Pictures of the Future – Power Transmission

The high-voltage alternating-power transmission systems whose towers dot landscapes do the job of connecting regional networks. The voltage alternates its polarity 50 to 60 times per second, just like power from a wall socket, but it is much higher because the losses produced by heating of the conductor climb as increasing amounts of current flow. Because the consumer is only interested in power – the product of voltage and current – these losses can be drastically reduced if the voltage is transformed to a much higher value. The current then sinks accordingly. There is just one problem: Over long stretches, the electrical oscillation phases of current and voltage begin to drift apart, which causes losses in the amount of useable electricity.

Such transmission losses restrict the kinds of energy sources that can be used. In Asia, for instance, major industrial areas are situated far from hydroelectric power reservoirs. And in Europe many people would like to use the solar radiation of the Sahara to produce electricity. But "alternating current networks that stretch over more than 1,000 km are not economical," notes Michail von Dolivo-Dobrowolsky, one of the pioneers of electrical engineering. Furthermore, this problem is even more acute for submarine cables, such as those that connect islands with the mainland. "Such cables, when operated using alternating current, are subject to substantial losses when they reach a length of 60 km," explains Asok Mukherjee from Siemens Power Transmission and Distribution (PTD) in Erlangen.

As a result, many countries are turning to a modern version of a technology that has been around for a long time: high-voltage direct-current transmission, or HVDCT. With HVDCT, direct current flows through a cable (without alternating) just as in a battery. The first electricity transmission in 1882 employed direct current. Back then power was transmitted from the town of Miesbach, Germany to an electricity exhibition in Munich. Sixteen years earlier, Werner von Siemens had built the first dynamo, which marked the birth of power-current technology.

But HVDCT has its price. It doesn't just need two transformers, as alternating current does. Instead, the current has to be rectified at one end of the connection and converted back into alternating current at the other. This is performed by converter valves, which switch through segments of the same polarity in rhythm with the three-phase current, thus converting alternating current into direct current. At the other end, they "chop up" the direct current in synch with the network frequency.

In 1933, Siemens' dynamo plant supplied the first commercially usable mercury-arc rectifier. A 4-MW test facility was subsequently opened in Berlin, and a commercial 60-MW link was built between Vockerode on the Elbe River and Berlin. The test facility was destroyed during the war, and Soviet forces dismantled sections of the other link for use in a test facility outside Moscow.

In Germany, researchers didn't resume work on HVDCT until 1963, when they began experimenting with new silicon-based converter valves. The first big contract came in 1969 when Portuguese engineers began considering how to transmit electricity produced at the Cahora Bassa hydroelectric plant in Mozambique to Johannesburg, South Africa, 1,420 km away. That was quite a feat. Even now, few transmission routes stretch that far.

Deciding on Thyristors. Siemens, which, among others, contributed to the project, suggested a risky approach. Its engineers wanted to use a recently developed semiconductor element, the thyristor, in place of mercury-arc valves. There were two basic reasons for the suggestion. Arc valves were expensive to produce and were not always trouble-free in operation. When control problems occurred, they could even destroy transformers and cable as a result of the huge amounts of energy involved. In fact, that was one reason for the power utility's doubts about high voltage direct current transmission. International experts rejected the plan at first. "They didn't even want to discuss the idea of thyristor valves," says Arnold Hofmann, then general representative for Siemens-Schuckertwerke, in his report. Only after Sweden's ASEA withdrew from the project were Siemens technicians allowed to introduce "their" semiconductor valves.

A total of 48 double valves rose into the sky, outfitted with 48,384 thyristors. Such a large number was necessary because of the relatively low load capacity of thyristors at the time. However, the engineers' boldness paid off. The system worked exceptionally well. Once Cahora Bassa entered service, people no longer wanted mercury-arc based valves for their HVDCT systems. And as the capacity of semiconductors rose, engineers were able to cut the number of thyristors needed. This, in turn, resulted in more HVDCT orders. Thus, in 1984 Siemens received an order from Canada and another in 1987 from the U.S. Today, the company is working on the 3,000-MW Gui-Guang project in China. The project is scheduled to be completed in 2005 and will require only 3,744 thyristors.

Back in the 1980s, HVDCT was an exotic creature for the energy utilities, which were accustomed to using alternating current. But rising energy prices and growing environmental awareness increased pressure to use all types of energy resources, especially hydro power. Since the late '90s, HVDCT has been experiencing a boom. Between 1993 and 2002, Siemens completed seven major projects in Europe, Asia and the U.S., including the 1,800-MW Tianshengqiao to Guangzhou link in China and the East-South Interconnector II in India, which transmits 2,000 MW over a 1,400-km network. In 2001, Northern Ireland was hooked up to Scotland via a 64-km submarine cable, and an HVDCT submarine cable is now being laid between Australia and Tasmania. "HVDCT took more than half a century to evolve from an exotic idea into a reliable product," says Mukherjee. For projects in the 100-million- to 300-million-€ range, HVDCT is now a profitable business, in which Siemens has captured about 40 % of the market.

Since 1995, Siemens has been backing another innovative technology: light-controlled semiconductors. "Our new thyristors are no longer operated using a current pulse," says Hans-Peter Lips, technical director at PTD. "We now use a 10-mW laser flash." Costly electromagnetic screening and control elements are no longer needed. The controller is located well away from the high-voltage section, to which it is connected by a fiber optic cable. "This has allowed us to cut the number of electronic parts in the valve by 80 %," says Lips. The valves are easy to maintain and have an expected life span of more than 30 years, which is why they are used in all of Siemens' new HVDCT facilities. And there's more good news. Transmittable power is expected to rise from today's maximum of 2,000 to 3,000 MW to as much as, 5,000 MW in the near future – if that is what the market requires.

Bernd Schöne