High-voltage direct current

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Bipolar system pylons of the Baltic-Cable-HVDC in Sweden

High-Voltage Direct Current (HVDC) is currently the most efficient and cost-effective system for bulk transmission of electric power over long distances, delivering electricity at low cost and with minimal electrical losses. As such, it is an essential element of the proposed super grid necessary for the realization of the DESERTEC Concept. By using a network of HVDC transmission lines, it is possible to balance supply and demand over long-distances and to create a stable power grid by balancing power from intermittent sources (e.g. wind power and photovoltaics) with dispatchable power (e.g. hydropower and concentrating solar-thermal power). The HVDC technology is proven and available.

Contents

Current status

HVDC systems have been in commercial use for over fifty years and today provide around 140 GW of power transmission capacity in some 145 projects worldwide[1] Until recently, the longest HVDC link in the world was a high-voltage direct current overhead electric power transmission line, with a transmission length of 1700 km in the Democratic Republic of Congo, linking the Inga hydroelectric complex at the mouth of the Congo River to mineral fields in Katanga. Now, China’s thirst for energy is driving further development and refinement of the technology. At the time of writing in July 2011, the longest HVDC line is the 2071 km, 6400 MW link connecting the Xiangjiaba Dam to Shanghai, in the People's Republic of China[2].

Whilst it has become the standard technology for transmitting electricity from remote sources of energy over long-distances, due to the logic of most power networks, HVDC is far less widely-used around the world than the near ubiquitous high-voltage alternating current (HVAC) systems. This is because, until now, most power plants have been situated close to major population centers and over shorter distances AC remains more cost-effective. But now, as a combination of economic and environmental factors makes renewable energy more desirable, it is HVDC that will enable the construction of the necessary supergrids, functioning like a highway system super-imposed on a network of country roads.

Future outlook

There are thousands of megawatts of HVDC lines scheduled to come online in the coming years.

For instance, in Europe the following HVDC interconnection projects are currently under construction: Nord E.ON 1, a 400 MW underground/subsea link to integrate Borkum, the world’s largest offshore wind farm, into the German grid; and Skagerrak 4, a 600 MW subsea cable between Southern Norway and Denmark, due to be commissioned in 2014. According to an article at PennEnergy, future interconnections may be installed between Norway and Germany, another between Norway and the United Kingdom, and even a ‘NorNed 2’ [3]. Beyond Europe, the technology is in increasing use across the world. In 2012, Brazil’s Rio Madeira link connecting the Amazonas to the São Paulo area will become the world’s longest HVDC link with a length of over 2500 km (1600 mi) [4].

Economic perspective

HVDC is a key technology in the DESERTEC Concept as it offers a cost-effective way to transmit large amounts of power over long distances from remote production sites to the centers of demand. With HVDC lines power losses are less than 3% per 1000 km, which is 30-40% lower than with HVAC lines[5]. However, HVDC links require rectifier stations that are considerably more expensive than the transformer stations for HVAC links [6]. Therefore HVDC's advantage of lower losses pays off only for long transmission distances while HVAC is cheaper for short distances. This relationship is illustrated in the graph on the right that compares the price per distance for 2000 MW overhead HVAC and HVDC lines [7].

Comparison of price per transmission distance 2000 MW HVAC and HVDC lines[7]

The German Aerospace Center (DLR) estimates in its TRANS-CSP study that, once capital investment, operating expenses and electrical losses incurred by HVDC transmission lines are factored in, then they would add around EUR 0.01-0.02 per kWh to the price of solar energy transported from the Middle East and North Africa to Europe, while generation costs will be greatly reduced due to the higher solar irradiation in deserts. The DLR also found that by 2020 the costs of an energy mix including the production and transport of concentrating solar-thermal power from North Africa to Europe, would be lower than those of conventional European power production technologies, many of which are subject to rising fuel prices and environmental costs [6].

For sea cable connections, once the transmission distance exceeds 80–90 kilometers, HVDC is more cost-effective and so is the standard solution for linking offshore wind parks far from the coast to the grid [8].

Environmental aspects

Many of the cheapest and most abundant sources of renewable energy are located far away from centers of demand. Long-distance HVDC lines make clean power from such sources accessible and thus enable the reduction of CO2 emissions in the power sector. For example, the Yunnan-Guangdong HVDC link in China brings 5000 MW of hydropower from the remote Yunnan province to the megacities Guangzhou and Shenzhen. According to Siemens "this will reduce annual CO2 emissions by over 30 megatons compared with equivalent fossil-fueled power plants in the power grid of Guangdong Province" [9].

In addition to integrating different types of clean power in a supergrid, HVDC transmission also enables much greater efficiency in energy networks by balancing supply and demand across a wider area. This means that instead of the more expensive option of building new power plants, better management of existing capacity can meet rising demand. This makes economic sense, as well as being good for the environment [10].

Finally, it is worth noting that HVDC transmission can be used with overhead lines underground cables and sea cables. Underground cables face less public opposition than overhead lines.

See also

External links


References

  1. Johnstone, H. 2011. Latest development in HVDC transmission. [ONLINE] Available at: http://www.geni.org/globalenergy/library/technical-articles/transmission/powergenworldwide.com/latest-development-in-hvdc-transmission/index.shtml. [Accessed 06 July 2011].
  2. ABB. 2011. Xiangjiaba - Shanghai ±800 kV UHVDC transmission project. [ONLINE] Available at: http://www.abb.com/industries/ap/db0003db004333/148bff3c00705c5ac125774900517d9d.aspx. [Accessed 06 July 2011].
  3. Johnstone, H. 2008. NorNed: giving Europe’s power trading a welcome boost. [ONLINE] Available at: http://www.pennenergy.com/index/power/display/349195/articles/power-engineering-international/volume-16/issue-10/features/norned-giving-europersquos-power-trading-a-welcome-boost.html. [Accessed 06 July 2011].
  4. ABB. 2011. Rio Madeira. [ONLINE] Available at: http://www.abb.com/industries/ap/db0003db004333/137155e51dd72f1ec125774b004608ca.aspx. [Accessed 06 July 2011].
  5. Siemens. 2011. UHVDC Transmission System: Benefits. [ONLINE] Available at: http://www.energy.siemens.com/hq/en/power-transmission/hvdc/hvdc-ultra/#content=Benefits. [Accessed 06 July 2011].
  6. 6.0 6.1 German Aerospace Center (DLR). 2006. TRANS-CSP Trans-Mediterranean interconnection for Concentrating Solar Power. [ONLINE] Available at: http://www.dlr.de/tt/desktopdefault.aspx/tabid-2885/4422_read-6588/. [Accessed 06 July 2011].
  7. 7.0 7.1 Rudervall, R., Charpentier, J.P., Sharma, R. 2000. High Voltage Direct Current (HVDC) Transmission Systems, Technology Review Paper. [ONLINE] Available at: http://library.abb.com/GLOBAL/SCOT/scot221.nsf/VerityDisplay/9E64DAB39F71129BC1256FDA004F7783/$File/Energyweek00.pdf. [Accessed 06 September 2011].
  8. Simonian, H. 2010. Taking Grids to the Next Level. [ONLINE] Available at: http://www.energy.siemens.com/us/pool/hq/energy-topics/living-energy/issue-3/Living_Energy_Issue3_Aug2010_SuperGrids.pdf. [Accessed 06 July 2011].
  9. ] Transmission&Distribution World. 2010. First 800-kV HVDC Link in China Now Fully Operational. [ONLINE] Available at: http://tdworld.com/overhead_transmission/siemens-china-hvdc-link-0610/. [Accessed 15 September 2011].
  10. ABB. 2011. Why HVDC?. [ONLINE] Available at: http://www.abb.com/industries/db0003db004333/1c28aeb520d854f2c125748100494768.aspx. [Accessed 06 July 2011].
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