Properties and Long-Term Performance of Internal Interfaces in Composite Insulators

Licentiate thesis, 2006

An important component that secures durability of composite insulators for high voltage applications is the quality of internal interfaces. Weak interfaces, in form of a reduced bonding or a loss of the adhesion between the insulation components, can be devastating for the long-term functionality of an insulator. A failure can take place years after the first signs of the appearance of ageing or weakening of the interfaces. The failure process may be accelerated by moisture ingress or even be initiated by partial discharge activity at the interfaces. The ambitions of manufacturers have therefore been directed towards designing interfaces with good and durable long-term properties that secure very low rate of ageing. However, there is insufficient knowledge about the ageing mechanisms of interfaces in composite insulators and knowledge on how the interfaces affect their durability in a long-term perspective. Moreover there is a lack of standardized ageing tests for evaluating quality of interfaces in composite insulation systems. In this work the aim has been to better understand the effects of ageing on properties and integrity of composite insulation systems containing interfacial defects. This was approached by investigating model interface objects with added artificial defects. Two different types of the objects were studied. In the first study, the inclined plane arrangement was used in a salt fog chamber to test samples produced in a manner that simulated eventual repairs performed on insulator housings of large hollow-core insulators. The interfaces studied were models of material joints obtained by two different techniques. The first type of joint was obtained by means of high temperature vulcanised silicone rubber, which, after the joint formation, was cured. The second type of joint was obtained by means of room temperature vulcanised silicone glue. To evaluate results of the tests, the changes of the tensile strength were monitored. It was found that the best long-term performance was achieved when the repair was carried out with the high temperature vulcanised rubber. The second study concentrated on evaluation of defects exhibiting insufficient adhesion at the interface between silicone rubber and epoxy resin substrate when exposed to partial discharges under dry and wet conditions. The investigations revealed that the epoxy substrate degraded since residues of epoxy resin decomposition were detected. In addition, surface cracking appeared on the silicone rubber facing the interface. This indicated formation of a silica layer on the exposed surface. Tendencies for defect growth, caused by a loss of adhesion outside of the defect area, were also observed, but this trend was not clear enough for drawing final conclusions about the effect.