The final, practical questions as to conductors in electrical transmission relate to their cost per kilowatt of maximum working capacity, and per kilowatt hour of delivered energy. If the cost of conductors per kilowatt of generator capacity is greater than that of all the remaining equipment, it is doubtful whether the transmission will pay. If fixed charges on the conductors more than offset the difference in the cost of energy per kilowatt hour at the points of development and delivery, it is certain that the generating plant should be located where the power is wanted. The great cost of conductors is often put forward as a most serious impediment to long-distance transmission, and the examples here cited will indicate the weight of this argument. In order to find the approximate cost of conductors per kilowatt of generator capacity for each of the transmission lines here considered, the price of bare copper wire is taken at 15 cents, and the price of bare aluminum wire at 30 cents per pound. In each case the weight of copper or aluminum conductor per kilowatt of generator capacity is used to determine their costs per kilowatt of this capacity at the prices just named. This process when carried out for the 15 transmission lines shows that their cost of conductors per kilowatt of generator capacity varies between $1.11 for the 4.5 mile line into Ludlow and $44.25 for the line of 83 miles from the Santa Ana River. It should be noted that the former of these lines operates at 2,555 and the latter at 397 volts per mile. The line into Madrid shows an investment in conductors of $31.80 per kilowatt of generator capacity with 625 volts per mile. That a long transmission does not necessarily require a large investment in conductors per kilowatt of generator capacity is shown by the line 65 miles long into Butte, for which the cost is $17.25 per kilowatt, with 769 volts per mile. For the transmission to St. Paul, a distance of 25 miles, at 1,000 volts per mile, the cost of conductors is $7.95 per kilowatt of generator capacity. The seven-mile line into Quebec shows an investment of $11.85 per kilowatt of generator capacity.
CHAPTER IV.
ADVANTAGES OF THE CONTINUOUS AND ALTERNATING CURRENT.
Electrical transmissions over long distances in America have been mainly carried out with alternating current. In Europe, on the other hand, continuous current is widely used on long transmissions at high voltages. So radical a difference in practice seems to indicate that neither system is lacking in points of superiority.
A fundamental feature of long transmissions is the high voltage necessary for economy in conductors, and this voltage is attained by entirely different methods with continuous and alternating currents. In dynamos of several hundred or more kilowatts capacity the pressure of continuous current has not thus far been pushed above 4,000 volts, because of the danger of sparking and flashing at the commutator. Where 10,000 or more volts are required on a transmission line with continuous current a number of dynamos are connected in series so that the voltage of each is added to that of the others. In this way the voltage of each dynamo may be as low as is thought desirable without limiting the total line voltage. There is no apparent limit to the number of continuous-current dynamos that may be operated in series or to the voltage that may be thus obtained. In the recently completed transmission from St. Maurice to Lausanne, Switzerland, with continuous current, ten dynamos are connected in series to secure the line voltage of 23,000. When occasion requires twenty or thirty or more dynamos to be operated in series, giving 50,000 or 75,000 volts on the line, machines exactly like those in the transmission just named, may be used. No matter how many of these dynamos are operated in series the electric strain on the insulation of the windings of each dynamo remains practically constant, because the iron frame of each dynamo is insulated in a most substantial manner from the ground. The electric strain on the insulation of the windings of each dynamo in the series is thus limited to the voltage generated by that dynamo. There is no practical limit to the thickness or strength of the insulation that may be interposed between the frame of each dynamo and the ground, and hence no limit to line voltage as far as dynamo insulation is concerned.
It is impracticable to operate alternating dynamos in series so as to add their voltages, and the pressure available in transmission with alternating current must be that of a single dynamo or must be obtained by the use of transformers. The voltage of an alternating may be carried much higher than that of a continuous-current dynamo of very large capacity, and in many cases pressures of 13,200 volts are now supplied to transmission lines by alternating dynamos. Just how high the voltage of single alternating dynamos will be carried no one can say, but it seems probable that the practical limit will prove to be much less than the voltages now employed in some transmissions. As the voltage of alternating dynamos is carried higher the thickness of insulation on their armature coils and consequently the size or number of slots in their armature cores and the size of these cores increase rapidly. The dimensions and weight of an alternating dynamo per unit of its capacity thus go up with the voltage, and at some undetermined point the cost of the high-voltage dynamo is greater than that of a low-voltage dynamo of equal capacity with raising transformers. To the voltage that may be supplied by transformers there is no practical limit now in sight. Lines have been in regular operation from one to several years on which transformers supply 40,000 to 50,000 volts; some large transformers have been built for commercial use at 60,000 volts, and other transformers for experimental and testing purposes have been employed in a number of cases for pressures of 100,000 volts and more.
Available voltages for continuous- and alternating-current transmissions are thus on a practically equal footing as to their upper limit. The amount of power that may be generated and delivered with either the alternating- or continuous-current system of transmission is practically unlimited. Single alternating dynamos may be had of 5,000 or even 8,000 kilowatts capacity if desired, but it is seldom that these very large units are employed, because the capacity of a generating station should be divided up among a number of machines. It is perhaps impracticable to build single continuous-current dynamos with capacities equal to those of the largest alternators, but as any number of the continuous-current machines may be operated either in series or multiple, the power that may be applied to a transmission circuit is unlimited.
At the plant or plants where the power transmitted by continuous current is received, a number of motors must be connected in series to operate at the high-line voltage. These motors may all be located in a single room, may be connected to machinery in different parts of a building, or may be in use at points miles apart. The vital requirement is that the motors must be in series with each other so that the line voltage divides between them. If simply mechanical power is wanted at the places where the motors are located, they complete the transmission system and no further electrical apparatus is required. Where, however, as at Lausanne, the transmitted power is to be used in a system of general electrical supply, the motors that receive the current at the line voltage must drive dynamos that will deliver energy of the required sorts. In the station at Lausanne four of the motors to which the transmission line is connected each drives a 3,000-volt three-phase alternator for the distribution of light and power. The fifth motor at this station drives a 600-volt dynamo which delivers continuous current to a street railway. A sixth motor in the same series drives a cement factory some distance from the station. Neglecting minor changes in capacity due to losses in the line and motors, this continuous-current system must thus include three kilowatts in motors and dynamos for each kilowatt delivered for general electrical distribution at the receiving station. In a case in which only mechanical power is wanted at the receiving station, the dynamos and motors concerned in the transmission must have a combined capacity of two horse-power for each horse-power delivered at the motor shaft. In contrast with these figures, the electrical equipment in a transmission with alternating current for mechanical power alone includes two kilowatts capacity in generators and motors, besides two kilowatts capacity in transformers for each corresponding unit of power delivered at the motor shaft unless generators and motors operate at the full line voltage. If a general electrical supply is to be operated by the alternating system of transmission, either motors and dynamos or rotary converters must be added to transformers where continuous current is required. An alternating transmission may thus include as little as one kilowatt in dynamos and one in transformers, or as much as two kilowatts capacity in dynamos, two in transformers, and one in motors for each kilowatt delivered to distribution lines at the receiving station.