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An EPRI White Paper

DC Power Production, Delivery and Utilization

High-voltage direct current (HVDC) transmission, continued

Table 1. Comparison of high voltage AC vs. DC solutions

AC solutions work best when: Lines are short and close to thermal rating Moderate boosts in capacity are sufficient New AC lines can be built quickly and cheaply Dynamic conditions require synchronous ties

Consider conversion to HVDC when: Lines are very long and well below thermal rating Very large boosts in capacity are needed New line construction is costly and time consuming Systems are weekly synchronized and could benefit from segmentation

Source: I Mod, Inc.5

HVDC may be the only feasible means of interconnecting power networks. HVDC can interconnect two separate electrical net- works so power can be exchanged between them.Two AC systems can be connected by installing a DC converter station in each sys- tem, with an interconnecting DC link between them, so it is pos- sible to transfer power even though the AC systems so connected remain asynchronous. For instance, an AC electric power system may not be synchronized to neighboring networks even though the physical distance between them is quite small. This is the case in Japan, which has asynchronous networks: half the country is served by a 60 hertz (Hz) network and the other half by a 50 Hz system. It is physically impossible to connect the two together by direct AC methods—only DC works.

HVDC can be a barrier to cascading blackouts. Perhaps one of the most vivid examples of a benefit of HVDC is related to its effect on grid reliability. HVDC lines cannot be overloaded and power flow can be controlled for grid stability. This was illustrated during the Northeast Blackout of 2003 blackout, when the Quebec/Canada grid was unaffected, since it is interconnected to the neighboring system with HVDC.As a result, more HVDC interconnections be- tween asynchronous networks such as those in eastern and west- ern U.S. and Texas have been advocated.Additional HVDC links are being considered, most notably the TransAmerica Generation Grid (TAGG), a connection of the eastern and western networks.

Figure 4. U.S. grid areas and DC energy bridges

TAGG—TransAmerica Generation Grid

Western Interconnection

DC Links

Powder River Basin,WY

Chicago, IL


Eastern Interconnection


Los Angeles, CA

Power Exchange as of 2004


Western - Eastern Western - ERCOT Eastern - ERCOT

700 MW 200 MW 800 MW

Three separate power systems exist in the U.S. HVDC links in the system enable power exchange, and further extensions of interconnections using DC are being considered. (Based on map from “Role of HVDC and FACTS in Future Power Systems,” CEPSI 2004 Shanghai paper by W. Breuer, et. al.)

June 2006

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