The results of an investigation into electrical tree growth in XLPE cable insulation using an embedded needle electrode are reported for a range of voltages from 9 kV rms to 27 kV rms. The partial discharge (PD) activity and tree structures were measured simultaneously throughout the tree growth and the trees were recorded from initiation up to and including the final runaway stage. A multifractal analysis was also performed on the tree structures as they propagated, and it was found that their fractal dimension increased and the distribution of embedded structures changed as small side channels were added to the tree as it grew. At 11 kV rms only branch trees were found and only bush (bush-branch) trees at higher voltages, but at 9 kV rms trees of three different shapes were formed. Observation of the tree shapes at 9 kV rms under reflected light followed by a detailed analysis using con-focal Raman spectroscopy, showed that the stagnated and branch-pine (monkey puzzle) tree shapes were due to the formation of a conducting graphitic deposit upon the walls on tree branches in the region of the needle electrode. This was not present in the branch trees produced at 9 kV rms. A simple scheme is presented for the formation of branch-pine trees and their corresponding PD activity based on the concept of conducting branch generation. The trees produced at 13 kV rms and above have a bush shape, which converts into a bush branch shape when a runaway branch grows from their periphery. This is shown to happen when the field at the bush tree periphery exceeded a voltage independent critical value, which was estimated to be 100 MV/m. The consequence of this result for the initiation of the runaway stage in branch trees is commented upon.

This paper reports on investigations aiming to understand the behavior of electrical trees formed in XLPE cable insulation at elevated temperatures. Samples cut from a real high voltage cable equipped with an embedded electrode system and composed of a needle separated by 2 mm from counter electrode were used. The temperature was varied between 50 and 90 °C and the range of voltage changed between 9 and 18 kV. The electrical treeing process was followed by means of microscopic observation from the moment of initiation until the leading branches connected the counter electrode and partial discharge (PD) activity was monitored simultaneously. The shape and fractal dimension of the resulting trees were analyzed. It was found that temperature and voltage level had pronounced effect on the process of electrical tree formation. At lower voltages different type of trees could be formed, whereas at higher voltages branch trees dominated. Increase of test temperature reduced the time for tree initiation, whereas the tree growth time was less affected, except for the lowest voltage level at which the slowest tree development was observed at the highest test temperature. It is postulated that the impeding effect of elevated temperature on tree growth is due to a change of tree channel sidewall conductivity, whereas the observed reduction of tree initiation time with the increase of temperature and voltage level is attributed to lowering of the threshold energy for damage at higher temperature as well as increasing of the injection current at higher voltage level.

The high-temperature superconducting (HTS) ac and dc cables are gradually finding their way toward commercialization due to their ability to transfer bulk electric power with negligible dielectric loss. In this context, this article critically reviews the different aspects of HTS cables used in various applications and related accessories. The operational, laboratory-scale, and prototype HTS cable projects are discussed in detail. Different dielectrics, electrical insulations, superconductors, cooling systems, cryogens, and upcoming projects of HTS cables are reviewed. The underlying challenges and future directions in ac and dc cables, electric aircraft and ships, associated costs, cryogenic insulation materials, cooling technologies, superconductors, and electrical insulations of HTS cables are discussed in detail. The space charge performance of presently used polypropylene laminated paper insulation and future insulation, i.e., Kapton material, is experimentally analyzed, to show the future challenges in the insulation design. The technology readiness level, market trends, and relevant test standards are explored. The research and market trends suggest that HTS cables can replace conventional transmission methods. However, their consistent growth and adaptation of HTS cables will depend on the design of cryocoolers, raw material cost of superconductors, and large-scale production.