The island of Gotland was originally self-sufficient in terms of electric energy. A thermal power station in Slite catered for the needs of the entire island. However production costs were relatively high and tariffs were double those on the Swedish mainland. It was therefore expensive to run industries on Gotland and there were problems with unemployment and depopulation.
In order to break this trend the Swedish parliament decided in 1950 to finance a transmission link from the mainland to Gotland. This link was designed to satisfy a greater demand for electricity and at the same time reduce the tariffs. The transmission was to use HVDC due to the long distance (90 km) across the sea.
Gotland 1
The submarine cable was laid in 1953 between Västervik on the mainland and Ygne, 10 km south of Visby on Gotland. In 1954 operations began with rated voltage of 100 kV and transmission capacity of 20 MW. It was the first commercial HVDC transmission in the world.
In 1970 the stations were supplemented with thyristor valves which were connected in series with the mercury-arc valves. This made it possible to raise the voltage to 150 kV and the transmission capacity to 30 MW. It was the first time thyristor valves were used in a commercial HVDC transmission in the world.
Gotland 2
In 1983 a new cable was laid between Västervik and Ygne. The rated voltage was 150 kV and transmission capacity 130 MW, and the converters were built up of thyristor valves. Gotland 2 and Gotland 1 operated independently of each other and the links could now meet all of Gotland’s power demands. It also meant that the oil-fired power station in Slite and the diesel sets in Visby could be halted, and function as reserve sources of energy.
Gotland 3
In 1985, Gotland recorded electricity needs of 147 MW, and it was assumed that demand would increase further. In order to meet this increased demand, as well as improving the reliability of supply to the island, a decision was taken to invest in yet another HVDC link, Gotland 3.
Gotland 3 usually works together with Gotland 2 to form a bipolar link but can also work completely independently. The total transmission capacity is thus 260 MW (max. 320 MW). The original cable and terminal equipment for Gotland 1 has been taken out of service and dismantled.
Gotland HVDC Light project
The push for renewable forms of energy has brought wind power plants to southern Gotland. The electrical system on Gotland, owned by Gotlands Energiverk AB (GEAB), normally has no energy production except from wind power. The power is provided from the mainland with frequency regulated by the HVDC link.
During recent years there has been a considerable increase in wind power production on the island. However, the infrastructure built for existing levels of consumption could not absorb the increased production.
This called for additional transmission capacity, as well as for better ways of maintaining good power quality, as the variable operating conditions of wind power plants result in flicker and in variations of reactive power.
Just as important were the great difficulties in getting the necessary permits to build an additional overhead transmission line.
HVDC Light’s ability overcome power quality problems in wind power plants and to transmit power using underground cables, encouraged the local utility GEAB to decide to build the world’s first commercial HVDC Light transmission. GEAB is a subsidiary of Vattenfall AB, which is financing the project together with the Swedish National Energy Administration.
The transmission link between the southern part of Gotland and the city of Visby is rated at 50 MW and became operational in June 1999. Two 70 km-long extruded 80 kV HVDC Light underground cables, laid in ground close to each other, connect the terminal stations. All equipment was mounted in enclosed modules in the factory and was fully factory tested, so that civil works, installation and commissioning were kept to a minimum.
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