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Abstract-Increased renewable energy integration and international power trades have led to the construction and development of new HVDC transmission systems. HVDC cables, in particular, play an important role in undersea power transmission and offshore renewable energy integration having lower losses and higher reliability. In this paper, the current commercial feasibility of HVDC cables and the development of different types of HVDC cables and accessories are reviewed. The non-uniform electric field distribution caused by the applied voltage, temperature dependent conductivity, and space charge accumulation is briefly discussed. Current research in HVDC cable for higher operation voltage level and larger power capacity is also reviewed with specific focus on the methodologies of space charge suppression for XLPE extruded cables.

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Memory effects on polypropylene systems with differents amounts of a nucleating agent (Dybenzylidene Sorbitol DBS) have been studied by Wide Angle X ray scattering methods. It has been observed that deformation applied in polypropylene melts, where even small amounts of DBS are included, is templated into the crystalline state. After crystallising a sheared polypropylene/DBS melt, an anysotropic texture is observed by X ray scattering, whereas the crystals produced from sheared pure PP melts are randomly distributed. This fact is directly addressed how the DBS is spread into the polypropylene melts. Different concentration and deformation conditions are explored allowing to conclude that, below a certain temperature, the DBS self-organize into a three dimensional network producing the gelation of the PP melt. When the deformation is applied in this gel state, it is templated into the crystals, whereas when it is applied at temperatures above this self organisation, there is not memory of the deformation process when the sample crystallise.

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The structure and chemistry of two electrical trees (designated Tree A and Tree B) grown in low density polyethylene have been studied by a combination of confocal Raman microprobe spectroscopy, optical microscopy and scanning electron microscopy. Despite being grown under similar conditions (A, 30 °C and 13.5 kV; B, 20 °C and 13.5 kV), these two trees exhibit very different structures. Tree A exhibits a branched structure while Tree B is more bush-like. In Tree A, the very tips of the structure are made up of hollow tubules, which exhibit just the Raman signature of polyethylene. On moving towards the high voltage needle electrode, fluorescent decomposition products are first detected which, subsequently, are replaced by disordered graphitic carbon. From the relative intensity of the graphitic sp2 G and D Raman bands, the constituent graphitic domains are estimated to be ∼4 nm in size, which leads to a local tree channel resistance per unit length of 1–10 Ω µm−1. These structures are therefore sufficiently conducting to prevent local electrical discharge activity. In Tree B, the observed fluorescence increases continuously from the growth tips to the needle. Here, the tree channels are not sufficiently conducting to prevent electrical discharge activity within the body of the tree. These results are discussed in terms of mechanisms of tree growth and, in particular, the chemical processes involved.

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