Overhead line (electricity)

“ In Plate VII. is seen an electric locomotive hauling a train of coal waggons in a colliery near the Tyne, and it will be seen that the overhead system is used, the trolley arm and wheel being replaced by sliding bows. In a colliery railway it is generally impossible to select the most favourable track from the railway constructor’s point of view, as the line must be arranged to serve certain points. This often means taking the line sometimes through low tunnels or bridges where the overhead wire must be low, and sometimes over public roads where the wire must be high ”

“ In the railway world there is a growing conviction that the adoption of electric traction on all suburban and inter-urban railways must be simply a matter of time. For main line traffic the possibilities of using electricity are as yet only an article of faith among electrical engineers. ”

“ The adoption of electric traction on such a route [Pg 123] as the Brighton main line would be a benefit in several ways. It would lead to a faster express service, as the high overload capacity of the electric motor enables it to take small account of gradients. ”

“ In any electric supply system the demand for current does not remain constant, but fluctuates more or less. For instance, in a system including an electric tramway, if a car breaks down and remains a fixture for a short time, all cars behind it are held up, and a long line of cars is quickly formed. ”

“ The prime advantage of an overhead line at high voltage is its comparatively small first cost, which is only a fraction of that of an underground or submarine cable in the great majority of instances. At very high voltages, like 40,000 to 50,000 or more, the overhead line must also be given first place on the score of reliability, since the lasting qualities of underground and submarine cables at such pressures is as yet an unknown quantity. ”

“ So some of these, with the aid of the electrical manufacturers, experimented and developed the alternating current of high voltage and low amperage, which is capable of being carried to distant transforming or sub-stations and there reduced to low voltage and high amperage. This alternating [Pg 438] current system, because of its great operating economies, is rapidly becoming the standard for the city railroad systems of metropolitan communities, as well as for the great trunk-line interurban electric roads that are beginning to gridiron the country. ”

“ A power-station will be located upon the site of the water-power, and the dynamos therein will generate electricity at, say, two thousand volts pressure. By means of step-up transformers this will be exchanged for a current of thirty thousand volts for transmission over a line of copper or aluminum wire to the distant consumption area. Here there will be a set of step-down transformers which will exchange the thirty-thousand-volt line current for one of so low a pressure as to be safe for local distribution to lamps, to motors, etc., either stationary or upon a railway. ”

“ An objection to operation with a single circuit in a transmission line that supplies widely separated sub-stations with lighting, power, and railway loads is the consequent difficulty of pressure regulation on the distribution lines at each sub-station. Such a transmission line necessarily delivers energy at different and fluctuating voltages at the several sub-stations, and these fluctuations are of course reproduced on the secondary side of the step-down transformers. ”

“ The overhead system has conquered because it is cheapest in first cost, cheapest to maintain, most economical in current, and most reliable in action. Later developments in surface-contact traction have run it very close on some of these points, but have not—for reasons which will be explained—affected the established position of the overhead system. ”

“ But by 1890 electric traction had become practical, and the issuing of patents for cable lines ceased as abruptly as it had begun. Before the close of the century practically every important cable line in the United States had changed its motive power to electricity. Thus in a brief quarter of a century this method of street-car traction had come into existence, revolutionized all hitherto known methods, and become obsolete. ”

“ Energy transmitted over long distances must sometimes pass through conductors that are underground or beneath water. In some other cases it is a question of relative advantages merely, whether portions of a transmission line go under water or overhead. Where the transmitted energy must enter a sub-station in the heart of a large city, it not infrequently must go by way of underground conductors without regard to the voltage employed. In some cities the transmission lines may be carried overhead, provided that their voltage is within some moderate figure, but not otherwise. ”

“ As the first essential of a successful electric railway is a plentiful supply of cheap power, development from the experimental stage of Portrush had to wait until engineers mastered the art of producing electricity from large generators. ”

“ This is particularly true in cases where energy transmitted over a single circuit is distributed for both incandescent lamps and large electric motors, because the starting and operation of such motors causes large fluctuations of amperes and terminal voltage on the transmission circuits. To hold such fluctuations within limits which a sub-station can readily compensate for, it is necessary that the loss in the transmission line be moderate, say often within ten per cent of the total voltage delivered to it at maximum load. ”

“ Electric trains are supplied with current from a central generating station, just as in the case of electric tramcars, but on passenger lines the overhead wire is in most cases replaced by a third rail. This live rail is placed upon insulators just outside the track rail, and the current is collected from it by sliding metal slippers which are carried by the cars. The return current may pass along the track rails as in the case of trolley tramcars, or be conveyed by another insulated conducting rail running along the middle of the track. ”

“ Generators for alternating current can be had that develop as much as 13,500 volts, but such generators cost more than others of equal power that operate at between 2,000 and 2,500 volts. These latter voltages are as high as it is usually thought desirable to operate distribution circuits and service transformers in cities and towns, so that if more than 2,500 volts are employed on the transmission line, step-down transformers are required at a sub-station. ”

“ In this fine building is contained a mighty specimen of what is called the three-phase distribution of electrical energy, the system being to create the power at one centre and distribute it over a wide area; that is, electricity is produced in the form of three-phase alternating current at a pressure of 6,500 volts, and sent on to five outlying sub-stations, where it is transformed to a potential of 310 volts, and then converted from alternating into continuous current at 500 volts, for working the cars. The total capacity of the main station is ”

“ There are a great number of magnificent water power sites, within very easy access of the surveyed route of the railway, all of which can be easily developed to furnish almost unlimited electrical energy. Hydro-electric energy could be developed at any one of these sites within a distance of thirty miles from the road, which would make transmission to points along it, or possibly operation of it actually, by electricity. It certainly assures cheap power for any manufacturing or milling centre which may develop. All these power locations must inevitably become very valuable. ”

“ The means by which this is accomplished is ingenious and surprising, since it means that the supply of electricity over a wire--seemingly the most subtle and elusive essence on earth--may be controlled like a stream from a cock, or the gas out of a burner. But this reduction of the current that makes the red glow in the clusters in a theater is by no means the only instance. The trolley-car, and even the common motor, may be made to start very slowly, and the unseen current whose touch kills is fed to its consumer at will. ”

“ In some transmission systems the distribution circuits for stationary motors are not fed by the same transmission lines that carry the lighting load, but draw their energy from lines that do no other work. This practice is certainly desirable, as it frees the lighting circuits from all fluctuations of voltage due to line losses with changing motor loads. ”

“ Three main factors are concerned in the cost of electric power transmission, namely, the transformers, the pole line, and the wire or conductors. These factors enter into the cost of transmitted energy in very different degrees, according to the circumstances of each case. The maximum and average rates of energy transmission, the total voltage, the percentage of line loss, and the length of the line mainly determine the relative importance of the transformers, pole line, and conductors in the total cost of delivered energy. ”

“ If, now, energy from the water-power is to be transmitted over a distance of many miles, a new set of costs is to be met. In the first place, it will be necessary to raise the voltage of the transmitted energy much above the pressure at the dynamos in order to save in the weight and cost of conductors for the transmission line. This increase of voltage requires transformers with capacity equal to the maximum rate at which energy is to be delivered to the line. ”

“ It was predicted by George Stephenson in 1825 that his system would supersede all other methods of conveyance in this country. Similarly can it now be prophesied that throughout the world electrical traction will ultimately supplant all other forms. An age of electricity is dawning, when “power” may be obtained direct from fuel or from the vast store of energy existing in the heated interior of the earth, or even from the atmosphere that surrounds us ”

“ The use of alternating current thus permits a large amount of energy to be economically distributed over a wide area from a single station, not only reducing the cost of the wiring, but securing greater economy by the use of one large station, instead of several small stations. ”

“ The greatest single power transmission in the world, that between Niagara Falls and Buffalo, is carried out with two pole lines, one of which is about twenty and the other about twenty-three miles long. The longer pole line, which is also the older, carries two three-phase circuits, each of which is made up of three 350,000 circular mil copper conductors. The shorter pole line carries a single three-phase circuit composed of aluminum conductors, each of which has an area in cross section of 500,000 circular mils. ”

“ But with electric traction the main item is the cost of the standing equipment—the power house, [Pg 26] mains, and overhead lines—and unless that equipment is adequately utilised the revenue will not cover the standing charges. A fifteen-minute service is, generally speaking, the lowest economic limit on an electric tramway. Every tramway manager tries to attract sufficient passengers for a more frequent service ”

“ As long as each electric system was confined to a single city or town a broken insulator pin could be quickly replaced, and any material interruption of service from such a cause was improbable. Where the light and power supply of a city, however, depends on a long transmission line, as is now the case in many instances, and where the line voltage is so great that contact between a wire and a cross-arm will result in the speedy destruction of the latter by burning, a broken pin may easily lead to a serious interruption of the service. ”

“ Power transmission with continuous current has the advantage that the cost of generators remains nearly the same whatever the line voltage, and that no transformers are required. Such transmissions are common in Europe, but have hardly a footing as yet in the United States. The reason for the uniform cost of continuous-current generators is found in the fact that they are connected in series to give the desired line voltage, and the voltage of each machine is kept under 3,000 or 4,000. ”

“ A number of rather long transmissions is carried out in Europe with continuous, constant current, and in such systems the line voltage varies directly with the load. As the loss of power in an electrical conductor depends entirely on its ohms of resistance, which are constant at any given temperature, and on the amperes of current passing through it, the line loss in a constant-current system does not change during the period of operation, no matter how great may be its changes of load. ”

“ The force of 11,000 volts will be much too powerful for direct application to the purposes of locomotion. It requires reducing by transformers and rotary converters into the safe and ordinary current of about 550 volts, which will be effected at sub-stations—Earl’s Court, South Kensington, Victoria, Charing Cross, Mansion House, and other places along the line. ”

“ It is open to the objection, however, that it in a measure legalizes the conveyance of electricity on conductors other than those designed for the purpose. In larger towns, with more than one power house and with car lines radiating from and circumscribing the business center, the attachment of conducting wires entails a ceaseless disturbance of the electrical equilibrium, curing the evil in spots and developing new danger points. ”