Transmission and distribution of electricity
Transmission and distribution of electricity
The Finnish electricity system consists of power plants, the main grid, regional networks, distribution networks and electricity consuming equipment.
Electricity generated at power plants is first transferred to the national main grid which has a voltage of 110, 220 or 400 kilovolts (kV). High voltages enable small energy losses. At power stations, electricity is transferred from the main grid to so-called medium voltage, or 20 kV, networks. At distribution substations, the voltage of the electricity obtained from the medium voltage lines is reduced to 400 volts and the power is transferred to customers through low voltage cables. The total length of the lines in the regional and distribution networks is approximately 25 times the length of the main grid lines. There are approximately 140,000 km of medium voltage lines and 220,000 km of low voltage lines.
The power grid in Finland is part of a Nordic system. In addition, there is a direct current (DC) connection from Estonia and Russia to Finland which enables power transmission between the different networks. The Nordic system is similarly connected to Central Europe through a DC system.
The national main grid, or power transmission network, enables the transmission of electricity through high voltage lines to all parts of Finland. The main grid comprises about 14,000 km of power lines and more than a hundred power stations. The main grid is owned by Fingrid Oyj. In addition, local power companies own 110 kV lines.
The maximum nominal voltage in the transmission lines is 400 kV. The other voltages used are 220 and 100 kilovolts. The power lines are either overhead lines suspended from electricity towers or underground cables. The higher the voltage, the higher the towers. Towers for 400 kV lines are always steel. The 110 kV towers are usually wooden, but sometimes the line runs through a location where the height or strength of a wooden tower is not sufficient, in which case steel towers are used. The best way to identify the voltage level is by the length of the insulator string used to suspend the wires and the number of insulating discs on the string. The higher the voltage, the greater the distance of the wires from the ground.
Lengths of power line insulator strings and number of insulating discs Power line voltage (kV)
|Lengths of power line insulator strings and number of insulating discs|
|Power line voltage (kV)||Length of insulator string||Number of insulating discs|
|110||1 metre, approximately||6–8|
|220||2 metres, approximately||10–12|
|400||4 metres, approximately||18–21|
The most common electricity tower type is the guyed portal tower with a cross arm, supported by steel guy wires. In portal towers, the conductors are at the same height from the ground. Free-standing portal towers can be used on arable fields, as they have no guy wires hampering agricultural work close to the tower. Free-standing steel pylons are used in places where space is limited, such as in urban areas. In that case, double circuit towers are used. The conductors of the two different circuits are at different heights from the ground. The picture below shows a guyed portal tower and a free-standing double circuit tower.
Transmission line area
The transmission line area comprises the transmission line corridor and the border zones on both sides of the corridor. The power line owner has a right of way in the transmission line area. The owner has the right to clear the transmission line corridor, restrict construction in the transmission line area, and limit the growth of trees in the border zones. The picture below shows the typical widths of the transmission line corridor and border zones.
Regional and distribution networks
The national grid includes the main 110 kV power lines. The 110 kV lines that do not belong to the national grid constitute the so-called regional network. It can be perceived as an intermediate between the main grid and the distribution network. The total length of all regional networks combined is about 7,500 kilometres. Distribution networks are electrical networks of less than 110 kV, i.e. medium- and low voltage networks owned by network companies. Medium voltage power lines are not suspended from insulator strings. Instead, they are fastened to fixed insulators on the cross arms of the tower. In cities and built-up areas, the distribution networks are usually implemented as underground cables. In rural areas, it is more common to use overhead lines.