Direct current transmission lines (HVDC)
Introduction
High voltage direct current "High voltage (electricity)") (HVDC or high voltage direct current, in English) is a system for transporting electrical energy used over long distances.
Alternating currents "Alternating current (physical)") are usually used for the transportation and domestic use of electricity, mainly because it can be converted with transformers from one voltage "Voltage (electrical)") to another. Thus, very high voltages are used for electrical transportation and lower, safer voltages for domestic use.
In the world, the high-voltage direct current electric transportation system is booming due to the growing needs for electrical interconnection of islands, offshore wind farms, oil platforms, the electrification of the transportation sector and others.
In Spain, the first submarine direct current line was launched in 2012 between the peninsula and the island of Mallorca (Project Rómulo), with a length of 237 kilometers and the capacity to transport 400 MW of electrical power at 250 kV voltage. The power line reaches a maximum depth of 1,485 meters under the sea. Another of the most notable projects was the construction in 2015 of a new power line between Spain and France to increase interconnection capacity with France. For this 62-kilometer-long underground line, the solution of direct current electricity transmission was also adopted, in this case at 320 kV voltage.[1].
The modern form of HVDC transmission uses technology developed in the 1930s by the Swedish company ASEA. One of the first commercial facilities was created in 1951: it was a 115 km line linking Moscow and Kashira (both in Russia). In 1954 another line of between 10 and 20 MW was created between the island of Gotland and the Swedish peninsula.
Currently, the longest line links the Inga dams with the Shaba copper mines in the Democratic Republic of the Congo.
Comparison with AC systems
Advantages
A long-distance, point-to-point HVDC transmission scheme generally has a lower overall investment cost and fewer losses than an equivalent AC transmission scheme. HVDC conversion equipment at terminal stations is expensive, but the total costs of long-distance DC transmission line are less than those of an AC line of the same distance. HVDC transmission requires less conductor per unit distance than an AC line, since there is no need to support three phases and the skin effect does not appear.