Towards an ultra-high voltage DC cable insulation – Adsorption of Charge Carriers in Polyethylene Nanocomposites
In previous PhD projects (New insulation materials for next generation of high voltage direct current cable)advanced nanocomposites as a new conceptual design of electrical insulation for high voltage direct current (HVDC) cables were evolved. Todays HVDC-cables are limited to the electrical properties of the insulating cross-linked polyethylene (XLPE), and this at a voltage that is not enough to facilitate efficient long-distance transmission (≈20000 km). The incorporation of metal-oxides in XLPE significantly improves the electrical properties, with higher breakdown strength and better space charge distribution. By careful tailoring of the interface of the incorporated nanoparticles, properties of the insulating nanocomposite may be reached that enables an increase in voltage in the HVDC-cables from todays 320 KV to 800 KV.
The previous projects went from A to Z, including synthesis of meta-oxide nanoparticles, surface modification of nanoparticles, compounding of nano-particles with low density polyethylene (LDPE) with thorough electrical and mechanical characterization. The nanoparticles are synthesized in-house to get control over crucial parameters such as; the size, morphology, low concentration of agglomeration, degree of contaminants, reproducibility and a complete knowledge about the particles history. The chosen metal-oxide, Zinc Oxide (ZnO), Magnesium Oxide (MgO) and alumina (Al2O3) are synthesized via wet precipitation; and furthermore the surface of nanoparticles with hydroxyl groups can be made multifunctional with known chemistry. The metal-oxides have different intrinsic properties such as band gap width, resistivity and crystal lattice, introducing different electrical properties to nanocomposites.
Key parameters for HVDC insulation materials, e.g. DC conductivity, are however not well understood for LDPE nanocomposites. In this project the developed LDPE composites will therefore be further characterized for a better understanding of the improved conductivity compared to neat LDPE. The influence of polar molecules, already present in HVDC cable insulation today due to crosslinking of LDPE, on the electrical properties will be investigated. In addition, the presence of water is also important, especially for composites, since the inorganic particles have a large surface area that can adsorb water. Furthermore, new LDPE composites will also be developed to continue the study about important parameters of the synthesized metal oxide nanoparticles for improved electrical properties.