[Trade Journal]
Publication: Electrical Age
New York, NY, United States
vol. 39, no. 11, p. 275-282, col. 1-3
Some Features of European High-
Tension Practice
By FRANK KOESTER
EUROPE, where high-tension, transmission systems originated, although not possessing as extensive systems as are common today in America, has many novel features, not only in transmission line construction, but also in switch gears. As many of these features are uncommon in American practice, the purpose of this paper is to give some points of European practice along these lines. However, it must not be taken for granted that all of the various features touched upon are of strictly European origin.
Starting with the wiring diagram, after which the general arrangement of the switching room is laid out, the systems are usually very complex, considered from an American point of view. One will find the greatest variations from the non-bus to double bars, or the double-ring bus system so commonly found in Swiss practice. To go directly to the point, Fig. 1 is submitted, which shows the wiring diagram of the Obermatt plant of the Lucerne-Engelberg, 27,000-volt transmission system, one of the recent and prominent Swiss installations.
Aside from a single generator set for railroad purposes, shown isolated at the left of the main diagram, the four generators supply current both for light and motor power, through a three-phase distribution system. Each of these four generators, designed to supply 1850 k.v.a. three-phase for power purposes, or 1380 k.v.a. single phase for lighting, generates at 300 rev. per min., 6000 volts at 50 cycles with 100 volts excitation.
The generator voltage is stepped up to 27,000, either by one single phase transformer, or a group of three for three-phase transformation. There are two groups for three-phase transformations, between which is a reserve transformer, used when one of the groups is out of commission. The current of any generator may feed the single-phase transformers, or may be sent through the three phase groups, for which purpose two ring systems are installed on either side of the transformer, one being single-phase, the other three-phase.
From the station lead three out-going lines, one for "Light Lucerne," one for "Power Lucerne" and the other for "Light and Power Unterwalden." The current for Lucerne is led to the substation Steghof, 27 miles away in the neighborhood of Lucerne. For both "Light Lucerne" and "Power Lucerne," two three phase lines are used; under ordinary conditions, one of the phases of the "Light" line is dead. The switching gear is so arranged that single- or three-phase current can be sent over either of the circuits, and the dead wire is in reserve in case one of the others is rendered inoperative.
The city of Lucerne demanded additional single-phase light free from fluctuations, which would not be given if motors were connected to the same circuit. To meet this condition the light and power loads are kept on in dependent circuits.
A similar system will be found in the power plant of the Vandoise Motor Power Co. on the Lakes Jonx and Ober, Switzerland. The five generators in this plant feed two three-phase ring systems and one single-phase.
Such arrangements may seem complicated on the surface, but after thoroughly studying them, the great flexibility is apparent, particular when one keeps in mind that the current is sup plied for various purposes by a few generator units. Further, when the nature of the future load cannot be ascertained, and the day and night load is irregular throughout the year; [in summer time the light load de creases, while the railroad load, particularly in mountainous countries. increases], the above wiring diagram copes admirably well with such conditions. However, such a system demands a large building to house the switching apparatus, which increases the first cost as well as that of maintenance.
There are many Continental plants which furnish good examples of the general layout of modern switching rooms. Fig. 2 shows a cross section of the switching room of the above mentioned Obermatt plant. Starting on the opposite side of the main generating floor, the compartment "MA" contains the circuit-breakers. On the gallery above are located the field rheostats, operated from the generator instrument column on the floor above, from whence the whole generating room is overlooked. Compartments "MO" contain the over load switches and buses for 6000-volt and 27,000-volt circuits. The transformer compartment "T" is provided with an elaborate circulating water piping system, controlled by an automatic signal device. "B" are horn lighting arresters, connected to water rheostats "WW." The same floor also contains the water-flow grounders "W S." and the choke coils "J."
This upper floor is devoted exclusively to apparatus for outgoing lines, and occupies only a small portion of the switching building. The switchboard for same is located in the rear of the instrument columns on the operating gallery. The two lower floors are divided up by partition walls into six separate rooms, with ample space between the various, buses, switches, etc. All bus bars are exposed, there being no partition between the individual phases. The only partition used is to separate the single and three-phase circuits. Such a lay-out is provided for both circuits, the 6000-volt and 27,000-volt. Of course, arrangement of this kind can be made only by having sufficient passageway for safety.
There are several Swiss hydroelectric installations, as well as sub stations, laid out on a similar scheme. This practice is not confined to hydraulic plants alone, but is also found in steam plants, an example of which is given in Fig. 3, showing the cross section of the generating and switching room of the St. Denis Power Plant, Paris. This plant is not only the largest one in Europe, but also the largest Parsons turbine station in the world, and is known to possess many novel features not found in American practice, particularly on the mechanical end, which, however, cannot be treated here, as this article is confined to switching practice only.
Commencing with compartment "D," which contains the generator leads, the current flows in alphabetical order, the apparatus being located as follows: "E" main generator switches; "F," main bus bars; "G." rheostats; "H," taking up the entire floor, contains the controlling bench and the outgoing feeder con trolling and switch-bench. No high tension current gets into the upper compartment, but is carried across to compartment "J," where the feeder switches are located. Compartment "K" contains the bus-bar junction switches, while "L" contains potential regulators. The last and lower compartment, "M," serves for the outgoing underground cables. There is a switchboard "C" located in the main generating room, beneath the staircase leading to the controlling bench room. This switchboard is equipped with instruments required for control of the exciter units, motor generator sets, booster and a polymorphic group.
This polymorphic group consists of four different machines, two motors and two generators, mounted on two shafts, coupled together. On either end is a 550-volt direct-current generator, between which is one three phase, 25-cycle, 10,250-volt alternator, and one two-phase, 42-cycle, 12,300 volt alternator, which can also supply 6150 volts. In the middle of the group is an electro-mechanical operated clutch coupling, making it possible to use the group in two sets. The two alternators, each having a capacity of 750 kw., may operate together under a load of 1500 kw. on the 550-volt service. It is also possible, with this group, to balance the load on the two alternating systems, by running the 25-cycle or the 42 cycle machines, or either one, or both of the direct-current machines as motors.
Controlling benches in Europe are not found in such general use as in America, but where they are they contain not only pilot switches, signal lamps, etc., but rheostat wheels and instruments necessary for the control of generators and outgoing feeders.
The structure of these benches is structural steel, forced with pressed or rolled steel plates. The sloping top is either of metal or white marble; slate or soapstone is little known in switch-gear practice. The dial-faced instruments are usually set flush with the top, while the edgewise instruments are set so they project above the surface. Although these benches are sometimes made entirely of metal, there is no danger to the operator, as the structure is well insulated.
Another novel feature in Continental switchboard practice is the wagon-panel system. In the Siemens Schuckert design, the entire panel and its equipment it built on a structural steel wagon, the rollers of which run on tracks. The design of the Allgemeine-Elektricitäts Gesellschaft consists of a small carriage containing the removable part, running on tracks on the structural steel frame of the switchboard. When the panel is to be removed a portable wagon is backed up, the latter is pulled out and the wagon removed. The movable panels in both systems are provided with locking devices, and cannot be withdrawn while they are operation. The electrical connections are made similar to knife-blade switches, consisting of heavy clips, which make and break the circuit when the panel is rolled in or out.
The principal advantage of the wagon-panel switchboard is that a panel can be withdrawn for inspection and repair, and a reserve panel shifted in without disturbing the operation of the remaining units; thus the dangers and delays otherwise encountered are eliminated.
The designs of switches greatly in many respects from those in American practice; however, there is no definite line of demarcation between the various types. Fig. 6 shows a 10,000-volt, 150-ampere, three-pole air-break switch, with automatic tripping device. In recent years it has found much favor in French and Swiss hydro-electric power plants. The make and break is made in a hollow porcelain cylinder of small bore. When the circuit is broken the arc formed expands the air so suddenly as to cause it to be extinguished by the induced draft. The application of this type of switch in a more simple design is seen in Fig. 7. Frequently these witches are placed on the floor below the switchboard, and operated from same by means of levers and steel cable.
Fig. 8 shows a 30,000-volt, 300 ampere oil switch, with an electro magnetic tripping device, a type common in equipments furnished by the Oerlikon Co., who also designed the previously mentioned type. A striking feature is the small oil cells. The switch in practice is set in an open concrete cell, the steel framework being removed.
A Siemens-Schuckert 6000-volt oil switch, with two current blowouts and one voltage relay, is shown in Fig. 9. They are usually set on a framework of the switchboard, or on the floor beneath same, and operated from the board by levers, sheaves and steel cables. They are installed for voltages up to 20,000; above this the phases are placed in separate oil tanks. In the latter case they are motor-operated, as will be seen in Fig. 10, showing the 35,000-volt switching room of the Heimbach plant, Germany. As already stated, most of the high tension apparatus is placed in open compartments exposed to view, and the contacts of many high-tension switches are placed in a single oil tank; such an arrangement is seen in Fig. 11, showing 11,000-volt and 50,000-volt oil switches in a substation at Castellanza, Italy. The types of transformers are practically the same as found in American practice; however, it may be of interest to give here some data on a high-voltage, air-cooled transformer as installed in an Italian substation at Lomazzo, where there are a number of 1250 k.v.a. 42,000- and 11,000-volt single-phase transformers. The peculiar feature of these transformers is that the cores are not encased, but placed in masonry compartments, through which air is forced from ducts beneath. The front of the compartments is cut off by a rolling shutter, and the air is discharged through the roof ventilators of each compartment. The reason given for this de sign is that ready inspection and re pair can be made, although when extensive repairs are required the transformers are removed to the repair shop. The manufacturers, Alioth, Münchenstein, Switzerland, guarantee the following efficiencies:
Regulation at 1.00 power factor full load 1% Regulation at 0.80 power factor full load 3% Regulation at short circuit.. 3% Efficiency, fill load… 97% . Efficiency, half load …85%
The operation of the blowers is included in these efficiencies.
The horn-type lightning arrester, so prominent in European practice, is always found in connection with auxiliary apparatus such as choke coils of the various types, oil and water rheostats, and especially water-flow grounders. The oil rheostat is similar in principle to the electrolytic arrester recently introduced into American practice.
Frequently horn-type lightning arresters are also equipped with other auxiliary devices; for instance, the Siemens-Schuckert relay horn lightning arrester has, in connection with it, condensers, Tesla transformers, automatic blowout, etc. The horns are placed three to four millimetres apart, which is the lowest practical setting, because dust or other particles may collect and cause it to discharge with a closer one. The gap of three to four millimetres will cause the arrester to discharge, under ordinary operating conditions, at 8000 volts, but with the use of the auxiliary apparatus it will discharge at 3000 volts and lower without changing the setting of the hours. This is accomplished by the discharge of an auxiliary gap set off by two condensers, as shown in Fig 12. The auxiliary discharge causes high frequencies in the Tesla transformer which starts the main gap.
Another application of the Siemens horn is shown in Fig. 13. It consists of a series of gaps connected to several layers of choke coils which, in turn, are connected to oil rheostats.
Water-flow grounders are very much used in Continental practice, and one will find them on practically every high-tension system. The water in a hydraulic plant is drawn from a penstock, while in other plants and substations it is supplied by a centrifugal pump to an elevated tank, from which it flows by gravity, or from city mains, or from springs when available. Fig. 14 shows a battery of lightning arresters as installed in the hydro-electric plant of the Vandoise Motor Power Co., Switzerland. It will be observed that the connections from the choke coils are attached to the funnel through which the water flows. The upper and lower tanks are connected by piping, so that there is a continuous circulation of water.
A grounder with the stream flowing upward against a baffle-plate is seen in Fig. 15. It was installed by the Alioth Co. in the 50,000-volt substation at Piattamala, Italy. The stream of water is 3/8 in. in diameter, 28 in. high, and allows a leakage of 0.1 ampere. The water is supplied by a spring under a head of 26 ft. Ammeters are inserted in the line connection to the apparatus, to detect failure in grounding.
In transmission line construction, European engineers are frequently handicapped by public service com missions, specifying the length of spans in street and railroad crossings, and that special structures or poles have to be put up for suspending guard wires, etc. With Heimbach 35,000-volt transmission system, Germany, much expenditure was incurred in this respect. In some crossings the public service commission was not satisfied with guard wire netting, but separate latticed steel construction had to be erected.
A good example of public service regulation in transmission line construction is shown in Fig. 17, giving a section of the 50,000-volt line between Kykkelerud and Hafslund, Norway. The close spacing of the towers was not alone sufficient; triangular steel frames had to be attached to the cross-arms so that in case of the breaking of a conductor the line is instantly grounded. A latticed structure used on the 50,000-volt Swiss-Italian transmission system (Brusio) is seen in Fig. 18. Besides having this expensive structure, the towers had to have a close spacing.
A novel arrangement of carrying transmission towers above the water will be found on the 27,000-volt Ober matt-Lucerne line, Fig. 19. Owing to the depth of the lake, the narrow street and the steep mountain on the other side, the only solution for placing the towers was on cantalever structures held in position by heavy concrete blocks. In order to protect the cantalever and the tower itself from boulders, heavy masonry abutments were placed on top of the concrete blocks; a passageway is provided to reach the tower. The total length of the cantalever is 30 ft., and the spacing of same 400 ft. The towers themselves are about 45 ft. high, measured from the cantalever to the lowest conductor. It will be observed, a practice common in continental line construction, that angle iron guards are placed to prevent a wire from falling to the ground in case of the breaking of an insulator, etc.
One of the latest and foremost transmission system in Europe is in the northern part of Italy, receiving its current from the hydraulic plant at Campocologno, located in the utmost southeastern part of Switzerland. The generator voltage (7000) is sent across the boundary through a tunnel, 1650 ft. long, to a substation at Piattamala, where it is stepped up to 50,000 volts. This tunnel is eight to Io ft. wide and 9.8 ft. high, the top being arched. Two separate circuits are placed, one on either side of the tunnel, and are provided with a re movable guard netting.
The tunnel scheme was chosen because the narrow valley through and over which the line would have to pass is frequented by storms, atmospheric discharges and great temperature differences.
The substation at Piattamala is built in the shape of a "T," accommodating 24 oil-cooled water-circulated transformers, 1250 kw. each. The transformers are arranged in two rows, between which are transfer tables for removing transformers to the repair room at the end of the crossleg of the "T." The 7000-volt leads enter the substation at the bot tom of the "T," and leave same at both ends of the crossleg.
The transmission line is in duplicate spaced from 13 to 16.5 ft. apart. The towers of latticed construction are about 40 ft. high up to the lowest conductor, and the average spacing is 393 ft. while the longest is 1280 ft., crossing the Gravina Valley at Colico. The lowest point of the transmission line is 640 ft. above sea level, crossing the Adda Valley; the highest point is at Palasco, 2130 ft. above sea level. The whole 50,000-volt transmission is 88.5 miles long.
From Lomasso, where the step down station is located, a 20,000-volt line runs to Como, a distance of 30 miles, while a 11,000-volt line runs 35 miles southward to a steam plant at Castellanza, for giving or drawing current from same. The bulk of the current goes to weaving mills.
The whole transmission line is divided into sections, varying from 8.5 to 25.5 miles in length. At the division points the line runs through section houses as seen in Fig. 20. The latter are equipped with sectionalizing switches, measuring apparatus and lightning arresters of the horn type, provided with choke coils and water flow grounders. As the line is in duplicate, disabled sections can be by passed at these section stations. At à distance of 65 ft. and parallel to the 50,000-volt line is a telephone and telegraph line for the exclusive use of the company.
Contrary to the average American practice, the wall outlets are of a very simple design. They consist of one or two sheets of plate glass with a porcelain bushing, though sometimes the latter is omitted. Insulators are placed on both sides of the panel so that this section of the conductor always remains straight. In some cases porcelain bushings are inserted in the wall, and the conductors lead directly to a distant tower. The hood common to American practice is practically unknown.
Insulators on European transmission lines are of an exceptionally small design; they are made either of one or two pieces.
Fig. 21 shows a four-petticoat, two piece insulator much used in Swiss and Italian practice, which is the result of many years' experience. The insulator shown has been used on a 50,000-volt Italian transmission line. It is 12 in. high and about 13 in. in diameter.
Particularly on the continent, insulators are frequently fastened to their pins by hemp and tow, with shellac or asphalt. Cement is not very much used, owing to the expansion of the cement cracking the insulators. For the same reason two piece insulators are frequently fastened together by hemp and shellac. Another binding material used is plaster of Paris.
