Topographical Impact on Space Charge Injection, Accumulation and Breakdown in Polymeric HVDC Cable Interfaces

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 strong impact of surface roughness, pinpointing the need for an accurate roughness evaluation.\ua0While 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 micro 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 (β) 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.\ua0Conclusively, 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

Measures of functional, real-world communication for aphasia: a critical review

Purpose: Aphasia is a language impairment caused by acquired brain damage such as stroke. For successful rehabilitation, a thorough understanding of naturalistic, real-world communication is imperative, as this is the behaviour speech and language therapy (SLT) ultimately aims to improve. In the field of aphasiology, there currently is a lack of consensus about the way in which communication should be measured. Underlying this is a fundamental lack of agreement over what real-world communication entails and how it should be defined. Method: In this critical review, we review the instruments that are currently used to quantify functional, real-world communication in people with aphasia (PWA). Each measure is checked against a newly proposed, comprehensive, theoretical framework of situated language use, which defines communication as: (1) interactive, (2) multimodal, and (3) based on context (common ground). Results: The instrument that best fits the theoretical definition of situated language use and allows for the quantification of communicative ability is the Scenario Test. Conclusions: This article provides a start in a more systematic and theoretically founded approach to the study and measurement of functional, real-world communication in aphasia. More work is needed to develop an instrument that can quantify communicative ability across different aphasia types and severities

Unlock the doors:Aggressive behaviour and seclusion on closed psychiatric wards

Seclusion of patients on psychiatric wards is a controversial intervention with serious safety issues for patients. Nurses have an important role in the decision to use seclusion when encountering aggressive behaviour. The aim of this thesis is to improve knowledge about the influence of nurses on the incidence of aggressive behaviour and the use of seclusion, in order to use that knowledge to prevent such events in the future. Patients and nurses share views on the factual cause of aggressive behaviour, but their perception of the severity differ. The attitude of nurses towards seclusion changed over the last decades from a therapeutic paradigm to a safety paradigm. In our study, teams with majority of male nurses were associated with less seclusion and aggressive behaviour. Teams with high scores on personality trait openness were associated with less seclusion and teams with high scores on personality trait extraversion were associated with patients’ aggression. There are no golden solutions to improve safety of psychiatric wards, but increased attention of development of nursing skills seems as an important step forward. By providing optimal nursing care, vulnerable patients on psychiatric wards receive the care they deserve

Characterization of Different Interface Types for HVDC Extruded Cable Applications

In extruded HVDC cable systems a variety of interface types can be found, and the interfacial properties depend on the application within the system. Such applications can be joints, terminations or the cable itself and they will all have different material combinations and manufacturing methods. To ensure at least 40 years of faultless operation of the cable system and its interfaces, proper design and quality control is essential. This requires detailed knowledge on how physical, measurable quantities of polymer surfaces relate to electrical characteristics such as electric field and space charge accumulation. This work aims to expand our understanding by assessing polymer surfaces created with different, industrializable preparation methods resulting in varying degree of surface roughness. Breakdown tests, charge decay measurements, optical profilometry and other characterization methods have been performed on cable peelings created from full-sized HVDC cables. It was found that, depending on preparation method, very different roughness levels and textures are created on the polymer surfaces. There were also other impacts of the preparation method found, such as altered surface morphology or the introduction of surface states. A clear relation between surface roughness and the localized field distribution was found, which mainly depended on the arithmetical surface height parameter Sa, but also to a minor degree on the shape of the surface texture. Field dependent bulk conductivity was found to reduce the tail in the surface field distribution histogram, but was not able to fully remove the distribution itself. The surface field distribution showed to have a clear impact on space charge injection, and was capable of creating a field threshold effect. This effect could not be accounted for by a fixed field parameter in the Schottky equation. In this work, charge injection was assumed to be the sum of Schottky and Fowler-Nordheim injection, which resulted in a very significant increase in charge injection above the threshold field. This led to bulk-limited charge injection above the threshold field, along with significant homocharge accumulation. Increased roughness was observed to reduce the breakdown field in the tested samples, which for the larger test objects scaled well with the calculated threshold fields. For the cable peelings, an impact of surface roughness was found as well, but at significantly higher field levels. The origin of this scaling effect is further discussed in this thesis. The threshold fields were also found for rough surfaces in the charge decay measurement. By utilizing rough ground electrodes, significantly higher decay currents were measured above the field thresholds. The increase in decay speed is related to opposite charge injection during the poling phase before the measurement. The findings of this work are believed to be applicable to many high voltage applications, especially in cases where solid insulation with low conductivity is used at high electric fields