Extra-high voltage networks are pure transmission networks. In Germany, they transmit extra-high voltages (EHV) of 220 kV and 380 kV over long distances of several hundred kilometers, mostly via overhead lines but also via underground cables. Overhead lines usually transmit alternating current, while underground cables transmit direct current. High-voltage direct current( HVDC) transmission reaches voltage values of 700 kV and 1,100 kV. These ultra-high voltages are referred to as ultra-high voltages( UHV).
High-voltage networks are connected directly to power plants and transmit the electricity via overhead lines to substations, where it is reloaded and fed into high-voltage networks and then into medium-voltage networks for supplying towns and cities.