Electricity networks generate electric and magnetic fields
Electricity is transmitted from power plants to consumers through the electricity networks. They consist of various electrical lines and cables, substations and distribution substations, and switchboards. Electricity is transmitted over long distances at high voltages and currents in the main grid and regional networks. Distribution takes place at lower voltages and currents in local distribution networks. Transmission and distribution generate electric and magnetic fields. Since the electric field depends on voltage and the magnetic field depends on current the main grid and regional networks generate higher fields than the distribution networks.
Finland's electricity networks are part of the Nordic electricity system. There are two direct current and alternating current links to Sweden, an alternating current link to Norway and two direct current links to Estonia. Electricity imported from neighbouring countries and generated by power plants is transmitted to throughout Finland in the main grid managed by Fingrid Oyj and in regional networks managed by local electricity network companies. Electricity is transmitted over overhead lines with a voltage of more than 100-kilovolt (1 kilovolt (kV) = 1,000 volts (V)), i.e., power lines, because energy losses in transmission are less at high voltages than at low voltages. In addition to power lines, electricity transmission networks have substations that distribute the transmission over the different lines and can be used also to convert the voltage between the different parts of the electricity network.
The high voltage of power lines is converted at local substations to the 20 kV medium voltage typically used in local distribution networks. The medium voltage is further converted to a low voltage (400 V) suitable for electrical equipment at distribution transformers located near consumers. The low voltage is distributed through the switchboards to the buildings' electricity networks.
Mostly overhead power lines are used in the main grid and regional networks, and as large structures, these power lines are the most visible part of the electricity network in the environment. At the end of 2021, there were approximately 5,200 km of 400 kV overhead lines, approximately 1,400 km of 220 kV overhead lines and approximately 14,750 km of 110 kV overhead lines. In addition, there were approximately 330 km of 110 kV underground cables, which are used in urban areas instead of overhead lines. In the near future, 400 kV underground cables will also be installed in urban areas. The direct current links from Sweden and Estonia have been established with submarine cables and with 400, 450 and 500 kV overhead lines of total length of approximately 60 km in Porvoo and Rauma.
In the distribution networks overhead lines are used in sparsely populated areas and underground cables in urban areas. At the end of 2021, there were approximately 89,000 km of medium-voltage overhead lines and approximately 66,000 km of underground cables. There were 112,000 km of low-voltage overhead lines and 144,000 km of underground cables. This means that the share of underground cabling was 42% in the medium-voltage networks and 56% in the low-voltage networks. To reduce power cuts caused by storms, underground cables are increasingly used in the distribution networks also outside urban areas.
An overhead line generates an electric and magnetic field in its vicinity, but an underground cable generates only a magnetic field. The highest electric and magnetic fields in overhead lines, approximately 10,000 volts per metre (V/m) and 10 microtesla (µT), are under 400 kV power lines. The fields decrease rapidly as the distance from the conductors increases. The electric field is attenuated in trees and bushes and in the structures of houses and does not penetrate into buildings as the magnetic field does. An underground cable generates a higher magnetic field above the ground than an equivalent overhead line but it decreases much faster than the magnetic field of an overhead line as the distance increases. The magnetic field of an underground cable decreases but the magnetic field of an overhead line increases as the height above the ground increases.