Abstract
To design reliable high voltage cables, clean materials with superior insulating properties capable of operating at high electric field levels at elevated temperatures are required. This study aims at the electrical characterization of a byproduct-free crosslinked copolymer blend, which is seen as a promising alternative to conventional peroxide crosslinked polyethylene currently used for high voltage direct current cable insulation. The characterization entails direct current (DC) conductivity, dielectric response and surface potential decay measurements at different temperatures and electric field levels. In order to quantify the insulating performance of the new material, the electrical properties of the copolymer blend are compared with those of two reference materials; i.e., low-density polyethylene (LDPE) and peroxide crosslinked polyethylene (XLPE). It is found that, for electric fields of 10–50 kV/mm and temperatures varying from 30 °C to 70 °C, the DC conductivity of the copolymer blend is in the range of 10−17–10−13 S/m, which is close to the conductivity of crosslinked polyethylene. Furthermore, the loss tangent of the copolymer blend is about three to four times lower than that of crosslinked polyethylene and its magnitude is on the level of 0.01 at 50 °C and 0.12 at 70 °C (measured at 0.1 mHz and 6.66 kV/mm). The apparent conductivity and trap density distributions deduced from surface potential decay measurements also confirmed that the new material has electrical properties at least as good as currently used insulation materials based on XLPE (not byproduct-free). Thus, the proposed byproduct-free crosslinked copolymer blend has a high potential as a prospective insulation medium for extruded high voltage DC cables.
Highlights
High voltage direct current (HVDC) cables play a crucial role in modern power transmission systems allowing for transporting bulk energy over long distances with low transmission losses.In parallel with the development of new HVDC technologies, cable insulations have undergone extensive material development to meet the requirements of increased operating voltage levels, which nowadays reach 640 kV [1,2,3,4]
It was found that the DC conductivity of the copolymer blend is on the same level as the conductivity of XLPE with an about 20% higher thermal activation energy
It was observed that the contribution of surface conduction and charge injection to the apparent conductivity in the copolymer is less or, at least, comparable to that for the other two materials (LDPE and XLPE)
Summary
High voltage direct current (HVDC) cables play a crucial role in modern power transmission systems allowing for transporting bulk energy over long distances with low transmission losses.In parallel with the development of new HVDC technologies, cable insulations have undergone extensive material development to meet the requirements of increased operating voltage levels, which nowadays reach 640 kV [1,2,3,4]. The activities in this area are mainly directed towards improving purity [9,10], adding organic additives/voltage stabilizers [11,12] and nanofillers [13,14,15,16] Other materials such as polyethylene-based nanocomposites [5,17,18], polypropylene (PP) [19,20,21,22], ethylene-propylene rubber (EPR) [23,24] and polymer blends [25,26,27] have received attention as potential substitutes for XLPE. Even though these materials have been found to exhibit some desired properties, e.g., ultralow conductivities [28], in-depth electrical characterization is essential before commercial HVDC applications [1,2] can be considered
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