Publication: American Electrician
New York, NY, United States
HIGH VOLTAGE POWER TRANSMISSION.
BY CLARENCE F FOWLER.
The commercial feasibility with which power can be economically transmitted from any one point to any other point depends broadly upon the intervening distance. Whether this distance is within the economical transmission radius of the source of power will depend largely on the difference between the price which can be obtained for delivered power as compared with the cost of producing it. This difference must cover the cost of operating, the price of energy lost in transmission, the cost of maintenance and repairs, the interest on the investment and finally the dividends payable on the stock.
It is well known that the greater the output of all generating and transforming apparatus (within certain limits) the less will be the cost per kilowatt and therefore the less will be the interest charge per kilowatt on the investment, and as the operating force need not be materially larger for the greater output it follows that as the output is increased the operating expenses per kilowatt will be diminished also.
Both of these facts have the same significance, namely, that the larger the output of a plant that can be concentrated at one point, the greater will be the economy in the generation of electrical energy. Moreover, the larger the market that exists or that can be created at the point to which power is to be transmitted, the greater may be the distance between this point and the generating source for the reason that a greater margin will be available for investment in transmission line, owing to the saving effected by the increased size of the generating plant.
The amount of material in the line conductors depends directly upon the amount of power transmitted, when the voltage line drop and transmission distance are fixed. In other words the cost per plant kilowatt for line conductors does not diminish with increasing output, as is the case with generating and transforming equipment. Consequently, for a given voltage, line drop and transmission distance, the cost of line conductors per kilowatt transmitted will be practically the same for all amounts of power.
Since for a given line voltage, drop and transmission distance, no gain may be expected in the cost of the line conductors per kilowatt transmitted no matter what amount of power may be transmitted, and as there appears to be a marked gain in operating expenses as well as in the interest charge on the investment per kilowatt of the generating and transforming equipment for large outputs, the transmission of power over any great distance will be commercially possible only when the market for power at the receiving end is large enough to permit such decrease in the cost per kilowatt of generating equipment as to more than offset an otherwwise prohibitive investment in transmission line.
Having taken into account some of the general economical aspects of the problem there remain some details to be considered, such as the selection of the line voltage; the arrangement and distance between line conductors; high-tension insulators, and the pole line.
Selection of Line Voltage. — Current practice seems to indicate the use of about 1,000 volts per mile of transmission, in order that the size of the line conductors may be kept within reasonable limits, but in some extreme cases two miles per thousand volts is allowed. In the matter of the selection of the proper line voltage, it is conceivable that the same may be raised to such a point that the excessive costs of transformers and line insulators will overbalance the saving effected by the reduction in size of the line conductors. The line voltage is also limited to a certain extent by losses which occur between wire and wire through the atmosphere and which experimental data have shown to be excessive after a critical voltage is reached. The losses below the critical voltage are trivial and have been proven to take place chiefly over the insulator surfaces and cross-arms. This loss is ilso affected to some extent by the diameter of the line wire — the smaller the wire the greater the loss which occurs with a given voltage. This seems to follow the well-known tendency of all high-potential discharges to jump or leak more freely between sharp edges and points than between rounded surfaces. In this connection the aluminum conductor (owing to its greater cross-section as compared with copper for the same conductively) would seem to present some advantage.
Arrangement of Line Conductors. — Owing to the economy in copper which the three-phase system offers it is presumed that this would be the only system considered in power transmission over any considerable distance. As to the arrangement of the three conductors, the best results are secured by locating the centres of these at the three points of an equilateral triangle, as this relation gives the least inductive reaction.
If a telephone circuit traverses the same pole line as the high-tension circuit, the former should be spiraled at intervals so that each wire of the high-tension circuit bears the same relation to the wires composing the telephone circuit in order to annul the effects of induction.
An important factor in the choice of the distance between different wires of a high-voltage transmission line is the atmospheric loss at the working voltage employed. Furthermore it is necessary to separate the line wire by such a distance that should a short-circuit be accidentally established it would not be possible for it to hold after the cause had been burned free. In this connection it must be borne in mind that an arc already established between line wires can be maintained by a much lower voltage than that required.to start the arc. The best practice in this particular is to employ a separation of line conductors of about 1 1/2 inches for each 1,000 effective volts.
High-Tension Insulators. — There are now several reliable makes of high-tension insulators on the market which are capable of taking care of working pressures up to 50,000 and 60,000 volts. There seems to be considerable difference of opinion as to whether glass or porcelain can be most advantageously employed for these higher potentials. Some of the advantages of glass insulators are: detection of a large proportion of the number of flaws by inspection; great dielectric strength; usually a somewhat more homogeneous structure than porcelain; less conspicuousness as targets on the cross-arms. The disadvantages of glass are its rather poor weathering properties and inferior mechanical strength.
The advantages of porcelain insulators are superior mechanical strength; somewhat better weathering qualities than those of glass; good dielectric strength when carefully vitrified. The disadvantages are that elaborate tests are necessary to discover flaws and the insulator forms a rather attractive target for mischievous persons. Porcelain insulators seem to be gaining in favor owing to improvements in the methods of manufacture. In the production of these insulators for high potentials, the great difficulty encountered heretofore was to produce an insulator of sufficient size in which the porcelain was homogeneous throughout. This trouble has been overcome by making insulators in as many as three distinct parts, these elements fitting inside one another when assembled and being held together by Portland cement, forming a composite insulator of porcelain of uniform density throughout and of sufficient size to possess the necessary surface leakage dimensions for the working voltage which it is intended to withstand.
In short, whether the insulator be glass or porcelain, it should have dielectric strength sufficient to prevent the current from leaking directly through the material of which it is composed and its dimensions should be large enough to prevent appreciable leakage of current over its surface.
Pole Line. —Where the amount of power transmitted is considerable and, therefore, reliability of service is of paramount importance, there seems to be a growing tendency toward abandonment of the wooden pole construction in favor of the more substantial steel towers. With increasing voltages it has also been found advisable to construct duplicate transmission lines on separate towers spaced from 30 to 50 feet apart.
These steel towers are usually constructed of angle iron riveted together in such a manner as to form a rigid structure in the form of a pyramid, the base of which is securely fastened to a substantial foundation. The towers are usually spaced so that there are from 12 to 15 per mile as contrasted with the usual hundred-foot spacing commonly used with wooden-pole construction.
Inasmuch as there are required only about one-fourth as many steel towers as wooden poles per mile of transmission, there is only one-fourth the number ofl insulating points to give trouble along the line when the tower construction is used. To effect this advantage the insulator has to possess high dielectric strength for the reason that the use of metal pins brings the ground potential up inside the bell; which would not be true in the case of an insulator supported on a paraffin-treated, hard-wood pin. Moreover, since the spans are longer the insulator also has to sustain a greater mechanical strain. It is therefore evident that the use of the steel tower for long-distance transmission has been largely dependent upon the evolution of a high-tension insulator possessing the proper electrical and mechanical qualities.
The chief disadvantage of the steel-tower construction is its high initial cost, but when reliability is considered the installation cost should not prove any great barrier to its use.
When the route of a transmission line presents a rather wavy profile, as in the case of mountainous country, it is usually thought to be good practice not to follow the slopes of the land closely, but by the use of short and long towers, or poles, as the case may be, the wires may be run on smooth grade and thus avoid undue mechanical strains on the insulators and pins. In passing over a summit, the top may be rounded off by the use of relatively long poles on either side of the pole on the crest of the hill, and in this manner distributing the compression strains among several insulators, instead of confining these locally to the crest insulator, as would be the case were the line allowed to follow the natural slope of the land. Furthermore, by utilizing relatively tall poles in the valley the depression may be reduced with consequent advantagous division and reduction of the tension strain on the insulators. This feature assumes importance where composite insulators are employed; as the elements of which these are composed may be in danger of being separated by undue upward strain. The foregoing are some of the features, briefly stated, that command attention in connection with the lay-out of a high-tension transmission system.
|Date completed:||September 14, 2009 by: Bob Stahr;|