As well as being a very pleasant place to visit, the island of Gotland (population 58 000) in the Baltic, off Sweden’s east coast, can claim a special place in the history of DC transmission technology.

It was here, in 1954, that the world’s first commercial HVDC link began operation. Some 96 km long, it connected Västervik on the mainland with Ygne on Gotland, 10 km south of Visby (a World Heritage site with medieval city wall, narrow cobbled alleys, and, drawing on personal experience, an excellent brewery).

This first link, Gotland 1, initially rated at 100 kV and 20 MW, was a monopolar system, with the sea used for the return current, and even in those early days extensive testing was done to determine the effects on fish, according to Gunnar Lunberg of Vattenfall, speaking at the 50th anniversary event in May.

The Swedish parliament decided in 1950 to fund a transmission link to the mainland because of concerns about the effects on the island’s economy of the high production costs at the oil-fired power station at Slite, the home of Sweden’s cement industry. At the time this power plant met the energy needs of the entire island.

HVDC was chosen because the long distance across water precluded AC.

The Swedish State Power Board (later Vattenfall) and Swedish electrical engineering company ASEA (the ‘A’ of ABB) had been carrying out joint transmission tests on HVDC since the mid 40s, including work on a trial link between Trollhättan and Mellerud.

A key enabling technology was the high voltage mercury arc valve (rectifier) that ASEA had been developing since 1929. This involved the insertion of intermediate “field-grading” electrodes, connected to an external voltage divider, between the anode and cathode and allowed the valves to operate at voltages many times higher than the rating of devices previously offered commercially.

Following the placing of the order by the Swedish State Power Board in 1950 for the HVDC link, ASEA stepped up development work on the valve technology, high voltage DC cable and other equipment. The basic principles of the technology developed in those four years, mainly at ASEA’s Ludvika site, are still essentially those in use today – although the power electronics has, of course, evolved somewhat.

Building on the success of Gotland 1, ASEA went on to supply a further seven HVDC projects employing mercury arc valves. The second order, placed in 1955, was for a 160 MW HVDC link across the English Channel. Other mercury arc valve based systems included Konti-Skan, linking Sweden and Denmark, allowing surplus Nordic hydro power to be sold to Denmark and Germany, as well as support to the Nordic system as needed. Other links were built between Sardinia and Italy, between the South and North islands of New Zealand and between Vancouver Island and the mainland.

There was also Sakuma, the first HVDC frequency converter, connecting 50 Hz and 60 Hz systems in Japan, to provide emergency support and energy exchange. The Sakuma installation was able to reverse from 300 MW in one direction, to 300 MW in the other, within 0.2s.

The last of the mercury arc valve based HVDC systems was for the ±400 kV 1440 MW Pacific Intertie in the USA, which started operating in 1970.

But by around then the decisive advantages promised by thyristors (originally invented in 1957) for HVDC systems – not least in terms of reliability and maintainability – had become abundantly clear, and, here, once again, Gotland played a pioneering role.

In 1967, one of the mercury arc valves in the Gotland 1 link had been replaced with an HVDC thyristor test valve (50 kV, 200 A, air-cooled), the first such application of thyristors.

After a one year trial Vattenfall ordered a set of thyristor valves for each Gotland 1 converter station. The thyristor valves were connected in series with the mercury arc valves, making it possible to raise the voltage to

150 kV (using the original subsea cable) and the transmission capacity to 30 MW.

The uprated system entered service in 1970, the first use of thyristors in a commercial HVDC link.

Overall, Gotland 1 operated successfully for 28 years, finally being decommissioned in 1986.

In 1983 a second link, Gotland 2, was constructed between Västervik and Ygne, with a rated voltage of 150 kV and a capacity of

130 MW. Keeping up Gotland’s tradition of innovation, this was the first use of second generation, suspended, thyristor valves, redundant digital control and protection and GIS in an HVDC application. It also employed second generation MIND (mass impregnated paper insulated cable).

This second link meant that the Slite power station, together with diesel sets in Visby, no longer needed to operate on a regular basis and could be relegated to reserve capacity.

In 1985, with demand on the island reaching 147 MW and rising, it was decided to invest in yet another HVDC link between Västervik and Ygne, Gotland 3, with a similar rating to Gotland 2, and employing the same technology. Gotland 3, which entered service in 1987, usually operates with Gotland 2 to form a bipolar link but can also work independently.

Meanwhile, from the early 70s onwards, the dramatic simplifications and improvements stemming from the use of thyristors had allowed HVDC to really take off around the world. The first thyristors were air cooled, oil cooling was then used, to be subsequently displaced by water cooling.

Other companies entered the market, but according to figures presented by Peter Smits, head of the Power Technologies Division, at the Gotland 50th anniversary event, ABB has supplied around 40 GW of the world’s HVDC transmission capacity, currently reckoned to be some 70 GW in over 90 projects.

Among ABB’s milestone HVDC projects were: Itaipu, Brazil, which can still claim to be the largest HVDC transmission scheme to date, 6300 MWe, with the highest voltage, ±600 kV DC; the two 3000 MW Chinese links connecting Three Gorges to Changzhou and to Guangdong, which have the world’s most powerful converters (1500 MWe per pole); Murraylink, Australia, which, at 177 km, is believed to be the world’s longest underground cable; and the 250 km, 600 MW, Baltic Cable linking Sweden and Germany, thought to be the world’s longest HVDC underwater connection. An order for a third 3000 MW Three Gorges link, to Shanghai, was placed in June 2004.

While light triggering of thyristors has been tested by ABB at Konti-Skan since 1988, the company elected not to go down that road (unlike other suppliers, notably Siemens). Instead, ABB has remained with electrical triggering, saying “we cannot see any good technical or commercial reasons to change over to a system that can do no more than our present system and which still requires electronics at each thyristor level for protection and monitoring.”

A major innovation in ABB’s “classical”, ie thyristor based, HVDC offering was however introduced in 1995, with the announcement of capacitor commutated converters (CCC). This has been described as “the first fundamental change to have been made to the basic HVDC system technology since 1954” (G Asplund, L Carlsson and O Tollenz, ABB Review, Special report, Power transmission).

The first commercial application of CCC was at the 2200 MW Garabi back to back HVDC station between Brazil and Argentina, which started operation in 2000.

Beyond thyristors

In parallel with this and other improvements to “classical” thyristor based HVDC (which will remain the predominant technology in bulk power transmission for the foreseeable future, albeit at steadily increasing voltages), ABB, around the mid to late 90s, was also working on a new generation of HVDC technology in which thyristors are replaced by voltage source converters. Such a change had already occurred in the area of industrial drives, and developments in this sector have tended to be a good pointer to what lies ahead in HVDC.

ABB focused its attention on VSCs employing insulated gate bipolar transistors and in 1997 the world’s first VSC based HVDC system, called HVDC Light, began transmitting power between Hellsjön and Grängesberg in Sweden.

And now we must return to Gotland once again, because it was here, in June 1999, that the world’s first commercial HVDC Light transmission system, rated at 50 MW, went into operation. It connects wind turbines in the south of the island to the load centre in the Visby area, employing two 70 km long extruded 80 kV HVDC Light underground cables.

The Gotland installation demonstrates many of the virtues claimed for HVDC Light, now available for ratings up to 350 MW, ±150 kV. These include HVDC Light’s ability to allow improvement in stability and reactive power control, and to cope with the intermittency of wind. It also avoided the considerable problems of trying to get permits to build an additional overhead transmission line in Gotland, and, because of its compactness, allowed the use of “building style” terminals.

Other more recent HVDC Light installations have included Murraylink (mentioned above), Troll A (Norway), the first HVDC supply to an offshore oil platform, and the 330 MW, 150kV, Cross Sound Cable between Connecticut and Long Island. ABB notes that during the August 2003 blackout emergency in the USA, energisation of the Cross Sound cable, having been delayed for a year by “political obstacles”, was carried out “overnight”, restoring power “quickly and efficiently to tens of thousands of consumers” and illustrating a further attractive feature of HVDC Light, its “black start” capability, enabling it to power up networks that have suffered a 100% failure.

Where now?

According to presentations during the Gotland event, two main factors favour an increased market share for HVDC in the future:

• security of supply concerns (heightened by the spate of blackouts in 2003) are suggesting that the size of synchronised grids should be limited, with asynchronous grids linked via HVDC back-to-back systems; and

• environmental imperatives, which are requiring utilities to maximise power through each transmission line, to minimise use of space, and to go underground where possible.

Generally, there is a pressing need for grids to be reinforced around the world, and HVDC has a good deal to offer.

In the USA, with its asynchronous networks, for example, HVDC back-to-back systems are very helpful.

In Europe the “Quick Start Programme” has identified about j1.7 billion worth of additional transmission investments which the EU thinks are needed within the next three or four years, the overwhelming majority – around j1.4 billion – to be in HVDC.

But this figure pales into insignificance when we consider the market potential in China, where HVDC might constitute around 10-15% of HV transmission. This market, with India close behind, is likely to play a key role in shaping the business and technology of HVDC over the next fifty years.

ABB and HVDC: a brief history

1929 ABB begins making static converters and mercury arc valves for voltages up to about 1000 V.

1945 ABB, together with the Swedish State Power Board (Vattenfall), sets up a test station at Trollhättan, Sweden. A 50 km power line is also made available.

1950 Swedish State Power Board places an order for equipment for the world’s first HVDC transmission link, between the island of Gotland and the Swedish mainland.

1954 The Gotland HVDC transmission link (20 MW) comes into service.

1955 ABB wins its second HVDC order when the British and French choose HVDC for a power transmission link across the English Channel (160 MW).

1965 Mercury arc valve HVDC projects commissioned: Konti-Skan (250 MW), Sakuma (300 MW) and New Zealand (600 MW).

Start of development work on HVDC valve technology based on thyristors.

1967 First generation HVDC thyristor test valve, 50 kV, 200 A (air cooled) in Gotland link commissioned.

1970 ABB’s last mercury arc valve project commissioned: Pacific Intertie (1440 MW, ±400 kV), USA.

Gotland link is extended with 10 MW, 50 kV thyristor valve bridge.

1973 Second generation thyristor test valve, 135 kV, 1050 A (air cooled) commissioned in Konti-Skan link, Sweden–Denmark.

1976 The world’s first HVDC transmission project with 12-pulse converters, pole 1 of the Skagerrak link, 500 MW, Norway–Denmark, commissioned.

1978 Third generation thyristor test valve, 133 kV, 2000 A (water cooled) commissioned in Pacific Intertie.

First HVDC transformer for 600 kV DC tested.

1979 Contract signed for the world’s largest HVDC transmission system: Itaipu, Brazil, 6300 MW, ± 600 kV.

1982 The world’s first project with 500 kV thyristor valves, the 560 MW Inga-Shaba system, Congo, commissioned. (Also the world’s longest line – 1700 km).

1983 Gotland 2 (130 MW) commissioned, with the world’s first suspended thyristor valves, redundant digital control and protection and GIS for HVDC.

1985 Itaipu bipole 1 (3150 MW, ±600 kV) commissioned.

The 200 MW Highgate back-to-back station commissioned.

Dynamically suspended thyristor valves are used for the first time in a seismically active area, in the Pacific Intertie HVDC upgrade project.

1986 Intermountain transmission, with the world’s most stringent reliability and availability requirements (1920 MW) commissioned.

1987 Itaipu bipole 2 (3150 MW, ±600 kV) commissioned.

The contract for the multi-terminal Quebec–New England transmission (2000 MW ± 500 kV) is signed.

1988 Commissioning of a light triggered thyristor (LTT) test valve, 135 kV, 1050 A in Konti-Skan 1.

1989 New world record for HVDC submarine cables: 400 kV, 500 MW, 200 km for Fenno-Skan.

World’s first HVDC transformers with extended delta commissioned in Vindhyachal, 2×250 MW back-to-back station in India.

1991 The world’s first active DC filter commissioned in Konti-Skan 2.

1992 The world’s first air insulated outdoor thyristor valve, 135 kV, 1050 A, commissioned in Konti-Skan 1.

The first multi-terminal HVDC transmission system, Quebec-New England (2000 MW, ± 500 kV) commissioned.

1993 The world’s first electronically controlled AC filter (ConTune) commissioned in Konti-Skan 2.

1994 New world record for HVDC submarine cables – 450 kV, 600 MW, 250 km – achieved in Baltic Cable, Sweden–Germany.

1995 The CCC (capacitor commutated converter) development is announced.

1997 HVDC Light: the world’s first voltage source converter (VSC) HVDC transmission based on IGBTs is commissioned in Sweden.

1999 The world’s first commercial HVDC Light system (50 MW) commissioned in Gotland, Sweden.

2000 The world’s first HVDC project with CCC commissioned at Garabi, Brazil, 2×550 MW back-to-back.

2002 The world’s longest land cable, the Murraylink HVDC Light transmission system (200 MW, 180 km) commissioned in Australia.

The world’s largest VSC (voltage source converter) HVDC transmission system (Cross Sound, 330 MW) commissioned between Long Island and Connecticut, USA.

Contract signed for the world’s first offshore platform HVDC transmission: Troll HVDC Light (2 x 42 MW), Norway.

2003 The Three Gorges – Changzhou transmission line in China starts operation. It includes the world’s largest HVDC converter, 1500 MW, 500 kV..

2004 The Three Gorges – Guangdong line is commissioned and the Three Gorges – Shanghai link is ordered (both 1500 MW, 500 kV).