Siemens will supply the first transformers with a transmission voltage of 1,100 kilovolts (kV) for a record-breaking power supply line in China. With a length of 3,284 kilometers and a transmission capacity of twelve gigawatts, the new HVDC transmission line between Changji in the northwest of China and Guquan in the east is the largest HVDC project in the world. By way of comparison, a modern nuclear power plant has an output of between one and two gigawatts. The 587 megavolt-ampere capacity of the project’s single-phase transformers from Siemens is also a record. When interconnected, three of these single-phase transformers will provide a capacity of 1761 megavolt-amperes (MVA) for one transmission stage.
Hair-Raising Transmission Voltages
Siemens is supplying transformers for an approximately 3,300-kilometer high-voltage direct-current (HVDC) transmission line designed to carry DC voltages of 1,100 kilovolts. These are the highest transmission voltages ever realized commercially for lines and transformers.
The HVDC link will be operated by the State Grid Corporation of China and is expected to enter service at the end of 2018. Siemens is manufacturing the transformers in its network of factories led by its transformer plant in Nuremberg, Germany, in cooperation with its HVDC transformer factory in Guangzhou, China, as well as with local partners.
HVDC technology reduces the loss of electrical power during transmission over long distances. At the beginning and end of every HVDC line, the direct current is converted into alternating current, and vice-versa, at converter substations. The transformers feed these substations and provide for the transformation of the high current and voltage.
In order to keep losses as low as possible over the extremely long distance of 3,000 kilometers, and to keep the transmission of electricity as efficient as possible, the electric power must be transmitted at the lowest current possible. This can only be done by using a high transmission voltage. The aim is therefore to attain a transmission voltage of ±1100 kV. The higher the transmission voltage, the lower the the transmission losses.
Size and Voltage Control
Siemens occupies a leading position in HVDC technology. In 2008, the company supplied the technology for the world’s first 800 kV HVDC line, likewise in China. For the 1,100-kV project, engineers had to design the transformers in such a way that they could handle the new maximum voltage and power. A few years ago, they used a prototype to demonstrate feeding the high voltages into a transformer.
The challenge here lies in controlling very large electrical fields. To achieve this, the transformer’s insulation had to be redesigned. In order to use conventional insulating materials based on cellulose (the solid materials) and mineral oil (the insulating liquid), the properties of these materials had to be studied in minute detail. An ingeneous arrangement of insulating materials now safeguards the operation and testing of the transformers. The components, such as the bushings that lead the current and voltage into the transformers, had to be adapted to the new requirements too. Another factor that had to be considered was electromagnetic stray fields. These cause losses in frame and tank components. To minimize these losses, Siemens experts have developed a special shielding.
World’s Largest Single-Phase Transformer
The result is the largest single-phase transformer ever built. It is more than 37 meters long, and when completely set up with installed bushings, it is about 13.5 meters high and a good five meters wide. Such a transformer initially weighs 490 tons. But after it has been filled with insulating liquid on-site and all the components, such as bushings, have been installed, it will weigh somewhat over 800 tons. Siemens is currently the only supplier that builds these transformers in such a way that they can be transported to their ultimate location as a single piece of heavy freight.