CHARACTERISTICS

Most transmission lines use alternating current (AC), where the current direction typically reverses itself 60 times per second. High Voltage Direct Current (HVDC) systems transmit power using direct current, which flows in (only) one direction. 80% of the losses occurring during transmission and distribution are due to resistance, which is inversely related to voltage–therefore, the higher the voltage, the lower the T&D losses. Electrical resistance losses in HVDC systems can be less than half of those in AC transmission lines, making HVDC well-suited for bulk transfer of electricity over large distances.

With lower losses, less electricity generation is required. With lower generation, there is also a reduction in GHG emissions when the transmitted electricity is generated by emitting sources.

Experience with HVDC dates to 1954 when the first HVDC line–with a power rating of 20 MW and 1900 kV over a distance of 96 km–was built in Sweden. Lines are now capable of larger loads–Brazil has a 6,300 MW and 600 kV line that spans 800 km. Current world HVDC capacity is approximately 63 GW, with plans for additional expansion underway. New methods of power generation that generate in direct current (thermoelectric, magnetohydrodynamic, fuel cells) will further improve the attractiveness of HVDC.

SIZE:
20 MW and 100 kV to 6,300 MW and 600 kV.

FEATURES:
Overhead lines can extend more than 800 km. Cables can be strung for more than 40 km. Transmission lines are perpetual, but the lifetime of HVDC components (rectifiers, invertors, thyristors and DC circuit breakers) is about 30 years.

COST:
Total cost of HVDC systems includes conductors, insulators, converters, tower and right-of-way costs. HVDC lines are less expensive than AC, but require converters at each terminal. HVDC is more economical than AC transmission for distances over 500 km for overhead transmission lines; 20-50 km for submarine cables; and 40-100 km for underground cables. These break-even costs do not include any credit for avoided emissions, or for avoided generation costs.

CURRENT USAGE:
In 1993, world HVDC capacity was 58,000 MW.

POTENTIAL USAGE:
An additional 9,000 MW planned (as of 1993).

ISSUES ASSOCIATED WITH IMPLEMENTING ACTION

• There is a lack of industry familiarity with HVDC technology, and environmental criteria are not well defined.
• No DC circuit breaker exists, restricting DC use to point-to-point. Switching or fault clearing cannot be accomplished without total outage of all connected DC lines.
• "Back-to-back" HVDC installations are needed to connect two alternating current systems. Need further development of DC breakers to increase HVDC system flexibility and development of lower cost converters at terminals.
• HVDC does not create an electromagnetic field.
• There is a limited ability to respond to large generator outages or system faults, resulting in potential system instability.

CLIMATE CHANGE IMPACT

EMISSION EFFECT:
    

CONDITIONS FOR EMISSIONS MITIGATION:

• Typical transmission line losses on the order of 7-10% in the United States and other developed countries could be reduced to 3-5%, resulting in a corresponding reduction in GHG-emitting generation demand. Transmission system line losses are often much higher in developing countries, affording the opportunity for even greater reductions.

EMISSION ESTIMATE:
Because DC transmission is more efficient than AC, use of HVDC reduces generation needed and the associated emissions of greenhouse gases.

COST-EFFECTIVENESS:
N/A

SECONDARY EFFECTS:
Use of more efficient DC transmission also minimizes associated emissions avoided from electricity generation.

RESOURCES

• Thallam, R.S. 1993. "High-Voltage Direct-Current Transmission," The Electrical Engineering Handbook, Dorf, R.C. (ed.), CRC Press, Boca Raton, FL (US).
• There is a partnership between the U.S. Department of Energy, (U.S.) Federal Marketing Authorities, the Electric Power Research Institute, several electric utilities and equipment manufacturers to develop and demonstrate HVDC. Projects have been conducted in the following areas: the DC Pacific Intertie, New England/Hydro Quebec Line, and HVDC Lines in the Mid-Continent Area Power Pool.
• HVDC transmission lines are installed from Sardinia-Corsica-Italy, Greece-Crete, Zaire-Egypt, Russia-Finland and Finland-Sweden, Brazil, India, and more.
• An HVDC interconnection project between TNB of Malaysia and EGAT (Thailand) is scheduled to be operational by mid 1999. With the development of the project, the present 132/115 kV AC interconnection will be upgraded in terms of power capacity and controllability which will further enhance system integrity, security and economic interchange between the two parties.
• An example transmission service agreement for the use of an HVDC line can be found on the www at http://www.nees.com/oasis/hvdcts.htm.
• The Electric Power Research Institute (EPRI) has and is developing HVDC support equipment including a revenue meter and an HVDC transmission line reference handbook.

CONTACTS

ABB Power T&D
Henry Chao
Raleigh, NC
Tel: (919) 856-2394
http://www.abb.se/pow/home.htm

Electric Power Research Institute
Mark Wilhelm
Director, Power Delivery Group
Palo Alto, CA
Tel: (650) 855-2771
mwilhelm@epri.com
http://www.epri.com/pdg/trans/

Electricité de France
Alain Le Du
Paris, France
Tel: +33 1 47 65 33 88
Fax: +33 1 47 65 32 51
Alain.Ledu@edfgdf.fr