The first three-phase power transmission line is solved by a long-term dispute: a constant or alternating current?

The year of electrification birth can be considered as 1891, when the general test of the three-phase system was held at the International Electrotechnical Exhibition in Frankfurt am Main.

The reason for organizing this exhibition were the difficulties of Frankfurt's Oberburger in choosing the type of current for the city's central power station.

The question of "permanent or alternating current?" Was the subject of scientific discussions during the 80s of the last century. An authoritative international commission set up at the request of the mayor did not come to an agreement and could not make any definite recommendations. Therefore, it was decided to organize an international electrotechnical exhibition and give an opportunity to all interested persons and firms to demonstrate their proposals and technical capabilities. An international test commission was organized under the auspices of the German scientist German Helmholtz. Among the members of the commission were the Russian engineer R. E. Classon. It was assumed that the commission will test all proposed systems and give an answer to the question of choosing the type of current and the prospective power supply system.

The organizers of the Frankfurt exhibition, on the initiative of the prominent German electrical engineer O. von Miller, offered AEG, in which Dolivo-Dobrovolsky worked at that time, to transmit, by electricity, the energy of the waterfall on the river. Neckar (near the town of Laufen) to the exhibition grounds in Frankfurt. The distance between these two points was 170 km. In Laufen there was a turbine at the disposal of the builders of the transmission, which gave a useful power of about 300 hp.

Until then, the transmission distance, not counting several experimental installations, did not exceed 15 km, and some competent experts believed that the efficiency of the installation could be lower than 50%.

The management board of the AEG company agreed to carry out power transmission, and Dolivo-Dobrovolsky was to design and build an induction motor with a capacity of about 75 kW and three-phase transformers with a capacity of 100-150 kVA within a year (!).

The manufacture of the generator was entrusted to the chief engineer of Swiss factory "Oerlikon" C. Brown, who collaborated with Dolivo-Dobrovolsky in the field of designing multiphase machines. The time was extremely short, and the tasks were very important: first, the new system of current was to be tested in the face of the representatives of the whole world; Secondly, the scale of the test was unprecedented.

Motors and transformers for such power have never been built. There was no question of experimental designs. Dolivo-Dobrovolsky wrote about the problem that arose: "If I did not want to incur an indelible shame on my three-phase current and subject it to a mistrust that I would hardly be able to disperse quickly, I had to take on this task and resolve it. The experiments of Laufen-Frankfurt and much that was then to develop on their basis, would go along the path of applying a single-phase current. "

In August 1891, the exhibition first lit 1000 incandescent lamps, powered by a current from the Laufen hydroelectric station. On September 12 of the same year, the Dolivo-Dobrovolsky engine activated a decorative waterfall. There was a kind of energy chain, a small artificial waterfall was powered by the energy of a natural waterfall, 170 km away from the first one.

What was this first three-phase line?

At the hydroelectric power plant in Laufen, the energy developed by the turbine was transmitted via a conical gear to the shaft of a three-phase generator of 230 kV, 150 rpm, winding the star. From the generator, copper buses led to the switchboard. At the latter, ammeters and voltmeters, lead fuses and a maximum-minimum current relay, which acted on the excitation circuit, were installed.

In Laufen and Frankfurt there were three three-phase transformers with a prismatic magnetic core. At the beginning of the tests, one transformer with a capacity of 150 kVA each was connected at each end of the line, with conversion ratios of 154 in Laufen and 116 in Frankfurt. Since there was no instrument for measuring high voltage, the secondary voltage was determined by simply multiplying the primary voltage by the transformation ratio. Transformers were immersed in tanks filled with oil.

The three-wire line was made on wooden supports with an average span of about 60 m. A copper wire with a diameter of 4 mm was fixed on pin-type porcelain-oil insulators. An interesting detail of the line was the installation of fuses from the high voltage side. At the beginning of the line, a 2.5 m long section was included in the break of each wire, consisting of two copper wires 0.5 mm in diameter each. To disconnect the line in Frankfurt by means of a simple adaptation, a three-phase short circuit occurred, the fuse-links burned out, the turbine began to develop a high speed, and the operator, noticing this, stopped it.

At the exhibition site in Frankfurt was installed a step-down transformer, from which at a voltage of 65 V, there were 1000 incandescent lamps located on a huge shield. There was also installed a three-phase asynchronous motor Dolivo-Dobogidravlichesky pump capacity of 100 hp. The engine was made inverted, that is, powered by the rotor.

Simultaneously with this powerful engine, Dolivo-Dobrovolsky exhibited an asynchronous three-phase motor with a power of about 100 W, with a fan on its shaft and a 1.5 kW engine with a DC generator sitting on its shaft, the latter fed incandescent lamps.

Before the power transmission, unexpected difficulties arose. The fact is that the line crossed the territories of the four German lands, and local authorities were very afraid of high tension. People were afraid of the wooden pillars with the plaques on which the skull was depicted. Knowledgeable people were confused by the fact that the equipment at the power plant was grounded, as was the neutral of the transformer. In this regard, very much feared breaking the wire and dropping it to the ground.

The exhibition committee conducted a lot of explanatory work, convincing local government officials that all hazards are provided and the line is reliably protected. Dolivo-Dobrovolsky had to conduct a dangerous but convincing experiment. Representatives of local authorities gathered at the border of the two lands. They turned on the line under tension and, in front of those present, artificially cut off the wire which, with a bright flash, fell on the rails of the railway. Dolivo-Dobrovolsky immediately approached and picked up the wire with his bare hands - he was so sure that the protection he designed would work reliably.

August 25, 1891 official launch of the line took place. Despite the fact that the line, cars, transformers, distribution boards were made in a hurry, that some details were given to the Dolivo-Dobrovolsky certificate for an hour, the entire installation, included without preliminary testing, immediately began to work quite well. Dolivo-Dobrovolsky, who became a famous inventor, said that among the uninitiated public there was an opinion that "in this exhibition waterfall" the real water from Neckar "gurgles, transmitted to Frankfurt by wire.

The transmission tests carried out by the International Commission produced the following results: the minimum efficiency of transmission (the ratio of power at the secondary terminals of the transformer in Frankfurt to the power on the shaft of the turbine in Laufen) was 68.5%, the maximum efficiency was 75.2%; The line voltage during the tests was about 15 kV.

The final conclusion of the commission is typical: "... the work of the line with alternating currents from 7500 to 8500 V (phase-to-auth.), Insulated oil, porcelain and air, more than 100 kilometers long, flowed at regular intervals, safely and without irregularities, like Work with alternating currents with a voltage of several hundred volts and with a line length of several meters. " A further test of the power line was also carried out at a higher voltage of 25.1 kV. The maximum efficiency was 78.9%.

The results of the Laufen-Frankfurt power transmission tests not only demonstrated the possibilities of electric power transmission, but also put an end to a long-standing dispute. The "alternating current" technique won in the struggle "permanent - alternating current". As a curious relapse of the ideas that have gone into the history of the struggle, one can point to the following testimony of M.O. Dolivo-Dobrovolsky: "In September 1889, Edison visited Berlin. With his proposal to inspect the new AC electric motor, he literally waved his hands: "No, no, alternating current is nonsense, not having a future. I not only do not want to inspect the AC motor, but also to know about it. " And he did not come!

This same power transmission has convincingly shown that among the AC systems, the advantages are on the side of the three-phase systems. The International Electrotechnical Exhibition and the International Congress of Electrical Engineers (September 7-12, 1981, Frankfurt am Main), opened this system of electric currents to a wide path to industry. The era of electrification began.

Shukhardin S. "Technology in its historical development"