Electric power transmission; a practical treatise for practical men

Preface To First Edition

) This volume is designed to set forth in the simplest possi-

ble manner, the fundamental facts concerning present practice in electrical power transmission.

Busy men have little time to spend in discussing theories of which the practical results are known, or in following the derivation of formulae which no one disputes. The author has therefore endeavored, in introducing such theoretical considerations as are necessary, to explain them in the most direct way practicable ; using proximate methods of proof when precise and general ones would lead to mathematical complications without altering the conclusion for the purpose in hand, and stating only the results of investigation when the processes are undesirably complicated.

In writing of a many-sided and rapidly changing art, it is impossible in a finite compass to cover all the phases of the subject or to prophesy the modifications that time will bring forth ; hence, the epoch of this work is the present, and the point of view chosen is that of the man, engineer or not, who desires to know what can be accomplished by electrical power transmission, and by what processes the work is planned and carried out. This treatment is not without value to the student who wishes to couple his investigations of electrical theory with its application in the hands of engineers, and puts the facts regarding a very great and important development of applied electricity in the possession of the general reader.

Such apparatus as is described is intended to be typical of the methods used, rather than representative of any particular scheme of manufacture or fashion in design. These last change almost from month to month, while the general conditions remain fairly stable, and the underlying principles are of permanent value.

January, 1897.

Preface To Fifth Edition

The art of electrical power transmission has changed but little in the past two years, since the fourth edition of this work went to press. There have been very many plants installed, few of them at all sensational in magnitude, voltage, or distance of transmission. The great bulk of such work is now rather commonplace. The upper limit of voltage has already risen to nearly 70,000 volts, and the next few years will assuredly see a very material increase over this figure. A few new pieces of apparatus have been recently brought into use, which have been noted in their appropriate places. Perhaps the most considerable impending changes are those in the resources of electric lighting which affect only those transmission systems which do their own distribution. These changes, however, bid fair to be on a very large scale and of very striking character within the next few years.

Juney 1907.

Contents

Chapter. Page.

I. Elementary Principles 1

II. General Conditions of Power Transmission ... 23

Chapter I. Elementary Principles

It has long been the fashion to speak of what we are pleased to call electricity as a mysterious ''force/*' and to attribute to everything connected with it occult characteristics better suited to mediaeval wizardry than to modern science. This unhappy condition of affairs has, in the main, come about through the indistinctness of some of our fundamental ideas and inexactitude in expressing them.

To speak specifically, there has been, even in the minds and writings of some who ought to know better, a tendency toward confusing the extremely hazy individuality of ''electricity" with the sharply defined properties of electrical energy. We have been so overrun by theories of electricity, two-fluid, one-fluid, and non-fluid — by electrically "charged" atoms and duplex ethers, that we have well-nigh forgotten the very great uncertainty as to the concrete existence of electricity itself. Even admitting it to be an entity, it most assuredly is not a force, mysterious or otherwise. Electrical force there is, and electrical energy there is, and with them we can freely experiment, but for most practical purposes "electricity" is merely the numerical factor connecting the two. It is related to electrical energy much as that other hypothetical fluid "caloric" was supposed to be related to heat energy. The analogy is not absolutely exact, but it nevertheless summarizes the real facts in the case.

The day has passed wherein we were at liberty to think of "electricity" as flowing through a material tube or as plastered upon bodies like a coat of paint. The things with which we have now to deal are the various factors of electrical energy.

It is the purpose of this chapter to treat of that form of energy which we denominate electrical, to discuss its relation to other forms of energy and some of the transformations which they may reciprocally undergo.

Speaking broadly, energy is power of doing work. The energy of a body at any moment represents its inherent capacity for doing work of some sort on other bodies. This, however, must not be understood as implying that the aforesaid energy is limited by our power of utilizing it. We may or may not be able to employ it to advantage or under possible conditions. As an example, take the massive weight of a pile driver. Raised to its full height it possesses a certain amount of gravitational energy — a possibility of doing useful work. This energy is temporarily unemployed and appears only as a stress on the supporting rope and frame- work. Under these circumstances, wherein the energy exists in static form, it is generally known as potential energy.

Now let the weight fall and with swiftly gathering velocity it strikes the pile and does work upon it, settling it deep into the mud. The energy due to the blow of the moving weight, energy of motion in other words, is called kinetic. But at the bottom of its fall the weight still has potential energy with reference to points below it, and we realize this as the pile settles lower and each successive blow becomes more forceful. At some point we are unable further to utilize the fall, and have then reached the limit of the available energy in this particular case.

We must not forget, however, that each time the weight was lifted, work had to be done against gravitation to give the weight its point of vantage with respect to available energy. This work was probably done by utilizing the energy of expanding steam — in other words, the energy of the steam was transformed through doing work on the piston into kinetic energy of the latter, which, through doing work against gravitation, has been enabled again to reappear as the energy of a falling body, and to do work on the driven pile. And back of the steam energy is the heat energy, by which work is done on the water in the boiler, and yet back of this the chemical energy of the coal, transformed into heat energy and doing work on the minute particles of iron in the boiler, for we know that heat is a species of kinetic energy.

Even the work done on our pile is not permitted to go untransformed into energy. Part is transformed into heat energy through friction and compression of the pile, part through friction of the water, and part raises ripples that may lift against gravity chips and pebbles on a neighboring shore. Other fractions go into the vibrational energy of sound; into heating the weight so that it gives out warmth — radiant energy — to the hand when held near it and to the surrounding air; and into electrical work done on the weight and neighboring objects, for the weight unquestionably receives a minute amount of electrical energy at each blow. Thus, a comparatively simple mechanical process involves a long series of transformations of energy.

No energy is ever created or destroyed, it merely is changed in form to reappear elsewhere, and work done is the link between one form of energy and another. And we may lay down another law of almost as serious import: No form of energy is ever transformed completely into any other.

On the contrary, the general rule is that with each transformation several kinds of energy appear in varying amounts, and among them we may always reckon heat. The object of any transformation is usually a single form of energy, hence practically no such thing as perfectly efficient transformation can be obtained. The energy by-products for the most part cannot be utilized and are frittered away in useless work or in storing up kinds of potential energy that cannot be employed.

The greatest loss is in heat, which is dissipated in various ways and cannot be recovered. The presence of unutilized heat always denotes waste of energy.

From what has gone before, we can readily appreciate that when we do work with the object of rendering available a particular kind of energy, the method must be intelligently selected, else there will result useless by-products of energy which will seriously lower the efficiency of the operation.

Whenever possible we utilize potential energy already existing in securing a transformation. Thus if heat is wanted, the easiest way of getting it is to burn coal, and to allow its energy to become kinetic as heat. If we want mechanical work done, we set heat energy to work in the most efficient way practicable. If electrical energy is desired, we set the energy of steam to revolving the armature of a dynamo. If the right method of transformation is not chosen, much of the energy will turn up in forms that we do not want or cannot utilize. Burning coal is a very bad way of getting sound, just as playing a cornet is but a poor means of getting heat, although a fire does produce a trifling amount of sound, and a cornet by continual vibration must be warmed to a minute degree.

These seem, and perhaps are, extreme instances, but when we realize that, somewhat to the discredit of human ingenuity, less than one-twentieth of the electrical energy supplied to an incandescent lamp appears in the form of light, the comparison becomes grimly suggestive.

Understanding now that in order to obtain energy in any given form (such as electrical), particular methods of transformation must be used in order to secure anything like efficiency, we may look a little more closely at various types of energy to discover the characteristics that may indicate efficient methods of transformation, particularly as regards electrical energy.

Speaking broadly, one may divide energy into three classes:

1st. Those forms of energy which have to do with movements of, or strains in, masses of matter. In this class may be included the ordinary forms of kinetic energy of moving bodies and the like.

2d. Those which are concerned with movements of, or strains in, the molecules and atoms of which material bodies are composed. In this class we may reckon heat, latent and specific heats, energy of gases, and perhaps chemical energy.

3d. All forms of energy which have to do with strains which can exist outside of ordinary matter, i.e., every kind of radiant energy and presumably electrical energy.

These classes are not absolutely distinct; for example, we do not know the relation of chemical energy to the third class, nor of gravitational energy to any class, but such a division serves to keep clearly in our minds the kind of actions to which our attention is to be directed.

It is only within the past few years that we have been able with any certainty to classify electrical energy, and even now much remains to be learned. For a very long while it has been known that light, i.e., luminous energy, must be propagated through a medium quite distinct from ordinary matter and possessing certain remarkable properties. It was well known that luminous energy is transferred through this medium by vibratory or wave motion. Even the period of the vibrations and the lengths of the waves were accurately measured, and from these and similar measurements it has been possible to classify the mechanical properties of this medium, universally called ''the ether," until we really know more about them than about the properties of many kinds of ordinary matter — a number of the rare metals, for example.

The next important step was the discovery, verified in the most thorough manner, that what had been known as radiant heat, such as we get from the sun or any very hot body, is really energy of the same kind as light. That is, it was found to be energy of wave motion of precisely the same character and in the same medium, differing only in frequency and wave length. It also has turned out in similar fashion that what had been called ''actinic" rays, that are active in attacking a photographic plate and producing some other kinds of chemical action, are only light rays of shorter wave length than usual, and so ordinarily invisible to the eye.

So much having been ascertained, it became clear that instead of three kinds of energy — "heat, light, and actinism," we were really dealing with only one — radiant energy, vibrating energy in the ether, varying in effect as it varies in frequency. Speaking in an approximate way, such wave energy has a frequency of six hundred thousand hillion vibrations per second and a velocity of propagation of about a hundred and eighty-five thousand miles per second, so that each wave is not far from one fifty-thousandth of an inch long. These dimensions are true of light waves; chemical action can be produced by waves of half the length, while so-called heat rays may be composed of waves two or three times as long as those of light. Such figures are starthng, but they can be verified with an accuracy greater than that of ordinary mechanical measurements.

We see that this radiant energy is capable of producing various disturbances perceptible to our senses, such as chemical action^ light, and heat, and that these different effects simply correspond to waves of energy having different frequencies and wave lengths. This being so, it is not imnatural to suppose that at still different frequencies other effects might be noted. This idea gains further probability from the experimental fact that waves of very different frequency traverse the ether with precisely the same velocity, showing no signs of slowing down or dying out, so that there seems to be no natural limit to their length.

During the past half dozen years it has been clearly shown that ''radiant energy" is capable of producing profound electrical disturbances, such as violent oscillations of electrical energy in conducting bodies, and that these effects exist whatever the frequency of the ether waves concerned. This verj' important fact was clearly foreseen by Maxwell more than twenty years ago, regarding light, and his prediction has been thoroughly verified through the persistent researches of the late Professor Hertz and others.

This discovery is often expressed by saying that radiant energy is an electro-magnetic disturbance, or that light is one kind of electrical action. It is more strictly accurate to say that radiant energy, just as it produces chemical disturbances on the photographic plate, affects the eye as light, and material bodies as heat, is also capable of producing electrical effects when transferred to the proper media. Most of our experiments on its electrical effects have been performed with waves many thousand times longer than those of light, but their general character has proved to be exactly the same.

A given substance may be differently related to waves of radiant energy of different lengths, but the phenomena are still essentially the same. For instance, a plate of hard rubber is thoroughly opaque to waves of a length corresponding to light, but is quite transparent to those of considerably greater length, such as can produce thermal or electrical effects. A plate of alum will let through light waves and very long waves, but will stop most of those which are efficient in producing heat. A thick sheet of metal is quite opaque to all known waves of radiant energy. Hence the fact noted long ago by Maxwell, that all good conductors are opaque to light, although the converse is not true.

The substance of all this is, that the same sort of disturbance in the ether which produces light is also competent to set up electrical actions in material bodies, and conversely, such actions may and do produce corresponding disturbances in the ether, which are thus transferred to other bodies. Such a transference corresponds to all that we know concerning the velocity with which electrical and electro-magnetic disturbances pass from body to body. It is equally certain that this velocity totally transcends anything we could hope to obtain from bodies having the dynamical properties of ordinary matter, while it does fit exactly the dynamical properties of the ether.

We are thus forced to the conclusion that when an electrical current, as we say, '^ passes along" a wire, whatever a ''current" may be, it is not siniply transferred from molecule to molecule in the wire as sound or heat would be, but that there is an immensely rapid transfer of energy in the neighboring ether that reaches all points of the wire almost simultaneously. It takes a measurable time for the electrical energy to reach and utilize the centre of the wire, although its progress along the surface, thanks to the free ether outside, is enormously rapid.

Thus takes place what is generally called a ''flow of electricity" along the wire. Looking at the process more closely, the nearest approach to flow is the transfer of energy along the wire by means of stresses in the ether which in turn set up strains in the matter along their course.

Whenever we cause in matter the particular stress which we call electromotive force for lack of a more exact name, the resulting strain is electrification, and if the stress be applied at one point of a conducting body, the strain is immediately transferred to other points by the stresses and strains in the surrounding ether. Wherever this transference of strain exists we have an electrical current, although this name is generally reserved for those cases in which there exists a perceptible transference of energy by the means aforesaid. If the conditions are such that energy must be steadily supplied to keep up the electromotive stress, we have such a state of things as we find in a closed circuit containing a battery.

To cause such a flow of energy we must first find means of setting up electromotive stress capable of being propagated through the ether. Now atoms and molecules are the only handles by which we can get hold of the ether. Only in so far as we can work through them can we do work on the ether.

As a matter of fact, we cannot do work of any kind on the molecules of a body without setting up electrical stresses of some sort. In most cases of mechanical work, which in the main produces stress on the molecules only by strains in the mass, the energy appears mainly as heat, and is only incidentally electrical, as for instance the energy wasted in a heated journal.

When, however, by any device we do work more directly on the molecules of a body, or on the atoms which compose the molecules, we are more than likely to transform much of this work into electrical energy. As a rough example of the two kinds of action just mentioned, pounding a body heats it without causing any considerable electrification, while on the other hand rubbing it rather gently, sets up a considerable electrification without heating it noticeably.

In fact, for many centuries, friction was the only known m.ethod of causing electrification. Later, as is well known, it was discovered that certain sorts of chemical action, which has to do directly with interchanges of energy between molecules, were very potent in electrical effects. With this discovery came the abihty to deal with steady transfers of electrical energy in considerable amount (electric currents), instead of the relatively slight and transitory effects previously known (electrification, ''frictional'' electricity).

To clear up the real nature of this difference it is well to consider what we mean by saying that a body is electrified, or has an electrical charge. In other words, what is electrification? Not very many years ago this question would have been answered by saying that a quantity of a substance, positive electricity (or negative as the case might be), had been communicated to the body in question; that this remarkable substance could reside only at the surface of the body and was able to produce in surrounding bodies exactly an equal quantity of negative electricity; that this ''charge" of electricity would repel another ''charge" of the same substance placed near it, or attract a charge of its opposite, the other substance called negative electricity; and much more to the same effect. All this was a very convenient hypothesis — it explained, after a fashion, the common facts and enabled investigators to discover many important electrical relations and laws. But it expressed much more than there was any reason to know. From the standpoint of our modern doctrines of energy, electrification is a very different thing.

Let an electromotive stress (from whatever source) be applied to a body, a metallic sphere for example, long enough to transfer to it a finite amount of energy. This energy appears as stresses and strains in the ether everywhere about the body under consideration and thence extends to the molecules and atoms of neighboring bodies, causing "induced charges." It is as if one were to fill a box with jelly, and then pull or push or twist a rod embedded in its centre. The result would be strains in the rod, the jelly, and the box, and in a general way the total stress on the box would equal that on the rod. By proper means we could detect the strain all through the substance of the jelly, but most easily by its variations from place to place.

We do not know exactly what sort of a strain in our ether jelly is produced by electromotive stress, but we do know that it possesses the quality of endedness, so that the strains in the matter concerned, i.e., in the ball and surrounding bodies, are equal and opposite.

In fact, the two "charges" are in effect the two ends of the same strain in the ether. They appear to us to be real attributes of the two opposed surfaces, because at these surfaces the dynamical constants, such as density, elasticity, etc., of the medium through which the strain is propagated, change in value, and differences in state of strain there become physically manifest.

In electric currents we have a very different state of things. The energy supplied by the electromotive stress, instead of becoming potential as electrostatic strain, and producing ''charge," does work and is transformed into other kinds of energ}^, thermal or chemical, mechanical or luminous.

When a stress of whatever kind is applied to a body, only a limited amount of energy can be transferred by it so long as the energy remains potential. Thus, in our box of jelly before referred to, a twist of given intensity applied to the stick, as for instance by a string wound around it and pulled by a given weight, can only transfer energy until the stresses produced in the jelly come to an equilibrium with it. On the other hand, if the box were filled with water and the stick were the axle of a sort of paddle wheel, the very same intensity of twist could go on communicating energy to the water as long as one chose to apply the necessary work.

This roughly expresses the difference between electric charge and electric current, viewed from the standpoint of energy. An electromotive stress applied to a wire charges it and then the transfer of energy ceases. If the same stress be applied under conditions that allow work to be done by it, energy will be transferred so long as the stress is kept up. In an open electric circuit we have a charge as the result of electromotive stress. When the circuit is closed, i.e., when a continuous medium is furnished on which work can be done, we have an electric current. The amount of this work and the flow of electrical energy that produces it depend on the nature of the circuit. Certain substances, especially the metals, and of metals notably copper and silver, permit a ready continuous transfer of energy in and about them. Such substances are called good conductors. The real transfer of energy takes place ultimately via the ether, but its amount is limited by the amount and character of the matter through which work can be done.

Whenever the strains in the ether, such as we recognize in connection with electrical charge, shift through space as when a current is flowing, other strains bearing a certain relation to the direction of flow are made manifest. Where there is a rapid and intense flow of energy, these strains are very great and important compared with any electrostatic strains that exist outside the conducting circuit. In other cases they may be quite insignificant. These strains are electro-magnetic, and with them we have to do almost exclusively in practical electrical engineering. They appear wherever there is a moving electrical strain, whether produced by moving a charged body or causing the charge upon a body to move.

Both kinds of strains exist in radiant energy, as in other cases of flowing energy. The stresses in electro-magnetic energy are at right angles both to the electrostatic stresses and to the direction of their motion or flow. If, for example,

Fig. 2.

we have a flow of electrical energy in a straight wire (Fig. 1), the electro-magnetic stresses are in circles about it.

If A be a wire in which the flow of energy is straight down into the paper, the electro-magnetic stresses are in circles in the direction shown by the arrow heads. If the wire be bent into a ring (Fig. 2), with the current flowing in the direction of the arrows, then the electro-magnetic stresses will be (following Fig. 1) in such direction as to pass downward through the paper inside the ring.

These electro-magnetic stresses constitute what we call a magnetic field outside the wire. The intensity of this field can be increased by increasing the flow of energy in the desired region in the systematic way suggested by Fig. 2. If, for example, we join a number of rings like Fig. 2 into a spiral coil shown in section in Fig. 3, in which the current flows

ELECTRIC TRANSMISSION OF POWER.

downward into the paper in the lower edge of the spiral, there will be produced a magnetic field in which the stresses have the direction shown by the arrows. Such a spiral constitutes a genuine magnet, and if suspended so as to be free to move would take up a north and south position with its right-hand end toward the north. In and about the spiral there exists a magnetic ''field of force," which is merely another way of saying that the ether there is under electro-magnetic stress. Its condition of strain is closely analogous to that about an electrified body, and, as in that case, there is no

I fbu b

Fig. 3.

work done on the ether after the strains are once established, since the energy then becomes potential. While this is being accomplished, work is done just as when a body is charged.

If, now, setting up such an electro-magnetic field requires energy to be spent by causing a current to flow in the spiral, we should naturally expect that if the same field could be set up by extraneous means, energy would momentarily be spent on the spiral in producing stresses and strains similar to those that set up the original field. This is found to be so, the process working backward as well as forward.

If, for example, we have two rings (Fig. 4), and by sending a current around one, transfer energy to the medium outside it, this energy will set up an electromotive stress in the other ring. The direction of this stress is not at once obvious, but we can get a very clear idea of it by considering the work done. If current is started in A (Fig. 4), in the direction shown, electro-magnetic stresses are produced in the direction of the arrow C. If these are to do work on B, the electromotive stress in the latter cannot have such a direction as to set up on its own account a magnetic field that would assist that of A, otherwise we could increase the field indefinitely without added expenditure of energy. Therefore, the electromotive stress in B, and hence the current, must be in a direction opposing the original current in ^, as shown in the figure. In like manner if the current in A be stopped and the field due to it therefore changes, there are changes in the electromagnetic stresses about B, that again set up an electromotive stress in it. If, however, this change of stress is to do work, the electromotive stress in B must be of such direction as to

Fig. 4.

oppose by its field the change in the field of A — i.e., it must change its direction and will now give us a current in the same direction as the original one in A. All this follows the general law, that if work is to be done by any stress it must be against some other stress. There can be no work without resistance.

In Fig. 4 we have the fundamental facts of current induction on which depend most of our modern methods of generating and working with electrical energy. Summed up they amount to saying that whenever there is a change in the electro-magnetic stresses about a conductor, work is done upon it, depending in direction and magnitude on the direction and magnitude of the change in the stresses.