Publication: Electrical World
New York, NY, United States
Electrical Power Transmission.—XVI.
BY LOUIS BELL PH. D.
Before taking up the practical task of line calculation it is necessary to consider somewhat at length the electrical difficulties that must be encountered and which impose limitations on our practically achieving much that in itself is desirable and useful. We have seen already that the secret of long distance transmission lies in the successful employment of very high voltages, and whatever the character of the current employed the difficulties of insulation constantly confront us. These are of various sorts, for the most part, however, those that have to do with supporting the conducting line so that there may not be a serious loss of current via the earth. Next in practical importance come those involved in insulating the conductor as a whole against, first, direct earth connections or short circuits in underground service, and second, ground or short circuits, if the line is an aerial one.
In a very large number of cases no attempt is made to insulate the wire itself by a continuous covering, and reliance is placed entirely on well-insulated supports. In most high voltage lines this is the method employed, partly for economy and partly because there is well-grounded distrust in the durability Of any practicable covering under varying climatic conditions end the constant strain imposed by high voltage currents.
So far as supports go, it is evident that while the individual resistance of any particular one may be very great, the total resistance of all those throughout the extent of a long line to which they are connected in parallel to the earth may be low enough to entail a very considerable total loss of energy. The possibility of such loss increases directly with the number of supports throughout the line. The most obvious way of reducing such losses would be to considerably increase the distance between supports. This process evidently cannot go on indefinitely, for mechanical considerations, and hence the greatest advance can be made in reducing the chance of loss in individual supports.
Most of the present practice consists merely of an extension of the methods that were devised for telegraphic work. These were quite sufficient for the purpose intended, but are inadequate when applied to modern high voltage work. The ordinary line consists, then, of poles bearing on pins of wood or metal secured to cress arms, bell-shaped glass or porcelain insulators. To grooves on or near the top of these the line wire is secured by binding wire. Loss of current to earth in a line so constituted takes place in two ways. First, the current may pass over the outer surface of the insulator, up over the interior surface, thence to the supporting pin and so to earth. Second, it may actually puncture the substance of the insulator and pass directly to the supporting structure.
The first source of trouble is the commoner and depends on the nature and extent of the insulating surface, and even more cm climatic conditions. The second depends on the thickness and quality of the insulating wall which separates the wire from the pin. To avoid leakage an insulator should be so designed that, first, the extent of surface shall be as long and narrow as possible; second, that this surface shall be both initially and continuously highly insulating. The first condition is met by making the insulator of comparatively small diameter and adding to the length of the path over which leakage must take place by placing within the outer bell of the insulator one or more similar hells (usually called petticoats). These not only help in the way mentioned, but they are likely to be tolerably dry even When the exterior surface is wet, and thus help to maintain the insulation.
A good glass or porcelain insulator made on these general lines gives excellent results with ordinary moderate voltages, say up to 3,000 volts. When the insulators are new and clean they will quite prevent perceptible leakage and for the voltage mentioned are satisfactory under all ordinary conditions. When higher voltages are employed the results may be at first good, but they are unlikely to stay so unless the climatic conditions are exceptionally favorable. Most glass permits a certain amount of surface leakage, even when new, although generally not enough to be of practical importance, but even the best of it weathers when exposed to the elements, so that in time the surface becomes slightly roughened and retains a film of dirt and moisture that is a very tolerable conductor. Even while perfectly free from this deterioration at first, it is generally hygroscopic, because it is in a trifling degree soluble even in rain water, and tends to retain a slight amount of moisture. Thus in damp climates glass is likely to give trouble when used on a high voltage line, As mortis temporary fall in infuriating properties a searching fog or drizzling rain is worse in its effects on insulators than a sharp shower or even a heavy rain, which tends to wash the outer surface free of dirt, and affect the comparatively clean interior but little.
Much cheap porcelain is also hygroscopic owing to the poor quality of the glaze, and it has the considerable further disadvantage of depending on this glaze for much of its insulating properties. Glass is homogeneous throughout its thickness, while porcelain inside the glaze is porous and practically without insulating value. Nevertheless porcelain which is thoroughly vitrified, the ordinary glaze being replaced by an actual fusing of the surface of the material itself, is decidedly preferable to glass, being tough and strong, quite non-hygroscopic and of very high insulating value. The surface does not weather and the insulation is well kept up under all sorts of conditions. If the vitrification extends, as it should, considerably below the surface, the insulator will resist not only leakage, but puncture, better than any glass. The process of making this quality of porcelain is somewhat costly, since the baking has to be at an enormous temperature and long continued, but the result is the most efficient insulating substance in use.
Surface leakage is more to be feared than puncture at all voltages, since the absolute insulation strength of .the material can be made high enough by careful attention to quality and sufficient thickness to withstand any voltage within our command, and that continuously barring mechanical injury. But leakage can never be depended on; it is a function of moisture, drifting dust, and things meteorological generally, besides which, it can take place in serious amount at voltages which otherwise would be very easy to work with.
As the result of all recent experiments, it may be confidently stated that danger from actual breaking down of the insulation by puncture need not he at all feared up to certainly 15,000 volts with any good modern high voltage insulator. With the best insulators at present attainable the voltage just mentioned can easily be doubled without serious danger of breaking through the material.
At such high electrical pressures, however, the difficulty of stopping surface leakage becomes formidable, especially in bad weather. Glass insulators become almost useless, and only the best porcelain is to be trusted—even this with some reservations. To reinforce the insulation of the surface it has been quite usual to take recourse in the extraordinary insulating properties of heavy mineral oil. Imagine the lower edge of the outer bell of any insulator folded inward so as to form a deep groove opening upward all around the insulator. Fill this hollow with oil and it is evident that if surface leakage takes place at all it must be across the surface of this oil. Various modifications of this plan will be shown later.
Such an oil insulator is quite free from surface leakage so long as the oil surface is kept clean and in good condition. This is, however, very difficult to do and there is great danger that the oil surface front the combined action of dirt and moisture will degenerate into a species of conducting slime. Where dust is very prevalent the oil is specially likely to give trouble; it is far better able to cope with moisture alone than with moisture plus dust. From what has been said it is clear that line insulation is largely a matter of climate. Where it is uniformly warm and dry almost any kind of insulator will suffice, provided the material has tolerable insulation strength and the insulator is of passably good design. With a climate foggy in summer and sleety in winter even the best porcelain insulators will be severely tried at high voltages.
The record of what has actually been done in the transmission of electrical energy at high voltages is comparatively short, and gives no very valuable data for the important work over long distances that must be done in the next few years. The experience with are light circuits at 5,000 volts or above, working over lines from 20 to 40 miles long, is already considerable: Seventy-five to 125 light dynamos are now common, the latter giving in the vicinity of 6,250 volts, and this front open-coil armatures of which the maximum voltage is roughly equal to that of an alternator of the same nominal voltage. The lines are usually, almost always, of wire having no very high insulating properties, supported on common glass insulators, often without an interior petticoat. One hundred and fifty light generators are in occasional use and now and then two machines of 75-light capacity or more are operated in series for considerable periods. This practice is commoner than is generally supposed.
Alternating circuits of 5,000 volts or more are now not at all uncommon either here or abroad. Several of them have been in operation long enough to be thoroughly tested. An 18-wile transmission at a little over 5,000 volts, to Guadalajara, Mexico; an 11-mile 5,000 volt line to Hartford, Conn.; another (three-phase) 14 miles to the works of the Oelikon Company, at 7,700 volts, and the Tivoli-Rome plant, 18 miles at 5,000 volts, have been in steady and successful operation for two years or more. A number of others, probably a score in all, of less prominence, owing to shorter length of lines and smaller capacity, are in regular operation at 5,000 to 6,000 volts. Most of the lines are of bare copper wire supported on oil insulators of glass or porcelain. They have been uniformly free from all serious trouble.
In the vicinity of 10,000 volts the experiments are fewer but none the less conclusive. Only two commercial plants have been regularly operated at such a pressure. One is the well-known Ferranti station working a 10,000-volt main from Deptford to London, about 11 miles, and using a concentric underground cable. The plant has experienced various vicissitudes and bait not been steadily operated, but the troubles are not generally chargeable to the mains, which have, however, been a little uncertain in their performance. The other case is the lighting plant in San Antonio Canyon, California. This consists of a single alternating generator of 150 kw at 1,000 volts. Raising transformers establish a line pressure of 10,000 volts, at which current is transmitted to Pomona, 16 miles distant, and to San Bernardino, 28 miles. At each of these places is a substation with reducing transformers and regulating apparat