Three-phase electric power
Three-phase electric power is a common method of alternating current electric power generation and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power, it is used to power large motors and other heavy loads. A three-wire three-phase circuit is more economical than an equivalent two-wire single-phase circuit at the same line to ground voltage because it uses less conductor material to transmit a given amount of electrical power. Polyphase power systems were independently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky, Jonas Wenström, John Hopkinson and Nikola Tesla in the late 1880s; the conductors between a voltage source and a load are called lines, the voltage between any two lines is called line voltage. The voltage measured across any one component is called phase voltage. In a symmetric three-phase power supply system, three conductors each carry an alternating current of the same frequency and voltage amplitude relative to a common reference but with a phase difference of one third of a cycle between each.
The common reference is connected to ground and to a current-carrying conductor called the neutral. Due to the phase difference, the voltage on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the remaining conductor; this phase delay gives constant power transfer to a balanced linear load. It makes it possible to produce a rotating magnetic field in an electric motor and generate other phase arrangements using transformers; the amplitude of the voltage difference between two phases is 3 times the amplitude of the voltage of the individual phases. The symmetric three-phase systems described here are referred to as three-phase systems because, although it is possible to design and implement asymmetric three-phase power systems, they are not used in practice because they lack the most important advantages of symmetric systems. In a three-phase system feeding a balanced and linear load, the sum of the instantaneous currents of the three conductors is zero.
In other words, the current in each conductor is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two phase conductors; as compared to a single-phase AC power supply that uses two conductors, a three-phase supply with no neutral and the same phase-to-ground voltage and current capacity per phase can transmit three times as much power using just 1.5 times as many wires. Thus, the ratio of capacity to conductor material is doubled; the ratio of capacity to conductor material increases to 3:1 with an ungrounded three-phase and center-grounded single-phase system. Constant power transfer and cancelling phase currents would in theory be possible with any number of phases, maintaining the capacity-to-conductor material ratio, twice that of single-phase power. However, two-phase power results in a less smooth torque in a generator or motor, more than three phases complicates infrastructure unnecessarily.
Three-phase systems may have a fourth wire in low-voltage distribution. This is the neutral wire; the neutral allows three separate single-phase supplies to be provided at a constant voltage and is used for supplying groups of domestic properties which are each single-phase loads. The connections are arranged so that, as far as possible in each group, equal power is drawn from each phase. Further up the distribution system, the currents are well balanced. Transformers may be wired in a way that they have a four-wire secondary but a three-wire primary while allowing unbalanced loads and the associated secondary-side neutral currents. Three-phase supplies have properties that make them desirable in electric power distribution systems: The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load; this makes it possible to reduce the size of the neutral conductor because it carries little or no current. With a balanced load, all the phase conductors so can be the same size.
Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations. Three-phase systems can produce a rotating magnetic field with a specified direction and constant magnitude, which simplifies the design of electric motors, as no starting circuit is required. Most household loads are single-phase. In North American residences, three-phase power might feed a multiple-unit apartment block, but the household loads are connected only as single phase. In lower-density areas, only a single phase might be used for distribution; some high-power domestic appliances such as electric stoves and clothes dryers are powered by a split phase system at 240 volts or from two phases of a three phase system at 208 volts. Wiring for the three phases is identified by color codes which vary by country. Connection of the phases in the right order is required to ensure the intended direction of rotation of three-phase motors. For example and fans may not work in reverse. Maintaining the identity of phases is required if there is any possibility two sources can be connected at the same time.
At the power station, an electrical generator converts mechanical pow
Sweden the Kingdom of Sweden, is a Scandinavian Nordic country in Northern Europe. It borders Norway to the west and north and Finland to the east, is connected to Denmark in the southwest by a bridge-tunnel across the Öresund, a strait at the Swedish-Danish border. At 450,295 square kilometres, Sweden is the largest country in Northern Europe, the third-largest country in the European Union and the fifth largest country in Europe by area. Sweden has a total population of 10.2 million. It has a low population density of 22 inhabitants per square kilometre; the highest concentration is in the southern half of the country. Germanic peoples have inhabited Sweden since prehistoric times, emerging into history as the Geats and Swedes and constituting the sea peoples known as the Norsemen. Southern Sweden is predominantly agricultural, while the north is forested. Sweden is part of the geographical area of Fennoscandia; the climate is in general mild for its northerly latitude due to significant maritime influence, that in spite of this still retains warm continental summers.
Today, the sovereign state of Sweden is a constitutional monarchy and parliamentary democracy, with a monarch as head of state, like its neighbour Norway. The capital city is Stockholm, the most populous city in the country. Legislative power is vested in the 349-member unicameral Riksdag. Executive power is exercised by the government chaired by the prime minister. Sweden is a unitary state divided into 21 counties and 290 municipalities. An independent Swedish state emerged during the early 12th century. After the Black Death in the middle of the 14th century killed about a third of the Scandinavian population, the Hanseatic League threatened Scandinavia's culture and languages; this led to the forming of the Scandinavian Kalmar Union in 1397, which Sweden left in 1523. When Sweden became involved in the Thirty Years War on the Reformist side, an expansion of its territories began and the Swedish Empire was formed; this became one of the great powers of Europe until the early 18th century. Swedish territories outside the Scandinavian Peninsula were lost during the 18th and 19th centuries, ending with the annexation of present-day Finland by Russia in 1809.
The last war in which Sweden was directly involved was in 1814, when Norway was militarily forced into personal union. Since Sweden has been at peace, maintaining an official policy of neutrality in foreign affairs; the union with Norway was peacefully dissolved in 1905. Sweden was formally neutral through both world wars and the Cold War, albeit Sweden has since 2009 moved towards cooperation with NATO. After the end of the Cold War, Sweden joined the European Union on 1 January 1995, but declined NATO membership, as well as Eurozone membership following a referendum, it is a member of the United Nations, the Nordic Council, the Council of Europe, the World Trade Organization and the Organisation for Economic Co-operation and Development. Sweden maintains a Nordic social welfare system that provides universal health care and tertiary education for its citizens, it has the world's eleventh-highest per capita income and ranks in numerous metrics of national performance, including quality of life, education, protection of civil liberties, economic competitiveness, equality and human development.
The name Sweden was loaned from Dutch in the 17th century to refer to Sweden as an emerging great power. Before Sweden's imperial expansion, Early Modern English used Swedeland. Sweden is derived through back-formation from Old English Swēoþēod, which meant "people of the Swedes"; this word is derived from Sweon/Sweonas. The Swedish name Sverige means "realm of the Swedes", excluding the Geats in Götaland. Variations of the name Sweden are used in most languages, with the exception of Danish and Norwegian using Sverige, Faroese Svøríki, Icelandic Svíþjóð, the more notable exception of some Finnic languages where Ruotsi and Rootsi are used, names considered as referring to the people from the coastal areas of Roslagen, who were known as the Rus', through them etymologically related to the English name for Russia; the etymology of Swedes, thus Sweden, is not agreed upon but may derive from Proto-Germanic Swihoniz meaning "one's own", referring to one's own Germanic tribe. Sweden's prehistory begins in the Allerød oscillation, a warm period around 12,000 BC, with Late Palaeolithic reindeer-hunting camps of the Bromme culture at the edge of the ice in what is now the country's southernmost province, Scania.
This period was characterised by small bands of hunter-gatherer-fishers using flint technology. Sweden is first described in a written source in Germania by Tacitus in 98 AD. In Germania 44 and 45 he mentions the Swedes as a powerful tribe with ships that had a prow at each end. Which kings ruled these Suiones is unknown, but Norse mythology presents a long line of legendary and semi-legendary kings going back to the last centuries BC; as for literacy in Sweden itself, the runic script was in use among the south Scandinavian elite by at least the 2nd century AD, but all that has come down to the present from the Roman Period is curt inscriptions on artefacts of male names, demonstrating th
Grängesberg is a locality situated in Ludvika Municipality, Dalarna County, Sweden with 3,481 inhabitants in 2010. The town was earlier dominated by iron-ore extraction at Grängesberg ore field from the 16th century to 1989. In January 1990 was the last ore-train from Grängesberg to Oxelösund. Grängesbergsbolaget had during a long period the world's largest iron-ore fleet and by 1899–1900 was it Sweden's most profitable company. During this time Grängesberg grew fast, during a 10-year period the town's population increased threefold. Today Spendrups is Grängesberg's biggest employer; the area is famous of its Railway Museum of Grängesberg
High-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be justified, due to other benefits of direct current links. HVDC uses voltages between 100 kV and 1,500 kV. HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz.
This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks. The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden and in Germany. Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, a 100 kV, 20 MW system between Gotland and mainland Sweden in 1954; the longest HVDC link in the world is the Rio Madeira link in Brazil, which consists of two bipoles of ±600 kV, 3150 MW each, connecting Porto Velho in the state of Rondônia to the São Paulo area. The length of the DC line is 2,375 km. In July 2016, ABB Group received a contract in China to build an ultrahigh-voltage direct-current land link with a 1100 kV voltage, a 3,000 km length and 12 GW of power, setting world records for highest voltage, longest distance, largest transmission capacity. High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, doubling the voltage will deliver the same power at only half the current.
Since the power lost as heat in the wires is directly proportional to the square of the current, doubling the voltage reduces the line losses by a factor of 4. While power lost in transmission can be reduced by increasing the conductor size, larger conductors are heavier and more expensive. High voltage cannot 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 transmission circuits. Transformers made voltage changes practical, 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. Practical conversion of power between AC and DC became possible with the development of power electronics devices such as mercury-arc valves and, starting in the 1970s, semiconductor devices as thyristors, integrated gate-commutated thyristors, MOS-controlled thyristors and insulated-gate bipolar transistors.
The first long-distance transmission of electric power was demonstrated using direct current in 1882 at Miesbach-Munich Power Transmission, but only 1.5 kW was transmitted. An early method of high-voltage DC transmission was developed by the Swiss engineer René Thury and his method was put into practice by 1889 in Italy by the Acquedotto De Ferrari-Galliera company; this system used series-connected motor-generator sets to increase the voltage. Each set was driven by insulated shafts from a prime mover; the transmission line was operated in a'constant current' mode, with up to 5,000 volts across each machine, some machines having double commutators to reduce the voltage on each commutator. This system transmitted 630 kW at 14 kV DC over a distance of 120 km; the Moutiers–Lyon system transmitted 8,600 kW of hydroelectric power a distance of 200 km, including 10 km of underground cable. This system used eight series-connected generators with dual commutators for a total voltage of 150 kV between the positive and negative poles, operated from c.1906 until 1936.
Fifteen Thury systems were in operation by 1913. Other Thury systems operating at up to 100 kV DC worked into the 1930s, but the rotating machinery required high maintenance and had high energy loss. Various other electromechanical devices were tested during the first half of the 20th century with little commercial success. One technique attempted for conversion of direct current from a high transmission voltage to lower utilization voltage was to charge series-connected batteries reconnect the batteries in parallel to serve distribution loads. While at least two commercial installations were tried around the turn of the 20th century, the technique was not useful owing to the limited capacity of batteries, difficulties in switching between series and parallel connections, the inherent energy inefficiency of a battery charge/discharge cycle. First proposed in 1914, the grid controlled mercury-arc valve became available for power transmission during the period 1920 to 1940. Starting in 1932, General Electric tested mercury-vapor valves and a 12 kV DC transmission line, which served to convert 40 Hz generation to serve 60 Hz loads, at Mechanicville, New York.
In 1941, a 60 MW, ±200 kV, 115 km buried cable
Jonas Wenström was a Swedish engineer and inventor. He invented the three-phase electric power system, the basis for ASEA. "It goes to conspiratorially speculate. Wenström writes: "Edison's new invention of electric light: a glowing carbon strip, is the same thing that I discovered a year ago... If I had his laboratory, resources, I would have done the same and better... a graphite strip between two mica plates provide a more effective light than Edison's."