Publication: Electrical World and Engineer
New York, NY, United States
20,000-Volt Three-Phase Plant in France.
BY P. LETHEULE.
HIGH-VOLTAGE transmissions are rare in France and though many projects have been proposed, they will probably await the commercial success of the plant forming the subject of this article. This plant is situated in the Department of Aude, in the extreme southeast of France, of which Carcassonne and Narbonne are the principal cities. The generating station is situated in the Corbieres, at the mouth of Saint Georges, near Quillan. The total head of water is about 330 feet. The dam is in the river Aude, about 35 miles from its source in the mountains.
The dam at Saint Georges is a solid masonry structure. A canal having a total length of 18,000 feet extends underground for some 13,000 feet, and is open for the remaining 5,000 feet. The open portion is built of cement supported by wood in a very similar manner to the construction used for the Simplon tunnel generating station. The size of the underground part of the conduit is 6 1/2 x 6 1/2 feet. The incline is 7 feet per mile. In the open part of the canal this incline is increased to 10 feet, and the section reduced proportionately.
The canal brings water to a forebay from which lead two iron conduits of 3 feet 3 inches diameter to the generating station. These conduits are supported every 10 feet of their length by masonry, on which they can slide, and an expansion joint is provided every 650 feet. In a length of 650 feet the loss of head is 3 1/2 feet.
The generating station is built over the iron conduits bringing the water to the turbines, and consists of a rectangular building of one floor, sufficient for eight groups of machinery, of which four are now installed and in service. At the end of the building is erected a platform, supporting the low-tension and transformer panels. In an extension of the building at the back is erected a high-tension switchboard and transformer room.
The generators deliver three-phase alternating current, at 2,900 volts, 50 cycles, to the low-tension panels in the generating room, and thence to the secondary low-tension side of the step-up transformers, the high-tension side of which delivers 20,000 volts.
The turbines are rated at 800 hp at 300 r. p. m. The wheels are of the Pelton type and were built by the Ateliers de Construction de Vevey. They are regulated either by hand or by an automatic regulator of the mechanical type. The distributor to the turbine offers four openings, which can be more or less regulated by a cylindrical valve controlled either by hand or by an automatic hydraulic regulator. The hydraulic regulator uses water under pressure from the conduits, which is previously cleaned by being passed through a filter. This regulator works perfectly, a sudden difference in load of 50 per cent., not affecting the speed above 4 per cent., and 100 per cent, variation, affecting the speed less than 10 per cent. It might be feared that the sudden difference in pressure would be dangerous to the conduits or the connections, but a regulator has been installed for preventing such a disastrous effect; it simply opens an outlet valve and discharges the water in case the pressure gets too high. The conduits conducting the water under pressure are also protected by tubes connecting with the open air at some distance above the station.
The efficiency of the turbines reaches 77 per cent, at full load. Their axis is feet above the level of the water on the lower side, but the corresponding power is not entirely lost, an exhaust tube being designed for admitting air and water, giving a kind of relative vacuum, and allowing a certain amount of the power to be regained.
The generators are Alioth three-phase machines. Their rating is 700 kw for a power factor of 100 per cent, or 540 kw for an 80 per cent, power factor. They operate at 50 cycles and 300 r. p. m., and generate 2,900 volts. The revolving part is entirely of iron with no windings. The fixed windings are star connected. The exciter is run on the end of the shaft, as in most German and Swiss installations. The exciting voltage is 50 to 60 volts, and the exciting current 25 amperes for full load, which gives .25 per cent, excitation loss. The efficiency at full load is 93 per cent. The drop in voltage is 5 per cent, for non-inductive load, and 16 per cent, for inductive load, 80 per cent, power factor. The weight of each machine is 35 tons, 12 tons of which are accounted for by the revolving part.
The cables from the machine to the low-tension switchboard in the gallery are provided with silver fuses. These cables are supported on porcelain insulators, and are provided with highly insulated connections.
The connections of the generating panels allow the machine to be run in parallel on one set of bus-bars, to which the low-tension secondaries of the step-up transformers can be connected. The voltage regulation of the alternators running in parallel is by hand, the field rheostats being mechanically coupled together and controlled by one single handle.
The high-tension step-up transformers which are between the high and low-tension switchboards consist of single-phase transformers, star-connected in the way well known in the United States, but which is quite exceptional with continental constructors. In spite of the high voltage, the transformers are not cooled with water or oil; the windings are simply exposed to the air, though it is proposed to employ mechanical air ventilation if necessary.
The high-tension side of the transformers is controlled by switches placed on the high tension switchboard, above the transformer room. The peculiar feature of these is the pneumatic or compressed air control from a distance, enabling the circuit to be opened or closed from the generating room by a man at the low-tension switchboard. There is a compressed air reservoir giving 90 lbs. pressure per square inch, which compressor is run by a low voltage, three-phase induction motor.
Reference to the accompanying map will show the extent of distributing network, which seems at first sight disproportionate to the amount of power distributed, which is only some 1,600 kw, though intended in the near future to reach a total of 3,600 kw. This is far from being a great amount of power, but France is not a country possessed of great sources of hydraulic power as are the United States and a few countries in Europe, such as Switzerland, Sweden or even Italy and Germany. The total length of lines reaches 240 miles, and will be extended to 360 miles. The distance from generating station to distributing center is 42 miles, and the main high-tension feeders distributing power from this center to the main transformer stations are from 18 to 24 miles long, so that the total distance from the central station to distributing points attains about 80 miles.