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An EPRI White Paper

DC Power Production, Delivery and Utilization

down. And because initial power systems were devoted to lighting loads and systems only generated power at times of high usage, the cost of energy was high—often more than $1 per kWh when adjusted for inflation to present dollars (2005) —compared to an average cost for residential electricity today of 8.58¢ per kWh.15

He demonstrated their ability to both raise and lower voltage by stepping up the 500-volt output of a Siemens generator to 3000-volts, lighting a string of thirty series-connected 100-volt incandescent lamps, and then stepping the voltage back down to 500-volts.

Engineers wanted to Interconnect systems to improve reli- ability and overcome the economic limitations of DC electri- cal systems. If one area’s power were out because of a problem at the generator, then the adjacent town would be available to pick up the load. In addition, by interconnecting isolated systems, a greater diversity of load was obtained, which would improve load factor and enable more economical operation of the generation plants.

Wires were run from his “central” generating station along Main Street in Great Barrington, fastened to the elm trees that lined that thoroughfare.A total of six step-down transformers were located in the basements of some Main Street buildings to lower the distribution to 100-volts.A total of twenty business establishments were then lighted using incandescent lamps.

Stanley’s demonstration of raising the generator voltage to

Another major driver was the desire to make use of hydroelec- tric power sources located far from urban load centers, which made long distance transmission essential, and therefore made alternating current essential.

Figure 24. The first AC generation plant,Ames, Colorado

Transformers transform the power delivery system

By using transformers, the voltage can be stepped up to high levels so that electricity can be distributed over long distances at low currents, and hence with low losses.

Transformers that could efficiently adjust voltage levels in different parts of the system and help minimize the inherent power losses associated with long-distance distribution were a critical enabling technology that led to today’s AC-dominated power distribution system. Transformers do not work with DC power.

Effective transformers were first demonstrated in 1886 by William Stanley of the Westinghouse company. According to the Institute of Electrical and Electronics Engineers (IEEE) History Center,16 Stanley first demonstrated the potential of transformers to enable AC transmission at Main Street in Great Barrington, Massachusetts:

Electricity produced here in the spring of 1891 was transmitted 2.6 miles over rugged and at times inaccessible terrain to provide power for operating the motor-driven mill at the Gold King Mine. This pio- neering demonstration of the practical value of transmitting electrical power was a significant precedent in the United States for much larger plants at Niagara Falls (in 1895) and elsewhere. Electricity at Ames was generated at 3000 volts, 133 Hertz, single-phase AC, by a 100-hp Westinghouse alternator.

Photo courtesy of The Smithsonian

June 2006

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