High-voltage direct current transmission (HVDC)

To date, electric energy has been transmitted from the power plant to the consumer almost exclusively through high-voltage lines carrying alternating current at a frequency of 50 hertz.

High-voltage direct current technology is now also to be used for expanding the power grid in the course of the energy transition. For technical and economic reasons, this technology is preferably utilised for connecting grid interface points far away from each other.

Technical advantages

High-voltage direct current transmission (HVDC) has several technical advantages over the long-established three-phase alternating current technology:

• HVDC lines have lower losses than three-phase alternating current lines.
• A power circuit generally requires only two conductors instead of three.
• Insulation can be designed in a less elaborate manner for the same nominal voltage.

Technical equipment

Just as with alternating current, high-voltage direct current can either be transmitted via overhead lines or underground cables.

In addition to electricity pylons and lines or underground cables, a converter is needed at each end of a high-voltage direct current transmission line route to convert alternating current to direct current and vice versa. The converter station connects a HVDC line to a three-phase alternating current network.

Electric and magnetic fields

Static electric and magnetic fields occur in the vicinity of HVDC lines. The direct current sides of the converters also produce static electric and magnetic fields.

Mainly time-varying alternating fields with the mains frequency of 50 hertz arise around three-phase alternating current connecting lines. Furthermore, electric and magnetic fields at other frequencies can be generated in the converters. Technical equipment (filters) on the three-phase AC and direct current sides ensure that these components are prevented from coming into contact with the connected lines as well as possible.

Field strength levels

The field strengths surrounding the individual technical equipment depend on several constructional and operational parameters and on the distances to the installation. They cannot be generally specified but have to be determined for each particular case.

It is currently assumed that the static magnetic fields of HVDC lines in the immediate vicinity of the transmission line route reach the magnitude of the Earth's natural magnetic field. In Germany, the latter has a flux density of about 45 microteslas. At present, little information is available about the electric field strengths of HVDC overhead lines; however, there is no limit value restriction for them as no direct health effects have been identified for static electric fields.

The electric fields generated by underground cables are shielded by cable insulation and the surrounding soil; only the magnetic field is present at the Earth's surface.

The highest static or low-frequency magnetic fields in converter stations are expected around the incoming and outgoing lines. In the vicinity of three-phase alternating current lines, magnetic alternating fields arise in the same order of magnitude as with other high-voltage lines. The walls of the converter halls shield against the electric fields generated by the particular plant components.

Potential health effects

Biological effects and thus direct health effects of static fields are only known to occur at very high magnetic field strengths. For this reason, no adverse health effects are expected from the low magnetic field strengths in the vicinity of HVDC lines or converters. Weaker magnetic fields might pose an indirect risk as they may exert forces on magnetisable objects and implants. This risk, however, is excluded by the limit value (see below).

The health effects of the low-frequency fields occurring in the vicinity of the converter do not differ from the effects of the fields around alternating current lines.

For high field strength levels of low-frequency fields the stimulation of muscle and nerve cells has been proven. Effects on the nervous system and an increased risk of childhood leukaemia are under scientific discussion. These effects might also occur at lower field strengths. However, a causal relationship with low-frequency magnetic fields has not been established.

Power supply: Limit values for static and low-frequency fields

In order to safely exclude the established health risks, limit values have been set in the 26th Federal Immission Control Ordinance (26th BImSchV) (see Limit values for stationary low-frequency and direct current installations).

The limit value for static magnetic fields has been determined so as to also avoid interference with implants. No limit value has been defined for static electric fields.

Corona discharges

Very high electric field strengths exist adjacent to the surface of the live parts of high-voltage overhead power lines (alternating current or direct current). Electric discharge processes (“corona discharges”) can generate electrically charged air ions and space charges. The active region close to the line where the air ions are generated is commonly referred to as "corona".

In the corona, small amounts of ozone and nitrogen oxides can be formed and pollutants in the air may change their electric state of charge. These substances can be dispersed by the wind. Whether this may have any adverse health effects on humans is the subject of discussion and research (see Further environmental effects of low-frequency fields).

Effects on animals and plants

Many, perhaps even all bird species are able to detect the static Earth’s magnetic field, and to use it to orient themselves. It is possible that the static magnetic fields of HVDC lines can be perceived by birds and that their behaviour is influenced by the fields in close proximity to the lines. The same also applies to species of mammals, such as bats, which orient themselves according to the Earth's magnetic field.

Harm to animals and plants caused by the electric and magnetic fields of high-voltage lines is not known and is also not expected from HVDC lines. However, direct effects of electricity such as electric shocks are possible.