War of Currents
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In the War of Currents era (sometimes, War of the Currents or Battle of Currents) in the late 1880s, George Westinghouse and Thomas Edison became adversaries due to Edison's promotion of direct current (DC) for electric power distribution over alternating current (AC) advocated by several European companies  and Westinghouse Electric based in Pittsburgh, Pennsylvania.
- 1 Background
- 2 Electric power transmission
- 3 Current wars
- 4 See also
- 5 References
- 6 Further reading
- 7 External links
During the initial years of electricity distribution, Edison's direct current was the standard for the United States, and Edison did not want to lose all his patent royalties. Direct current worked well with incandescent lamps, which were the principal load of the day, and with motors. Direct-current systems could be directly used with storage batteries, providing valuable load-leveling and backup power during interruptions of generator operation. Direct-current generators could be easily paralleled, allowing economical operation by using smaller machines during periods of light load and improving reliability. At the introduction of Edison's system, no practical AC motor was available. Edison had invented a meter to allow customers to be billed for energy proportional to consumption, but this meter worked only with direct current. The transformation efficiency of the early open-core bipolar transformers was very low. Early AC systems used series-connected power distribution systems, with the inherent flaw that turning off a single lamp (or the disconnection of other electric device) affected the voltage supplied to all others on the same circuit. The direct current system did not have these drawbacks as of 1882, giving it significant advantages.
Alternating current had first developed in Europe due to the work of Guillaume Duchenne (1850s), Ganz Works (1870s), Sebastian Ziani de Ferranti (1880s), Lucien Gaulard, and Galileo Ferraris.
The prototype of the high efficiency, closed core shunt connection transformer was made by the Hungarian "Z.B.D." team (composed of Károly Zipernowsky, Ottó Bláthy and Miksa Déri) at Ganz Works in the autumn of 1884. The new Z.B.D. transformers were 3.4 times more efficient than the open core bipolar devices of Gaulard and Gibbs. Transformers in use today are designed based on principles discovered by the three engineers. Their patents included another major related innovation: the use of parallel connected (as opposed to series connected) power distribution. Ottó Bláthy also invented the AC electricity meter to complete the competition of AC and DC technology. The reliability of the AC technology received impetus after the Ganz Works electrified a large European metropolis: Rome in 1886.
In North America one of the believers in the new technology was George Westinghouse. Westinghouse was willing to invest in the technology and hired William Stanley, Jr. to work on an AC distribution system using step up and step down transformers of a new design in 1886. After Stanley left Westinghouse, Oliver Shallenberger took control of the AC project. In July 1888, George Westinghouse licensed Nikola Tesla's US patents for a polyphase AC induction motor and transformer designs and hired Tesla for one year to be a consultant at the Westinghouse Electric & Manufacturing Company's Pittsburgh labs. Westinghouse purchased a US patent option on induction motors from Galileo Ferraris in an attempt to own a patent that would supersede Tesla's. But with Tesla's backers getting offers from another capitalist to license Tesla's US patents, Westinghouse concluded that he had to pay the rather substantial amount of money being asked to secure the Tesla license. Westinghouse also acquired other patents for AC transformers from Lucien Gaulard and John Dixon Gibbs.
The "War of Currents" is often personified as Westinghouse vs. Edison. However, the "War of Currents" was much larger than that: It involved both American and European companies whose heavy investments in one current type or the other led them to hope that use of the other type would decline, such that their share of the market for "their" current type would represent greater absolute revenue once the decline of the other current type enabled them to expand their existing distribution networks.[not specific enough to verify]
Electric power transmission
The competing systems
The DC distribution system consisted of generating plants feeding heavy distribution conductors, with customer loads (lighting and motors) tapped off them. The system operated at the same voltage level throughout; for example, 100 volt lamps at the customer's location would be connected to a generator supplying 110 volts, to allow for some voltage drop in the wires between the generator and load. The voltage level was chosen for convenience in lamp manufacture; high-resistance carbon filament lamps could be constructed to withstand 100 volts, and to provide lighting performance economically competitive with gas lighting. At the time it was felt that 100 volts was not likely to present a severe hazard of fatal electric shock.
To save on the cost of copper conductors, a three-wire distribution system was used. The three wires were at +110 volts, 0 volts and −110 volts relative potential. 100-volt lamps could be operated between either the +110 or −110 volt legs of the system and the 0-volt "neutral" conductor, which carried only the unbalanced current between the + and − sources. The resulting three-wire system used less copper wire for a given quantity of electric power transmitted, while still maintaining (relatively) low voltages. However, even with this innovation, the voltage drop due to the resistance of the system conductors was so high that generating plants had to be located within 1.6 km or so of the load. Higher voltages could not so easily be used with the DC system because there was no efficient low-cost technology that would allow reduction of a high transmission voltage to a low utilization voltage.
Since direct current could not easily be converted to higher or lower voltages, separate electrical lines had to be installed to supply power to appliances that used different voltages, for example, lighting and electric motors. This required more wires to lay and maintain, wasting money and introducing unnecessary hazards. These hazards, for example, proved fatal to a number of people in the Great Blizzard of 1888, with their deaths being attributed to collapsing overhead power lines in New York City.
Edison considered the need for many local power plants in the direct current system more democratic. Each locale could build electrical plants to suit its need and would not have to rely on a large monopoly to supply electricity. The proponents of AC counter-argued that building a local plant would be too costly for rural areas, leaving them with no electrical supply at all.
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In the alternating current distribution system power could be transmitted more efficiently over long distances at high voltages, around ten times that of the loads, using lower current. For a given quantity of power transmitted via DC or AC, the wire cross-sectional area is inversely proportional to the voltage used. Alternatively, the allowable length of a circuit, given a wire size and allowable voltage drop, increases approximately with the square of the distribution voltage. With AC current, a transformer is used to down step the (relatively) high voltage to low voltages for use in homes and factories. This had—and still has—the practical significance that fewer, larger generating plants can serve the load in a given area. Large loads, such as industrial motors or converters for electric railway power, can be served by the same distribution network that fed lighting, by using a transformer with a suitable secondary voltage.
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The advantage of AC for distributing power over a distance is due to the ease of changing voltages using a transformer. Available electric power is the product of current × voltage at the load. For a given amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have an almost fixed electrical resistance, some power will be wasted as heat in the wires. This power loss is given by Joule's first law and is proportional to the square of the current. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, high-current, low-voltage transmissions will suffer a much greater power loss than low-current, high-voltage ones. This holds whether DC or AC is used.
Converting DC power from one voltage to another requires a large spinning rotary converter or motor-generator set, which was difficult, expensive, inefficient, and required maintenance, whereas with AC the voltage can be changed with simple and efficient transformers that have no moving parts and require very little maintenance. This was the key to the success of the AC system. Modern transmission grids regularly use AC voltages up to 765,000 volts. Power electronic devices such as the mercury arc valve and thyristor made high-voltage direct current transmission practical by improving the reliability and efficiency of conversion between alternating and direct current, but such technology only became possible on an industrial scale starting in the 1960s.
Alternating-current transmission lines have losses that do not occur with direct current. Due to the skin effect, a conductor will have a higher resistance to alternating current than to direct current; the effect is measurable and of practical significance for large conductors carrying thousands of amperes. The increased resistance due to the skin effect can be offset by changing the shape of conductors from a solid core to a braid of many small (isolated) wires. However, total losses in systems using high-voltage transmission and transformers to reduce the voltage are very much lower than DC transmission at working voltage.
Edison's publicity campaign
Edison carried out a campaign to discourage the use of alternating current, including spreading disinformation on fatal AC accidents, publicly electrocuting animals, and lobbying against the use of AC in state legislatures. Edison directed his technicians, primarily Arthur Kennelly and Harold P. Brown, to preside over several AC-driven killings of animals, primarily stray cats and dogs but also unwanted cattle and horses.  Acting on these directives, they were to demonstrate to the press that alternating current was more dangerous than Edison's system of direct current. He also tried to popularize the term for being electrocuted as being "Westinghoused". Years after DC had lost the "war of the currents," in 1903, his film crew made a movie of the electrocution with high voltage AC, supervised by Edison employees, of Topsy, a Coney Island circus elephant which had recently killed three men.
Edison opposed capital punishment, but his desire to disparage the use of alternating current led to the invention of the electric chair. Harold P. Brown, who was being secretly paid by Edison, built the first electric chair for the state of New York to promote the idea that alternating current was deadlier than DC.
When the chair was first used, on August 6, 1890, the technicians on hand misjudged the voltage needed to kill the condemned prisoner, William Kemmler. The first jolt of electricity was not enough to kill Kemmler, and only left him badly injured. The procedure had to be repeated and a reporter on hand described it as "an awful spectacle, far worse than hanging." George Westinghouse commented: "They would have done better using an axe."
Willamette Falls to Niagara Falls
In 1889, the first long distance transmission of DC electricity in the United States was switched on at Willamette Falls Station, in Oregon City, Oregon. In 1890 a flood destroyed the Willamette Falls DC power station. This unfortunate event paved the way for the first long distance transmission of AC electricity in the world when Willamette Falls Electric company installed experimental AC generators from Westinghouse in 1890. That same year, the Niagara Falls Power Company (NFPC) and its subsidiary Cataract Company formed the International Niagara Commission composed of experts, to analyze proposals to harness Niagara Falls to generate electricity. The commission was led by Sir William Thomson (later Lord Kelvin) and included Eleuthère Mascart from France, William Unwin from England, Coleman Sellers from the US, and Théodore Turrettini from Switzerland. It was backed by entrepreneurs such as J. P. Morgan, Lord Rothschild, and John Jacob Astor IV. Among 19 proposals, they even briefly considered compressed air as a power transmission medium, but preferred electricity. But they could not decide which method would be best overall.
International Electro-Technical Exhibition
The International Electro-Technical Exhibition of 1891 featured the long distance transmission of high-power, three-phase electric current. It was held between 16 May and 19 October on the disused site of the three former “Westbahnhöfe” (Western Railway Stations) in Frankfurt am Main. The exhibition featured the first long distance transmission of high-power, three-phase electric current, which was generated 175 km away at Lauffen am Neckar. It successfully operated motors and lights at the fair.
When the exhibition closed, the power station at Lauffen continued in operation, providing electricity for the administrative capital, Heilbronn, making it the first place to be equipped with three-phase AC power.
Many corporate technical representatives (including E.W. Rice of Thomson-Houston Electric Company, what became General Electric) attended. The technical advisors and representatives were impressed.
AC deployment at Niagara
In 1893, NFPC was finally convinced by George Forbes to award the contract to Westinghouse, and to reject General Electric and Edison's proposal. Work began in 1893 on the Niagara Falls generation project: power was to be generated and transmitted as alternating current, at a frequency of 25 Hz to minimize impedance losses in transmission (changed to 60 Hz in the 1950s).
Some doubted that the system would generate enough electricity to power industry in Buffalo. Tesla was sure it would work, saying that Niagara Falls could power the entire eastern United States. When finished, the first Niagara River hydraulic tunnel would have a capacity to develop 75 MW. None of the previous polyphase alternating current transmission demonstration projects were on that scale of power:
- The Lauffen-Neckar demonstration in 1891 had the capacity of 225 kW
- Westinghouse successfully used AC in the commercial Ames Hydroelectric Generating Plant in 1891 at 75 kW (Single phase)
- The Chicago World's Fair in 1893 exhibited a complete 11,000 kW polyphase generation and distribution system with multiple generators, installed by Westinghouse 
- Almirian Decker designed a three-phase 250 kW AC system at Mill Creek California in 1893 
On November 16, 1896, electrical power was transmitted to industries in Buffalo from the hydroelectric generators at the Edward Dean Adams Station at Niagara Falls. The generators were built by Westinghouse Electric Corporation using Tesla's AC system patent. The nameplates on the generators bore Tesla's name. To appease the interests of General Electric, they were awarded the contract to construct the transmission lines to Buffalo using the Tesla patents.
As a result of the successful field trial in the International Electro-Technical Exhibition of 1891, three-phase current, as far as Germany was concerned, became the most economical means of transmitting electrical energy.
In 1892, General Electric formed and immediately invested heavily in AC power (at this time Thomas Edison's opinions on company direction were muted by President Coffin and the GE board of directors). Westinghouse was already ahead in AC, but it only took a few years for General Electric to catch up, mainly thanks to Charles Proteus Steinmetz, a Prussian mathematician who was the first person to fully understand AC power from a solid mathematical standpoint. General Electric hired many talented new engineers to improve its design of transformers, generators, motors and other apparatus.
In Europe, Siemens & Halske became the dominant force. Three phase 60 Hz at 120 volts became the dominant system in North America while 220-240 volts at 50 Hz became the standard in Europe.
Alternating current power transmission networks today provide redundant paths and lines for power routing from any power plant to any load center, based on the economics of the transmission path, the cost of power, and the importance of keeping a particular load center powered at all times. Generators (such as hydroelectric sites) can be located far from the loads.
Remnant and existent DC systems
Some cities continued to use DC well into the 20th century. In central Helsinki, there was a DC network in existence up until the late 1940s, and in the 1960s, Stockholm's dwindling DC network was eliminated. A mercury arc valve rectifier station could convert AC to DC where networks were still used. In 1942, the Greenwich Village neighborhood in New York City used DC. Parts of Boston, Massachusetts along Beacon Street and Commonwealth Avenue still used 110 volts DC in the 1960s, causing the destruction of many small appliances (typically hair dryers and phonographs) used by Boston University students, who ignored warnings about the electricity supply. New York City's electric utility company, Consolidated Edison, continued to supply direct current to customers who had adopted it early in the twentieth century, mainly for elevators. The New Yorker Hotel, constructed in 1929, had a large direct-current power plant and did not convert fully to alternating-current service until well into the 1960s. This was the building in which AC pioneer Nikola Tesla spent his last years, and where he died in 1943. In January 1998, Consolidated Edison started to eliminate DC service. At that time there were 4,600 DC customers. By 2006, there were only 60 customers using DC service, and on November 14, 2007, the last direct-current distribution by Con Edison was shut down. Customers still using DC were provided with on-site AC-to-DC rectifiers. The city of San Francisco, California featured a DC power grid to supply power for pre-1940s winding-drum elevators. Around the end of 2010, the DC grid was divided into 171 separate islands with each island supplying 7 to 10 customers.
The Central Electricity Generating Board in the UK continued to maintain a 200 volt DC generating station at Bankside Power Station on the River Thames in London as late as 1981. It exclusively powered DC printing machinery in Fleet Street, then the heart of the UK's newspaper industry. It was decommissioned later in 1981 when the newspaper industry moved into the developing docklands area farther down the river (using modern AC-powered equipment). The building was converted into an art gallery, the Tate Modern.
Electric railways that use a third-rail system generally employ DC power between 500 and 750 volts; railways with overhead catenary lines use a number of power schemes including both high-voltage AC and high-current DC.
High-voltage direct current (HVDC) systems are used for bulk transmission of energy from distant generating stations, or for interconnection of separate alternating current systems. These HVDC systems use electronic devices like mercury arc valves, thyristors, or IGBTs that were unavailable during the War of Currents era. Power is converted to and from alternating current at each side of the HVDC link. An HVDC system can transmit more power over a given right-of-way than an AC system, which is an advantage in overall cost. HVDC systems allow better control of power flows in transient and emergency conditions, which helps prevent blackouts. HVDC is an alternative to AC systems for long-distance, high-load transmission, see List of HVDC projects for example projects.
DC power is still common when distances are small, and especially when energy storage or conversion uses batteries or fuel cells. These applications include:
- Electronics, including integrated circuits, high-power transmitters and computers
- Vehicle starting, lighting, and ignition systems
- Hybrid and all-electric vehicle propulsion with internal power-supply
- Telecommunication plant power (wired and cellular mobile)
- Uninterruptible power for computer systems
- Utility-scale battery systems
- "Off-grid" isolated power installations using wind or solar power
In these applications, direct current may be used directly or converted to alternating current using power electronic devices. In the future, this may provide a way to supply energy to a grid from distributed sources. For example, hybrid vehicle owners may rent the capacity of their vehicle's batteries for load-levelling purposes by the local electrical utility company.
DC in data centers
A computer data center may have hundreds or thousands of processors in operation. Since such centers typically are vital to the operation of a business or institution, they require highly reliable power distribution. The energy consumed by a data center is a significant part of its operation cost, and heat dissipated by power supplies must be removed by air conditioning equipment, resulting in additional energy consumption. Studies by the Electric Power Research Instititute (EPRI) have suggested that the multiple levels of power distribution in a data center can be replaced by a 380 volt DC distribution system. Multiple levels of AC transformation and uninterruptible power supply (UPS) can instead be replaced by a building-wide 380 volt DC system connected directly to the processor power supplies. A minimum of 7 or 8% annual energy consumption can be saved by eliminating the multiple stages of conventional AC power distribution. A great increase in reliability can also result, since the inverter output stages of UPS are the source of many data center failures. The 380 V building DC network could be directly connected to batteries to provide uninterruptible power. A local DC microgrid could also offset utility energy purchase with local generation from solar panels, wind turbines, or other distributed generation sources.
The 380 volt level was selected because it greatly reduces the size of conductors compared with a 48 volt distribution system (the standard in the telecommunications industry.) The system would be operated with a split supply, at +190 V and -190V with respect to ground, to minimize the shock hazard to people. The 380 volt level is compatible with the typical ratings of components now used in computer power supplies. 
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