question: why is Alternating Current superior to Direct Current for long distance transmission?
comment: For very long distances, DC is more efficient. URL_0
passages: High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is to be transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead of alternating current. For a very long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the additional cost of the required converter stations at each end.
   AC became the dominant form of transmission of power with innovations in Europe and the US in electric motor designs and the development of engineered "universal systems" allowing the large number of legacy systems to be connected to large AC grids.
   The first long distance AC line was long, built for the 1884 International Exhibition of Turin, Italy. It was powered by a 2 kV, 130 Hz Siemens & Halske alternator and featured several Gaulard secondary generators with their primary windings connected in series, which fed incandescent lamps. The system proved the feasibility of AC electric power transmission on long distances.
   High voltage cannot readily be used for lighting or motors, so transmission-level voltages must be reduced for end-use equipment. Transformers are used to change the voltage levels in alternating current (AC) transmission circuits. Transformers made voltage changes practical, and AC generators were more efficient than those using DC. Because of this, AC became dominant after the conclusion of the War of Currents in 1892. The "War of Currents" was a competition fought in the US between the DC system of Thomas Edison and the AC system of George Westinghouse and Nikola Tesla.
   Beginning about 1887 alternating current generators came into extensive operation and the commercial development of the transformer, by means of which currents of low voltage and high current strength are transformed to currents of high voltage and low current strength, and vice versa, in time revolutionized the transmission of electric power to long distances. Likewise the introduction of the rotary converter (in connection with the "step-down" transformer) which converts alternating currents into direct currents (and vice versa) has effected large economies in the operation of electric power systems.
   In 1882, the German Miesbach–Munich Power Transmission used 2kV DC over a distance of . 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 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. 
   Basically transmission can be of two types :
  
  BULLET::::- HVDC Transmission (High Voltage Direct Current transmission)
  
  BULLET::::- HVAC Transmission (High Voltage Direct Current Transmission)
  
  Section::::Overview.:HVDC transmission.
  
  High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is to be transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead of alternating current. 
   The late 1870s and early 1880s saw electricity starting to be generated at power stations. These were initially set up to power arc lighting (a popular type of street lighting) running on very high voltage (usually higher than 3000 volt) direct current or alternating current. This was followed by the wide spread use of low voltage direct current for indoor electric lighting in business and homes after inventor Thomas Edison launched his incandescent bulb based electric "utility" in 1882. Because of the significant advantages of alternating current over direct current in using transformers to raise and lower voltages to allow much longer transmission distances, direct current was replaced over the next few decades by alternating current in power delivery. In the mid-1950s, high-voltage direct current transmission was developed, and is now an option instead of long-distance high voltage alternating current systems. For long distance underseas cables (e.g. between countries, such as NorNed), this DC option is the only technically feasible option. For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current.
   Section::::Comparison with AC.:Advantages.:Overhead line systems.
   With the widespread establishment of electrical grids, power transmission is usually associated most with electric power transmission. Alternating current is normally preferred as its voltage may be easily stepped up by a transformer in order to minimize resistive loss in the conductors used to transmit power over great distances; another set of transformers is required to step it back down to safer or more usable voltage levels at destination.
   The late 1880s and early 1890s would see the financial merger of smaller electric companies into a few larger corporations such as Ganz and AEG in Europe and General Electric and Westinghouse Electric in the US. These companies continued to develop AC systems but the technical difference between direct and alternating current systems would follow a much longer technical merger. Due to innovation in the US and Europe, alternating current's economy of scale with very large generating plants linked to loads via long distance transmission was slowly being combined with the ability to link it up with all of the existing systems that needed to be supplied. These included single phase AC systems, poly-phase AC systems, low voltage incandescent lighting, high voltage arc lighting, and existing DC motors in factories and street cars. In what was becoming a "universal system", these technological differences were temporarily being bridged via the development of rotary converters and motor-generators that would allow the large number of legacy systems to be connected to the AC grid. These stopgaps would slowly be replaced as older systems were retired or upgraded.
   A long-distance, point-to-point HVDC transmission scheme generally has lower overall investment cost and lower losses than an equivalent AC transmission scheme. HVDC conversion equipment at the terminal stations is costly, but the total DC transmission-line costs over long distances are lower than for an AC line of the same distance. HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases and there is no skin effect.
   Section::::Comparison with AC.:Advantages.:Asynchronous connections.
   In the early days of commercial electric power, transmission of electric power at the same voltage as used by lighting and mechanical loads restricted the distance between generating plant and consumers. In 1882, generation was with direct current (DC), which could not easily be increased in voltage for long-distance transmission. Different classes of loads (for example, lighting, fixed motors, and traction/railway systems) required different voltages, and so used different generators and circuits.
   By 1890 the power industry had flourished and power companies had built thousands of power systems (both direct and alternating current) in the United States and Europe – these networks were effectively dedicated to providing electric lighting. During this time a fierce rivalry in the US known as the "War of Currents" emerged between Edison and Westinghouse over which form of transmission (direct or alternating current) was superior. In 1891, Westinghouse installed the first major power system that was designed to drive an electric motor and not just provide electric lighting. The installation powered a synchronous motor at Telluride, Colorado with the motor being started by a Tesla induction motor. On the other side of the Atlantic, Oskar von Miller built a 20 kV 176 km three-phase transmission line from Lauffen am Neckar to Frankfurt am Main for the Electrical Engineering Exhibition in Frankfurt. In 1895, after a protracted decision-making process, the Adams No. 1 generating station at Niagara Falls began transmitting three-phase alternating current power to Buffalo at 11 kV. Following completion of the Niagara Falls project, new power systems increasingly chose alternating current as opposed to direct current for electrical transmission.
   In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor. However, there are a number of advantages to bipolar transmission which can make it an attractive option.
  
  BULLET::::- Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return. This reduces earth return loss and environmental effects.
   Section::::Early transmission.:Availability of large-scale generation.
  
  Availability of large amounts of power from diverse locations would become possible after Charles Parsons' production of turbogenerators beginning 1889. Turbogenerator output quickly jumped from 100 kW to 25 megawatts in two decades. Prior to efficient turbogenerators, hydroelectric projects were a significant source of large amounts of power requiring transmission infrastructure.
  
  Section::::Early transmission.:Transformers and alternating current.
   (and possibly in other cooperating cities along the transmission route). Such a system could be less prone to failure if parts of it were suddenly shut down. One example of a long DC transmission line is the Pacific DC Intertie located in the Western United States.
  
  Section::::Capacity.
   By the 1890s, single and poly-phase AC was undergoing rapid introduction. In the U.S. by 1902, 61% of generating capacity was AC, increasing to 95% in 1917. Despite the superiority of alternating current for most applications, a few existing DC systems continued to operate for several decades after AC became the standard for new systems.
  
  Section::::History of electrification.:Steam turbines.
   The comparison between the dangers of alternating current at typical power transmission frequences (i.e., 50 or 60 Hz), and direct current has been a subject of debate ever since the War of Currents in the 1880s. Animal experiments conducted during this time suggested that alternating current was about twice as dangerous as direct current per unit of current flow (or per unit of applied voltage).
   Due to this specialization of lines and because transmission was inefficient for low-voltage high-current circuits, generators needed to be near their loads. It seemed, at the time, that the industry would develop into what is now known as a distributed generation system with large numbers of small generators located near their loads.
  
  The transmission of electric power with alternating current (AC) became possible after Lucien Gaulard and John Dixon Gibbs built what they called the secondary generator, an early transformer provided with 1:1 turn ratio and open magnetic circuit, in 1881.
   Initially electricity was generated using a technique that produced direct current to DC. This type of current had the unfortunate property of being difficult to transmit long distances over wires. In 1897 Westinghouse established a manufacturing facility in Hamilton and began producing heavy alternating current (AC) generators and AC motors (based on Serbian-American inventor Nikola Tesla's induction motor) for the Canadian market. AC had an advantage in that it was more amenable to transmission over long distances. The competitor of that company, General Electric of Canada (1892), with production facilities in Peterborough, Ontario eventually followed suit. AC technology has been used for electrical transmission since that time.
   The AC power systems was developed and adopted rapidly after 1886 due to its ability to distribute electricity efficiently over long distances, overcoming the limitations of the direct current system. In 1886, the ZBD engineers designed the world's first power station that used AC generators to power a parallel-connected common electrical network, the steam-powered Rome-Cerchi power plant. The reliability of the AC technology received impetus after the Ganz Works electrified a large European metropolis: Rome in 1886.
   Alternating current had been under development with contributions by Guillaume Duchenne (1850s), the dynamo work of Zénobe Gramme, Ganz Works (1870s), Sebastian Ziani de Ferranti (1880s), Lucien Gaulard, and Galileo Ferraris. Starting in the 1880s alternating current gained its key advantage over direct current with the development of functional transformers that allowed the voltage to be "stepped up" to much higher transmission voltages and then dropped down to a lower end user voltage for business and residential use. The high voltages allowed a central generating station to supply a large area, up to long circuits. 
   The concept that is the basis of modern transmission using inexpensive step up and step down transformers was first implemented by Westinghouse, William Stanley, Jr. and Franklin Leonard Pope in 1886 in Great Barrington, Massachusetts, resorting also to European technology. In 1888 Westinghouse also licensed Nikola Tesla's induction motor patent giving AC a much needed usable motor. This system was developed into the modern 3-phase system by Mikhail Dolivo-Dobrovolsky and Allgemeine Elektricitäts-Gesellschaft and Charles Eugene Lancelot Brown in Europe, starting in 1889. 
  
raw_presuppositions: Alternating Current is superior to Direct Current for long distance transmission.
raw_corrections: Direct Current is more efficient for long distances.