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Pictures of the Future


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Pictures of the Future
The Magazine for Research and Innovation

The Future of Energy

Giving the Grid an Advanced Backbone

HVDC PLUS converter modules for 1000 MW in a converter station of the HVDC transmission link between Baixas, France, and Santa Llogaia Spain.

With the introduction of HVDC PLUS transmission technology, Siemens is laying the foundation for a renewable energy-based electricity supply in the coming decades.

Ideally, energy from renewable sources should be used where it is generated, however, that is not always possible. Electricity produced at offshore wind farms, for instance, has to be transported to coastal areas. This usually requires the use of high-voltage direct-current transmission (HVDC), which offers the most efficient solution when cable lengths in excess of 80 kilometers are involved. HVDC systems use a converter station to transform alternating current (AC) generated by offshore wind farms into direct current, after which the direct current is converted back to alternating current on land. For example one of the offshore DC connection in the North Sea provides power with transmission losses less than four percent.

Electricity produced on land by solar power facilities and at wind and hydroelectric power plants also often needs to be transported over long distances to metropolitan areas. For example, new transmission lines with a total length of 3,800 kilometer are now being planned for the transport of power generated mainly by wind farms, from northern Germany to the country’s southern regions. DC is the current of choice for power transmission here as well. “These electricity highways will form the backbone of the energy network of the future,” says Jörg Dorn, Head of Product Development for HVDC converters at Siemens Energy.

Final assembly of an HVDC PLUS transport unit at a Siemens factory in Nuremberg.

That is not surprising, given the fact that power losses in a three-phase 2,500-MW AC transmission system amount to around nine percent over a distance of 800 km. DC transmission can reduce these losses by anywhere from 30 to 50 percent. Since 2010, a Siemens HVDC system in China has been demonstrating how a DC transmission line can be operated successfully. This system, which has a transmission capacity of 5,000 megawatts, supplies the large cities in Guangdong Province with electricity produced 1,400 kilometers away at hydroelectric power plants in Yunnan Province. Instead of using electricity generated with additional coal power plants, Guangdong Province is now producing around 30 million fewer tons of CO2 per year.

Finally, HVDC transmission offers the unique possibility of linking three-phase alternating current grids that are technically incompatible due to differing grid frequencies, for example. It is therefore only logical that Siemens continues to refine HVDC technology. The company is one of the leading manufacturers of HVDC systems, with a market share of approximately 40 percent at the moment.

Thanks to a Siemens HVDC line, Guangdong Province is now producing around 30 million fewer tons of CO2 per year.

Ensuring that Faults Remain Local

Siemens has developed HVDC solutions designed for applications where the use of traditional HVDC technology would require major effort and expense. HVDC PLUS technology, for instance, was used for the first time in 2010 to link a gas-fired power plant in Pittsburg, California, with downtown San Francisco via a cable 85 kilometers long running through the East Bay region.

Shoebox-sized devices are bundled into converter towers like this. Such towers help to stabilize power transmission systems.

HVDC PLUS facilities offer several benefits. For example, unlike conventional HVDC systems, they typically do not require the use of AC filters in the grid. That is because the insulated gate bipolar transistor (IGBT) modules needed for current conversion can be switched on and off so precise and in such an intelligent manner in an HVDC PLUS system that virtually ideal current and voltage waveforms are produced by the conversion process. This eliminates the need for filters and thus saves space, thereby making HVDC PLUS a highly suitable solution for offshore wind farms, for example. Siemens also continuously improves power electronics systems, of course. For example, at the International Council on Large Electric Systems (CIGRE) trade show in August 2016 in Paris, Siemens unveiled IGBT modules that offer performance that is twice as good as previous versions. This means that fewer modules are needed to achieve the required performance, which saves even more space.

If a power plant fails completely, the new technology’s ability to generate the mains voltage reduces the risk of a blackout.

And that is not all, because although conventional HVDC systems require an existing mains voltage to convert from AC to DC, HVDC PLUS facilities can produce this voltage by themselves. This offers a benefit in that if the voltage in the transmission line is interrupted, or if a power plant fails completely, the new technology’s ability to generate the mains voltage enables a “black start,” thereby reducing the risk of a blackout.

What is more, if lightning strikes an overhead line, a new converter generation with full-bridge technology enables the system to launch several restarts within just a few hundred milliseconds, which ensures any faults remain local and do not spread.

Converter hall of the converter station Baixas at the French side of the HVDC transmission link between France and Spain.

Underground or Overhead

Whereas conventional HVDC facilities are the product of more than 40 years of development work, HVDC PLUS technology is still relatively new. However, major steps have already been taken to develop it further. The most powerful HVDC PLUS system at the moment is a 65-kilometer connection between Baixas in France and Santa Llogaia in Spain with two systems that run through two underground cables, each of which has a capacity of 1,000 megawatts. Up until now, HVDC PLUS technology has been used for underground cable links. However, thanks to the new IGBT modules with full-bridge technology, the 340-kilometer Ultranet power link from northern to southern Germany that is scheduled to be completed by 2022 will be able to operate with a flexible and reliable overhead line connection with a capacity of 2,000 megawatts. The Ultranet project is just the initial step towards a future European-wide DC electricity grid based on renewable energy sources that is envisioned for 2050 by the European Network of Transmission System Operators for Electricity. These and other concepts are designed to address energy supply requirements for the coming decades.

Hubertus Breuer