Polyethylene Blends, a Material Concept for Future HVDC-cable Insulation

Sammanfattning: High-voltage cables are a critical component of tomorrow’s power grids that seamlessly integrate hydro, wind and solar power. Further improvements in transmission capacity of both high-voltage alternating- and direct-current, HVAC and HVDC, cables are likely reached through improved insulation materials. A number of approaches to improve insulation materials are currently being considered. This thesis explores two concepts for future insulation materials: (i) polymer:metal oxide nanoparticles, and (ii) polymer:polymer blends.

To investigate the former, nanocomposites containing Al2O3 nanoparticles dispersed in a low-density polyethylene (LDPE) matrix are considered. The addition of nanoparticles is found to improve the DC insulation properties, i.e. reduces the residual electrical conductivity, but increases the risk for electrical breakdown under AC conditions. This first part of the thesis leads to the conclusion that the use of nanocomposites can require a trade-off between AC/DC behaviour.

In the second part of this thesis polyethylene blends are investigated as an alternative to nanocomposites. The addition of minute amounts of high-density polyethylene (HDPE) to LDPE reduces the DC electrical conductivity by one order of magnitude. Moreover, trace amounts of HDPE did not appear to influence the dielectric strength under AC conditions.

Besides an improvement in electrical performance polyethylene blends display superior thermo-mechanical properties. Additive-like amounts of HDPE are able to prevent creep above the melting temperature of LDPE, which offers an alternative to crosslinking. It can be anticipated that such thermoplastic insulation instead of commonly used crosslinked polyethylene (XLPE) would considerably ease cable manufacture.

The thermo-mechanical properties are rationalised with a favourable blend microstructure. In particular, complete melt miscibility is found to give rise to a fine distribution of HDPE lamellae that, through tie chains, maintain a continuous network in molten LDPE. The extent of creep correlates with the molecular weight of HDPE.

In summary, this thesis demonstrates that the use of polyethylene blends is a promising avenue, which may lead to insulation materials with improved electrical and mechanical performance.