SIEMENS

About 12 m² of transparent pink plastic wrap enshrouds electrical equipment at Siemens that is about to set new records. Dr. Hartmut Huang, who heads the Technology and Innovation Department at Siemens Power Transmission Solutions, lifts the wrap and displays a swimming-pool sized steel frame to which a multitude of multicolored, bucket-sized capacitors, coils and high-power transistors are fitted. "What you’re looking at," he says, "is a completely assembled converter module for high-voltage, direct-current transmission (HVDCT). At present, we assemble hundreds of such modules every year." From Nuremberg, Germany, they are shipped to nearly every continent — to Australia, Asia, the Americas and increasingly also to other European countries. Stacked to the height of a multistory tower, arrays of such modules are used to convert alternating current (AC) from enormous power plants into direct current (DC) for transmission — and at the far end of the line, back to AC.
"HVDCT contributes less severely to climatic change because power losses are much lower than in conventional AC transmission," Huang explains. With DC transmission it therefore becomes worthwhile to even tap into very remote renewable energy sources. Next year in China for instance, HVDCT converters from Siemens will begin to transmit green power from remote hydroelectric plants in the southwest to the large coastal cities over a line spanning 1,400 km (see article "China's River of Power"). "Those converters will consist of 192 modules and handle up to 5,000 MW at 800,000 V DC," Huang reports. That’s a world record." And these are volumes that can help to satisfy the voracious energy hunger of megacities. According to Huang’s information, at about five percent, power losses are expected to be less than half as high as those associated with AC transmission.
The reason for the high power losses in alternating current transmission is the constant reversal of the current flow. This gives rise to electric and magnetic fields around transmission lines that either augment or impede the current flow. The otherwise strictly synchronous current and voltage waves are thrown out of phase, which reduces useful electric power. This effect is referred to as "losses due to reactive power" and is particularly intense in the case of submarine cables. Here, the power line and the ground conductor are located close together, so that strong electric fields are generated between them. Even a cable that is just 80 km long delivers practically no useable power without countermeasures. "HVDCT via submarine cables already pays for itself at distances of 60 km," says Huang. He adds that in overhead transmission lines, in which the ground line and power line are meters apart, reactive power losses are much smaller: "HVDCT then pays for itself starting at distances from 500 to 800 km."

CD-Sized Thyristors. There is no difference between the basic components of converter modules for submarine or overhead lines. Huang explains that the core element of the modules is a high-power thyristor the size of a compact disk, but which weighs about a kilogram. "In each module, about two dozen such thyristors are connected in series," he says. With sophisticated controllers, these chop up the DC in phase with the net frequency, converting it into AC. In the opposite application, they single out the directionally-identical portions of the AC and consolidate them into DC. The thyristors are controlled by laser light. "In HVDCT applications, that’s a worldwide first," Huang says. He adds that the advantages of this configuration are that, "The laser control is much less susceptible to electromagnetic interference than traditional electronic triggering systems. What’s more, it needs much less space." (see Pictures of the Future, Spring 2006, Green Power for Victoria and Testing with Simulation).
But thyristor converters have a drawback. "On the AC side, they draw large quantities of reactive power, which they need to generate electric and magnetic fields, from the grid," Huang explains. Since the direction of these fields continually reverses in phase with the alternating current, without countermeasures reactive currents would flow back and forth between the converter and the grid, thus stressing line voltage. To prevent this, the converters are combined with meter-high capacitor systems. "This causes the reactive currents to flow back and forth between the converter and the capacitor banks without impacting the connected AC grid," says Huang. Such reactive power compensation systems, however, may amount to one-eighth of the whole system’s footprint.

Compact Converters. For about two years now, Siemens has had a solution for this problem as well: a miniature version — about the size of a suitcase — of the standard HVDCT converter. It is named HVDC PLUS. Depending on the power involved, it consists of 2,000 to 3,000 modules. These devices, which are also produced in Nuremberg, are switched not by thyristors, but by commonly used IGBTs (Insulated Gate Bipolar Transistors). "They can be controlled faster and more accurately, and can terminate the current in any module with millisecond accuracy. As a result, they generate nearly perfect current and voltage curves, which are synchronous. What’s more, they do so without the need to include additional elements for reactive power compensation," says Huang (see article "SVC Plus — the Perfect Wave" on the left).
The compact dimensions of HVDC PLUS amount to a significant benefit in large, densely built-up cities. For instance, in San Francisco, a 400-MW HVDC PLUS system is currently under construction, where it will facilitate electric power transmission. But according to Huang, HVDC PLUS systems are also suitable for use on offshore oil platforms and wind parks. These propeller-driven power plants derive special advantages from the superb regulation capabilities of the new technology — which isn’t surprising, given that their power output can vary sharply depending on whether conditions are calm or stormy.
With its Insulated Gate Bipolar Transistor-based converters in the high voltage field, Siemens has achieved a worldwide first, as these devices were previously used mainly in electric motor drives or as inverters for smaller solar systems. However, despite its advantages, HVDC PLUS technology is not likely to replace the well-proven thyristor converter in the short term. "When it comes to maximum buildable power, it still falls short of the classic variant. In addition, it also looses more power during conversion," says Huang.
Both the conventional and the innovative modules are processed on an assembly line before being thoroughly quality-checked in a steel cube test cell, whose interior is completely protected by screens. There, every single module component — including the frame and mounting supports — is tested at up to 100 kV and the resulting current values observed. "That’s our voltage- proof test," Huang explains. Voltage arc-overs are an indication of defective contacts or insulation that needs to be corrected. In extreme cases they are audible as a hum, but experienced testers usually recognize them by the resulting, inappropriate current values.
Once the modules have passed all the tests, they are carefully packaged as ocean freight before being transported to the harbor for shipping. "Every week we ship eight conventional and about 100 HVDC PLUS converter modules out of here. At present, the destinations are an HVDCT line that will connect the U.K. and the Netherlands, and an HDVC PLUS line in the U.S.," says Huang. Due to increasing demand in recent years, the assemblers have had to increase their pace substantially despite the economic crisis.

Firewall for the Power Grid. This trend is expected to continue, not only because of the world’s increasing demand for electric power, but also because systems involving two converters with a short DC buffer stage between them are useful as coupling stations. Such systems can also be used between grids carrying different frequencies — for example, in the U.S. and Japan. "In the future, such stations could also be used within uniform grids to protect individual sections against mains faults, much as a firewall protects a computer against viruses," says Huang. Among other benefits, this would further improve the reliability of electricity supplies in Europe. Due to the uniform nature of the European power grid, it is fairly easy for a network fault to spread, as occurred on November 4, 2006 to produce a blackout. Back then, a disconnected high-tension line across the Ems River combined with excess supply from wind power plants in northern Germany to cause a chain reaction of line overloads and automatic shutdowns that turned off millions of lights in many parts of Europe. The effects were even felt in parts of France and Spain.
"DC coupling stations could at least diminish the spread of such power failures," says Huang. Power at the wrong frequency — in 50-Hz Europe, the maximum permitted variance is only one half of a cycle — would be converted into DC at all coupling stations, which would thus eradicate the fault. At the other end, each such station would transmit reliable power at a stable, specified frequency.
While discussions about the possible use of converter-supported firewalls continue, scientists in Huang’s team are already working on new world records. "We’re in the process of testing even larger thyristors for conventional converter modules," says Huang. "Our aim is to increase their power, and thus also the power of the converter. The researchers are also optimizing the control of IGBTs for HVDC PLUS systems. Their objective here is to reduce the number of modules in a system for a given power level, and thus to make high-power conversion even more economical.