Evaluating Resistance of Materials for Outdoor Insulator Housings to Corona and Ozone
Paper in proceedings, 2009
It took a few decades before polymeric composite insulators became broadly accepted on the market as an alternative solution to the traditional porcelain and glass counterparts. In the struggle on the way to develop composite insulators to the stage they are today, researchers and engineers tested numerous ideas and solutions. Many of these attempts were concentrated on elaborating design criteria and on selecting material solutions, which together would allow standing severe working electric stresses, thus securing their reliable long term performance.
Properties required for ceramic materials to be useful within high voltage outdoor insulator applications have been specified since long (IEC 60672). The situation regarding requirements for polymeric materials has been entirely different. Despite of the long experience, an adequate standard have not existed until now. However, serious manufacturers have used own selection criteria of best materials, which caused that for vast majority of composite insulators available on the market the required quality can be assured. Some utilities have also elaborated own specifications defining material properties to be proven by insulator suppliers. This situation has become even more complicated by the fact that manufacturers and users of insulators have sometimes different opinions regarding the significance of specific material properties and their limits. As a result CIGRE working group WG D1.14 concentrated during recent years on defining the physical parameters important for the use of polymeric materials in outdoor insulation and on checking if relevant test methods are available today. Twelve properties have been identified , whereas standardized test methods and minimum requirements have been available for eight of them. Since 2004, the work of WG D1.14 has therefore focused on developing new test methods and on setting minimum requirements for the remaining four properties, among which the resistance to corona and ozone was listed as being of great importance.
Under the action of corona housing material surfaces are simultaneously subjected to a mixture of energetic and reactive species as well as radiations. Diverse chemical reactions take place during the exposure. For example, the most important ones in case of silicone based polymers include (i) an increase of the oxygen content at the surface by formation of silanol and hydroxyl groups, (ii) oxidative crosslinking, and (iii) degradation of the polymer network structure resulting in the formation of low molecular mass compounds. These by modifying material surface, yield changes of mechanical and electrical properties of the housing.
This report describes work performed at Chalmers University of Technology within the activities of CIGRE WG D1.14 on the development of methodology for testing the resistance to corona and ozone. The main goal of this work concentrated first on designing the necessary equipment and then on defining test conditions. These are presently used by a number of research groups within a Round Robin Test (RRT) procedure. Thereafter, samples of different materials were treated and the resulting changes of electrical, mechanical and structural properties are evaluated.