100 years of high voltage DC links

Even by 1907, one hundred years ago, the transmission of electric power from distant power plants to large cities was becoming a routine necessity.

Arguments about the pros and cons of alternating current and direct current as the better medium were already exercising the minds of engineers in the young, expanding, and still obscure electric power community. Very early on the advantages of HVDC for long distance power transmission were being recognised: however the means for its implementation were not at that time available.

The use of constant current HVDC transmission was explored in Europe as early as the year 1888. A system had been designed by the Swiss engineer René Thury (photo, right) and is still referred to as the ‘Thury system’. It consisted of several DC generators connected in series to feed a high-voltage transmission line. The loads consisted of DC motors (also connected in series) which could drive machinery directly or drive other types of generators to supply whatever electric power was required locally. The series connection of generators and motors eliminated the need to design these machines to withstand the high transmission voltage. Between the years 1888 and 1912, Thury transmission systems were installed in Italy, Switzerland, Spain, Hungary, Russia and France. An early example was installed in 1889 in Italy by the Society Acquedotto de Ferrari-Gallieri. This system transmitted 630 kW at 14 kV DC over a distance of 120 km, but in other systems transmission voltages as high as 100 000 V with line currents up to 150 A were utilised.

Figure 1. René Thury

The Moutiers – Lyon HVDC transmission line

The heyday of Thury HVDC systems came in 1906 when the largest HVDC transmission line up to that time was installed. It was laid between the power station at Moutiers and the city of Lyon to supply electric power to the town’s railway terminal. Most of the route was covered using overhead lines but in Lyon itself a pair of mass-impregnated HVDC underground cables were laid over a 4 km route.

Information on the route length and the voltage differs among authors, but Monsieur Thury stated that the upgraded system had an operational voltage of up to 125 kV DC. The total route length was more than 180 km. It was convenient that the cable manufacturer, Le Societé des Câbles de Lyon, was situated around the corner. The cables had an 18 mm paper insulation, impregnated with resin-oil and contained within a double lead sheath. The 75 mm2 aluminum conductor allowed the transmission of up to 30 MW in both cables. This cable is believed to be the first truly commercial HVDC cable, and this year marks the 100th anniversary of the date it first went into service.

The insulation thickness of 18 mm is deserving of some comment. Although today we can handle voltages of 300 - 350 kV DC with this same insulation thickness, we should bear in mind that the manufacturers probably had to run the cable several times through the (presumably short) lapping line of those early days to build up 18 mm of insulation. All this without the assistance of sensors, computers and automatic processes – not at all a bad effort. Perhaps the moderate stress of approximately 16 kV/mm was the reason for its long years of trouble-free operation. Electrical engineers of the nineteen-forties would later call this insulation 'rather thick', and opted for operational stresses up to 40 kV/mm

Different sources report different dates for decommissioning of the Lyon transmission line. However, according to all sources the cable remained in trouble-free service well into the thirties, that is, about 30 years. Another Thury cable transmission was installed over the Epine mountain pass. It operated at 150 kV phase-to earth voltage and reportedly worked well despite the very severe steep sections. Unfortunately, detailed information about this cable could not be discovered.

Despite the success of the Thury DC system in the first decades of the last century, AC systems developed rapidly around the world. 220 kV was achieved in the twenties, and 287 kV in 1936 (Boulder Dam, Colorado, USA). Although the advantages of HVDC were not forgotten, the Thury system proved inadequate for higher power ratings, and due to this and to the absence of efficient converter units, HVDC transmission fell into disuse. But not for long.

The rise of AC

Mercury vapour rectifiers and controlled converters were developed for the benefit of electrified railway systems and to supply the power demands of huge radio transmission valves (vacuum tubes) in Germany, Sweden, UK, USA, and other countries. Power engineers realised early on that these converter valves could be used for HVDC transmission, and a number of prototype and demonstration arrangements were constructed.

Some German companies were in the frontline of development and urged state authorities to subsidise a larger commercial HVDC transmission project. After a great deal of lobbying on behalf of a variety of proposals the German Reichsluftfahrtministerium and Reichswirtschaftsministerium abruptly ordered major companies in the country to construct the Elbe-Berlin HVDC transmission scheme.

The Elbe-Berlin Project

The Elbe-Berlin transmission line would bring power from the power plant Vockerode on the river Elbe to the Reich capital. Although the distance was 115 km over land, and raw materials extremely scarce in those days of World War II, the Reich authorities ordered that the line be built as a pair of underground cables. Perhaps it is not so far-fetched an assumption that the government wanted to hide the transmission line from allied bomber planes. The history and properties of the transmission scheme are described in detail by Tröger (Entstehung der 440 kV Gleichstrom-Hochspannungs-Übertragung Elbe-Berlin, ETZ 69, 1948).

Three German cable manufacturers were contracted to supply the HVDC cables, Siemens-Schuckert, AEG, and Felten & Guillaume.

Table 2 lists the cable’s properties, although Menge (ETZ 69 (1948) Vol. 2, p 37-44 and Vol. 3, p 83-89) suggests somewhat different values.

The conductor was made from aluminum, a less precious material in time of war than copper. It had a three-sector conductor core and a second layer of flat wire. To save scarce material the armouring was made from flat steel wires laid with spacing. (Figure 2).

Despite the war, the Elbe-Berlin transmission line was completed in April 1945, but after the defeat of the Nazi regime the cable system was uninstalled as part of war reparation payments. Although the German utilities asked for a short period of test runs the Russian authorities dismantled the entire system immediately and took it away.

Figure 2. Elbe-Berlin cable.

Figure 3. Bipole MI cable for the NorNed link.

Moscow - Kashira

The ‘liberated’ Elbe-Berlin HVDC system was re-installed as the Moscow – Kashira transmission line which served both as industrial power transmission and as an experimental installation for of HVDC research. The cable covered 112 km and was commissioned in 1950. Besides the cable from the Elbe-Berlin line, some cable of Russian manufacture was also used. The system was at first put into partial operation with ±100 kV between the poles, and, later, with 200 kV between one pole and ground. Operating at up to 31 kV/mm the system suffered from many breakdowns, possibly because the cable brought from Germany was damaged during recovery and transport.

The mercury arc

The Germans and Soviets were not the only ones working on the development of HVDC systems with mercury valves. General Electric in the USA and ASEA in Sweden also created small scale experimental HVDC transmissions. However, testing of the valves revealed a number of technical limitations which made them difficult to use for high voltage. At ASEA, Uno Lamm, one of the pioneers of modern HVDC, filed a patent in 1928 that introduced a substantial improvement.

Eventually after almost 20 years of relentless but sometimes frustrating research and experimentation the technology was considered to be sufficiently safe for its first commercial application. The Swedish government decided in 1950 to connect the island of Gotland electrically to the Swedish mainland. The limited transmission power and moderate voltage made this project an ideal starting point.

The submarine cable deployed for the Gotland link featured a solid 90 mm2 copper conductor and 7 mm mass-impregnated paper insulation designed for 100 kV DC. After 16 years of service the cable was upgraded to 150 kV, an amazing increase of 50%.

After more than 30 years of service the cable was recovered and replaced by another submarine link of greater capacity, but the Gotland link nonetheless marked the beginning of modern power transmission with HVDC cables. Encouraged by its superior operational record, utilities all over the world began to appreciate the potential of HVDC cables and the new opportunities they might provide.

Today, more than 50 years later, HVDC cables cross state borders, island straits, and shallow seas. They can connect countries, and may one day cross the oceans to connect continents. The latest contribution is the NorNed link connecting the Netherlands to Norway, an unprecedented cable length of 580 km. This project had been proposed before in 1933 but, as before, the technical means for its implementation were not available. A large part of the distance is covered with a novel two-conductor cable made by ABB (Figure 3). Many features of the 1954 Gotland cable can still be found in the 2005 NorNed cable, but improved and perfected with newly developed materials and modern manufacturing techniques.

Since the Moutiers – Lyon HVDC transmission line went into service in 1907 mass-impregnated HVDC cable has developed into one of the most reliable and longest living components available to power technology. Meanwhile new technologies, for example ABB's HVDC Light, have made their way into the power cable arsenal. 'Light' is made from an extruded insulation providing a cable that is simpler to manufacture and more rugged than mass-impregnated cables. In the last 10 years, more than 1300 km of extruded HVDC Light cables have been installed worldwide. Such cables are not yet available as long submarine connectors operating at 400-500 kV, but it will surely take less than 100 years to get there.