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An EPRI White Paper

DC Power Production, Delivery and Utilization

High-voltage direct current (HVDC) transmission, continued

Figure 2. HVDC thyristor valves

Figure 3. Intermountain HVDC transmission

The Los Angeles Department of Water and Power (LADWP) operates an HVDC transmission system that brings power from a coal-fired plant in Utah to California.

Map and photo courtesy of ABB

Photo courtesy of ABB

HVDC applications

HVDC can be more economical and more reliable for certain long- distance transmission needs, depending on multiple factors, as out- lined in Table 1.3

HVDC can be the most economical option in some cases. Depend- ing on the distances involved, an overhead DC transmission line and towers may be less expensive per unit of length than an overhead AC line. Although DC converter stations are more costly than AC line terminating stations, in some cases transmission lengths as low as 400 miles are sufficient to make the DC system more economi- cal.The “breakeven” distance needed for economical deployment of HVDC may be less when submarine or underground cable trans- mission is involved. In these cases,AC cable transmission can be no longer than about 30 miles, but distances of 180 miles have been achieved with HVDC cables, and systems covering over 700 miles are in planning stages.

HVDC cables may make it easier and more economical to site high- voltage lines.Although power demand in certain areas may increase, installing high-voltage lines, or power plants near load centers, typically meets with public opposition. Using compact HVDC un- derground cables as city infeeds from remote areas may be a means to overcome political problems associated with building such new systems. Moreover, in some circumstances, by upgrading or replac- ing existing AC transmission lines with HVDC, the power transfer capability of existing rights of way may be substantially increased.

Greater power flow control may be afforded to marketers and system operators.Various restructuring and deregulation schemes have forced the power system to operate in ways for which it was not designed.The power system was, in effect,“designed to operate as a private toll road, and is now expected to operate as an open ac- cess highway, handling millions of transactions daily.”4 AC networks do not easily accommodate desired power flow control for today’s system, so power marketers and system operators may require the power flow control capability provided by HVDC transmission.

June 2006

Page 7

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