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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Transmission of electricity over long distances. History of invention and production
Directory / The history of technology, technology, objects around us Power transmission line (TL) - one of the components of the electrical network, a system of power equipment designed to transmit electricity through electric current. Also, an electric line as part of such a system that extends beyond the power plant or substation.
In the last third of the XNUMXth century, the energy problem became very acute in many large industrial centers of Europe and America. Residential buildings, transport, factories and workshops demanded more and more fuel, which had to be brought from afar, as a result of which the price of it was constantly growing. In this regard, here and there they began to turn to the hydropower of rivers, which is much cheaper and more accessible. At the same time, interest in electrical energy was growing everywhere. It has long been noted that this type of energy is extremely convenient: electricity is easily generated and just as easily converted into other types of energy, easily transmitted over a distance, supplied and crushed. The first power stations were usually an electric generator connected to a steam engine or turbine, and were intended to supply electricity to individual objects (for example, a workshop or a house, in extreme cases, a quarter). From the mid-80s, central city power stations began to be built, providing current primarily for lighting. (The first such power plant was built in 1882 in New York under the direction of Edison.) The current was generated by powerful steam engines. But by the beginning of the 90s, it became clear that the energy problem could not be solved in this way, since the power of the central stations located in the central part of the city could not be very large. They used the same coal and oil, that is, they did not remove the problem of fuel delivery. It was cheaper and more practical to build power plants in places with cheap fuel and water resources. But, as a rule, areas where it was possible to obtain cheap electricity in large quantities were removed from industrial centers and large cities by tens and hundreds of kilometers. Thus, another problem arose - the transmission of electricity over long distances. The first experiments in this area date back to the very beginning of the 70s of the XIX century, when they used mainly direct current. They showed that as soon as the length of the connecting wire between the current generator and the motor consuming this current exceeded several hundred meters, a significant reduction in power was felt in the motor due to large energy losses in the cable. This phenomenon is easy to explain if we remember the thermal effect of the current. Passing through the cable, the current heats it. These losses are greater, the greater the resistance of the wire and the strength of the current passing through it. (The amount of heat released Q is easy to calculate. The formula looks like: Q=RI2, where I is the strength of the passing current, R is the cable resistance. Obviously, the resistance of the wire is greater, the greater its length and the smaller its cross section. If in this formula we take I=P/U, where P is the power of the line, and U is the current voltage, then the formula will take the form Q=RP2/U2. From this it can be seen that the heat losses will be the smaller, the greater the voltage.) There were only two ways to reduce losses in the power line: either increase the cross section of the transmission wire, or increase the voltage. However, an increase in the cross section of the wire greatly increased its cost, because quite expensive copper was then used as a conductor. Much more winning promised the second way. In 1882, under the leadership of the famous French electrical engineer Despres, the first direct current power line was built from Miesbach to Munich, 57 km long. The energy from the generator was transferred to an electric motor that powered the pump. In this case, the losses in the wire reached 75%. In 1885, Despres carried out another experiment, carrying out a power transmission between Creil and Paris over a distance of 56 km. In this case, a high voltage was used, reaching 6 thousand volts. Losses decreased to 55%. It was obvious that by increasing the voltage, it was possible to significantly increase the efficiency of the line, but for this it was necessary to build high-voltage direct current generators, which was associated with great technical difficulties. Even with this relatively low voltage, Despres had to constantly repair his generator, in the windings of which a breakdown occurred every now and then. On the other hand, a high voltage current could not be used, since in practice (and primarily for lighting needs) a very small voltage, about 100 volts, was required. In order to lower the DC voltage, it was necessary to build a complex converter system: the high voltage current drove the motor, which, in turn, rotated the generator, which gave a lower voltage current. At the same time, losses increased even more, and the very idea of transmitting electricity became economically unprofitable. Alternating current seemed more convenient in terms of transmission, if only because it could be easily transformed, that is, its voltage could be increased and then decreased over a very wide range. In 1884, at the Turin exhibition, Golyar carried out power transmission over a distance of 40 km, raising the voltage in the line to 2 thousand volts with the help of his transformer. This experience gave good results, but it did not lead to a widespread development of electrification, since, as already mentioned, single-phase AC motors were inferior to DC motors in all respects and did not have distribution. Thus, it was unprofitable to transmit single-phase alternating current over long distances. In the following years, two systems of multi-phase currents were developed - Tesla's two-phase and Dolivo-Dobrovolsky's three-phase. Each of them claimed a dominant position in electrical engineering. Which way should electrification go? At first, no one knew the exact answer to this question. In all countries there was a lively discussion of the advantages and disadvantages of each of the systems of currents. All of them had their ardent supporters and fierce opponents. Some clarity on this issue was achieved only in the next decade, when a significant breakthrough was made in electrification. The Frankfurt International Exhibition of 1891 played a huge role in this. At the end of the 80s, the question arose of building a central power plant in Frankfurt am Main. Many German and foreign firms offered the city authorities various options for projects involving the use of either direct or alternating current. The mayor of Frankfurt was clearly in a difficult position: he could not make a choice where even many specialists could not do it. To clarify the controversial issue, it was decided to arrange a long-planned international electrical exhibition in Frankfurt. Its main goal was to be a demonstration of the transmission and distribution of electrical energy in various systems and applications. Any company could demonstrate its success at this exhibition, and an international commission of the most authoritative scientists had to subject all the exhibits to a thorough study and answer the question of choosing the type of current. By the beginning of the exhibition, various companies had to build their power transmission lines, and some were going to demonstrate the transmission of direct current, others - alternating current (both single-phase and multi-phase). The company AEG was asked to carry out the transmission of electricity from the town of Laufen to Frankfurt over a distance of 170 km. At that time it was a huge distance, and many considered the idea itself fantastic. However, Dolivo-Dobrovolsky was so confident in the system and the possibilities of the three-phase current that he persuaded director Rothenau to agree to the experiment. When the first reports about the Laufen-Frankfurt power transmission project appeared, electrical engineers around the world were divided into two camps. Some enthusiastically welcomed this bold decision, others treated it as a noisy but groundless advertisement. Possible energy losses were calculated. Some believed that they would be 95%, but even the biggest optimists did not believe that the efficiency of such a line would exceed 15%. The most famous authorities in the field of electrical engineering, including the famous Despres, expressed doubts about the economic feasibility of this undertaking. However, Dolivo-Dobrovolsky managed to convince the company's management of the need to take on the job offered. Since there was very little time left before the opening of the exhibition, the construction of the power line took place in a great hurry. For six months, Dolivo-Dobrovolsky had to design and build an asynchronous 100 hp motor of unprecedented power. and four transformers for 150 kilowatts, despite the fact that the maximum power of single-phase transformers was then only 30 kilowatts. There could be no question of experimental designs: there was simply not enough time for this. Even the built engine and transformers could not be tested at the plant, since there was no three-phase generator of the appropriate power in Berlin (the generator for the Laufen station was built in Erlikson). Consequently, all elements of power transmission had to be turned on directly at the exhibition in the presence of many scientists, representatives of competing firms and countless correspondents. The slightest mistake would be unforgivable. In addition, all responsibility for the design and installation work during the construction of power lines fell on the shoulders of Dolivo-Dobrovolsky. Actually, the responsibility was even greater - after all, the question was being decided not only about the career of Dolivo-Dobrovolsky and the prestige of the AEG, but also about which path the development of electrical engineering would take. Dolivo-Dobrovolsky perfectly understood the importance of the task before him and wrote later: “If I did not want to bring indelible shame on my three-phase current and expose it to distrust, which would hardly be able to quickly dissipate later, I was obliged to take on this task and resolve it. Otherwise, the Laufen-Frankfurt experiments and much that was to be developed on their basis later would have gone along the path of using a single-phase current. A small hydroelectric power station was built in Laufen in a short time. 300 hp turbine rotated a three-phase current generator, designed and built, as already mentioned, at the plant in Erlikson. From the generator, three heavy-gauge copper wires led to the switchboard. Ammeters, voltmeters, lead fuses and thermal relays were installed here. From the switchboard, three cables went to three three-phase "prismatic" type transformers. The windings of all transformers were connected in a star. It was supposed to conduct power transmission at a voltage of 15 thousand volts, but all calculations were made for work at 25 thousand volts. To achieve such a high voltage, it was planned to include two transformers at each end of the line, so that their low voltage windings were connected in parallel, and the higher voltage windings were connected in series. From the transformers in Laufen, a three-wire line began, suspended on 3182 wooden poles 8 and 10 m high with an average span of 60 m. There were no switches on the line. In order to quickly turn off the current if necessary, two original devices were provided. Near the Laufen hydroelectric power station, two supports were installed at a distance of 2 m from one another. Here, a fusible insert, consisting of two copper wires with a diameter of 5 mm, was included in the gap of each wire of the line. In Frankfurt and near railway stations (part of the line ran along the railway track), so-called corner closures were installed. Each of them was a metal bar suspended by a cord on an L-shaped support. It was enough to pull the cord, and the beam fell on all three wires, creating an artificial short circuit, which caused the fuses in Laufen to burn out and the entire line to be de-energized. In Frankfurt, the wires went to step-down transformers (they were at the exhibition in a special pavilion), which reduced the output voltage to 116 volts. 1000 incandescent lamps, 16 candles (55 watts) each, were connected to one of these transformers, and a large three-phase Dolivo-Dobrovolsky motor, located in another pavilion, was connected to the other. The line voltage of the generator at Laufen was 95 volts. The step-up transformer had a transformation ratio of 154. Therefore, the operating voltage in the power line was 14650 volts (95×154). For that time it was a very high voltage. The governments of the lands through which the power line passed were alarmed by its construction. Some had a feeling of fear even in front of wooden poles, on which tablets with skulls were fixed. Of particular concern was the possibility of a wire breakage and its fall onto the railroad tracks. The exhibition committee and the line builders had to do a lot of explanatory work to convince government officials that all possible dangers were foreseen and that the line was reliably protected. The administration of Baden still did not allow to connect the section of the already completed line on the Baden border. In order to remove the last obstacles and dispel the doubts of the local authorities, Dolivo-Dobrovolsky conducted a dangerous but very convincing experiment. When the line was first energized, one of the wires on the border of Baden and Hesse was artificially cut off and fell on the railroad tracks with a bright flash. Dolivo-Dobrovolsky immediately came up and picked up the wire with his bare hands: he was so sure that the protection he had designed would work. This "method" of proof turned out to be very illustrative and removed the last obstacle before testing the line. On August 25, 1891, at 12 noon, 1000 electric lamps, powered by the current of the Laufen hydroelectric power station, flashed for the first time at the exhibition. These lamps framed the shields and the arch above the entrance to that part of the exhibition, the exhibits of which belonged to the Laufen-Frankfurt transmission line. The next day, a 75 kilowatt engine was successfully tested, which powered a ten-meter waterfall for the first time on September 12. Despite the fact that the line, machines, transformers, switchboards were made in a hurry (some details, according to Dolivo-Dobrovolsky, were thought out in just an hour), the entire installation, switched on without preliminary testing, to the surprise of some and to the delight of others, immediately began to work well. The waterfall made a special impression on the visitors of the exhibition. However, people more knowledgeable in matters of physics and electrical engineering rejoiced that day not at a huge waterfall sparkling with thousands of glass splashes, illuminated by dozens of multi-colored lamps. Their delight was connected with the understanding that this beautiful artificial waterfall is powered by a spring located 170 km away on the Neckar River near the town of Laufen. They saw before them a brilliant solution to the problem of power transmission over long distances. In October, an international commission began testing the Laufen-Frankfurt transmission line. It was found that the transmission losses were only 25%, which was a very good figure. In November, the line was tested at 25 volts. At the same time, its efficiency increased, and losses decreased to 21%. The vast majority of electricians from all over the world (more than a million people visited the exhibition) appreciated the significance of the Laufen-Frankfurt experiment. Three-phase current received a very high appraisal, and from now on the widest path to industry was opened to it. Dolivo-Dobrovolsky immediately became one of the world's leading electrical engineers, and his name became world famous.
Thus, the main energy problem of the late XNUMXth century was resolved - the problem of centralizing the production of electricity and transmitting it over long distances. It became clear to everyone the way in which a multi-phase current could be brought from a distant power plant to each individual workshop, and then to an individual machine. The immediate consequence of the emergence of multi-phase current technology was that in subsequent years, in all developed countries, the rapid construction of power plants and the widest electrification of industry began. True, in the early years it was still complicated by a fierce struggle between competing companies seeking to introduce one or another type of current. Thus, in America, the Westinghouse company first took over, which, having bought up Tesla's patents, tried to distribute a two-phase current. The triumph of the two-phase system was the construction in 1896 of a powerful hydroelectric power station on Niagara Falls. But three-phase current was soon universally recognized as the best. Indeed, a two-phase system required four wires, and a three-phase system only three. In addition to greater simplicity, it promised significant cost savings. Later, Tesla, following the example of Dolivo-Dobrovolsky, proposed to combine two return wires together. In this case, the currents were added, and in the third wire a current flowed approximately 1,4 times greater than in the other two. Therefore, the cross section of this wire was 1 times larger (without this increase in the cross section, overloads occurred in the circuit). As a result, the costs for two-phase wiring still turned out to be more than for three-phase, while two-phase motors were inferior to three-phase ones in all respects. In the 4th century, the three-phase system was established everywhere. Even the Niagara power station was eventually converted to three-phase current. Author: Ryzhov K.V.
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