Following successful final acceptance testing at the Siemens transformer factory in Nuremberg, the world’s first 800-kV transformer for extra-high-voltage DC transmission systems HVDC is ready for delivery. The transformer will be used in the “Yunnan-Guangdong” high-voltage DC transmission system (HVDC) in China, one of the two HVDC systems with the highest rated capacity in the world. Siemens will deliver 10 of the 800-kV HVDC transformers for this system, and another ten units with a DC insulation voltage of 600 kV. Starting in mid-2010, this HVDC system will transport 5000 MW of power over 1400 km (870 mi) between Yunnan province in the south west of China and the province of Guangdong in the south of the country.

As a technology leader, Siemens is opening up a new dimension of HVDC technology with the construction of these systems. The system in China will be the first in the world to transport electricity at a DC transmission voltage of +/- 800 kV. As a core component of every HVDC transmission system, the converter transformers must also transform the voltage of the power grid to a voltage of 800 kV. “With this transformer we have taken over the role of technology leader in this industry. The innovations introduced here will benefit the entire product range,” said Udo Niehage, CEO of the Power Transmission Division in the Siemens Energy Sector.

To increase transmission capacity, the operating voltages have been increased to 800 kV, 60 percent higher than the peak DC transmission voltage of 500 kV normal for today’s HVDC transmission systems. Using many innovative solutions based on its many years of experience in the construction of converter transformers, Siemens therefore redesigned the first 800-kV converter transformer in the world in all its essential elements. “Thanks to enormous technical efforts we have been able to increase the efficiency of these high-tech transformers by 20 percent,” said Bertram Ehmann, CEO of the Transformers Business Unit in the Siemens Energy Sector.

The research and development work focused primarily on meeting the exceptionally high standards demanded for the insulation systems of these high-tech transformers operating at an extra-high voltage of 800 kV. For example, because of the necessary insulating clearances in air, the two valve bushings through which the current flows from inside the transformer into the converters are 14 m long. Another important consideration was the factory testing facility, which had to be adapted to handle the huge increase in the voltage necessary to test this new kind of transformer and to be able to perform the final testing.

The first of the 10 converter transformers using 800-kV technology for the HVDC system in China will be quickly followed by others. As part of an additional order, Siemens will also deliver ten extra-high voltage converter transformers, five of them rated at 800 kV, for the converter station on the generator side of the future high-voltage DC transmission link from the Xiangjiaba hydroelectric power station in the south west of China to Shanghai on the country’s east coast. With a transmission capacity of 6400 MW and a length of more than 2000 km (1250 mi), the planned extra-high voltage DC transmission link is currently the longest system of its kind in the world with the highest rated capacity.