China's economy is growing rapidly, as is the corresponding demand for more and more electricity to sustain its glistening megacities. To help the metropolises become cleaner, even more renewable energy will be used in the future. But the favorable conditions needed to generate it are found far away from the urban areas in the east. Instead, they are found in the sparsely populated western part of the country, such as Xinjiang province. There, it is very windy and there are many hours of sunshine, as well as numerous rivers to generate hydropower. The challenge is to transport the energy over great distances to the east while keeping losses at a minimum.
The most powerful transformers in the world for a 1,100 kV HVDC line in China
Siemens supplied 28 transformers for the converter station of a high-voltage direct current transmission (HVDC) project in China. And they included tap changers and composite hollow insulators from Reinhausen.
The most powerful transformers in the world for a 1,100 kV HVDC line in China
Siemens supplied 28 transformers for the converter station of a high-voltage direct current transmission (HVDC) project in China. And they included tap changers and composite hollow insulators from Reinhausen.
Energy transmission with direct current over thousands of kilometers
The solution to this challenge is high-voltage direct-current (HVDC) transmission. To allow electricity to be transported across great distances with the lowest possible losses, the power has to be transmitted with a low current and a high transmission voltage. For long transmission paths, this is most economical with direct current. Therefore, at the beginning of the HVDC transmission line, the alternating current is transformed into direct current in a converter station and then transformed back to alternating current at the end of the line.
There are already numerous HVDC lines in operation around the world, but the line that connects the city of Changji in Xinjiang province with Guquan in Anhui province surpasses all previous records. At 3,284 kilometers long, it has a transmission capacity of twelve gigawatts. This roughly corresponds to the power of twelve conventional large power stations. 1,100 kilovolts of direct-current voltage is required for this transmission. By comparison, the highest transmission capacity to date is eight gigawatts over a distance of 2,210 kilometers with a transmission voltage of 800 kilovolts.
3284 km
length
12 GW
transmission power
12 ≙
conventional large power stations
1100 kV
DC voltage
One converter station; 28 Siemens transformers
The Chinese energy company State Grid Corporation of China commissioned Siemens to design 28 transformers for the converter station at the end of the line in Guquan. The transformers, which were manufactured at two Siemens sites and two partner plants in China, have also set a new record: Each of the 1,100 kilovolt DC single-phase transformers is 37.5 meters long, 14.5 meters high, 12 meters wide and weighs 909 metric tons when in operation. With a transmission capacity of 587.1 megavolt-amperes each, they are the most powerful HVDC transformers in the world. This level of power places enormous demands on the transformers and on the reliability of the technology.
„The vacuum on-load tap-changer VACUTAP® VR® is the most suitable option for the extra-high-voltage range and HVDC applications.“
If the line were to fail, millions of people would be without electricity. When it came to the on-load tap-changers, Siemens therefore used the VACUTAP® VR® from Maschinenfabrik Reinhausen (MR). As Silke Sprissler, Portfolio Manager for tap changers at MR, explains, "The vacuum on-load tap-changer VACUTAP® VR® is the most suitable option for the extra-high-voltage range, and therefore for HVDC applications. It is also very dependable in operation." The VACUTAP® Advanced Arc Control System facilitates the reliable quenching of the switching arc, and the VACUTAP®Step Protection System® offers protection against overvoltages. Another advantage is that the VACUTAP®VR® is extremely economical, as it is capable of performing 300,000 tap-change operations without any maintenance. The diverter switch insert can be used for 1.2 million tap changes before it needs to be replaced. "The converter stations are located in very remote regions, which means that they need to be low-maintenance," explains Schwarzwälder.
The record transformer in numbers
1100 kV
DC single-phase transformers
37.5 m
length
14.5 m
height
12 m
width
909 t
weight
587.1 MVA
transmission power
The longest high-voltage bushings in the world
The vast transformer dimensions also presented other suppliers with enormous challenges. This included the global market leader in high-voltage bushings – HSP Hochspannungsgeräte GmbH from the German city of Cologne. 19 meters long and weighing 15.5 tons, the bushings were produced by HSP for the HVDC transformers in a plant built specifically for the bushings because the existing production hall was too small. For the insulation, HSP used composite hollow insulators from Reinhausen France SAS. Their length of 15 meters was also a challenge in production. Record after record was set, but they will surely be broken for the production and transmission of large amounts of energy around the world in the future!
