The market for high voltage DC (HVDC) transmission systems has increased dramatically the last few decades. This is mainly due to the economic, electrical, and environmental advantages for bulk power transmission over long distances. Also, in some cases, such as for long subsea cable links, HVDC is the only option. HVDC cables have traditionally used oil-paper for insulation, with the most common type being the mass-impregnated, nondraining cable. The oil or mass-impregnated cables have for a century shown to have a very high reliability and are currently used for voltages and power ratings up to 525 kV and about 1 GW. Today, it is still the preferred technology for the highest voltage levels [1]. However, in the last two decades, extruded polymeric insulation has become more popular, and the highest voltage level currently installed is +320 kV used for grid interconnections and for offshore wind farms. HVDC cable systems using cross-linked polyethylene (XLPE) are available for voltages up to +600 kV and 3 GW and polypropylene based for +600 kV and 3.5 GW.
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