DOI:10.1049/jiee-1.1928.0191

出版商:IEE    

年代:1928

数据来源: IET

DOI:10.1049/jiee-1.1928.0192

出版商:IEE    

年代:1928

数据来源: IET

DOI:10.1049/jiee-1.1928.0193

出版商:IEE    

年代:1928

数据来源: IET

摘要:

A theory of the losses in the lead covering of an insulated cable is developed which combines reasonable accuracy with simplicity. Formulae are obtained for the induced sheath voltages and for the losses with bonded sheaths. It is shown that when the cables of a 3-phase system are arranged in a plane the sheath losses occurring in the two outer cables are not equal, although previous writers have usually assumed this to be the case. A numerical example is given in Appendix 2 to show the possible differences which may exist. Experimental work, verifying the results obtained both for single-phase and for 3-phase currents, with two arrangements of cables, symmetrical and in a plane, is given in Appendix 1.The formulae obtained are given in a convenient form for reference in Section (5).The examples of the numerical values of the sheath voltages and sheath losses, given in Appendix 2, have been carefullychosen to show the method of applying the formulae developed in the paper, and also to give an indication of the magnitude of the effects involved. It is difficult, with formulae containing so many variables, to give a few numerical examples which will cover the whole range of conditions met with in practice, but the following examples will give an idea of the importance of the effects considered. The frequency assumed is 50 cycles per second.(A) Sheath voltages. (Sheaths bonded together at one end only.)—These voltages are directly proportional to the line current and to the length of the line for all arrangements of cables. The only other variable, at a given frequency, is the ratio of the spacing between axes of adjacent cables to the mean diameter of the lead sheath. The voltages are directly proportional to the logarithm of this ratio. As an illustration of the magnitude of the effects, values are given below for a line current of 1 000 amperes and a line length of 1 mile. Two spacings are considered, the first with the cables almost in contact, and the second with their axes 5 diameters apart.The diameter referred to is the mean diameter of the sheath.(i) Single-phase.—The voltage between sheaths varies from about 150 volts, when the sheaths are almost touching, to 470 volts when the cables are 5 diameters apart.(ii) Three-phase—Cables arranged at the corners of an equilateral triangle.—The voltage between sheaths varies from about 130 volts, when the sheaths are almost touching, to 400 volts when the cables are 5 diameters apart.(iii) Three-phase—Cables arranged in a plane with the middle cable equidistant from the two outer cables.—The voltage between the sheaths of the two outer cables varies from about 250 volts, when adjacent sheaths are almost touching, to 530 volts when adjacent cables are 5 diameters apart.The corresponding figures for the voltage between the sheath of each outer cable and the sheath of the middle cable are 150 volts and 410 volts respectively.(B) Sheath losses. (Sheaths bonded together at both ends.)—The ratio of these losses to the copper losses does not directly depend on the line current, but it increases as the size of the cable is increased. In general, the losses do not become serious until the line current exceeds 500 amperes. The losses, also, do not increase quite in proportion to the spacing of the cables, that is, the curve of variation of sheath losses with spacing is slightly convex to the spacing axis in all cases, but for spacings up to 5 diameters the variation is approximately linear. The convexity of the curve increases as the size of the cables is increased and also as the spacing is increased.The figures given below are for a cable with a core resistance of 0-04 ohm per mile and with a sheath resistance of 0.3 ohm per mile. Such a cable would be capable of carrying a current of about 1 000 amperes.(i) Single-phase.—With the cables in contact the sheath losses are equal to 55 per cent of the copper losses, and with the axes of the cables spaced 5 diameters apart the sheath losses are equal to 280 per cent of the copper losses.(ii) Three-phase—Cables arranged at the corners of an equilateral triangle.—With the cables in contact the sheath losses are equal to 60 per cent of the copper losses, and with the axes of the cables spaced 5 diameters apart the sheath losses are equal to 280 per cent of the copper losses.(iii) Three-phase—Cables arranged in a plane.—With the cables in contact the sheath losses are equal to 100 per cent of the copper losses, and with the axes of adjacent cables spaced 5 diameters apart the sheath losses are equal to 310 per cent of the copper losses.The sheath losses are divided equally between the sheaths in the single-phase system, and also in the 3-phase system when the cables are arranged at the corners of an equilateral triangle. When the cables are arranged in a plane the division is unequal, so that the loss in the middle sheath is 17 percent of the total sheath losses, the loss in one outer sheath is 37 per cent, and the loss in the other outer sheath is 46 percent when the cables are in contact. When the axes of adjacent cables are spaced 5 diameters apart, the loss in themiddle sheath is 26 per cent, the loss in one outer sheath is 32 per cent, and the loss in the other outer sheath is 42 percent of the total sheath losses.

DOI:10.1049/jiee-1.1928.0194

出版商:IEE    

年代:1928

数据来源: IET

摘要:

It is usual, when single-conductor cables are used, to transpose them at regular intervals in order to minimize the disturbance to other circuits.If the cables are arranged in any other formation than at the corners of an equilateral triangle, it is desirable to transpose regularly for the additional reason of equalizing the impedance-drops in each line, which will be different without transposition, due to the asymmetrical arrangement of the cables. It is sometimes convenient to lay the cables in a plane with the middle cable equidistant from the two outer cables, and the impedance in this case will be considered, when the cables are not transposed and when the sheaths are earthed at one point only.It is shown in an Appendix that earthing the sheaths at both ends, so that part of the sheath currents can return through the earth, will not appreciably alter the impedance-drops. The method is readily applicable to any irregular arrangement of cables, but the formulae become rather lengthy and it is only proposed to deal here with the arrangement of the cables in a plane.An example is worked out fully to show the method, and the results of experimental work carried out to verify the theory are given at the end of the paper.In the example chosen, it is shown that in order to deliver a balanced 3-phase load of 500 amperes at a power factor of 0.8 (lagging) and with voltages between lines of 6 000 volts, when the cables are arranged in a plane without transposition, it is necessary at the sending end(5 miles distant) to have a voltage between one outer line (A) and the middle line (B) of 7260 volts, between the other outer line (C) and the middle line (B) a voltage of 7 000 volts, and between the two outer lines a voltage of 7 340 volts, when the sheaths are insulated from each other.If the sheaths are bonded together, but earthed at one end only, the voltages necessary are 7 290 volts between the outer line (A) and the middle line (B); 7 i70 volts between the outer line (C) and the middle line (B); and 7 500 volts between the two outer lines.The unbalance in both cases may be seen to be considerable.If the sheaths are earthed at both ends, and the earth resistance is zero, the voltages required at the sending end become 7 270, 7 170 and 7 500 volts respectively. These figures are very little different from the figures given above, namely, 7 290, 7 170 and 7 500 volts, for the case of bonded sheaths with only one earth point.Results of example worked out in Section (3).Phase sequence of lines is A, B, C, where A and C are the two outer lines.At receiving end, current = 500 amperes, power factor = 0.8. Voltage between lines = 6 000 volts.If, with the cables arranged in a plane, regular transposition of the cables is effected, then the impedance-drop in each of the three lines is the same over an integral multiple of three transpositions, and is less than the average of the three impedance-drops in the untransposed lines. The difference is usually small, but may amount to 10 per cent when the cables are laid close together. The difference diminishes as the cables are spaced further apart.When the cables are regularly transposed, the estimation of the impedance-drops presents no difficulties, for the system is then equivalent to a symmetrically arranged system with axes of cables3√2 times as far apart as the axes of adjacent cables in the plane arrangement.It follows from this that if the cables are laid close together the impedance-drops will always be greater for the plane arrangement with regular transposition than for the symmetrical arrangement. The difference is about 20 per cent when the cables are in contact. The difference diminishes as the cables are spaced further apart. The sheath currents modify these results slightly, but, unless the sheath losses are very large, no appreciable error will be introduced by assuming an equivalent symmetrical arrangement.

DOI:10.1049/jiee-1.1928.0195

出版商:IEE    

年代:1928

数据来源: IET

摘要:

The preliminary section of the paper deals with the early history of research on selenium, and its sensitivity to light. In Section 1, dealing with the construction of selenium cells, different types of cells are enumerated and described. It is emphasized that the cell must have little inertia to light-changes, and for this reason the selenium layer must be thin so that as much as possible is affected by the light. The properties of selenium cells are dealt with in Section 2. The author's experiments have shown that the change in conductanceG(due to a given illuminationE) varies as some power of the illuminationE, i.e.G∞Ex. The index valuexvaries from type to type of cell, and even amongst cells of the same type small variations inxoccur. Thus no two cells, even of the same type, have in general exactly the same quantitative reaction towards a given illumination. It is shown, however, that these empirical relations are quantitatively reproducible to a high degree of accuracy, provided the experimental conditions are reproduced.A brief résumé of the many factors influencing the conductance of a selenium cell is followed in each case by the results of recently published research. Some experiments by the author on the decay of the conductance of selenium after exposure to illumination show that the internal state of the selenium, as determined by the change in conductance, is, throughout a large portion of the visible spectrum, independent of the quality of the stimulus. Some filter experiments are described, from which it appears that the change in conductance of selenium cells is dependent, not on the number of foot-candles incident on the cell, but rather on the amount of radiant energy received. Section 2 concludes with a short note on the properties of selenium crystals.The chemical, electrolytic and electronic theories of the action of light on selenium are reviewed in Section 3. The recent researches of Gudden and Pohl suggest a return to the physico-chemical theory. It appears that the problem of explaining the observed phenomena is not one of photo-electricity but rather of conductivity in non-homogeneous media containing a large number of bounding crystal surfaces.In Section 4 the practical applications of selenium cells are described in some detail under the following headings:—(a) Application of the selenium cell to photometry, and to relay problems.(b) The Optophone.(c) The Photophone.(d) The talking film.(e) Television.The conclusion sums up the present state of our knowledge of the action of light on selenium.

DOI:10.1049/jiee-1.1928.0196

出版商:IEE    

年代:1928

数据来源: IET