Surface tracking in the inter-phase region of large transformers

Transmission and distribution operators throughout the world manage significant populations of ageing large transformers which use mineral oil and paper as the dielectric insulation. Many of these transformers are reaching the end of their projected service lives and subject to various end-of-life failures. One failure mode occurs in the inter-phase region which leaves evidence of surface tracking along the barrier boards. The failure mode is thought to occur in three stages but the propagation mechanism is unclear. This project focuses on this failure mode in particular and considers surface tracking on the oil-pressboard interface in general. Interaction between mechanisms forms the overall theme.The thesis reviews the current research into condition monitoring, ageing and internal electrical discharge in large liquid filled transformers and places the research in context with the industrial requirement for asset life extension. The project is experimentally based and resulted in the development of a unique test facility which formed a significant part of the work. The test facility permits high voltage from two independent sources to be applied to a model of the inter-phase barrier under controlled conditions of moisture and temperature.The development of the test facility and a new approach to the study of surface tracking on the oil-pressboard interface are described along with results from experiments conducted on new and service aged pressboard. The experiments have revealed the role of the interfacial layer in the transport of charge across the surface of the oil-pressboard interface. It is found that the interaction between the electric fields from adjacent voltage coils enhances the polarisation of the interfacial layer at the oil-pressboard interface and plays a role in the mechanism for creeping discharge. Finally, the interfacial layer is also the mechanism which decreases the bulk voltage withstand of the liquid dielectric medium across barrier surfaces