Transmission of electricity over large distances
In the last third of the XIX century in many major industrial centers of Europe and America has become a very serious problem feel the energy. Residential buildings, transport, factories and workshops have demanded more fuel to bring up that comes from far away, so that the price has steadily increased. In this regard, here and there we began to turn to hydropower rivers, much cheaper and more available. However universally increasing interest in electric energy. It has long been noted that this type of energy is extremely easy to use: easy to generate electricity and also is easily converted into other forms of energy is transmitted over a distance, is crushed and fed without difficulty.
The first power station is typically a generator connected to the steam engine or turbine, and intended to supply electricity to individual objects (eg, shop or at home, in the extreme case, the quarter). Since the mid-80s were built central city power plant, which provided current is primarily for lighting. (The first such power plant was built in 1882 in New York under the direction of Edison.) Talk to them elaborated powerful steam engines. But by the early '90s it became clear that the way to solve the energy problem is not as central power stations located in the central part of the city, could not be very large. They used the same coal and oil, that is, not solve the problem of fuel delivery.
Cheaper and more practical was to build power plants in places with cheap fuel and water resources. But as a rule, the area where it was possible to obtain a large amount of cheap electricity, have been removed from industrial centers and large cities in the tens or hundreds of kilometers. Thus, there is another problem - the electricity transmission over long distances.
The first experiments in this area relate to the beginning of 70-ies of the XIX century, when used in the main DC. They showed that as soon as the length of the connecting wire between the alternator and the engine consumes the current exceeds a few hundred meters, there is a significant reduction in the power of the engine due to the large energy losses in the cable. This phenomenon is easily explained if we remember the Joule effect. Passing through the cable, the current heats it. This loss is greater, the greater the resistance wire and current flowing through it. (Number of the released heat Q can easily calculate formula is:.. Q = R • I2, where I - the power flowing current, R - resistance of the cable is obvious that the resistance of the wires is greater, the greater its length and the smaller the cross section of If. this formula, take I = P / U, where P - power lines, and U - voltage current, the formula takes the form Q = R • P2 / U2 this shows that the loss of heat will be the smaller, the larger the voltage).. there are only two ways to reduce the losses in the transmission line, either the transmitter section to increase the wire or the voltage to increase. However, increasing the wire section greatly increases the cost of it, because as a conductor if used too expensive copper. Much more winning promised second path.
In 1882, under the leadership of the French electrical Despres first DC power line was built from Miesbach to Munich, the length of 57 km. Energy transmitted from the generator to the motor drives the pump. Thus losses in the wire reached 75%. In 1885, Despres had another experiment by exercising power between Creil and Paris over a distance of 56 km. This high voltage is used, up to 6 thousand volts. The loss dropped to 55%. It was obvious that increasing the voltage can significantly increase the efficiency of the line, but that it was necessary to build a high-voltage direct current generator, which was associated with great technical difficulties. Even with a relatively small voltage Despres had to constantly repair your generator windings which every now and then there is a breakdown. On the other hand, the high current can not be used because, in practice (and especially for the needs of the lighting) required very little stress on the order of 100 volts. In order to lower the DC voltage, the converter had to build a complex system: high voltage current is driven by a motor, and that in turn rotates the generator, which gave a current of lower voltage. This loss is further increased, and the idea of power transmission itself becomes uneconomical.
Alternating current in the transfer seemed more comfortable if only because it can be easily transformed, that is, in a very wide range to increase, and then to lower its voltage. In 1884, at the Turin exhibition Golyar realized power over a distance of 40 km, lifting with your transformer line voltage up to 2 thousand volts. This experience has given good results, but it has not led to extensive development of electrification, because, as already mentioned, the single-phase AC motors in all respects inferior to the DC motor and had spread. Thus, a single-phase alternating current was unprofitable to transfer over long distances. In the following years, two currents of multiphase systems were developed - a two-phase and three-phase Tesla Dolivo-Dobrovolsky. Each of them claimed to be a dominant position in electrical engineering. By what way it was to go electrification? The exact answer to this question at first no one knew. In all the countries went a lively discussion of the advantages and disadvantages of each of the current systems. All of them had their ardent supporters and fierce opponents. Some clarity on this issue was reached only in the next decade, when a significant breakthrough in the electrification was taken. A huge role in this was played by the Frankfurt International Exhibition in 1891.
In the late 80s there was a question about the construction of the central power station in Frankfurt. Many German and foreign companies offered to city authorities the various options for projects involving the use of a DC or AC. Lord Mayor of Frankfurt was clearly in a difficult situation: he could not make a choice where it was not under force even to many specialists. To clarify the issues and it was decided to organize in Frankfurt had long planned international electrotechnical exhibition. Its main purpose was to be a demonstration of electrical power transmission and distribution in the various systems and applications. Any company could show in this exhibition its successes, and the International Commission of the most respected scientists had to be subjected to all the exhibits scrutiny and to answer the question about the choice of the current type. By the beginning of the exhibition various firms had to build their lines of power transmission, with some going to show the DC transmission, the other - the variable (such as single-phase and multiphase). Firm AER was invited to carry out the transmission of electricity from the village of Laufen in Frankfurt at a distance of 170 km. In those days it was a great distance, and many thought the idea of fiction. However Dolivo-Dobrovolsky was so confident in the system and the possibilities of three-phase current, that persuaded the director Rotena to agree to an experiment.
When the first reports of the project power Laufen - Frankfurt, electrical divided into two camps in the world. Some of them enthusiastically welcomed the courageous decision, while others treated it as a noisy but groundless advertising. Counted the possible loss of energy. Some thought that they amount to 95%, but even the biggest optimists did not believe that the efficiency of such a line exceeds 15%. The best-known authorities in the field of electrical engineering, including the famous Despres, have expressed doubts about the feasibility of this venture. However Dolivo-Dobrovolsky was able to convince the company's leadership necessary to undertake the proposed work.
Since before the opening of the exhibition was very little time, the construction of transmission lines was held in a big hurry. During the six months Dolivo-Dobrovolsky had to design and build an unprecedented power induction motor 100 hp and four transformer 150 kilowatt, while the maximum power of single-phase transformers, was then only 30 kilowatts. There could be no question of experimental designs: it is simply not enough time. Even built motor and transformers could not be tested at the factory because it was not in Berlin appropriate power alternator (generator to Laufenovskoy Erliksone station built in). Consequently, all the elements of power had to be inserted immediately at the exhibition in the presence of many scientists, representatives of competing companies and numerous correspondents. The slightest mistake would have been fatal. In addition, on the shoulders of Dolivo-Dobrovolsky lay the entire responsibility for the design and installation works in the construction of power lines. In fact, the responsibility was even greater - because it was a question not only of a career Dolivo-Dobrovolsky and prestige of the AER, but also about which path electrical development. Dolivo-Dobrovolsky was well aware of the importance of its tasks and later wrote: "If I did not want to bring my three-phase current disgrace and subject him to distrust, which is unlikely to be able to then rapidly dissipate, I had to take over this task and solve it. Otherwise experiences Laufen-Frankfurt and more, that then had to develop on this basis, would have gone on the way of application of single-phase current. "
In Laufen has in a short time built a small hydroelectric power plant. The turbine output of 300 hp rotated three-phase AC generator, designed and built, as already mentioned, at the plant in Erliksone. From the generator three copper wires of large cross section led to the switchboard. There have been installed ammeters, voltmeters, lead fuses and thermal relays. From switchboard three cables going to the three-phase transformers "prismatic" type. The windings of transformers connected in a star. It was supposed to conduct power at a voltage of 15 thousand volts, but the calculations were made on the work of 25 thousand volts. To achieve such a high voltage was planned to include two transformers at each end of the line so that they were of low voltage windings are connected in parallel, and higher winding - sequentially.
From transformers in Laufen began three-wire line suspended on 3182 wooden poles and a height of 8 to 10 m with an average span of 60 m. There are no switches on the line was not. In order to, if necessary, you can quickly turn off the current, provides two original tools. Near Laufenskoy hydroelectric two supports at a distance of 2 to 5 m installed from one another. Here, in the gap of each conductor lines include fuse, consisting of two copper wires with a diameter 0 15 mm. In Frankfurt and near the railway stations (part of the line was along the railroad tracks) have been established so-called angular contactors. Each of them was a metal bar suspended by a cord to the L-shaped support. It was enough to pull the cord, and the bar fell on all three wires, creating an artificial short-circuit, causing the fuse inserts in Laufen and de-energize the line. In Frankfurt wire approached step-down transformer (they were on show in a special pavilion), which reduced the output voltage to 116 volts. To one of these transformers were connected incandescent lamps 1000 16 candles (55 watts), each to the other - a large three-phase motor Dolivo-Dobrovolsky, located in another pavilion.
Linear voltage of the generator in Laufen was 95 volts. Up transformer turns ratio was equal to 154. Therefore, the operating voltage power line was 14650 volts (95 • 154). For that time it was very high voltage. Government land, through which the transmission line, were alarmed by its construction. Some had the feeling of fear, even in the wooden pillars, which were reinforced plates with skulls. Of particular concern is the possibility of wire breakage and drop it on the railroad tracks. Exhibition Committee and to construct lines of businesses had to spend a huge outreach, to convince government officials that all possible hazards are provided and that the line is secure. Baden The administration is still not allowed to connect site already finished lines in Baden border. In order to remove the last obstacles and dispel doubts of the local authorities, Dolivo-Dobrovolsky had a dangerous, but very convincing experiment. When the line was first included under stress, one of the wires on the border of Baden and Hesse she was artificially cut short and bright flash fell on the railway track. Dolivo-Dobrovolsky immediately walked over and picked up the wire with their bare hands: so he was confident that the work constructed them protection. This "method" of the evidence was very clear and removed the last barrier before testing line.
August 25, 1891 at 12 am at the exhibition for the first time broke out in 1000 electric bulbs fed current Laufenskoy hydroelectric. These lamps framed panels and arch over the entrance to the part of the exhibition, the exhibits that relate to the power Laufen - Frankfurt. The next day was successfully tested the engine power of 75 kilowatts, which is September 12 for the first time led by a ten-meter waterfall. Despite the fact that the line machines, transformers, switchboards were produced in a hurry (some of the details, according to Dolivo-Dobrovolsky, were thought out just for an hour), the entire installation included without prior testing, to the surprise of some and to the joy of others, immediately I began to work well. The special impression on visitors made waterfall. However, those more knowledgeable in matters of physics and electrical engineering, rejoice in this day a huge waterfall, sparkling with thousands of glass spray, illuminated by dozens of multi-colored lights. Their enthusiasm has been associated with the understanding that this beautiful man-made waterfall driven by a source located at a distance of 170 km on the Neckar River near the village of Laufen. They saw a brilliant solution to the power transmission problems over long distances.
In October, the International Commission has started tests Laufen-Frankfurt power line. It was found that the transmission system losses account for only 25%, which is a very good indicator. In November line was tested at a voltage of 25 thousand volts. At the same time it has increased efficiency and reduced losses to 21%. The vast majority of electricians all over the world (the exhibition was visited by over a million people) will appreciate the value of Laufen-Frankfurt experiment. Three-phase current was much appreciated, and it is the wide path into the industry was now open. Dolivo-Dobrovolsky immediately became one of the leading electrical engineers the world, and his name has become world famous.
It was resolved the main energy problem of the late XIX century - the problem of centralization of power generation and its transfer over long distances. All became clear the way in which a multi-phase current can be supplied from a distant power plant to each individual shop, and then a separate machine. The immediate consequence of the emergence of a multi-phase current technology was the fact that in the coming years in all developed countries began the rapid construction of power plants and the electrification of the broadest industry. However, in the early years, it is still complicated by a bitter struggle between competing companies seek to implement some type of current. For example, in America, first took up the company Westinghouse, which is buying up patents Tesla tried to extend the two-phase current. Triumph two-phase system was the construction in 1896, the powerful hydropower plant on the Niagara Falls. But the three-phase current was soon widely recognized as the best. Indeed, the two-phase system required of the four wires, and three-phase - only three. In addition to greater simplicity, she promised significant cost savings. Later, Tesla, for example Dolivo-Dobrovolsky, proposed to combine the two reverse wires together. This was accompanied by the addition of the currents in the third conductor current flowed at about 1 and 4 times greater than the other two. Therefore the cross section of the wire was 1 to 4 times more (without increasing the cross section of the overload occurred in the chain). As a result, the cost of a two-phase wiring still turned out more than three-phase, whereas the two-phase motors in all respects inferior to the three-phase. In XX century, three-phase system is established everywhere. Even Niagara power plant was eventually converted to a three-phase current.