Mechanisms of partial discharges in polymeric HVDC power cables

Research Project, 2018 –

Polymeric high voltage direct current (HVDC) cables are used today for various purposes, in particular, for transmitting bulk power generated by renewable energy sources to the grid (e.g., from offshore wind farms to the shore), as interconnections between AC grids, connections to remote generation and loads, etc. High voltage electrical insulation of such cables is usually made of cross-linked polyethylene, which is characterized by high purity, extremely high electric resistivity and low losses. Despite of the excellent dielectric properties of the material, it is however practically impossible to avoid formation of micro-defects (voids, cavities) in it during manufacturing process (extrusion). Therefore certain partial discharge (PD) activity can be always recorded even in a fresh cable. During operation, the intensity of PDs may increase due to high electrical and thermal stresses in the insulation and, in long perspective, lead to accelerated ageing of the material shortening cable’s life time. To propose means for restraining partial discharges in polymeric insulating materials, understanding of mechanisms of their formation and development is crucial. Therefore, the aim of the project is to explore physical processes leading to initiation, growth and relaxation of partial discharges in micro-voids in a highly resistive polymeric material exposed to a strong DC electric field. The study will be conducted by combining computer simulations and experiments, the results of which will be used to validate the computational approach. The developed model will be further utilized for predicting PD characteristics in the material under various electric field levels and temperatures. The results of the study will contribute to the development of a global model of electrical treeing in polymeric high voltage cables initiated with the support of the Chalmers AoA Energy in 2013 (project “Transport of electric charges in polymeric insulation of high voltage cables’’). It is expected that the project will create a basis for an extended application to be submitted to external funding authorities. The industrial stakeholders with potential interest in the research are Borealis (Stenungsund), NKT Cables (Karlskrona), Nexans (Halden).