HVDC Systems Gotland: the HVDC pioneer

Fifty years ago, the first ever commercial HVDC link was commissioned on Gotland. PEi visited this Swedish island to trace the history of the technology, and to find out what the next step in its development will be.

Siàƒ¢n Green, Managing Editor

In 1950, high energy prices on the Swedish island of Gotland were affecting the local economy and resulting in depopulation and unemployment. In order to reverse this trend and boost the island’s industrial sector, the Swedish government decided to finance a transmission link from Gotland to the mainland.

This decision resulted in the commissioning of the world’s first commercial high voltage direct current (HVDC) transmission link in 1954. Fifty years on, HVDC technology has evolved considerably and plays an important role in electricity networks all over the world.

That first link between Gotland and the Swedish mainland was a 20 MW, 150 kV link. Around 70 000 MW of HVDC transmission capacity is now installed around the world in more than 90 projects.

Figure 1. HVDC technology can help to lower the cost of energy and improve grid reliability
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A growing market

Developments in the world’s power markets over the last ten years ” from the first steps towards liberalization to the blackouts in North America and Europe in 2003 ” together with rapidly growing power demand in countries such as China and India, are converging to create a strong, diverse and growing market for HVDC technology.

In today’s increasingly competitive power markets, cost, reliability and the environment are high priorities. As was achieved with the first Gotland link, HVDC technology can help to lower the cost of energy by interconnecting two neighbouring networks and allowing them to share resources. Such interconnections also help to improve grid reliability and prevent blackouts. HVDC can also be used where efficient transmission over long distances is required as power losses from DC lines are much lower compared with those from conventional AC (alternating current) lines. Such ‘bulk transmission’ applications have been used in China, Australia and Brazil.

HVDC is also being used to support the growth of renewable energy and its integration into electricity grids. It is particularly suited to harnessing wind power generated offshore onto onshore grids, and for connecting offshore oil platforms to mainland grids.

The growth in importance of HVDC is in turn driving further development of the technology. Semiconductor technologies in particular enabled the development of HVDC Light, an ABB-patented technology designed for underwater and underground transmission. HVDC Light has black start capability, enabling it to power up networks that have suffered a complete failure, and has improved stability and reactive power control at each end of the network compared with conventional HVDC technology.

Figure 2. Thyristor valves at Ygne converter station, Gotland
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Pioneering projects

As with the commissioning of the world’s first ever HVDC link, Gotland again played a part in the pioneering development of HVDC Light. In 1999, Gotland became the site of the world’s first HVDC Light transmission system, linking a wind farm on the southern tip of the island to the town of Visby, 70 km further north. This link was the fourth HVDC project to be commissioned on the island.

The submarine cable for the first Gotland HVDC link ” Gotland 1 ” was laid in 1953 between Vàƒ¤stervik on the Swedish mainland and Ygne, Gotland. Cable laying for the 96 km link took just 24 weeks, in 1954 operations began with a rated voltage of 100 kV and a transmission capacity of 20 MW. The link employed mercury arc valves, mass impregnated paper insulated cables, and a vacuum tube-based control system.

Although the development of mercury arc valves was essential to the success of the first HVDC link, the technology had limitations. The invention of the thyristor in 1957 presented new opportunities, however, and in 1967 one of the mercury arc valves at Gotland was replaced with a thyristor valve. After a year-long trial, a complete thyristor valve group was installed in each converter station, representing the first time that thyristor valves were used in a commercial HVDC link. This development allowed the link’s voltage to be increased to 150 kV and its transmission capacity to 30 MW.

Development of thyristor technology continued throughout the 1970s and 1980s, enabling converter stations to be simplified. Second-generation, water-cooled thyristors were developed, and used in the second Gotland-mainland HVDC link, for which a new cable was laid between Vàƒ¤stervik and Ygne. The 150 kV, 130 MW Gotland 2 link was commissioned in 1983, and was the first in the world to feature a fully redundant digital control and protection system and gas insulated switchgear (GIS) for HVDC.

Gotland 1 and Gotland 2 operated independently of each other and between them could meet all of the island’s power needs.

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). Gotland 3 was commissioned in 1987, while the original cable and terminal equipment for Gotland 1 was taken out of service.

Figure 3. Mercury arc valve at Ygne, Gotland
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The lighter alternative

Development of HVDC components continued into the 1990s, with a particular aim being to improve the level of control over the current through the thyristors and further develop the cable technology. ABB realised that in industrial drives, voltage source

convertors (VSCs) with semiconductors that can be switched off as well as on, had already replaced thyristors, and could be applied to transmission systems. The company therefore concentrated its development work on VSCs to develop HVDC Light. In 1997, the world’s first VSC HVDC transmission system began transmitting power between Hellsjàƒ¶n and Gràƒ¤ngesberg in Sweden.

Due to its ability to overcome power quality problems, HVDC Light is particularly suited to applications where the infeed of small scale or renewable generation to the grid is required. For this reason, it was chosen to link a wind farm on the southern tip of Gotland to the town of Visby and this project, commissioned in 1999, was the world’s first HVDC Light transmission system installation.

The 50 MW, 80 kV Gotland HVDC Light link consists of VSCs located at the terminal stations and two extruded HVDC Light cables. The use of VSCs in combination with underground DC cables allows rapid control of both active and reactive power, keeping the AC voltage stable in the surrounding AC network. This ensures a high level of power quality in spite of the intermittent nature of wind power generation. HVDC Light cables are made of polymeric material and are lightweight and oil-free.

New developments

Conventional HVDC technology is proving its worth in projects such as the Three Gorges links in China, which evacuate power to major load centres in Guangdong and Shanghai. HVDC Light has been used in projects such as the Cross Sound Cable link in the USA, which was instrumental in restoring power quickly to consumers after the blackout of August 2003.

In the USA, expenditure on transmission systems is expected to double following the August 2003 blackout, and $12 billion is needed just to relieve major bottlenecks in the networks, according to FERC. Cross-border interconnection projects are also likely to increase in Europe in order to help meet the goals of increasing renewable generation and grid reliability while keeping costs low. In China, annual investments of $10 billion in transmission systems are being made.

This increased demand for advanced transmission systems is driving continued development of HVDC technology. According to Peter Smits, head of ABB Power Technologies, the focus of their research is on increasing the voltage capabilities of HVDC transmission systems. Most links are 500 kV, although 600 kV has been used in Brazil, and for the very long distance transmission projects ABB is aiming for 700-800 kV, says Smits. This, says Georg Schett, head of technology, ABB Power Technologies, would enable the length of bulk power transmission lines to reach 2000 km and would particularly suit projects in China and India.

However, conventional HVDC is a mature technology with little potential for further development, says Schett. “We can make certain improvements to classical HVDC but we generally expect that it will stay the same for years to come,” he notes. HVDC Light, on the other hand, has considerable potential for further development. Its main disadvantage is that produces a relatively high level of losses compared to conventional HVDC, and this means that it is not suitable for long distance bulk transmission projects. To overcome this, ABB is working towards improving the performance of the converters, predominantly by improving the transistor technology and the converter layout.

In addition, ABB is aiming to increase the level of power that can be transmitted by HVDC Light systems by increasing the voltage levels. This, says Schett, is a long term development plan, with an an initial goal of reaching 300 kV.

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