The world's longest undersea cable reaches Tasmania

Eucalyptus trees, green pastures, blackberry hedges and thistles dominate the hilly countryside of southeastern Australia. Shy koala bears hide in trees, while curious kangaroos explore a nearby open-pit lignite mine.

White steam rises from the cooling towers of the Loy Yang power plant, where lignite is fired to generate electricity for the Melbourne area some 165 km west of the plant. Since the spring of 2006, that lignite power has been supplemented by a green source of energy produced on the island of Tasmania.

There, the license plates bear the slogan "Your natural state"—which is not surprising, as Tasmania is rich in forests, large ferns, marshes and canyons. What's more, Tasmania, which is around the size of Ireland, covers 90 % of its energy needs from hydropower, and is now providing some of that power to neighboring Victoria. The power is carried by a 290-km undersea interconnector cable 70 m beneath the Bass Strait.

Alternating current (a.c.) was not an option here, as transmission losses would have been too great. Instead, the "Basslink," as the interconnector is known, uses high-voltage direct-current transmission, or HVDC (see Pictures of the Future, Fall 2003,? Power Transmission). "This is the only way to economically transmit large amounts of electricity over great distances," says Erwin Teltsch, an HVDC expert at Siemens Power Transmission and Distribution (PTD). "HVDC begins to pay off when above-ground lines reach a length of 600 km; with undersea cable, the threshold is 60 km."

The cable being used for the Basslink is 15 cm thick. It resurfaces on Ninety Miles Beach in Victoria. There, it runs through a duct under the beach, continues for a few kilometers as an underground cable, and finally emerges as an above-ground line running 70 km to Loy Yang. There, the d.c. is converted into a.c. with the help of power converter valves. "Only then can it be fed into the three-phase power system," says Dr. Günther Wanninger. "On the other side, in George Town, Tasmania, a similar station transforms the a.c. generated there into d.c." Wanninger is an electrical engineer at PTD and head of the Basslink project, for which Siemens supplied the rectifier stations and overhead lines. Consortium partner Prysmian Cables & Systems, a former Pirelli subsidiary, provided the undersea cables.

The interconnector makes it possible to send up to 600 MW of power from Tasmania to Victoria. Transmission works in the opposite direction as well, however, which means Tasmania is able to tap into the continental power grid during dry periods when its rivers do not contain enough water to fill its dams. Another advantage of HVDC systems is that they require only two cables as opposed to the three needed for three-phase current transmission. As a result, an HVDC overhead line also requires less space.

Basslink is not only the world's longest HVDC undersea cable link; it also has several other impressive features. For example, semiconductor elements—thyristors—act as power converters, which are controlled by 10-mW laser flashes via glass fibers. These thyristors, which have a diameter of 100 mm, were produced by Infineon, and are made of silicon, molybdenum and copper. To achieve a d.c. voltage of 400 kV, several dozen thyristors per converter valve are connected in series and suspended from the ceiling of an 18-m-high hall to secure them against earthquakes. All of these thyristors must trigger within 1 µs in order to ensure that none are overloaded or damaged.

Siemens is the only HVDC supplier to use such laser-controlled converters. Conventional technology relies on electrically-triggered thyristors, which require a pulse with a power of several watts. The pulse is generated by a complex electronic system located at each thyristor. "You don't need such a system with the direct light pulse," says Teltsch. "As a result, the control electronics for the thyristor valves requires around 80 % fewer components. That not only saves on space; it also increases reliability."

And there's another benefit for National Grid Australia, which operates the system. "The customer also gets to work with our new Win-TDC control technology," says Wanninger. "This system displays a high degree of integration, which means the hardware takes up less space in the converter station." Whereas the switchgear cabinets used in previous control systems were 20 m long, today's cabinets have a length of only around 10 m. All control, regulation and protective functions are carried out by a Simatic-TDC system that has already proved itself in rolling mills. What's more, the Simatic WinCC visualization system simplifies operation. For example, if the user wishes to change a setting, this can be done easily using the Windows user interface. "The standardized software and hardware platform reduces the number of spare parts needed, but that's not all," says Wanninger. "It also simplifies troubleshooting."

HVDC sea cables are also being used for a similar Siemens project on the other side of the world—but one where curious kangaroos are unlikely to be seen, as the location is in the New York-New Jersey metropolitan area. The project involves an HVDC link between Sayreville, New Jersey and Long Island that will be used for power transmission starting in mid-2007. Siemens is supplying the rectifier stations, and Prysmian is again providing the 105-km-long power cable, through which 750 MW of electricity will flow at a direct voltage of 500 kV. That should certainly be enough power to help Long Island cope with hot summer months.