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An EPRI White Paper

DC Power Production, Delivery and Utilization

AC vs. DC Power:An Historical Perspective

ers, retailers, buyers, and users want to mitigate risk and cost, which requires investment in product development, system integration, professional training—and time.

Edison’s concept for electrification of the U.S.—which includ- ed royalties from his patents on direct current systems—was to deploy relatively small scale, individual DC plants to serve small areas—such as the Pearl Street Station (Figure 23), which powered a part of New York City’s financial district.

Early power systems developed by Thomas Edison generated and delivered direct current (DC). However, DC power systems had many limitations, most notably that power typically could not be practically transmitted beyond a distance of about one mile.

But George Westinghouse’s polyphase alternating current (AC) power system—invented by Nikola Tesla and used with trans- formers developed by William Stanley, Jr., who also worked for Westinghouse—proved to be far superior technically and eco- nomically. The voltage of AC could be stepped up or decreased to enable long distance power transmission and distribution to end-use equipment.

Moreover, because changing the voltage of DC current was extremely inefficient, delivery of power with direct current in Edison’s time meant that separate electric lines had to be in- stalled to supply power to appliances and equipment of dif- ferent voltages, an economically and physically impractical approach. Another limitation was that DC current incurred considerable power losses.

Edison fought vociferously against the use of alternating cur- rent-based systems, which he claimed would be dangerous because of the high voltage at which power would need to be transmitted over long distances.

My personal desire would be to prohibit entirely the use of alter- nating currents.They are unnecessary as they are dangerous.

Figure 23.

  • Thomas Edison, 1889, Scientific American

He even went so far as to demonstrate the danger of AC by us- ing it to electrocute a Coney Island elephant named Topsy who had killed three men. He also electrocuted numerous cats and dogs procured from neighborhood boys. But despite proving that alternating current could be an effective means of electro- cuting these hapless creatures, its superiority to DC for trans- mission and distribution was compelling.

June 2006

Page 27

Edison’s Pearl Street Station entered service on September 4, 1882, serving 85 customers with 400 lamps. This early electric distribution system on direct current power delivery which could not extend over about a mile.

Picture Courtesy of Con Ed

Alternating current can be produced by large generators, and the voltage of alternating current can be stepped up or down for transmission and delivery. The distance limitation of direct current and the difficulties of changing voltages proved criti- cal factors in abandoning DC systems in favor of those based on AC. With DC systems, power had to be generated close to where it was used. This resulted in problematic reliability and economics. If the local plant failed, the entire system was

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