The development of cross-borders lines started earlier than many people think. In Europe it started by 1920, mainly to take advantage of Swiss hydropower. In most of continental Europe, cross-border interconnections took place before the creation national networks. The process of national interconnection slowed down and was restricted to the radial operation of power plants from one country to another.
After the Second World War the process of cross-border lines restarted. In 1949 there were 3 main systems, isolated from each other and operating 220 kV radial systems in France, Spain, Portugal, Italy, Netherlands, Belgium, Germany, Austria and Czechoslovakia. ; Switzerland hydropower was divided between the three blocks.
In 1958 synchronous operation for two blocks was achieved with a common point in Switzerland, moving towards a full synchronous operation in the early 1960s. Former Serbia Montenegro joined in 1975, followed by Greece and Albania in 1985.
In order to assist the process of integration UCPTE was founded in 1951. Later in 1997 UCPTE changed into UCTE and is now the organisation of Transmission System Operators.
For obvious geographical reasons, the Northern part of Europe started the process of integration of their national systems later but it became the most significant model was Nordel, combining international exchange in a free international market. In this relatively small region there is a huge diversity of power sources. In the early 1990s Norway was and still is almost 100% reliant on hydro power, while its neighbour Sweden used hydro power for approximately half of its generation, the rest being split between thermal and nuclear power. Finland’s generation was 60% thermal, with the rest split about evenly between hydro and nuclear power, while Denmark relied on thermal power for 90% of its electricity. In the hydro dependent countries, when water levels were low it made sense for Norway and Sweden to be able to draw on cheap thermal power from Denmark and Finland, and when water levels were high it was equally sensible to sell cheap hydro power to Denmark and Finland. And so Nordel was born, to facilitate the exchange of cheap power across national borders. It involved a change in mind set, from perceiving electricity as a closely guarded strategic national resource, to an international commodity to be bought and sold in the market place