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The PhD defence can be accessed through Zoom, and the it will open shortly before 13:15. We would kindly ask you to keep the video off and mute the microphone during the seminar. At the end of the session there will be an opportunity to ask questions through Zoom. In case there will be any updates about the event, these will be posted on this website.
Espen Doedens is an industrial PhD student in the Power grids and components group
Faculty opponent is Docent Severine Le Roy, Laplace Laboratory, University of Toulouse
Examiner is Professor Stanislaw Gubanski from the Power grids and components group
Extruded HVDC cable systems feature a variety of interface types, for which physio-chemical properties will depend on the type of application. Such applications can be joints, terminations or the cable itself, all introducing different material combinations and manufacturing methods. To ensure beyond 40 years of faultless cable system operation, the interface’s design and quality control procedures are essential. Interfacial control requires detailed knowledge on how measurable physio-chemical properties of polymer surfaces relate to their electrical performance, through features such as localized electric field strength, space charge injection and breakdown strength. This work aims to expand such understanding by assessing polymer surfaces created with different, industrialized preparation methods, featuring different degrees of surface roughness. Surface preparation was carried out on real HVDC cable prototypes, from which cable peelings were extracted, ensuring replication of the material’s bulk and interfacial natures into the small-scale tests. Also, DC breakdown tests on medium voltage cables revealed a strong impact of surface roughness, pinpointing the need for accurate roughness evaluation.
While chemical characterization assessed certain features brought about in the preparation, physical assessments such as optical profilometry quantified the surfaces’ topographies. It was found that the topography, featuring micrometer and sub-micrometer geometrical variation, could be readily adopted in a mesoscopic modelling approach. Thereby, the geometric impact on local quantities of field strength, charge density and injection current density was estimated. Also, a set of roughness enhanced charge injection equations were derived for charge injection types such as Schottky, Fowler-Nordheim and hopping injection mechanisms. Such equations, featuring surface-specific field (ß) parameters, were employed in a one-dimensional bipolar charge transport model. Through careful model calibration against the results of space charge measurements, the parameters for roughness enhanced charge injection, together with parameters for charge transport, trapping, detrapping and recombination, were estimated. This calibration verified roughness enhanced injection and generated a description of the density of states in the material’s bulk. Furthermore, DC breakdown tests performed on the cable peelings for establishing the relationship between surface roughness and breakdown strength. An adopted multi-scale simulation approach, based on the calibrated parameter set, estimated local field strength, charge density and other quantities in the surface domain.
Conclusively, surface topography causes a local redistribution of the electric field, in turn locally increasing charge injection due to its strong field dependency at the rough asperities. Ultimately, coinciding high field strength and high charge density, at repeated positions along the surface, yields a lower breakdown strength. Such knowledge allows for tailoring the methodologies of surface preparation and quality control in HVDC cable systems, and other HV apparatuses. Control over mesoscopic surface effects will allow engineers to design ever more advanced and long-lasting HV components, meeting humanity’s renewable energy transmission needs for decades to come.
24 November, 2020, 13:15
24 November, 2020, 16:15