DC Power: Back to the Future?

By Jonathan Marshall

Readers old enough to remember the great standards war between VHS and Betamax videotapes know that Sony’s loss triggered the eventual decline of that once indomitable electronics giant. By the same token, Microsoft’s clever entrenchment of Windows as the standard operating system for PC software made Bill Gates the richest man in the world.

Nikolai Tesla, above, promoted AC power, while Thomas Edison was the big proponent of DC power.

But the biggest standards war of modern times was the War of Currents. This epic contest at the turn of the 20th century between direct current (DC) and alternating current (AC) pitted the great inventor Thomas Edison against George Westinghouse and his allied genius Nikola Tesla.

The campaign was replete with highly paid lobbyists and public relations stunts — even electrocuting an elephant and a large dog — to convince customers and utilities to adopt one standard or the other.

DC systems were installed in a number of cities, from New York to Helsinki. In downtown San Francisco, hundreds of customers still buy DC power from PG&E to drive their once state-of-the-art winding-drum elevators. PG&E has invested a great deal of money and technology in recent years to upgrade the safety and reliability of its unusual DC grid and better integrate it with the surrounding AC system.

Around the world, engineering trials eventually proved the general superiority of AC. Unlike DC, AC circuits could use inexpensive transformers, with no moving parts, to step voltages up or down. AC transmission lines could operate with low losses at high voltages, then reduce voltages to levels suitable for local use by businesses and households. DC required much more expensive and clumsy equipment to accomplish the same.

A surprising comeback for DC power

Thanks to modern power conversion electronics, however, DC power is having a surprising comeback. Some enthusiasts even predict that it could capture as much as half the grid within a few decades.

The reason? At very high voltages, DC has even lower losses than AC, making it ideal for very long transmission lines—for example, those carrying renewable wind power from the North Sea to continental Europe. One market survey predicts that China will use DC for up to 40 percent of its new transmission lines.

PG&E hosts in its service area one of fewer than two-dozen high-voltage DC transmission lines in the United States. The Trans-Bay Cable is a 53-mile, 200-kilovolt DC line that carries power under the Bay from Pittsburg to San Francisco. Two other DC lines, the 500-kilovolt Pacific DC Intertie and the 500-kilovolt Path 27 from Utah, serve Southern California.

“Historically, DC transmission has usually proven cost effective only over distances of more than 500 miles,” said Bangalore Vijayraghavan, PG&E’s manager of transmission planning. “But as new technology is developed, we may see that breakeven point come down.”

Earlier this month, the large power equipment company ABB, a champion of DC power, claimed a technology breakthrough—a fast and cost-effect circuit-breaker for high-voltage DC circuits.

“This historical breakthrough will make it possible to build the grid of the future,” Joe Hogan, CEO of ABB, said in a statement. “Overlay DC grids will be able to interconnect countries and continents, balance loads and reinforce the existing AC transmission networks.”

Futurists see growing for DC power

Futurists have taken note of the huge number of digital devices that rely on direct current, and the growing number of distributed generators like rooftop solar panels that produce direct current, to suggest that DC will be reborn as a standard.

“Within the next 20 years we could definitely see as much as 50 percent of our total loads be made up of DC consumption,” said Greg Reed, director of the Power & Energy Initiative at the University of Pittsburgh. “It’s accelerating even more than we’d expected.”

The Intermountain DC Intertie brings power to Southern California. The DC power on the right has only two conductors, while the AC lines have three.

A few years ago, researchers at Lawrence Berkeley National Laboratory showed that energy-intensive computer datacenters could achieve power savings of about 10 percent—a huge amount–by feeding server racks with DC power rather than AC.

Today NTT, Japan’s telecommunications giant, has at least five facilities in Tokyo using DC power, and a San-Ramon-based industry group is promoting a new 380-volt DC standard for data centers and commercial buildings.

Email Jonathan Marshall at jonathan.marshall@pge.com.




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