which still need to be standardized. In this context, the investigations reported in this thesis concentrated on elaborating adequate test methodology for evaluating the resistance of housing materials to AC corona/ozone treatments. The work contributed this way to an international effort to check the effectiveness of the proposed methodology by participating in a CIGRE round robin test (RRT) activities. The test procedure has been based on a use of multi-needle corona source for treatment of polymeric material samples in a dry and ventilated discharge chamber. The treatment conditions were thoroughly investigated and set; the elaborated procedure was thereafter checked by performing treatments and measuring resulting changes of electrical and mechanical properties on five different types of housing material samples delivered by collaborating industrial partners. Results obtained from the study showed that the mostly affected parameter, on nearly all the materials investigated, was surface resistivity. At the same time, the effects of the treatment on volume resistivity, dielectric permittivity and dissipation factor, all being the properties of the materials bulk, remained rather weak. Apart from that, the changes in mechanical properties, including breakdown strength and elongation at break, were also noticeable. These could mainly be attributed to surface oxidation, as revealed by scanning electron microscopy (SEM) as well as Fourier transform infrared spectroscopy (FTIR) analyses. Surface cracking that appeared during the treatment on some of the materials as well as the accompanying it reduction of mechanical strength allow believing that the presence of corona and ozone in the vicinity of polymeric insulators may especially influence their mechanical integrity when continuously remaining under tension. In the second group of investigations the elaborated in this thesis test methodology was applied for determining differences and similarities in the effects of AC and DC corona/ozone treatments on housing polymeric materials. The tests were in this case performed on two types of commercially available high temperature vulcanized (HTV) silicone rubber materials. Less pronounced effects of the treatment were found during DC corona/ozone exposure. This is a result of DC corona intensity reduction, attributed to electrostatic charging of the polymeric surfaces and subsequent reduction of electric field strength in the discharge regions. Among the investigated dielectric properties, surface resistivity still remained the mostly affected parameter. At the same time, the polymer matrix in the bulk remained practically untouched. The results of tensile tests did not exhibit clear regularities and the observed reduction of the mechanical performance was weak. Furthermore, the intensity of surface oxidation, as revealed by FTIR and X-ray photoelectron spectroscopy (XPS) analyses, could in turn be correlated with the measured during the treatments ozone concentrations. The AC corona exposure, with higher discharge intensity than under the DC treatment, produced higher doses of ozone and yielded a more severe surface oxidation. Hydrophilic groups (e.g. OH), formed on the oxidized surfaces of the HTV samples, determine therefore the loss of hydrophobic property and the rate of its recovery. One may conclude that the less pronounced degradation of silicone rubber materials during DC corona/ozone exposure provides positive indication for successful service of polymeric insulators in HVDC installations. The investigations presented in this thesis included also evaluation of the resistance to corona/ozone treatment of two batches of model nanocomposite materials, e.g. epoxy and silicone (PDMS) nanocomposites. The obtained results were, in general, similar to those found on the commercial materials. It was clearly observed that the introduction of the nano-filler had retarded deeper degradation on the surface of the nanocomposites.