Influence of Ageing on Material Properties at Interfaces in Composite Insulators

Doktorsavhandling, 2009

High quality of internal interfaces in composite insulators for high voltage applications is necessary for securing durability of their functionality. Weak macroscopic interfaces between the glass fibre reinforced (FRP) epoxy core and the silicone rubber (SIR) housing, such as spots with reduced bonding or a loss of the adhesion as well as presence of foreign contaminants, can appear to have an accelerating effect on the evolution of ageing processes, though it may take years before the first signs of processes endangering the insulator function appear. Among factors that may strongly contribute to the development of the ageing are moisture ingress and appearance of partial discharge activity at the interfaces. Manufacturers of composite insulators, especially those producing large hollow core insulators for the highest voltage levels, pay a lot of attention to control the design of interfaces with good and durable long-term properties that secure a very low rate of ageing. However, there is still insufficient knowledge about the ageing mechanisms themselves. Moreover there is a lack of standardised ageing test methods for evaluating quality of interfaces in composite insulation systems. Therefore the aim of the work presented in this thesis has concentrated on elucidating these issues. Original experimental methods have been developed with a purpose to age materials at or in the vicinity of an internal macroscopic interface. Two different main types of interfaces with an added artificial defect were investigated. The first type represented cured or glued joints within repaired insulator housings, whereas the second type modelled presence of defects between the fibreglass reinforced epoxy core and the silicone rubber housing in an insulator. Metallic inclusions as well as areas of reduced adhesion were the defects introduced between the core and the housing. The investigations on repaired joints revealed that performance of contaminated surfaces was similar in their vicinity, though mechanical tests indicated that the cured interface had better long-term stability compared to the glued ones. The former would therefore be a more suitable method when repairing damaged insulator sheds in field conditions. The tests on models of the adhesion defects included partial discharges as the stress factor. Infrared spectroscopy and microscopic observations were applied to detect chemical and structural changes appearing during the tests. Signs of material degradation were found on both epoxy and silicone rubber in areas surrounding the discharge spots. Among them was evidence for a build-up of silica-like layer on silicone rubber surface. Further investigations did not confirm a formulated hypothesis that the layer may impede humidity transfer. A standard tracking test was used to investigate interfaces with inserted metallic inclusions. The combination of severe surface contaminations and distorted electrical field distribution by the inclusion resulted in lowered time to track compared to samples without defects. An important final conclusion arising from the whole study is that some interfacial defects, depending on their location and size as well as on the surrounding environment, can significantly influence local electric field distribution in their vicinity, which later may cause diverse electric discharges to take place. This, in turn, may enhance material degradation not only in the vicinity of the defect but also in a larger volume around it, yielding serious consequences for the performance of composite insulators.