The birth of commercial HVDC

2014-07-09 - Sweden in the 1950s was ripe for the development of new electrical transmission technology. Electricity consumption was doubling with each passing decade, but the major hydro reserves in the north lay more than 1,000 kilometers from the major load centers in the south.

Power engineers were looking for the most economical, reliable way to transmit electrical power over long distances. This was also true on the Swedish island of Gotland, about 90 km from the mainland in the Baltic Sea.

At the time, Gotland generated all of its own electricity from a single thermal power station located at the town of Slite. But the cost of production was high, and island electricity tariffs were twice as high as those on the Swedish mainland. As a result, by the early 1950s the economy was stagnating. It was expensive to operate industries on the island, so the businesses went elsewhere. Unemployment rose and the threat of depopulation and decline loomed.

Gotland was the only part of Sweden that completely lacked hydro resources, and it was too far out in the Baltic to install an AC connection to the mainland. This made the island a prime candidate for a new kind of long distance transmission technology that had been under development by the Swedish electrical conglomerate, ASEA, since the 1920s.

A key challenge was finding a reliable and economic valve capable of converting high voltage electrical currents from AC to DC, and then back again. Determined to stay in the frontline of electrical innovation, ASEA early on began to develop grid-controlled mercury-arc valves for very high voltage transmission, what we today call high-voltage direct current (HVDC).

The development of this and other equipment required the study of completely unknown fields, where earlier technical experience could only be applied to a limited degree. For a number of years it was an open question whether any solution existed to the challenges. Yet one by one they were overcome, and HVDC technology was proven to work in field tests. Even so, a commercial link had never been attempted.

Fortunately, Gotland was a moderately-sized power consumer, and the owner of the island’s power system was open to new solutions. But even a 20 megawatt (MW) transmission line required major development efforts, including system layout and design, a high-voltage converter valve, main circuit components, control systems and a 100 kilovolt (kV) submarine cable.

Finally, a 20 MW, 100 kV HVDC link was commisioned between Gotland and the Swedish mainland in 1954, the first commercial link of its kind in the world. It was a great success and paved the way for many more HVDC links to be built in different parts of the world. Another advantage that would support the growth of HVDC over the years was its efficiency in overhead, underground and subsea transmission. Moreover, HVDC could also connect AC grids and at the same time, stabilize them.

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