High-voltage direct current (HVDC)-landlines are high-voltage power transmission lines which transmit electric energy via high DC voltage. Static electric fields and static magnetic fields occur as a by-product. However where underground cables are concerned, there is only a static magnetic field, because the static electric field is almost completely shielded by the earth and the cable material. The kind of the magnetic field and the level of the magnetic flux density correspond to those of the geomagnetic field (Natural static fields).
High-voltage direct current power lines are mainly used for the energy transfer for distances from 750 km upwards. An example of a large-scale HVDC landline is the HVDC Québec-New England line between Canada and the US with a total length of 1480 km. In Germany, the so-called Südlink-electricity link is planned. It is a project between the German energy network operators TenneT TSO and TransnetBW to establish an 800 km long HVDC power line in the framework of the “German Netzentwicklungsplan” (energy grid development plan). The aim of the new power line is to transport the current generated by wind plants in the Northern part of Germany to Southern industrial regions.
Ionized air molecules and corona discharge
Charged („ionized“) air molecules and aerosols (i.e. a colloid of fine solid particles or liquid droplets in air) are a separate topic in the discussion of health-related effects of power lines (alternating current and direct current). The air ions are produced in the so-called corona-zone (a region with a partial electrical breakdown in the air) in a distance of several cm away from the transmission line. The ionized air molecules, also called corona ions, occur during normal operation directly at the current-carrying transmission lines via corona discharge processes. Clouds of electrically charged air molecules (space charge clouds) can be drifted sideways by the wind. This effect is remarkably more pronounced with direct current power lines than with alternating current transmission lines, because the continuous charge reversal of the alternating current neutralize the ionized molecules faster. That way, only direct current overhead power lines cause a significant drifting effect. Additionally, air pollutants (e.g. ozone, nitric oxide) can be generated by the formation of free radicals near the corona. Those air pollutants are normally neutralized very quickly by chemical reactions or binding to other contents of the air. Thus, they can’t reach long distances.