1911 Encyclopædia Britannica/Power Transmission/Electrical
IV.—Electrical
Though the older methods of power transmission, such as wire ropes, compressed air and high-pressure water, are still worked on a comparatively small scale, the chief commercial burden has fallen upon the electric generator and motor linked by a transmission line. The efficiency of the conversion from mechanical power to electrical energy and back again is so high, and the facility of power distribution by electric motors is so great, as to leave little room for competition in any but very exceptional cases. The largest single department of electrical power transmission-that is, transmission for traction purposes -is at present almost wholly carried on by continuous currents. The usual voltage is 500 to 600, and the motors are almost universally series-wound constant-potential machines. The total amount of such transmission in daily use reaches probably a million and a half horse power. In long distance power transmission proper continuous currents are not used to any considerable extent, owing mainly to the difficulty of generating continuous currents at sufficient pressure to be available for such work, and the difficulty of reducing such pressure, even if it could be conveniently obtained, far enough to render it available for convenient distribution at the receiving end of the line. Single continuous current machines have seldom been built successfully for more than about 2000 to 3000 volts, if at the same time they were required to deliver any considerable amount of current. About 300 to 500 kilowatts per machine at this voltage appears to be the present limit, although it is by nomeans unlikely that the use of com mutating poles and 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 inasmuch as there is as yet no entirely successful method of automatic voltage regulation on very large units; and the less hand regulation the better. Moreover, the design which secures this result also tends to secure stability of wave form in the electromotive force, a matter of even greater importance. There has been much discussion as to the best wave form for use on alternating circuits, it having been conclusively shown that for a given fundamental frequency the sinusoidal wave does not give the most economical use of iron in the transformers. For transmission work, however, particularly over long lines, this is a matter of inconceivably small importance compared with the stability and the freedom from troubles from higher harmonics that result from the use of a wave as nearly sinusoidal as can possibly be obtained. In every alternating circuit the odd harmonics are considerably in evidence in the electromotive force, either produced by the structure of the generator or introduced by the transformers and other apparatus. These are of no particular moment in work upon a small scale, but in transmission on a large scale to long distances, or especially through underground cables, they are, as will be seen in the consideration of the transmission line itself, a serious menace. Inasmuch as the periodicity of an alternating circuit must be maintained sensibly constant for successful operation, great care is usually exercised to secure such governing of the prime movers as will give constant speed at the generators. This can now be obtained, in all ordinary circumstances, by several forms of sensitive hydraulic governors which are now in use. The matter Of absolute periodicity has not yet settled itself into any final form. American practice is based largely upon 60 cycles per second, which is probably as high a frequency as can be advantageously employed. Indeed, even this leads to some embarrassment in securing good motors of moderate rotative speed, and the tendency of the frequency is rather downward than upward. An inferior limit is set by the general desirability of o erating incandescent lamps off the transmission circuits. For this purpose the frequency should be held above 30 cycles per second; below this point, flickering of the lamps becomes progressively more serious, especially with lamps having the very slender metallic filaments now commonly employed—so serious, indeed, as practically to prohibit their successful use—and plants installed for such low frequencies are generally confined to motor practice, or to the use of synchronous converters, which are somewhat easier to build in large units at low than at high periodicities. Occasional plants for railway and heavy motor service operate at as low as 15 ~, and more at 25 ~. Nearly all the general work of power transmission, however, is carried on between 30 and 60 ~. The inferior limit at which it is possible successfully to operate alternating arc lamps is about 40 ~; and if these are to be an important feature in transmission systems the indications are that practice will tend towards a periodicity above 40 ~, at which all the accessory apparatus can be successfully operated. European practice is based generally upon a frequency of 50 ~, which admirably meets average conditions of distribution.
Transmission Lines.—Power transmission lines differ from those used for general electric distribution principally in the use of higher voltage and in the precautions entailed thereby. The economic principles of design are precisely the same here as elsewhere, save that the conductors vary less in diameter and far more in length. Inasmuch as transmission systems are frequently installed prior to the existence of a well-developed distribution system the conditions of load and the market for the power transmitted can seldom be predicted accurately; consequently, the cases are very rare in which Kelvin's law can be applied with any advantage; and as it is at best confined to determining the most economical conditions at a particular epoch this law is probably of less use in power transmission than in any other branch of electric distribution. A superior limit is set to the permissible loss of energy in the line by the difficulty attending regulation for constant potential in case the line loss is considerable. The inferior limit is usually set by the undesirability of too large an investment in copper, and lines are usually laid out from the standpoint of regulation rather than from any other. In ordinary practice it seldom proves advantageous to allow more than 15% loss in the line even under extreme conditions, and the cases are few in which less than 5% loss is advisable. These few cases comprise those in which the demand for power notably overruns the supply as limited by the maximum power available at the generating station, and also the few cases in which a loss rea ter than 5% would indicate the use of a line wire too small iom a mechanical standpoint. It is not advisable to attempt to construct long lines of wire smaller than No. 2 American wire-gauge (-257 in. diameter), although occasionally wire as small as No. 4 (°204 in. diameter) may safely be employed. Smaller diameter than this involves considerable added difficulty of insulation in lines operated at voltages in excess of about 50,000. The vast majority of transmission lines are composed of overhead conductors. In rare instances underground cables are used. In single-phase work these are preferably of concentric form, which, however, gets too complicated in the three-phase lines generally employed to secure economy in copper; for the latter, triplicate cables, lead sheathed, laid in glazed earthenware ducts, seem to give the best results. On account of the cost and the difficulty of repair of such lines they are not extensively used, and cables have not yet been produced for the extremely high voltages desirable in some very long circuits, although they are readily obtainable for voltages up to 30,000 or 40,000. As to the material of the conductors, copper is almost universally used. For very long spans, however, bronze wire of high tensile strength is occasionally employed as a substitute for copper wire, and more rarely steel wire; aluminium, too, is beginning to come into use for general line work. Bronze of high tensile strength (say 80,000 to 100,000 lb per square inch) has unfortunately less than half the conductivity of copper; and unless spans of many hundred feet are to be attempted it is better to use hard-drawn copper, which gives a tensile strength of from 60,000 to 65,000 lb to the square inch, with a reduction in conductivity of only 3 to 4%. As to aluminium, this metal has a tensile strength slightly less than that of annealed copper, a conductivity about 60% that of copper, and for equal conductivity is almost exactly one-half the weight. Mechanically, aluminium is somewhat inferior to copper, as its coefficient of expansion with temperature is 50% greater; and its elastic limit is very low, the metal tending to take a permanent set under comparatively light tension, and being seriously affected at less than half its ultimate tensile strength. Joints in aluminium wire are difficult to make, since the present methods of soldering are little better than cementing the metal with the flux; in practice the joints are purely mechanical, being usually made by means of tight-fitting sleeves forced into Contact with the wire. With suitable caution in stringing, aluminium lines can be successfully used, and are likely to serve as a useful defence against increase in the price of copper. Whatever the material, most important lines are now built of stranded cable, sometimes with a hemp core to give added flexibility.
With respect to line construction the introduction of high voltages, say 40,000 and upwards, has made a radical change in the situation. The earlier transmission lines were for rather low voltages, seldom above 10,000. Insulation was extremely easy, and the transmission of any considerable amount of power implied heavy or numerous conductors. The line construction therefore followed rather closely the precedents set in telegraph and telephone construction and in low tension electric light service. In American practice the lines were usually of simple wooden poles set 40 to 50 to the mile, and carrying wooden cross-arms furnished with wooden ins carrying insulators of glass or porcelain. The poles were little larger than those used in telegraph lines, a favourite size being a 40-ft. pole about 8 in. in diameter at the top and 15 in. at the butt, set 6 to 7 ft. in the earth. Such poles commonly bore two cross arms, the lower and longer carrying 4 pins, and the shorter upper arm 2 pins, so disposed that the upper pin on each side of the pole would form with the nearer .pins below an equilateral triangle 18 to 24 in. on the side. The poles therefore carried two three phase circuits one on either side, one or both circuits being spiralled. In European practice iron poles have been more frequently used, again following rather closely the model of telegraph practice, with similar spacing of poles, and with insulators, usually of porcelain, somewhat enlarged and improved over telegraph and electric light insulators, and spaced somewhat more widely. As between wooden and steel poles, the latter are of course the more durable and much the more costly. The difference in cost depends largely on the locality, and ultimately on the life of the wooden poles. This ranges from two or three up to ten or fifteen years, the latter figures only in favourable soils and when the lower ends of the poles have been thoroughly treated with some preservative. Under such conditions wood is often ultimately the cheaper material.
The use of very high voltages results in, for all moderate powers, the use of small and consequently light wires and in the necessity for heavy, large and costly insulators. For security against leakage and failure it becomes desirable to reduce the numer of insulation points, and with the resulting lengthening of span to design the line as a mechanical structure. A transmission line is subject to three sets of stresses. The most considerable are those due to the longitudinal pull of the catenary depending on the weight and tension of the wires. Under ordinary conditions these strains are balanced and come into play only when there is breakage of one or more wires and consequent unbalancing. It has been the common practice to give the poles sufficient strength to withstand this pull without failing. The maximum amount of the pull may be safely taken at the sum of the elastic limits of the wires, since it is unsafe so to design the spans as to be subject to larger stresses. There is also lateral stress on a line due to wind acting upon the poles and wires, the latter amounting to little 'unless their diameter is increased by a coating of sleet, a condition which gives maximum stresses on the line. Wind then tends to push the line over, and it also increases the longitudinal stresses, being added geometrically to the catenary stress. The actual possibility of wind pressure is very generally over-estimated, and has resulted in much needlessly costly construction. In the first place, save for actual tornadoes, for which no estimates can be given, even the highest winds at the level of any ordinary transmission line are of modest actual velocity. It is probable that no transmission line save on mountain peaks at a very high elevation is ever exposed to an actual wind velocity of 75 m. per hour, and only at intervals of years is a velocity of even 60 m. reached near the ground level. Further, the maximum wind velocities are practically never reached at very low temperatures when the line is under its maximum catenary stress, and sleet formation, which takes lace only within a very limited temperature range, is practically unknown under conditions of maximum wind. The relation of wind velocity to pressure in case of a suspended wire or cable may be approximately expressed by the equation P=0-0025V2, where P is the pressure per square foot of projected area of cable, and V is the actual wind velocity in miles per hour. Except for sleet conditions the wind pressure is, then, a matter of little concern. At times sleet may accumulate on bare wires to a thickness of half an inch to an inch. Even under these conditions the lateral stability of the line is a matter of less concern than the added component of stress in the catenary. The third element of line stress, the actual crushing stress of the wire load, is of no consequence in high voltage transmission work. In scientific line design the best example has been set by the Italian engineers, who, realizing that the longitudinal strains, which are very severe in case of breakage of spans rigidly supported from pole to pole, are immediately relieved by a slight increase in catenary drop, have introduced the principle of longitudinal flexibility. The poles or towers of structural steel are so designed as to be fairly stiff against lateral pressure and are given secure foundation against overturning, but are deliberately designed to deflect lengthwise the line in the extreme case of breakage of wires so as at once to relieve the catenary tension without passing their elastic limit. In this way complete security is attained with a miniinum of material and expense.
In recent construction both in America and Europe the tendency is to use steel poles or towers of ample height, 40 to 60 ft. and spans ranging from 300 to 600 ft., occasionally more. The catenary drop allowed is considerable, often 3 to 4% of the span length. Crossarms and pins, when used, are commonly of iron or steel, and the interiors of the insulators are therefore fairly at earth potential. The insulators are of dense and hard-baked porcelain, built up of three or four shells cemented together to form a whole, with several deep petticoats to protect the inner surfaces from wettin . Such insulators may be 12 to 18 in. in diameter over all, and iom top groove to base a little more. If well designed and made, insulators of this type can endure even under very heavy precipitation alternating voltages of 60,000 to 100,000 effective without Hashing over, and double these figures when dry. For line voltages above 60,000 to 70,000 it is apparent that the insulating factor of safety would be seriously reduced, and some recent lines have been equipped with suspension insulators. These are in effect porcelain bells from 10 in. diameter upward strung together like a string of Japanese gongs. The bells are all the same size and are spaced about a foot apart, the suspensions being variously designed. These insulating groups can be as large as need be, and it is easy to push the aggregate insulation resistance, both dry and wet, far beyond the figures just mentioned. This suspension requires higher poles than the ordinary, but allows a considerable amount of longitudinal back lash, in case a wire burns off. T00 extensive slip along the line is checked by guys fitted with strain insulators, like the suspension ones, at suitable intervals. The suspension insulator gives promise of successful use of voltages much higher than 100,000 volts. The wires on high voltage systems are generally widely spaced: very seldom less than 2 ft. between centres, and for the higher voltages something like 1 ft. for each 10,000 volts.
Voltage.-The most important factor in the economy of the conducting system is the actual voltage used for the transmission. This varies within very wide limits. For transmissions only a few miles in length the pressures employed may be from 2000 to 5000 volts, but for the serious work of power transmission less than 10,000 volts are now seldom used. This pressure, under all ordinary conditions and in all ordinary climates, can be and is used with complete success, and apparently without any greater difficulty than wouldbe encountered at much lower voltage. It is regarded as the standard transmission voltage in American practice for short distances up to 10 or 15 m. Beyond this, and sometimes even on shorter lines, it is greatly increased; up to 20,000 volts there seems to be no material difficulty whatever in effecting and maintaining a sufficient insulation of the line. In the higher voltages there were in 1908 more than fifty plants in regular operation at 40,000 volts and above. Of these more than a score are operated at 60,000 volts and above. The highest working voltage employed in 1909 was 110,000 volts, which was successfully used in two American plants: that of the Grand Rapids-Muskegon (Michigan) system, and in the transmission work of the Central Colorado system. These both employ suspension insulators with five bells in series, and operate with no more trouble than falls to the lot of systems using ordinarily high voltages. The Rio de ]aneiro transmission system, operates at 88,000 volts with large porcelain insulators, I7-5 in. in over-all diameter and 19-75 in height, carried on steel pins; the Kern River (California) plant at 75,000 volts with similar construction; the Missouri River Power Co. (Montana) at 70,000 volts, using glass insulators on wooden pins saturated with insulating material. There is no especial difficulty in building transformers for still higher pressures, the real problem lying in the insulation of the line. Taken as a whole these high voltage lines have given good service, those near the upper limit doing apparently as- well as those near the lower, owing to more careful precautions in construction. Likewise the distances of transmission have steadily risen. There are, all told, nearly a score of power transmissions over 100 m. in length, the longest distance yet covered being from De Sabla to Sausalito (California), a distance of 232 m. This, like most other long American transmissions, is at 60~, and it is interesting to note that even over such distances there seems to be very little evidence of trouble due to frequency. In point of fact, those who have had the most experience with long distance transmission are the last to worry about the difficulties of using alternating current. Some unusual phenomena turn up in high voltage work, but they are rather interesting than alarming. The lines become self-luminous from “ coronal ” discharge at a little above'20,000 volts, and at 40,000 or 50,000 volts the phenomenon, which is sometimes aggravated by resonance, becomes of a striking, not to say startling, character. At above 100,000 volts this coronal discharge must be given serious consideration. Resonance, in substance, is due to synchronise of the periodic electromotive force, or a harmonic thereof, with the electro-magnetic time-constant of the system. The frequency of the currents actually employed in transmission work is so low that resonance with the fundamental frequency must be extremely rare; resonance with the harmonics is, however, common-much commoner than is generally supposed. In every electromotive force wave the odd harmonics are more or less in evidence, particularly the third, fifth and seventh. If the electromotive force wave departs notably from a sinusoidal form, traces of harmonics up to at least the 15th may generally be found; the third, seventh and the alternate higher harmonics are manifest in iiattening the crest of the wave. Supposing, what is seldom quite true, that the harmonics are symmetrically disposed in phase with the fundamental, all the harmonics tend somewhat to elevate the shoulders of the wave; a wave, therefore, with peaked shoulders and a depression in the centre is certain to be affected by harmonics, while if it has a high central crest, there is evidence of great predominance of the fifth and higher harmonics. Generally the harmonics are slightly out of phase with the fundamental, so that the wave is both deformed and unsymmetrical. As to the amplitude of these harmonics, the third is usually the largest, and may sometimes in commercial machines amount to as much as 20% of the amplitude of the fundamental, and frequently 10%. In machines giving nearly sinusoidal waves it is of course much less, but it is not difficult to find even the seventh and higher harmonics producing variations as great as 5%. Since, other things being equal, the rise in electromotive force due to resonance is directly proportional to the magnitude of the harmonics, and the chance of getting it increases rapidly with the presence of those of the higher orders, the desirability of using the closest possible approximation to a sinusoidal wave is self-evident. The greater the inductance and capacity of the system and the less its ohmic resistance, the greater the chance of getting serious resonance. As regards the distributed capacity and inductance due to the line alone, the ordinary conditions are not at all formidable; the general effect of such distributed capacity and inductance is to produce in the system a series of static waves, their length varying inversely with the frequency. At commercial frequencies the wave length is very great, so great that even in the longest lines at present employed only a small fraction of a single wave length appears; the total length of the line is generally much less than one quarter the complete wave length, and the only notable effect is a moderate rise of potential along the line. The time-constant of the alternating circuit is T=~00629x/ (LC), where L is the absolute self-induction in henrys and C the capacity in micro farads; and if the frequency, or a marked harmonic thereof, coincide with this time-period, resonance may safely be looked for, and resonance of the harmonics may appear conspicuously in lines of ordinary lengths. The following table gives the values, both L and C, per mile of three-phase circuit, of the sizes (American wire-gauge) ordinarily employed for transmission circuits, the wires being assumed to be strung 48 in. apart and about the height already indicated:- 000
4
1
0
nch
IO
2
-2
-2
-22
-204
o-00322
0-003,28
o-oo336
o-oo338
O'O0347
0-00351
0~0164
0'O160
0-0157
0-0154
0-0151
O'O148
Y Size No. Diameter. L. C.
oooo 0-460 O'0O3I2 0-0167
0-4
oo 0-365
0 0-3 5
1 0 89
2 0 58
3 0 9
o-oo358
O'O145
In cases where underground cables form a part of the system, the above values of C are very largely increased, and the probability of resonance is in proportion enhanced. A still further complication is introduced by the capacity and inductance of the apparatus used upon the system, which may often be far greater than that due to the entire line, even if the latter be of considerable length. In point of fact, it is altogether probable that resonance due to the distributed capacity and inductance of the overhead line alone is of rare occurrence and generally of trivial amount, while it is equally probabld
that resonance due to localized capacity and inductance other than that of the line conductors may, and often does, cause very serious disturbances upon the system. The subject has never been adequately investigated, but the tendency towards formidable sparking and arcing at various points on long-distance transmission systems is generally far greater than can be accounted for by consideration of the nominal voltages alone. The conditions may be still further complicated by the effect of earths or open circuits, which sometimes may produce, temporarily, appalling resonance phenomena, through bringing into action the capacity and inductance of the apparatus and introducing surges. In ordinary working the resonance of the harmonics is not very conspicuous, and the fact that it occurs not systematically, but only in special ways and under special conditions, indicates more strongly than anything else that the vital point is not the time-constant of the line alone, but those of the apparatus connected thereto. A definite and persistent tendency towards resonance may sometimes be effectively checked by the introduction of suitable inductance in the parts of the system most seriously affected, but the best general policy is to avoid as far as possible the presence of the higher harmonics, which are the chief sources of dan er.
Ciosely allied to and connected with resonance is the phenomenon known as “ surging, ” which is due to the discharge of the electromagnetic energy stored in a circuit containing inductance and capacity when that circuit is broken. This discharge is an oscillatory one, going on with decreasing amplitude until it is frittered away by resistance and other sources of loss. Its frequency is that of the system affected, and the surge may get reinforcement from resonance proper. It is sufficiently serious on its merits, however, since the resulting rise of voltage increases directly with the current and may produce terrific results when the break comes as the result of a short circuit. Minor surging occurs when there is a sudden and violent change in the conditions of the circuit even without an actual break. Such a change produces an impulsive redistribution of energy that may give a sharp rise in voltage. Every point of abrupt variation in the electrical constants on the system is liable to be affected by minor surges. Such disturbances when trivial are commonly referred to as “ static.” Surging, depending as it does on the current ruptured, may, and indeed often does, give particularly formidable effects on circuits of moderate voltage, while on high voltage transmission circuits the usually moderate current and the large margin of safety in the insulation are important ameliorating influences.
Maintenance.-Transmission lines are, when practicable, laid out through open country, and along roads which furnish easy access for inspection and repairs. The chief sources of danger in temperate climates are mechanical injury from the falling of branches of trees across the circuits, sleet and wind storms, and lightning. The first mentioned difficulty may be avoided by keeping clear, so far as possible, of wooded country, and it should be remembered that, at the voltages customarily used for transmission, a twig the size of a lead-pencil falling across the wires may set up arcin, and it will end by burning the wires completely off-not directfy by fusion, but by persistent arcing. A properly constructed overhead line is practically safe against all storms, save those of most extraordinary violence, and with care may be made secure even against these. As a matter of practice, interruptions of service upon transmission systems are very rarely due to trouble upon the main line itself, but are far more likely to occur in some part of the distributing system. The most dangerous combination of circumstances is a sleet storm sufficient to coat the wires with ice, followed by heavy winds; if the line, however, is constructed with proper factors of safety, bearing this particular danger in mind, there need be very little fear of serious results. Lightning is a much more formidable enemy. The lightning discharges observed upon electric circuits are of two general descriptions: first, a direct discharge of lightning upon the line, more or less severe, and always to be dreaded; and secondly, induced discharges due to lightning flashes which do not hit the line, or to static disturbances which may or may not produce actual lightning. Discharges of the former class are vastly more severe than those of the latter, and, fortunately, are somewhat rare. They may actually shatter the line, or may distribute themselves along it for a considerable distance, leaping from wire to pole, and thence to earth, without actually damaging the line to any marked degree. The induced discharges are felt principally in the apparatus, causing many of the burn-outs observed in transformers and generators. There is no complete protection against the effects of lightning upon the apparatus. Even the best lightning arr esters are palliatives rather than preventives. If, however, a number of arr esters are put in parallel, with reactance coils between them on the way towards the apparatus, the vast majorit of lightning discharges, to whatever cause they may be due, will be deflected harmlessly to earth. Moreover, the apparatus itself has a considerable power of resistance, due to its high insulation. The ends of the line should be very thoroughly protected by such lightning arr esters, and other points, such as prominent elevations along the line, should receive similar additional protection. In some cases a substantial steel-wire cable stretched along the tops of the poles several feet above the line wires and well grounded at frequent intervals has been found very advantageous. With the best protection at present available. lightning is not a serious menace to continuity of service, and the apparatus of the distributing system is far more difficult to protect than the main line and its apparatus.
Sub-stations.-In most long-distance transmission work the transmission line itself terminates in a sub-station, which bears to the general distribution system precisely the same relations which are borne by a central electric supply station to its distributing lines. Such a sub-station should be treated, in fact, as a central station, receiving its electric energy from a distance instead of employing local generators driven by prime movers. The design of the substation, however, is somewhat different from that of the ordinary central station. The transmission lines terminate generally in a bank of reducing transformers, bringing the voltage from the 10,000 or higher voltage employed upon the line to the 2000 or more generally used in the distribution. These transformers are usually large, and their magnitude should be determined by the same considerations which apply to determining the size of the units to be employed in a generating station. The general rule to be followed is that the separate units shall be of such size that one of them may be dispensed with without serious inconvenience. In the case of transformers, the unit in two- or three-phase working is the bank of transformers, which must be used together. In Continental practice three-phase reducing transformers are frequently made to include all three phases in a 'single structure; this practice is less frequently followed in American plants, separate transformers being more often used in each phase. In this case, two or three transformers, according as the two- or three-phase system is used, constitute a single transformer unit in the sense just mentioned. If a change is to be made from three-phase line to two-phase distribution, the change is made by the appropriate vector connexion of the transformers. The f ull-load efficiency of large sub-station transformers is commonly 97 to 98 %. In any case, the sub-station is furnished with voltage regulating appliances, to enable the voltage upon the distribution lines to be held constant and uniform. These regulators are, in practice, transformers with a variable transformation ratio. This is obtained in divers ways-sometimes by changing the inductive relations of the primary and secondary coils, sometimes by changing the relative number of effective turns in primary and secondary. Sets of these inductive regulators enable the voltage to be controlled over a sufficiently wide range to secure uniform potential on the system, and with a degree of delicacy that obviates any undesirable changes in voltage. The regulation is usually manual, no automatic regulator yet having proved entirely satisfactory. In very large systems it is worth noting that the so-called “ skin effect " in alternating current conductors may become conspicuous. In the transmission circuits themselves the wires are, in practice, never large enough to produce any sensible difference in conductivity for continuous and for alternating currents. In the heavy omnibus-bars of a large sub-station this immunity may not be continued, but in such cases fiat strips are frequently employed. If these are not more than, say, a centimetre in thickness, the “ skin effect ” is practically insignificant for all frequencies used commercially. Not infrequently the sub-station also contains devices for the changing of alternating to continuous current, usually synchronous converters feeding either traction system or electric lighting mains. Beyond these converters the system becomes an ordinary continuous-current system, and is treated as such. When very close regulation is necessary, motor generators are often preferred to synchronous converters. Series arc lighting from transmission circuits is a much more serious problem. At the present time two methods are in vogue: first, the operation of continuous-current series-arc machines by synchronous or induction motors driven from the transmission system; and, secondly, series alternating apparatus for feeding alternating arcs. This apparatus consists either of constant current transformers with automatically moving secondaries, or of inductive regulators, also automatic in their action, supplemented by transformers to supply them with the necessarily rather high voltage employed for arc distribution. As between these two systems practice is at present divided; electrically, the alternating apparatus gives a rather higher real efficiency, but involves the use of alternating arcs, which are somewhat less efficient, watt for watt, as light producers than the continuous-current arcs. The apparatus, however, requires practically no care, while the arc machines, driven by motors, require the same amount of care as if they were driven by other power. Arc light transformers, however, are likely to have low power factors, hardly above 0-8 at full load, and rapidly falling off at lower loads. Synchronous rectifiers chan ing the alternating current into a unidirectional current, suitable for use"with arc lights, have been employed with some success, but not to any considerable extent. They are satisfactory in avoiding the use of alternating currents in the arc, and consume but little energy in the transformation from one form of current to the other, but involve the use of static transformers automatically giving constant current, which are somewhat objectionable on the score of lowpower factor. Mercury rectifiers are now used rather extensively and give excellent results, although they are as yet of somewhat uncertain life, and, like the synchronous rectifiers, require special transformers when worked at constant current. In Continental practice arc lights are almost universally worked off constant potential circuits, and hence the difficulties just considered are for the most part peculiar to American systems.
Distances of Transmission.—The ultimate determining factor in the distance to which power can be commercially transmitted is the economic side of the transmission, the maximum distance being the maximum distance at which the transmission will pay. As a mere engineering feat the transmission of power to a distance of many hundred miles is perfectly feasible, and, judging from the data available, the phenomena encountered in increasing the length of lines have not been of such character as to cause any hesitation in going still farther, provided the increase is commercially feasible. In American practice, it is within the truth to say that nearly all transmissions of reasonable size (say a few hundred kilowatts) to distances of twenty miles, or less, are pretty certain to pay. At distances up to fifty miles, in a large proportion of cases power can be delivered at prices which will enable it to compete with power locally generated by steam. From fifty to one hundred miles (on a large scale-several thousand kilowatts) the chances for commercial success are still good. The larger the amount of ower transmitted, the better on the whole is the commercial outllbok. The longest one yet operated has already been noted, and may be regarded as a commercial success. In certain localities where the cost of fuel is extremely high, transmissions of several hundred miles may prove successful from a commercial as well as an engineering standpoint, but the growth of industry, which indicates the necessity for such a transmission, may go on until, through improved facilities of transport, the cost of fuel may be greatly lowered and the economic conditions entirely changed. Such a modification of the conditions sometimes takes place much more quickly than would be anticipated at first sight, so that when very long distance transmissions are under consideration, the permanence of the conditions which will render them profitable should be a very serious subject of consideration. (L. Bl.)