Page:EB1911 - Volume 22.djvu/248

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234
[ELECTRICAL
POWER TRANSMISSION


other improvements may considerably increase these figures. For distances at which more than this very moderate voltage is desirable one must either depend on alternating currents or use machines in series. In American practice the former alternative is universally taken. On the continent of Europe a very creditable degree of success has been achieved by adopting the latter, and many plants upon this system are in use, mostly in Switzerland. In these generators are worked at constant current, a sufficient number in series being employed to give the necessary electromotive force.

Power Transmission at Constant Current.-In this system, which has been developed chiefly by M. Thury, power is transmitted from constant current generators worked in series, and commonly coupled mechanically in pairs or larger groups driven by a single prime mover. The individual generators are wound for moderate currents, generally between 50 and 1 50 amperes, and deliver this ordinarily at a maximum voltage of 2000 to 3 500, the output per armature seldom being above 300 kw. For the high voltages needed for long distance transmission as many generators as may be required are thrown in series. In the Moutiers-Lyons transmission of 110 m., the most considerable yet installed on this system, there are four groups, each consisting of four mechanically-coupled generators. The common current is 75 amp., and the maximum voltage per group is about 15,000 volts, giving nearly 60,000 volts as the transmission voltage at maximum load. In the St Maurice-Lausanne transmission of about 5 5 m. the constant current is 150 amp. and the voltage per armature is 2300, tive pairs being put in series for the maximum load voltage of 23,000.

Regulation in such plants is accomplished either by varying the field strength through an automatic governor or by similarly varying the speed of the generators. Either method gives sufficiently good results. The transmission circuit is of the simplest character, and the power is received by motors, or for local distribution by motor generators, held to speed by centrifugal governors controlling field varying mechanism. For large output the motors, like the generators, are in groups mechanically coupled and in series. In the Moutiers-Lyons transmission motor-generators are even designed to give a three-phase constant potential distribution, and in reverse to permit interchange of energy between the continuous current and several poly phase transmission systems.

The advantages of the system reside chiefly in easier line insulation than with alternating currents and in the abolition of the difficulties due to line inductance and capacity. It is probably as easy to insulate for 100,000 volts continuous current as for 50,000 volts alternating current. Part of the difference is due to the fact that in the latter case the crest of the E.M.F. wave reaches nearly 75,000 volts, and in addition static effects and minor resonant rise of voltage must be reckoned with. There is some possibility, therefore, of the advantageous use of continuous current in case very great distances, requiring enormous voltages, have to be covered. In addition, a constant current plant is at full voltage only at brief and rare periods of maximum load instead of all the time, which greatly increases the average factor of safety in insulation.

On the other hand, the constant current generators are relatively expensive and of inconveniently small individual output for large transmission work, and require very elaborate precautions in the matter of insulation. Their efficiency is a little less than that of large alternators, but the difference is partially off-set by the transformers used with the latter for any considerable voltage. A characteristic advantage of the constant current system is the extreme simplicity and cheapness of the switching arrangements as compared with the complication and cost of the ordinary switch-board for a poly phase station at high voltage. Comparing station with station as a whole it is at least an open question whether the poly phase system would have any material advantage in cost per kw. in an average case. The principal ains of the alternating systems appear in the relative simplicity of the distribution. In dealing with a few large power units the constant current system has the best of the argument in efficiency, but in the ordinary case of widespread distribution for varied purposes the advantage is quite the other wav.

The high-voltage constant-current plant lends itself with especial ease to operation, at least in emergency, over a grounded circuit. In some recent plants, e.g. Moutiers-L ons, provision is made at the sub-stations for grounding the centrallpoint of the system and either line in case of need, and in point of fact the voltage drop in Working grounded is found to be within moderate and practicable limits. The possibilities of improvement in the system have by no means been worked out, and although it has been overshadowed by the enormous growth of poly phase transmission it must still be considered seriously.-Transmission

by Alternating Current.-The alternating current has' conspicuous advantages. In the first place, whatever the voltage of transmission, the voltage of generation and that of distribution can be brought within moderate limits at a very high degree of efficiency by the use of transformers; and, in the second place, it is possible to build alternating-current generators of any required capacity, and for voltages high enough to permit the abolition of raising transformers except in unusual circumstances. At present such generators, giving 10,000 to 13, 500 volts directly from the- armature windings, are in common and highly successful use; and while the use of raising transformers is preferred by some engineers, experience shows that they cannot be considered essential, and are probably not desirable for the voltages in question, which are as great as at the present time seem necessary for the numerical majority of transmission plants. Polyphase generators, especially in large sizes, can be successfully wound up to more than double the figures just mentioned. The plant at Manojlovac, Dalmatia, has been equipped with four 30,000 volt three-phase generators, giving each about 5000 kw. at 42 ~with 420 revolutions per minute, the full load efficiency being 94%. But for very large transmission work to considerable distances where much higher voltages are requisite such transformers cannot be dispensed with. Alternating currents are practically employed in the poly phase form, on account both of increased generator output in this type of apparatus and of the extremely valuable properties of the poly phase induction motors, which furnish a ready means for the distribution of power at the receiving end of the line. As between two- and three-phase apparatus the present practice is about equally divided; the transmission lines themselves, however, are, with rare exceptions, worked three-phase, on account of the saving of 25% in copper secured by the use of this system. Inasmuch as transformers can be freely combined vectorially to give resultant electromotive forces having any desired magnitudes and phase relations the passage from twophase to three-phase, and back again, 'is made with the utmost ease, and the character of the generating and receiving apparatus thus becomes almost a matter of indifference. As regards such apparatus it is safe to say that honours are about even: sometimes one system proves more convenient, sometimes the other. The difficulty of obtaining proper single-phase motors for the varied purposes of general distribution has so far prevented any material use of single-phase transmission systems. Generators for Power T transmission.—The generators are usually large two- or three-phase machines, and in the majority of instances they are driven by water-wheels. Power transmission on a large scale from steam plant has, up to the present, made no substantial progress, save as the networks of large electrical supply stations have in some cases grown to cover radii of many miles. The size of these generators varies from IO0 or 200 kw. in small plants, up to 10,000 or more in the larger ones. Their efficiency ranges from 92% or thereabouts in the smaller sizes up to 96% or a fraction more in the largest, at full load. The voltage of these generators varies greatly. When raising transformers are used it is usually from (500 to 2500 volts; without them the generators are usually woun for 10,000 to I '§ ,500 volts. Intermediate voltages have sometimes been employed, but are rather passing out of use, as they seem to fulfil no particularly useful purpose. The tendency at the present time whatever the voltage, is towards the use of machines with stationary armatures and revolving field magnets, or towards a pure inductor type having all its windings stationary. At moderate voltages such an arrangement is merely a matter of convenience, but' in high-voltage generators it is practically a necessity. Low-voltage machines are usually provided with polyodontal windings, these windings having several separate armature teeth per pole per phase, while the high-voltage machines are generally monodontal; in both classes the edges of the pole pieces are usually chambered away in such form as to produce at least a close approximation to the sinusoidal form for the electromotive force. For this purpose, and to obtain a better inherent regulation under variations of load, the field magnets are, or should be, particularly powerful. In the best modern generators the variation of electromotive force from no load to full load, non-inductive, is less than 10% at constant field excitation. Closeness of inherent regulation is an important matter in generators for transmission work 1