FIRST LECTURE r t, fVHtrM LABORATORY. \ GENERAL REVIEW I~" N ITS economical application, electric power passes through the successive steps : generation, transmission, ■^ conversion, distribution and utilization. The require- ments regarding the character of the electric power imposed by the successive steps, are generally different, frequently contradictory, and the design of an electric system is therefore a compromise. For instance, electric power can for most pur- poses be used only at low voltage, no to 600 volts, while economical transmission requires the use of as high voltage as possible. For many purposes, as electrolytic work, direct current is necessary; for others, as railroading, preferable; while for transmission, alternating current is preferable, due to the great difficulty of generating and converting high voltage direct current. In the design of any of the steps through which electric power passes, the requirements of all the other steps so must be taken into consideration. Of the greatest importance in this respect is the use to which electric power is put, since it is the ultimate purpose for which it is generated and transmitted ; next in importance is the transmis- sion, as the long distance transmission line usually is the most expensive part of the system, and in the transmission the limitation is more severe than in any other step through which the electric power passes. The main uses of electric power are : General Distribution for Lighting and Pozver. The relative proportion between power use and lighting may vary from the distribution system of many small cities, in which 10 GENERAL LECTURES practically all the current is used for lighting, to a power distribution for mills and factories, with only a moderate lighting load in the evening. The electric railway. Blectro chemistry. For convenience, the subject zvill he discussed under the subdivisions: I. General distribution for lighting and power. Long distance transmission. Generation. Control and protection. Electric railway. Electrochemistry. Lighting. Character of Electric Power. Electric power is used as — a. Alternating current and direct current. b. Constant potential and constant current. c. High voltage and low voltage. a. Alternating current is used for transmission, and for general distribution with the exception of the centers of large cities; direct current is usually applied for railroading. For power distribution, both forms of current are used; in electrochemistry, direct current must be used for electrolytic work, while for electric furnace work alternating current is preferable. The two standard frequencies of alternating current are 60 cycles and 25 cycles. The former is used for general distri- bution for lighting and power, the latter for conversion to direct current, for alternating current railways, and for large powers. GENERAL REVIEW ii In England and on the continent, 50 cycles is standard frequency. This frequency still survives in this country in Southern California, where it was introduced before 60 cycles was standard. The frequencies of 125 to 140 cycles, which were standard in the very early days, 20 years ago, have disappeared. The frequency of 40 cycles, which once was introduced as compromise between 60 and 25 cycles is rapidly disappear- ing, as it is somewhat low for general distribution, and higher than desirable for conversion to direct current. It was largely used also for power distribution in mills and factories as the lowest frequency at which arc and incandescent light- ing is still feasible; for the reason that 40 cycle generators driven by slow speed reciprocating engines are more easily operated in parallel, due to the lower number of poles. With the development of the steam turbine as high speed prime mover, the conditions in this respect have been reversed, and 60 cycles is more convenient, giving more poles at the same generator speed, and so less power per pole. Sundry odd frequencies, as 30 cycles, 33 cycles, 66 cycles, which were attempted at some points, especially in the early days, have not spread; and frequencies below 25 cycles, as 15 cycles and 8 cycles, as proposed for railroading, have not proved of sufficient advantage — at least not yet — so that in general, in the design of an electric system, only the two standard frequencies, 25 and 60 cycles, come into considera- tion. b. Constant current, either alternating or direct, that is, a current of constant amperage, varying in voltage with the load, is mostly used for street lighting by arc lamps; for all other purposes, constant poteatial is employed. 1 2 GENERAL LECTURES c. For long distance transmission, the highest permis- sible voltage is used ; for primary distribution by alternating current, 2200 volts, that is, voltages between 2000 and 2600; for alternating current secondary distribution, and direct current distribution, 220 to 260 volts, and for direct current railroading, 550 to 600 volts. I. General Distribution eor Lighting and Power. In general distribution for lighting and power, direct current and 60 cycles alternating current are available. 25 cycles alternating current is not well suited, since it does not permit arc lighting, and for incandescent lighting it is just at the limit , where under some conditions and with some genera- tor waves, flickering shows, while with others it does not show appreciably. The distribution voltage is determined by the limitation of the incandescent lamp, as from 104 to 130 volts, or about no volts, no volts is too low to distribute with good regu- lation, that is, with negligible voltage drop, any appreciable amount of power, and so practically always twice that voltage is employed in the distribution, by using a three-wnre system, with no volts between outside and neutral, and 220 volts between the outside conductors, as shown diagrammatically in Fig. I . By approximately balancing the load between the two circuits, the current in the neutral conductor is very small, the GENERAL REVIEW 13 drop of voltage so negligible, and the distribution, regarding voltage drop and copper economy, so takes place at 220 volts, while the lamps operate at no volts. Even where a separate transformer feeds a single house, usually a three-wire distribu- tion is preferable, if the number of lamps is not very small. When speaking of a distribution voltage of no, some voltage anywhere in the range from 104 to 130 volts is employed. Exactly no volts is rarely used, but the voltages of distribution systems in this country are distributed over the whole range, so as to secure best economy of the incan- descent lamp. This condition was brought about by the close co-oper- ation, in this country, between the illuminating com- panies and the manufacturers of incandescent lamps. The constants of an incandescent lamp are the candle power — for instance t6; the economy — for instance 3.1 watts for hori- zontal candle power; and the voltage — for instance no. By careful manufacture, a lamp can be made in which the filament reaches 3.1 watts per candle power economy at 16 c. p. within one-half candle-power: but the attempt to fulfill at the same time (the condition, that this economy and candle power be reached at no volts, within one-half volt, would lead to a considerable percentage of lamps which would fall outside of the narrow range permitted in the deviation from the three con- stants; and so, if the same distribution voltage were used throughout the country, either a much larger margin of varia- tion would have to be allowed in the product, that is, the lamps would be far less uniform in quality — as is the case abroad, — or a large number of lamps would not fulfill the requirements, could not be used, and so would increase the cost of the rest. 14 GENERAL LECTURES Therefore, all the efforts in manufacture are con- centrated on producing the specified candle power at the required economy, and the lamps are then sorted for voltage. This arrangement scatters the lamps over a considerable voltage range, and different voltages are then adopted by different distribution systems, so as to utilize the entire product of manufacture at its maximum economy. The result of this co-operation between lamp manufacturers and users is, that the incandescent lamps are very much closer to requirements, and more uniform, than would be possible otherwise. The effect however is, that the distribution is rarely actually no, and in alternating current systems, the primary distribution voltage not 2200, but some voltage in the range between 2080 and 2600, as in step-down transformers a constant ratio of transformation, of a multiple of 10 -r- i, is always used. In the following, therefore, when speaking of no, 220 or 2200 volts in distribution systems, always one of the voltages within the range of the lamp voltages is understood. In this country, no volt lamps are used almost exclu- sively, while in England, for instance, 220 volt lamps are generally used, in a three-wire distribution system with 440 volts between the outside conductors. The amount of copper required in the distribution system, with the same loss of power in the distributing conductors, is inversely proportional to the square of the voltage. That is, at twice the voltage, twice the voltage drop can be allowed for the same distribution efficiency; and as at double voltage the current is one-half, for the same load twice the voltage drop at half the current gives four times the resistance, that is, one-quarter the conductor material. By the change from the 220 volt distribution with no volt lamps, to the 440 volt distribution with 220 volt GENERAL REVIEW 15 lamps, the amount of copper in the distributing conductor, and thereby the cost of investment can be greatly reduced, and current supplied over greater distances, so that from the point of view of the economical supply of current at the customers' terminals, the higher voltage is preferable. However, in the usual sizes, from 50 to 60 watts power consump- tion and so 16 candle power with the carbon filament, and correspondingly higher candle power with the more efficient metallized carbon and metal filaments, the 220 volt lamp is from 10 to 15^ less efficient, that is, requires from 10 to 15% more power than the no volt lamp, when producing the same amount of light at the same useful life. This differ- ence is inherent in the incandescent lamp, and is due to the far greater length and smaller section of the 220 volt filament, compared with the no volt filament, and therefore no possibil- ity of overcoming it exists ; if it should be possible to build a 220 volt 16 candle power lamp as efficient — at the same useful life of 500 hours — as the present no volt lamp, this would simply mean, that by the same improvement the efficiency of the no volt lamp could also be increased from 10 to 15%, and the difference would remain. For smaller units than 16 candle power, the difference in efficiency is still greater. This loss of efficiency of 10 to 15%, resulting from the use of the 220 volt lamp, is far greater than the saving in power and in cost of investment in the supply mains ; and the 220 volt system with no volt lamps is therefore more efficient, in the amount of light produced in the customer's lamps, than the 440 volt system with 220 volt lamps. In this country, since the early days, the illuminating companies have accepted the responsibility up to the output in light at the customer's lamps, by supplying and renewing the lamps free of charge, and the system using no volt lamps is therefore universally 1 6 GENERAL LECTURES employed while the 220 volt lamp has no right to existence; while abroad, where the supply company considers its responsi- bility ended at the customer's meter, and the customer is left to supply his own lamps, the supply company saves by the use of 440 volt systems — at the expense of a waste of power in the customer's 220 volt lamps, far more than the saving effected by the supply company. In considering distribution systems, it therefore is unnecessary to consider any other lamp voltage than no volts (that is, the range of voltage represented thereby). In direct current distribution systems, as used in most large cities, the 220 volt network is fed from a direct current generating station, or — as now more frequently is the case — from a converter substation, which receives its power as three-phase alternating, usually 25 cycles, from the main generating station, or long distance transmission line. In alternating current distribution, the 220 volt distribution cir- cuits are fed by step-down transformers from the 2200 volt primary distribution system. In the latter case, where con- siderable motor load has to be considered, some arrangement of polyphase supply is desirable, as the single-phase motor is inferior to the polyphase motor, and so the latter is preferable for large and moderate sizes. COMPARISON OF ALTERNATING CURRENT AND DIRECT CURRENT M the low distribution voltage of 220, current can economically be supplied from a moderate distance only, rarely exceeding from i to 2 miles. In a direct current system, the current must be supplied from a generating station or a converter substation, that is, a station containing revolv- ing machinery. As such a station requires continuous atterv- GENERAL REVIEW 17 tion, its operation would hardly be economical if not of a capacity of at least some hundred kilowatts. The direct cur- rent distribution system therefore can be used economically only if a sufficient demand exists, within a radius of i to 2 miles, to load a good sized generator or converter substation. The use of direct current is therefore restricted to those places where a fairly concentrated load exists, as in large cities; while in the suburbs, and in small cities and villages, where the load is too scattered to reach from one low tension supply point, sufficient customers to load a substation, the alternating current must be used, as it requires merely a step- down transformer which needs no attention. In the interior of large cities, the alternating current system is at a disadvantage, because in addition to the voltage consumed by resistance, an additional drop of voltage occurs by self-induction, or by reactance ; and with the large conduc- tors required for the distribution of a large low tension current, the drop of voltage by self-induction is far greater than that by resistance, and the regulation of the system therefore is serious- ly impaired, or at least the voltage regulation becomes far more difficult than with direct current. A second disadvantage of the alternating current for distribution in large cities is, that a considerable part of the motor load is elevator motors, and the alternating current elevator motor is inferior to the direct current motor. Elevator service essentially consists in starting at heavy torque, and rapid acceleration, and in both of these features the direct current motor with compound field winding is superior, and easier to control. Where therefore direct current can be used in low tension distribution, it is preferable to use it, and to relegate alternat- ing current low tension distribution to those cases where direct I pliOPERVY OF ELCCTRICAL LABORATOKY, ] I FACULTY OF AfPLieO SCItNCE. J 1 8 GENERAL LECTURES current cannot be used, that is, where the load is not sufficiently concentrated to economically operate converter substations. The loss of power in the low tension direct current system is merely the i^r loss in the conductors, which is zero at no load, and increases with the load; the only constant loss in a direct current distribution system is the loss of power in the potential coils of the integrating wattmeters on the customer's premises. In the direct current system therefore, the efficiency of distribution is highest at light load, and decreases with increasing load. In an alternating current distribution system, with a 2200 volt primary distribution, feeding secondary low tension cir- cuits by step-down transformers, the i^r loss in the conductors usually is far smaller than in the direct current system, but a considerable constant, or "no load", loss exists; the core- loss in the transformers, and the efficiency of an alternating current distribution is usually lowest at light load, but increases with increase of load, since with increasing load the transformer core loss becomes a lesser and lesser percentage of the total power. The iV loss in alternating current systems must be far lower than in direct current systems: 1. Because it is not the only loss, and the existence of the "no load" or transformer core loss requires to reduce the load loss or iV loss, if an equally good efficiency is desired. With an alternating current system, each low tension main requires only a step-down transformer, which needs no atten- tion ; therefore many more transformers can be used than rotary converter substations in a direct current system, and the i^r loss is then reduced by the greatly reduced distance of second- ary distribution. 2. In the alternating current system, the drop of voltage in the conductors is greater by the self-inductive drop than the GENERAL REVIEW 19 ir drop ; the ir drop is therefore only a part of the total voltage drop; and with the same voltage drop and therefore the same regulation as a direct current system, the i^r loss in the alternat- ing current system would be smaller than in the direct current system. 3. Due to the self-inductive drop, smaller and therefore more numerous low tension distribution circuits must be used with alternating current than with direct current, and a separ- ate and independent voltage regulation of each low tension cir- cuit— ^that is — each transformer, therefore usually becomes im- practicable. This means that the total voltage drop, resistance and inductance, in the alternating current low tension distribu- tion circuits must be kept within a few percent., that is, within the range permissible by the incandescent lamp. As a result thereof, the voltage regulation of an alternating current low tension distribution is usually inferior to that of the direct cur- rent distribution — in many cases to such an extent as to require the use of incandescent lamps of lower efficiency. While there- fore in direct current distribution 3.1 watt lamps are always used, in many alternating current systems 3.5 watt lamps have to be used, as the voltage regulation is not sufficiently good to get a satisfactory life from the 3.1 watt lamps.