EIGHTH LECTURE GENERATION For driving electric generators the following methods are available : 1. The hydraulic turbine in a water power station. 2. The steam engine. 3. The steam turbine. 4. The gas engine. COMPARISON OF PRIME MOVERS I. The advantages of water power, compared with steam power, are: a. Very low cost of operation : no fuel, very little attend- ance. The disadvantages are : a. Usually the cost of development and installation is far higher than with steam power. b. The location of the water power cannot be chosen freely, but is fixed by nature; therefore the power cannot be used where generated, but a long distance transmission line is required. c. Usually lower reliability of service, due to the depend- ence on a transmission line, and on meteorological conditions : the river may run dry in summer, ice interfere with the opera- tion in winter. The speed of the water in the turbine depends upon the head of water, and is approximately, in feet per minute, 480 Vh, where h is the head, in feet. The peripheral speed of the turbine, and so its revolutions, depends upon the speed and therefore upon the head of the water. At high heads of 500 to 102 GENERAL LECTURES 2000 feet, as are found in the West, the electric generators are thus high speed machines, of good economy and moderate size and cost. At low heads, however, such as are usual in the East- ern States, direct connection to a turbine leads to slow speed generators of many poles and large size and cost ; while indir- ect driving, by belt or rope, is mechanically undesirable. Very low head water powers of less than 20 to 30 feet head there- fore are of little value and their development is economical only where electric power is valuable. Of the two types of turbines, the reaction turbine runs approximately at the speed of the water, and the action or impulse turbine at half the speed of the water. At the same head and thus the same speed of the water, the reaction turbine gives higher speed, and is therefore used in water powers of low and medium heads, where the speed of the water is low; while the impulse (turbine, as the Pelton wheel, is always used at very high heads, at which the reaction turbine would give too high speeds. Where water power is not available, the power has to be generated by the combustion of fuel. In this case, a greater freedom exists in the choice of the location of the plant ; and it is located as near to the place of consumption as considera- tions of the cost of property, the availability of condensing water for the engines, the facilities of transportation, etc., per- mit. Transmission lines therefore are less frequently used, but in steam stations of large power, high potential distribution cir- cuits of 6600, 11,000 or 13,200 volts, commonly underground by cables, are used in supplying electric power from the main generating station, to the substations as centres of secondary distribution (New York, Chicago, etc.). As source of power is available then : The steam engine. The steam turbine. The gas engine. GENERATION 103 Comparison of the steam turbine with the steam engine: Some of the advantages of the steam turbine over the steam engine are : a. High efficiency at low loads, and a flatter efficiency- curve; that is, the turbine efficiency remains high at partial loads, and at overloads, where the steam engine efficiency falls off greatly; so that the superiority of the steam turbine in efficiency, while marked at rated load, is still far greater at partial load, light load and overload. b. Smaller size, weight and space occupied. c. Uniform rate of rotation, therefore decreased liability of hunting of synchronous machines, and decreased necessity of heavy foundations to withstand reciprocating strains. d. Greater reliability of operation and far less attend- ance required. The steam turbine reaps a far greater benefit in economy than the steam engine from superheat of the steam, and from a high vacuum in the condenser. Some of the disadvantages of the steam turbine are : a. It is a new type of machine, developed only within the last ten years, and operating engineers and attendants are therefore less familiar with it than with the reciprocating engine ; and the steam turbine is replacing the steam engine in electric power plants so rapidly, that it is difficult to get suf- ficient men to intelligently install and operate them. It is therefore of greatest benefit in a steam turbine instal- lation that the user familiarize himself with the machine, so as not to depend upon the manufacturer in every minute detail, but take care of minor troubles just as he would do with a steam engine. As the steam turbine is a very simple apparatus this is not difficult. 104 GENERAL LECTURES The speed characteristic of the steam turbine is similar to that of the constant voltage direct current shunt motor, or the polyphase induction motor ; while that of the reciprocating steam engine is similar to that of the series motor. That is, to produce the same torque, the steam turbine requires approxi- mately the same amount of steam, irrespective of the speed; therefore its efficiency is highest at a certain speed, or rather range of speed, but falls off with the speed ; while the steam consumption of the reciprocating engine, at constant torque, is approximately proportional to the speed, that is the number of times the cylinders are filled per minute. Or in other words, the torque per pound of steam used per minute is approximately constant and independent of the speed in the turbine (just as the torque per volt-ampere is approximately constant for all speeds in the induction motor), while in the reciprocating en- gine the torque per pound of steam used per minute is approxi- mately inversely proportional to the speed, or at least greatly increases with decrease of speed (just as in the series motor the torque per volt-ampere input increases with decrease of speed). The steam turbine therefore would not be suitable for directly driving a railway train in rapid transit service, but is suitable for driving the ship's propeller. Just as in the induction motor a series of economical speeds can be produced by changing the number of poles, so in the siteam turbine a series of economical speeds can be pro- duced by changing the number of expansions. For driving electrical machinery this, however, is of no importance. Comparison of the gas engine with the steam turbine and the steam engine. The leading and foremost advantage of the gas engine, a|;id the feature which gives it the right of existence, is its GENERATION 105 high efficiency. That is, the same amount of coal, converted to gas and fed to a good gas engine, gives far more power than when burned under the boilers of the most efficient steam turbine. The cause is that the gas engine works over a far greater temperature range than the steam engine and even the steam turbine — although the latter, by its ability to economic- ally utilize superheat and high condenser vacuum, gets the benefit of a larger temperature range over the steam engine. If therefore the gas engine were not so very greatly handi- capped in every other respect, it would long have superseded the steam engine and the steam turbine. The disadvantages of the gas engine in every respeot but efficiency are such, however, that in spite of its existence of over half a century it has not made a serious impression on the industry; while the steam turbine in the last ten years of its development has practically replaced the steam engine in large electric generating plants. The cause of the disadvantages of the gas engine is the high maximum temperature and the high maximum pressure compared with the mean pressure in the cylinders, which is necessary to get the greater temperature range and thus the efficiency, therefore is inherent in this type of apparatus. The output depends upon the mean pressure in the cylinder, which is low; the strains on the maximum pressure, which is very high ; and the gas engine therefore must be very large, and its moving parts very strong and heavy, for its out- put. The impulse due to the rapid pressure change is very jerky — almost of the nature of an explosion — and the steadi- ness of the rate of rotation is therefore very low, requiring for electric driving very heavy flywheels and numerous cylinders. Compared with the steam engine, the disadvantages of the gas engine so are : io6 GENERAL LECTURES a. Lower reliability; higher cost of maintenance in attendance, repairs, and greater depreciation. b. Larger size and space occupation for the same output. c. Less ease to start. d. In general, lower steadiness of the rate of rotation. The advantage of the gas engine is, that it requires no boiler plant ; the compensating disadvantage, that it requires a gas generating plant. This latter disavantage disappears where gas is available as fuel — in the waste gases of blast furnaces of steel plants and in the natural gas districts — and in those cases gas engines have found their introduction. They have also been installed for smaller powers, where low cost of fuel is un- essential, but the operation of a steam boiler is objectionable, as in isolated plants using city gas or liquid fuel (gasolene, etc.). In general, however, with the exception of thoise special cases, the gas engine does not yet come into consideration in the electric power generating station. ELECTRIC GENERATORS In general, considerations of economy make it desirable to generate the electric power in the form in which it is used. In most cases, however, this is not feasible, but a higher voltage or even a different form of power (alternating instead of direct) is necessary in the generating station than that re- quired by the user, to enable transmission and distribution; and then usually three-phase alternating current is generated. I. For isolated plants, and in general distribution of such small extent as to be within range of 220 volt distribution, 220 volt direct current generators are used, operating a three- wire system, either two no volt machines, supplying the two sides of the system, or 220 volt machines, deriving the GENERATION 107 neutral by equalizer machines, or by connection to a storage battery, or by compensator and collector rings on the 220 volt generator. That is, two diametrically opposite (electrically) points of the armature winding are connected to collector rings, (so giving an alternaiting current voltage on those col- lector rings), an alternating current compensator (transformer with a single winding) is connected between the collector rings, and the neutral brought out from the center of the compen- sator, as shown diagrammatically in Fig. 24. This arrange- ment is now most commonly used. Fig. 24 For direct current distribution in larger cities, such generating stations have practically disappeared, and have been replaced by converter substations, receiving power from a 6600, 11,000 or 13,200 volts, and usually 25 cycles. 2. For street railway, 600 volt direct current generators main generating station, as three-phase alternating current of are still used to a considerable extent, where the railway system is of moderate extent. In large railway systems, and roads covering greater distances, as interurban trolley lines, io8 GENERAL LECTURES direct generation of 600 volts direct current is also disappear- ing before the railway converter substation, receiving power as three-phase alternating from transmission lines or high voltage distribution cables. 3. For general distribution by alternating current, with a 2200 volt primary system, direct generation is still largely used, as the use of 2200 volt permits the system to cover a very large territory, and substations are mainly used only where the power can be derived from a long distance trans- mission line, or where the 2200 volt distribution is only a part of a large system of electric generation; as in the suburban diistribution of large cities, using converter substations for the interior. In this case, where the transmission line or the main generating station is at 60 cycles, large station transformers are used for the supply of the 2200 volt distribution; where the power supply is at 25 cycles, either frequency converters, or motor generators change to 60 cycles, 2200 volts. 4. For special use, as for electrochemical work, where the electric power is generated directly, different voltages, etc., may be used to suit the requirements. Where the power cannot be generated in the form in which it is used, and that is the case in all larger systems, three- phase alternators are almost universally used. The single-phase system has the disadvantage that single- phase induction and synchronous motors and converters are inferior to polyphase machines, and single-phase alternators larger and less efficient, and for lighting, where single-phase is preferable, single-phase lighting circuits can be operated from polyphase alternators. Two-phase also is gradually going out of use, since it offers no advantage over the three-phase, and the three-phase is GENERATION 109 preferable for transmission, requiring only three conductors, while two-phase requires four. In polyphase alternators the flow of power is constant, that is, at any moment adding the power of all phases gives the same value, while in single-phase alternators the power is pul- sating. In a polyphase machine the armature reaction also is con- stant, in a single-phase machine, pulsating; in the latter therefore, in machines of very large armature reaction, as turbo-alternators, pulsations of the magnet field, and thereby loss in efficiency, and heating may result. An alternator has armature reaction and self-induction. The armature reaction is the magnetic action of the arma- ture current on the field, that is, the armature current demag- netizes or magnetizes the field according to its phase, and so lowers or raises the voltage. Armature reaction therefore is expressed in ampere turns. Self-induction is the action of the armature current in producing magnetism in the armature, which magnetism does not go through the field. This magnetism induces an e. m. f. in the armature, which opposes or assists the e. m. f . produced by the field magnetism, according to the phase of the armature current, and so lowers or raises the voltage. Self-induction, or "armature reactance" therefore is expressed in ohms. Armature reaction and self-induction therefore act in the same manner, lowering the voltage with lagging and raising the voltage with leading current. In calculating alternators, either the armature reaction and the self-induction can both be considered, which makes the calculation more complicated; or the armature reaction may be neglected and the self-induction made so much larger as to allow for the armature reaction. This self-induction is then no GENERAL LECTURES called the "synchronous reactance" and, combined with the armature resistance, the "synchronous impedance" of the ma- chine. Or the self-induction may be neglected and only the armature reaction considered, but which is increased to allow for the self-induction. The last way (armature reaction), is used in designing machines; the second way (synchronous reactance) in calcula- tions with machines and systems. In the momentary short circuit current of alternators, however, the armature reaction and the self-induction must be considered separately, since they act differently. In the moment of short circuiting an alternator, the self- induction acts immediately in limiting the current, but not so the armature reaction, because it takes time before the arma- ture current demagnetizes the field, that is, the field exciting winding acts as a short circuited secondary around the field poles, and retards the decrease of field magnetism resulting from the demagnetizing action of the armature current by inducing a current in the field winding, which tends to main- tain the field magetism. Therefore in the first moment after the short circuit the armature current is limited by self-induction only, and is therefore much larger than afterwards, when self-induction and armature reaction both act. In machines of low armature reaction and high self- induction, as high frequency alternators, the momentary short circuit current is not much larger than the permanent short circuit current. In machines of low self-induction, that is, of a well distributed armature winding, but high armature reac- tion, (that is, very large output per pole, as in steam turbine alternators,) the momentary short circuit current may be many GENERATION , in times greater than the permanent value of the short circuit cur- rent, which is reached after a few seconds. In the moment of short circuiting such an alternator, the field current rises to several times its normal value, and becomes pulsating, of double frequency. Gradually the armature cur- rent and the field current die down to their normal values. By inserting non-inductive resistance in the field circuit of the alternator, the field current, which is induced in the moment of short circuit, can be forced to die out more rapidly, and the armature short circuit current made thereby to reach its final value more quickly, that is, the duration of the excessive momentary short circuit current may be reduced. By inserting reactance, as choke coils or reactive coils, in the armature circuit of the alternator, its momentary short cir- cuit current can be reduced, and this is advisable in such machines in which the current otherwise would reach danger- ous values. Since the regulation of such alternators mainly depends upon the armature reaction, which is very large com- pared with the self-induction, even a considerable external self- induction inserted as reactive coil for limiting the momentary short circuit current does not much increase the combined effect of armature reaction and self-induction ; that is, does not seriously affect the regulation.