ELECTRICAL POWER TRANSMISSION. The trans mission of electrical energy has now been so developed as to supply inexpensive and reliable power over distances as great as several hundred miles. Its service in making cheap water power available is obvious. A great share of all power is steam generated, and the function of transmission is equally essential here. This is not because it may permit locating generating sta tions at the coal mines, a plan that is frequently discovered to he uneconomical, but because steam power generation may be economically concentrated in large central stations if that power can be transmitted to the user. Such stations can be located where adequate supplies of cold water are available for condensing steam —an operation necessary to high efficiency. Larger generating units (inherently more efficient) can be used and greater refine ments adopted to increase the efficiency of generation. (See ELECTRIC POWER GENERATION.) The number of attendants in a large station is about the same as in a small one. but one large station replaces many small ones with a consequent saving in wages. Further, the equipment necessary for a given generating capacity can be furnished more cheaply in large units than in small. This is true not only for prime movers and generators but for boilers, auxiliary apparatus and even buildings. Actually, however, more expensive apparatus is used in large stations in order to obtain very high efficiency. The second fundamental reason for the economies available through power transmission is as follows: When small isolated stations are used, each station must be large enough to supply the maximum demand for power in its area. The total generating capacity required in the whole district is equal to the sum of the maximum demands. These de mands, however, do not all occur at once. There is a certain diversity, as it is called. If one large station supplies the district, its capacity need only be as great as the maximum simultaneous demands, or total load from all the areas. With the maximum demands from the areas occurring at different times the load is maintained longer on the large station and the output for the whole day is larger in relation to the generating capacity, thus making the load factor higher. It is for this reason that the efficiency of a large station is so important. A third advantage that transmis sion offers is that of increased reliability. This applies chiefly where several generating stations are concerned, because when trouble occurs in one station, power may be transmitted from others to replace the output of the affected station. Further economies also are possible through such interconnection of stations and will be explained later.
At one time hope centred around the scheme of connecting the lights in series, allowing the same current to flow from one light into the next, the total voltage being the sum of the voltages used across all the lights. Thus small current and high voltage might be obtained; but though the voltage across each light would be small, the potential of the circuit would be unsafe, and the connection very inflexible because it was necessary that all lights should be burning at once. The problem thus took form as one of transmitting at high voltage but distributing at low voltage with lights, motors, and heating devices connected in parallel and operating independently. Up to this time only direct-current systems had been used. In such a system the current flows con tinuously in one direction around the circuit, and one generator terminal remains positive in voltage, the other negative. The fruitlessness of early efforts to develop a satisfactory transmis sion system with direct current turned the attention of investi gators to alternating current. In an alternating-current circuit the direction both of current around the circuit and of voltage between the generator terminals reverses periodically and smoothly so that if their values are plotted against time a sine wave results. A complete change from positive to negative and back again is called a cycle. Alternating-current power is ordinarily generated at 25, 5o or 6o cycles per second. It is customary to represent alternating voltages diagrammatically by means of rotating arrows or vectors. Thus in fig. i, A and B are the terminals of a gener ator. The voltage between A and B is represented by the vector AB, rotating counter-clockwise about A. The voltage from A to B is positive while the rotating arrow points upward, and negative while it points downward. The distance of the arrow head above the horizontal line gives the actual positive or negative value of the voltage. All problems in alternating current design may be solved by means of such a vector diagram.
Following the shift of interest to alternating current, the trans f ormer was developed by such pioneers as Stanley, in America, and Zipernowski, Deri and Blathy, in Hungary, in 1886. The trans f ormer consists of an iron core surrounded by two coils. One coil—the primary—is connected to a source of alternating voltage which tends to force current through the coil, first in one direction, then in the other. This change of current sets up a magnetic flux in the iron linking the two coils; the flux induces a voltage in each coil which has the same value per turn in both coils. Enough current flows to induce in the primary coil a voltage opposite and almost equal to the impressed voltage. Since the voltage in each turn is the same, the voltage induced in each coil is proportional to the number of turns ; but the voltage induced in the primary is nearly equal to the impressed voltage. Therefore the voltage induced in the secondary bears practically the same ratio to the impressed voltage as the number of secondary turns does to the primary. The voltage available across the secondary can be made whatever desired by winding the proper number of turns into the coil. Thus the problem of a high transmission and a low distribu tion voltage is solved. When a voltage higher than the generator voltage is desired a transformer can be used to step up the voltage ; at the other end of the transmission line transformers can step the voltage down again. Alternating current is necessary because the electro-magnetic induction in the transformer is caused only by a change of magnetic flux, which in turn depends on the increase and decrease of current. Mechanical motion can produce the same effect, but one of the great advantages of the transformer is that there are no moving parts, a fact which makes it most efficient.
The alternating current, however, introduces certain minor difficulties, the chief of them being this very induction that is essential in the transformer. That characteristic of the circuit which produces induction, the inductance, acts upon the current like inertia. Due to the inductance the current does not change direction or arrive at its positive and negative maxima as soon as the voltage. Engineers say the current lags behind the voltage, and represent the relations with vectors. Thus in fig. 2, the vector E, rotating counter-clockwise, represents the voltage. It may be imagined as "caught" by a camera in the position shown at a certain instant. The vector I, representing the current flowing from A to B was also caught at the same instant as it follows along behind the voltage vector. It will be seen that the current, measured by the distance of the head of the arrow from the horizontal line (fig. 2) changes from one direction to the other and reaches its positive and negative maxima after the voltage. The amount of lag depends on the inductance, though the current never lags the voltage by more than a quarter of a cycle or 90° difference in vector position. When the current lags, it is impos sible to supply the maximum power over a given system. The power at any instant equals the product of instantaneous voltage and current. If the voltage is a maximum when the current is zero and vice versa, that is, if the angle of lag is 90°, the power flows out and then back in equal amounts so that the net power delivered is zero. The more nearly the lag approaches 90° the less is the net power transmitted to the load by a given current and voltage. But, independently of whether the lag is zero and the power delivered a maximum for a given current and voltage, or whether the lag is 90° and the power a minimum, the maximum voltage and current are limited by the capacity of the generating and transmitting apparatus because excess voltage would result in insulation failure and excess current would cause a high tem perature and extreme electromechanical forces, either of which might cause an interruption to service. The net power delivered, which of course is the only fraction of the power that does use ful work, is equal to the product of the current and voltage multiplied by the power factor, a factor equal to the cosine of the angle between the voltage and current vectors. The net power delivered is measured in kilowatts or active kilovolt amperes.
As said before, the maximum power which the generating and transmitting system can deliver is reduced when the power factor is low, that is to say—when the angle of lag is large. The excess generating capacity represented by this reduction in maximum net power delivered serves merely to pump energy into the system during part of the cycle and to receive it back during another part. This energy is called the reactive power and is found by multiplying the product of volts and amperes by the reactive factor. Thus there are active and reactive kilovolt amperes to be supplied. There are, however, two types of reactive power; that due to a lag of current behind the voltage, already described, and that when the current leads the voltage. The latter tends to neutralize the former so that with equal amounts of leading and lagging reactive power there would be left only the true active power. Such an ideal situation rarely exists, however.
The leading current arises from capacitance, the ability of parts of the circuit to store electrical energy in a static condition (condenser effect), just as the lagging power factor is caused by inductance, the ability of the circuit to store electrical energy in a magnetic condition. Electric lights cause no lag of current, but most motors and other apparatus, in which there are magnetic or capacitive effects, draw a lagging current, so that usually the power to be sent over a transmission line is at lagging power factor, thus giving rise to a certain inefficiency which cannot be avoided, but can only be reduced to a minimum by proper design. The transmission line itself possesses both inductance and capaci tance, factors which create several transmission problems. In the first place the line inductance of itself causes a further lag of current and requires more lagging kilovolt-amperes from the generating source. Furthermore, the inductance is related to the resistance of the line. In absorbing reactive power it causes a drop of voltage similar to the drop of voltage caused by the resistance in absorbing active power, except that the reactive voltage drop is a maximum when the current is a minimum, pro ducing the effect of a lag of reactive voltage drop of 90° behind the current. As a result, if the current is in phase with the im pressed voltage the reactive drop is a maximum when the impressed voltage is zero, or the reactive drop lags 90° behind the impressed voltage. The effect is to delay the time when the voltage arrives at the far end of the line. If the current is just a quarter cycle behind the impressed voltage the reactive drop subtracts directly from the impressed voltage. Conversely, if the current leads the impressed voltage by a quarter of a cycle the reactive drop adds to the impressed voltage producing a higher voltage at the end of the line than if this drop did not occur. There are two principal effects of line capacitance. One effect is that it requires a so-called line charging current at leading power factor. When there is no lagging current required by the load the generator must supply the leading charging current; when the load does require lagging current, the line charging current partially neutralizes it so that the generator must supply only the net reactive power. The second effect has already been alluded to ; when the generator must supply a net leading current to charge the line, some of this current flows out to the farther part of the line, causing a rise of voltage.
The values of the line capacitance and inductance depend chiefly on the size of wire and the distance between wires. Capacitance increases with the diameter of the wire and the closeness of the spacing. Inductance decreases with these factors.
Following the invention of the transformer came another con tribution of great importance to the alternating current system. This was the development of the three-phase system by Tesla and others about 189o. The original single-phase system consisted of only two wires through which the current flowed. The three phase system combined three phases into one system, so arranged that if the voltage of one phase should be at say a positive maxi mum at a certain time, that of another would be at a positive maximum a third of a cycle later, and the third voltage two thirds of a cycle later than the first ; this would institute a complete cycle and would be followed in turn by a second positive maximum of the voltage in the first phase. All three phases could be sent over only three wires. Mathematical analysis showed that almost twice the power could be transmitted over 1•5 times the copper, as with the single-phase system with a resultant important saving. Any two of the wires could be used independently to form one phase.
Since the advent of the three-phase system there has been growth in size, power and voltage of transmission systems but no radically new developments (in fact there are some few systems in Europe, known as Thury systems, where direct current is still used) .
Before the latter stages of this work are begun the transmis sion voltage must be decided on. Although this is usually about 1,00o volts per mile, the actual value depends on the amount of power to be transmitted and various other factors as well as the distance. Against the cost of the conductor, decreasing as the voltage is increased, must be balanced the increased cost of insulation, spacing, and towers throughout the line. Of equal importance are the means to maintain service in case of a failure on the system.
Since corona does not begin until a certain voltage is reached the current flowing to supply the corona loss flows only while the voltage is near its maximum value in the cycle. A current flowing in this way is the equivalent of a triple frequency cur rent. Corona is not the only phenomenon affecting the con ductor. As the size of the conductor increases the current tends to concentrate near the surface. Because of this "skin effect", as well as for the suppression of corona, it is desirable to make the conducting portion of the conductors of hollow cylindrical cross section.
The mechanical stresses are relatively simple and may be mentioned first. Normally there is the weight of conductors to be supported. At certain points there must be tension sufficient to keep the conductors from sagging too far. The wind of ten exerts heavy side pressures on conductors and towers. However, the worst stress occurs when sleet freezes around the conductor, creating a cylindrical mass 3 or 4 in. in diameter. The dead weight is very much increased and the increased area augments wind stresses. In localities where storms are frequent and severe, copper conductors with steel cores are sometimes used to give a high tensile strength.
The electrical stresses are most acute on the insulators hold ing the conductors at each support and in the apparatus con nected to the line. The thousands of insulators used all along the line must have sufficient dielectric strength to resist the continuously applied voltages. Dirt may collect on them, rain may wet them, but still they must continue to insulate. In the early days this was a hard requirement to meet. The type of insulator chiefly used on high voltage lines is shown in Plate II., fig. 2. Formerly the chief cause of failure, aside from poor ma terials or design, was that the voltage stress did not divide evenly over the insulator string. The disc nearest the line was subject to excess stress. To eliminate the concentration of stress, grading shields were introduced (shown at ends of the arc, Plate II., fig. 2). This device divides the stress evenly over the string. The shield has the further advantage that if an arc does occur it strikes through air instead of cascading along the surface of the in sulators and weakening them.
Lightning, however, need only strike in the vicinity of a trans mission line to cause abnormal voltages. When a cloud bearing an electric charge passes over a transmission line, the charge on the cloud induces a charge on the transmission line, the charge on the line being of opposite sign to that on the cloud. So long as the charge remains on the cloud the line charge is held in place by attraction and is known as a bound charge. The line voltage also is held to low values. However, when the light ning strikes to ground or to another cloud, the cloud charge is drained off in a few millionths of a second. The charge on the line is suddenly released and its voltage increased. It starts travelling out in both directions along the line putting increased stress on all the insulation. Lightning voltages have been studied on transmission lines and in the laboratory, by means of a genera tor which produces sudden voltages of the same sort as lightning. Voltages as high as 5,000,000 volts have been employed for such tests by the General Electric company in its laboratory at Pitts field. Plate II., fig. 2 shows a discharge produced by this "light ning generator." Miniature towns, transmission lines, and clouds have been built to scale—likewise devices called surge recorders have been used on transmission lines to measure the voltage set up by lightning.
From these investigations a considerable amount of data has been collected, and as a result apparatus highly resistant to light ning has been built and protective measures for life and property have been devised. It appears from measurements of the in duced voltage on transmission lines, and of the length of light ning strokes, that the voltage is of the order of ioo,000,000 volts. This voltage exists between clouds or between cloud and earth until the moment of discharge, then it is drained off in a few micro-seconds. The maximum current may be perhaps ioo,000 amperes and the total energy of the discharge 4 kilowatt hours. The voltage appearing on the line will vary as the height of the line. It frequently attains a value of 50,000 volts per foot height of line, but may approach ioo,000 volts per foot of height. Such voltages are far in excess of normal operating voltages and may cause arcs to strike across the line insulators. Such an arc may constitute either a short-circuit or an arcing ground and is very undesirable in either case. The lightning voltage stresses the apparatus even more than the line.
The voltage may be reduced in several ways. In the first place the high voltage causes corona which drains the energy of the surge and the high current flowing through the line resistance drains more energy. The loss in energy is accompanied by a de crease in voltage. The voltages can also be reduced to approxi mately one half by stringing a wire, called a ground wire, along the tops of the line supports and connecting it solidly to ground.
Devices known as lightning arresters are usually connected between each line wire and ground, close to the transformers at each end of the line. Plate I., fig. 2 shows a group of light ning arresters. Their function is to drain off the energy repre sented by the freed lightning charge. At ordinary operat ing voltages they do not allow line currents to pass, but where lightning surges traverse the line, the arresters become con ducting and thus discharge the high potential charges harm lessly to ground. Their characteristics are such that they do not permit line current to follow the lightning currents, but immediately restore their high resistance by automatic action. This action is, unfortunately, not characteristic of line insula tor breakdowns, and consequently disastrous sustained power arcs may follow lightning disturbances if the insulators are not large enough.
But the danger of the lightning voltage is much greater than indicated by its mere magnitude. This increased danger lies in its rapid rate of increase or impulse character. Ordinary 6o cycle voltage rises from zero to a maximum in a quarter of a cycle or 210 of a second, but a lightning impulse rises to maxi mum value in perhaps one millionth of a second, and may be all over when a 6o cycle voltage is just beginning to rise. When the voltage is of this type, much higher values are required to break down the line insulators and apparatus because it lasts so short a time and is not repeated. On the other hand the effect of such impulses is to concentrate all of the voltage across the first few turns of a transformer. For this reason these turns are specially insulated and shields have been developed to dis tribute the stress evenly over the whole winding. Such care is taken in building the transformer because, however undesirable it may be for a line insulator to break down, a transformer break down causes a greater damage. A short-circuit or ground results in either case but a breakdown of the transformer places it out of service for some time and incurs heavy expense. Also an in sulator flashover or breakdown can often be taken care of by removing the voltage from the transmission line.
This problem demands careful co-ordination of transformer and line insulation. If the transformer insulation is weaker than the line insulation the transformer fails; if the line insulation is weak the excess voltage flashes over a line insulator and the arc may remain to form a short-circuit. The best solution seems to be offered by using a transformer insulation which is stronger than the line insulators in its immediate vicinity. Suitable pro tective devices are also generally desirable. Another problem is whether to operate with a certain point in the system, called the "neutral," connected to ground. If such a connection is not made, an accidental grounding of one line, or an insulator flashover, will produce a very high transient voltage and probably a short-circuit will result ; if the system is grounded an accidental line ground causes a short-circuit directly, but there are no extreme voltages to injure the insulation permanently. Heavy third harmonic or triple-frequency currents are more likely to result when the system is grounded.
Oil Switches.—However imperative it is to maintain service, there may be mentioned short circuits and grounds. Under such conditions it is necessary to disconnect the transmission line from the generator. As will be explained later, special circuit arrangements make continuous service possible even when there is a short-circuit, if the affected portion of the line can be removed from the system. Although there are other reasons for desiring switches, it is the problem of short circuits that has developed a particular form of switches known as oil circuit breakers. The problem lies in the fact that under short-circuit conditions the heavy current makes it difficult to break the circuit. The in creasing voltages, too, have had their effect in increasing the difficulty of making proper circuit breakers. A switch group is shown in Plate II., fig. 5. The switch itself, immersed in a special oil possessing very high insulating properties, is made so that the contacts will open as quickly as possible, making it difficult for the arc to follow. Also, the space for the arc is restricted so that the gas pressure evolved by the arc forces the contacts apart and blows out the arc itself.
Transmission and Distribution Systems.—There are sev eral types of transmission systems. A generating station may de liver power over one circuit to a distant point. As a rule, how ever, two circuits at least will be used so that if one fails the other may be kept in service. Such an arrangement would be typical of a water power station feeding a distant centre. At the generating station there would be the oil circuit breakers, trans formers and lightning arresters. At the receiving end, besides the step down transformers, switches and arresters, there might be a synchronous condenser. This is a rotating machine similar to a generator which supplies reactive power at either leading or lagging power factor as required. Sometimes when the load power factor is low, capacitors are used which take a leading current in the same manner as the line capacitance and raise the power factor.
The station at the receiving end is commonly called a sub station. There the power is transformed to a lower voltage. Out from the substation, supply lines or "feeders" radiate at reduced voltage, ordinarily 2,3oo, 4,000, 6,900, i i,000 or even up to a 5,00o volts. The loads are supplied from the secondary windings of transformers whose primaries are connected across the feeders, located near the buildings supplied. The early scheme was to run the wires from a particular transformer secondary, to the building or buildings supplied. Thus a group of buildings receives its power from a certain definite trans former.
In recent years another plan has been adopted for cities where the load in a given area is heavy. The secondaries of all the transformers are connected together, making a secondary net work. Several transformers may feed to a concentrated load and if one transformer fails the service is not interrupted. One de velopment of this scheme employs several feeders covering ap proximately the same area. When the load is light one or more of the feeders is switched off at the substation, whereupon switches on the secondaries of the transformers associated with the disconnected feeders are opened, removing the transformers from the system and saving the losses which they occasion even when supplying no load. The secondary switches referred to, and other automatic devices necessary to control the flow of power in case of a failure, constitute the most serious difficulty presented by this arrangement.
The substation as an intermediate step between the transmis sion and distribution systems may seem an unnecessary compli cation, but in fact, it is economical. A small transformer designed for direct connection to a' high voltage system would cost several times as much as one for connection to a relatively low voltage feeder. Since the higher voltage does not make so much dif ference in a large transformer it is cheaper to step down the voltage from the transmission voltage in a few large units, and to use lower voltage feeders in which the power is divided and the current small and to which the cheaper distribution transformers can be connected.
After this brief description of distribution systems the more complicated transmission systems may be considered. There is, first, the radial system where a central generating station must supply several substations surrounding it. Such a system is similar to that previously described except that several lines radiate from one station. The more developed system is of the ring type, in which two lines start from the generating station and after being tapped at several substations, meet again. The advantage is that power can flow to any station in either of two directions.
Finally, there are transmission networks which are made by a combination of the radial and ring systems and offer several paths for power flow to each station. The gain from having more than one path is twofold. Power can be fed over the most eco nomical paths and in case of trouble on one line, others can be used to supply power.
A final stage is the interconnection of generating systems, so that each system can be fed by several stations. The advantages are manifold. When there is trouble anywhere, there are many directions for feeding the desired power and many sources that may be used. With so many generating sources, the amount of spare capacity is reduced as compared to that which an isolated station would have to have to maintain service in case of a gen erator failure. The area covered is larger and the diversity greater, so that the total capacity required is less.
The interconnection of water-power and steam power stations is very advantageous. Ordinarily in a water-power plant the investment cost is higher, the operating cost low ; in a steam plant the ratio reverses. By interconnecting, the water-power station can be operated at nearly full capacity all the time. For such operation the total cost is only a little more than if opera tion were only at, say, I o% capacity. Only when the peak or maximum load comes on is the power taken from the steam sta tion. Thus steam power, with its high operating cost, is used as little as possible, while hydro-electric power, with its high fixed investment charges, is used as much as possible.
Such interconnection involves many problems, however. Be fore operation can be effected at all, all of the apparatus on the system must be held to exactly the same frequency of alternation and to definite phase relations. When one station speeds up a little it takes more power and the others less. This tends to slow down the faster station, but the force may often be too small, and if the generators once pull out of step large amounts of power surge back and f orth and there is a probability that the whole system will be stalled. Even when the stations do not pull out of step the directions in which the power is forced to flow may be undesirable for economic reasons.
Aside from mere mechanical excellence of the speed regulat ing devices a supervisory control of the whole system must be exercised to control the exchange of power both active and re active. This is done by the chief load dispatcher, who has in front of him a plan of the whole system dotted with indicating devices to show the position of switches, generators and other apparatus. (See ELECTRIC POWER GENERATION.) The load dispatcher must receive information from all points on the system. This is often carried out by means of carrier current telephony—that is, telephony making use of radio fre quency which is sent over the transmission lines themselves. The telephone circuits at both ends of the transmission line are coupled to the line electrically by means of capacitance, or, less frequently, inductance. Plate II., fig. 4 shows a capacitor used for this purpose. .
However, the control which the dispatcher can exercise over the flow of power is limited by the circuit constants and the voltages that must be held at certain points. In general the flow of active power depends on the lag of voltages with respect to each other; the flow of reactive power on voltage mag nitudes. The supply of reactive power costs little in steam and may best be supplied from relatively inefficient stations of earlier design. This leaves the supply of active power to the most efficient stations; i.e., modern steam stations or hydro-electric stations where "fuel" is cheap per kilowatt-hour generated.
Ordinarily the transformer does not allow any change in the ratio of voltage during operation, but such a change may be desirable to control the flow of reactive power. It is obtained by a special transformer arrangement known as load ratio con trol, which allows the changing of the number of active turns in the windings and hence of the voltage during operation.
Other very complicated aspects of transmission should be mentioned. Oil switches are necessary, but useless unless properly controlled. If when trouble occurs it were necessary to locate it by tests and to open switches by hand, the system would be out of operation or burned up long before anything could be done. Schemes dependent on devices called relays, have been developed which automatically locate all sorts of trouble and then open just those switches which disconnect the affected part and leave the rest of the system in operation. These relays may be operated by temperatures, under- or over-voltage, over current, over-power, reverse-power, unbalance of currents, etc. They may have various sorts of time delay. They may even be actuated by carrier current.
Telephone Interference.--A problem, which in a way is extraneous, is that of acoustic interference with telephone cir cuits. Telephone lines are conveniently placed parallel to power lines. So placed, they are exposed to induction from the power line, that is, the heavy current in the power lines sets up a mag netic flux which links the telephone wires, thus inducing a volt age in them. This voltage acts on the telephone receivers and confuses the sound of the voices. If the power current is very heavy the noise may be so great as to give a listener a severe acoustic shock.
If the three power conductors could be so arranged that they were equidistant from both telephone wires there would be no interference, because in a three-phase system the relations of the currents are such that one is always the equal and opposite of the other two. The voltage induced by any one power line would be nullified by the other two. As it is impractical to arrange the conductors with the desired symmetry at any point, an approach is made to this by transposition, that is by rearranging the con ductors on the poles regularly after a certain distance has been traversed.
Transposition cannot always be made completely effective, particularly when it is triple frequency (third harmonic) cur rents rather than those of fundamental (operating) frequency which cause the interference. Triple frequency currents occur with certain transformer connections and flow out along all three conductors, returning through the ground. Transposition between ground and lines is impossible, so the telephone wires offer the only opportunity for transposition, but one that it is difficult to make effective. Circuits made of inductances and capacitances and called "filters" are frequently used to filter out these unde sired voltages from the telephone circuits.
Large amounts of power per line introduce a problem which must always be considered. The greater the flow of current over a given transmission line with a given amount of inductance, the greater is the lag of the receiver voltage behind the generator volt age. Therefore, when it is attempted to increase the power trans mitted over a line by increasing the current flowing, the lag of the receiver voltage behind the generator voltage becomes greater. But the maximum power that can be transmitted over any given line under present circumstances is obtained when the receiver voltage lags the generator voltage by about 9o°. Longer trans mission lines mean more inductance, and hence more lag. Be yond the go° point, therefore, the maximum power decreases, or, in other words, the longer the line the less power can be trans mitted.
Another factor, namely the increase of charging current with added length of line, also tends to cut down the maximum power carrying capacity of the line. This line charging current, being 90° in the lead of the generator voltage, and reacting within the generator operates to raise the generator voltage. In order to keep the voltage down to normal, the magnetic field supplied to the machine by the exciter (see ELECTRIC POWER GENERATION) must be cut down. Therefore, the larger the charging current re quired of the machine, the less the field that can be carried. But it is only the current produced by the action of this field on the generator windings that gives useful power. So, to keep down the voltage, the power output of the machine must be sacrificed. To allow the voltage to rise would ultimately result in destruction of the insulation of the system.
This whole situation is termed "stability," and is one of the limiting factors in long distance power transmission. Methods of increasing the stability limits of a system have been devised, and the problem constitutes one of the principal fields of study for transmission engineers to-day.
The stability problem on long lines is accompanied by other serious conditions brought about during short circuits. The principal f actors here are the increased line voltage drop due to the exceedingly heavy currents flowing, and the inability of the generators and other synchronous machines to adjust them selves promptly to the short-circuit condition. The machines are no longer held to the same speed because the reactions which occur are not strong enough to hold the system stable. Increas ing the speed of response of the electrical machine, however, overcomes these difficulties in considerable measure. All of these problems are called problems of stability.
As a whole, the requirement that a system be stable tends to limit both the distance of transmission and the amount of power at present voltages, but the problem is being solved through the use of specially designed apparatus. Higher voltages involve an increase of cost as compared to present voltages, unless the amount of power and the distance of transmission are very much increased.
(F. W. P.; X.)