Electrothermal Characterisation and Advancements of Insulation Systems for HVDC Cable Joints by Simulation and Experiment

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For the ongoing green energy transition, underground high-voltage direct current (HVDC) cable systems are deployed instead of more conventional overhead transmission lines, as they require less space and have less impact on the landscape, leading to greater public acceptance. The development of the HVDC technology is one of the greatest challenges of our time for the HV engineering community. This research proposal supports this endeavour by intertwining experiment and simulation towards a deeper understanding of HVDC equipment, focusing on cable joints as prominent examples. The principle of field grading is of paramount importance for cable joints, as it ensures electric and thermal stability under regular and adverse operating conditions. This project aims at a full characterisation of insulations and field grading systems. The main challenge is to understand and quantify the complex interplay between materials, geometry and fields, in bulk domains and at material interfaces. The project will combine electrothermal field simulation and HV measurement techniques in order to characterise electrothermal field distributions affected by nonlinear field- and temperature-dependent materials, polarisation and charge accumulation. Therefore, existing and newly developed procedures for measuring such insulation materials under relevant electrothermal exposure are applied. The data is converted into behavioural material curves, containing all dependencies and accounting for the occurring uncertainties. Measurement campaigns and transient electrothermal field simulations investigate electric and thermal field effects in cable joints. Furthermore, uncertainty quantification and sensitivity analyses yield an improved understanding of field grading, field deflection, and thermal robustness, being the major principles of HVDC technology. Moreover, the project studies the effect of slow (heating, charge accumulation) and fast transients (polarity reversal, lightning) of HVDC devices, thereby assessing the robustness of cable joints under realistic loads and hazards. Finally, the project demonstrates the developed tool chain by dimensioning a new cable joint including graded materials. By simulation, a large design-parameter space can be explored whereas by measurements on a prototype part, fine-tuning can be carried out, which brings an innovative cable joint design within range. The research will lead to a significantly improved understanding of field grading and HVDC cable joints, paving the way towards a simulation-aided design strategy for decisively robustified 21st century's HVDC equipment.