Showing papers in "Journal of the Institution of Electrical Engineers in 1929"

An investigation of short waves

Abstract: In this paper an account is given of investigations of shortwave transmission effects made during the past two years. These include, firstly an investigation of scattering, multiple signals and signal mutilation, which supplies perhaps the main bulk of the paper, secondly the results of a nine months' direction-finding interception of short-wave commercial stations, and finally a discussion, in the light of these results, of the problems of short-wave transmission and such cognate subjects as the nature and constitution of the Heaviside layer, fading and polarization effects.An opportunity is afforded for the revision of conclusions arrived at in the author's previous paper.* The chief alteration is in the estimated height of the lowest levels of the Heaviside layer in daytime. The accumulation of evidence from the time-lag experiments of Breit and Tuve, and the interference experiments of Appleton and those of Hollingworth (which incidentally both give the group time and consequently an over-estimate of the height) have led the author to a revision of the mathematical result by which he obtained from long-wave measurement the effective height of the daylight ionized layer, and in doing so he has found an error which makes the estimated height just double that previously obtained, for inseatd of the relationd0 = h2/λ† we have d0 = h2/λ, making the estimated height lie between 80 km for summer and 97 to 100 km for winter.Previously the value of τ, the mean time between collisions, was found to lie between the limits 0.5 × 10−6 and 0.5 × 10−7 (on the basis of magnetic storm effects). Calculations based on the kinetic theory would imply a height of between 76 and 92 km for these values of τ in fairly close agreement with the above, and there is no necessity to make the rather forced assumption previously made that the electronic mean free path was some 10 to 50 times greater than that calculated from the normal kinetic theory on the assumption that the electron may penetrate the outer rings of the molecules without being appreciably deflected. Apparently this effect occurs only in inert gases, such as helium, and is not likely to occur in the upper atmosphere.With regard to scattering, the glancing angle for transmission, great-circle bearings of the main rays, and the daylight attenuation, the author has found the previous conclusions to be on the whole well-founded.Some modification of the lower wave-length limit for night transmission is required, and the author finds that this lower limit depends on the time of the night and season. Late-night regions behave very differently from early-night regions, thus confirming Appleton's results showing a progressive change in the Heaviside layer during the hours of darkness. The short-wave daylight limit appears to be close to 10m, but sporadic long-distance transmissions have been observed on wave-lengths shorter than this.The interpretation of many observations of short-wave transmission depends upon a previous experimental knowledge of scattering. A discussion of scattering will therefore be given first, and later the general results of long-distance and short-distance direction -finding will be considered. Fading and magnetic storms will next be discussed, then “skip effects”, and finally a general theoretical discussion in the light of the results obtained will be given.

Direct generation of alternating current at high voltages

Abstract: The authors begin by pointing out, with the help of a mechanical analogy, the true place which the alternating current transformer takes in the group of plant considered to-day to be standard for the generation, transmission and distribution of electric power on a large scale.They propose the use of high-voltage alternators and the partial abolition of transmforers. In support of these proposals, the authors discuss the gradual growth of conditions favourable to the introduction of the high-voltage alternator, and describe the obstacles in the way of further development at lower voltages.This discussion leads naturally to the advantages to modern alternator design to be derived from the acceptance of the principle of direct generation at high voltages.This discussion leads naturally to the advantages to modern alternator design to be derived from the acceptance of the principle of direct generation at high voltages.Apart from the design of the alternator itself, including its cable leads, there are many ways in which the adoption of such an alternator would effect financila economy in capital outlay and running expenses, and these matters the authors proceed to set out.They then taken a brief survey of what has already been done in the past in the field of direct generation at 30000 volts, and describe in detail a 33000-volt, 25000-kW, 3000-r.p.m. alternator of unique design, built for the North Metropolitan Electric Power Supply Co., for installation in the new power station at Brimsdown, North London. The operating experiences on site since the setting to work of the alternator in August 1928 are stated.As a matter of interst to those engineers and designers engaged in alternator construction and development, and outline is given of some of the experimental research carried out by the authors in the working out of the practical details of the new design, and of the tests made at works on the completed alternator.The paper concludes with some reference to the future possibilities of the extended use of direct generation at high voltages, and the authors ask for a candid expression of opinion from their critics.

A critical study of a three-phase system of unarmoured single-conductor cables, from the standpoint of the power losses, line constants and interference with communication circuits

Abstract: This paper has been prepared as a guide to the selection of methods of laying three separate unarmoured ? singleconductor cables for the transmission of three-phase power. A wide extension of the use of cables of this type is contemplated for transmission at 66 kilovolts and upwards. There are two general practicable methods which may be adopted for laying such cables. They may be laid in a symmetrical manner, the axes of the cables being equidistant from one another; or they may be laid alongside one another with their axes in a plane. The electromagnetic effects when carrying three-phase alternating current are materially different in the two cases. The principal factors affected are :? (1) The voltage which may be generated in the sheaths. (2) The energy loss in the sheaths. (3) The impedances of the different cables. (4) The influence on communication circuits. The arrangement in a plane leads to asymmetry and inequality in several of the factors involved, such as voltage generated in the sheath, power loss, impedance and temperature-rise; and in addition the effects are materially greater on the whole than when the same current is transmitted by cables arranged symmetrically. The current-carrying capacity of the two systems for a given temperature-rise has been given some consideration in view of the possibly better cooling conditions of the plane arrangement; but it is found that, in general, the increased losses by separation or substitution of the flat arrangement substantially offset the better cooling conditions. The general conclusions are that, as regards the electromagnetic and heating effects, the symmetrical triangular arrangement, with spacing as close as possible and with cross bonds and transposition at every joint, should be adopted as standard The difference between the effects of triangular spacing and of flat spacing with regular transposition is small and the latter is permissible when special circumstances render it desirable. These conclusions apply to cables laid both direct in the ground and in ducts.

Recent developments in turbo-generators

Abstract: The subject of ventilation is first dealt with, the different systems used in the past, and recent developments, being reviewed. Radial, axial, radial-axial and multiple-inlet systems are referred to, and internal and external fans are discussed. The elimination of eddy currents is next considered, the various methods, past and present, being reviewed. These include: Twisted and transposed windings; twisted and transposed slot conductors; twisting and transposition of end-connections; combination of transposed slot conductors and end-connections. The paper then deals with the magnetic field of stator endconnections and discusses the following matters: Local heating produced in end of core, stator end-plate and stator endconnections; measurements by exploring coils under different load conditions, i.e. leading load, lagging load and shortcircuit; effect of squirrel cage located between stator winding and rotor coil-retaining rings of single-phase generator; neutralization of end field by copper screening plate. In the next section the subject of insulation is treated, the following being considered: Vibration of conductors; swelling of insulation; smooth core design; expansion and contraction of armature conductors; crinkling of insulation; bakelized conductors; clamping of stator winding. Rotor forgings are discussed, especially in connection with the relative advantages of oil-quenching and annealing. Finally, the subject of coil-retaining rings is dealt with, and the method of support, difficulties due to rigidly supported rings, and non-magnetic rings are discussed.

Electrical insulating papers for the manufacture of power cables

Abstract: The work of previous authors on the subject of paper as a cable-insulating material is briefly reviewed. It is shown that, in the past, attention has been directed too exclusively to the fibre stock, and the relation of the physical structure to the electrical properties has been relatively neglected. The characteristics which a cable-insulating paper should possess are then outlined. Briefly, these are high electric. strength, high oil-absorption, low electrical conductivity, high mechanical strength, and permanence.The results of tests designed to determine the relation of the paper structure to these qualities are then quoted and discussed. From these tests it is concluded that a satisfactory paper for power-cable insulation should have the fibres closely packed to give low bulk while still retaining high absorbency. The fibre packing should not be obtained by heavy surface calendering.The methods of test employed and the influence of the manufacturing processes on the product are briefly discussed in appendices.

An introduction to researches on circuit breaking

Abstract: This paper is an introduction to the Electrical Research Association's researches on arc rupture which have been in progress since 1922.An introductory section describes the genesis and aims of the research. The problem of circuit breaking is far more complicated than has been generally supposed, and can be solved only by separate study of the numerous variables involved. The apparatus, controls, and general lay-out of the testing station at Carville are described.The investigators aimed at means for preventing the arc from re-striking, rather than attacking the arc when heavy current is flowing, which may cause heavy energy release.It is shown that, in oil switches, the arc is often broken while the contacts are still within the gas bubble, so that, in such cases, arc extinction is not due to the quenching effect of the oil. Experiments have proceeded along two lines: (1) Study of the gas formed and of its arc-rupturing properties as compared with other gases, with a view to its utilization to full advantage in arc rupture; and (2) a study of the best methods of utilizing the good features of oil for like purpose, the general aim being the utilization of each method separately or in combination in improved or new types of design.Novel methods of gas-sampling during arcing are described, and the results are given of gas collection and analysis covering a wide range of currents and powers.The present paper deals mainly with arcing in gas. For given gaseous conditions in the gap between the contacts at current pause, the adequacy of the gap depends on the instantaneous value of the voltage gradient appearing across it at current zero, and it is found that, for arcs in hydrogen, the. length of critical gap is approximately proportional to the sine of the angular displacement between current and “virtual” voltage zeros. This differs radically from Bauer's conclusions. In arcs in oil there are superimposed phenomena, but in general the same law applies.With asymmetrical currents in gas and oil there is a phase-displacement effect which affects the available voltage in addition to the phase angle due to circuit conditions.Experimental results on arc-voltage characteristics are given which indicate that Norberg's application of Steinmetz's formula, if used over the range considered in the present paper, would lead to large errors in the results obtained for arc energy, whereas his experimental results, though few in number, are consistent with the results here described.A formula for arc energy when arcing takes place in gas is developed, based on simplified assumptions arising from experiment and theory. This formula illustrates the great complexity, more especially of certain known features of the time and frequency factors, which may in certain circumstances play a highly important part in affecting arc length and energy production; these appear to have been omitted in the previously published formulae of Bauer and others.The above work leads to certain practical applications.Certain effects of arc lengthening by mechanical and electromagnetic means are discussed. This section concludes with some description of the principles of operation of a new device for arc rupturing by gas blast, which has been developed as a direct outcome of the investigation of arcing in gas. Models tested show great improvement on recognized methods of arc rupture in oil, and show promise of a satisfactory solution of the problem of arc rupture in gas, which is the main subject of this introductory paper.In spite of the added experimental difficulties due to the presence of oil in oil circuit-breakers, great advance has been made and further details will be given when the patent situation permits.

The testing of porcelain insulators

Abstract: The paper discusses the technique of porcelain insulator testing. The ground covered includes the usual electrical and mechanical tests called for in the various national specifications, and also special tests of more recent origin.

Note on sheath losses in single-core single-phase cables

Abstract: The conditions which govern the eddy currents in the lead sheaths of a pair of single-core cables undergo a change a s the sheaths are moved from the position of intimate contact. The consequent changes in e.m.f. and eddy losses have been the subject of considerable discussion. The following notes record an attempt to settle the matter experimentally by tests on a cable with a Drysdale potentiometer. The conclusions reached are that while the sheath circuit e.m.f. appears to be proportional to log d/r, the change in the losses as this ratio passes through the value 2 is rendered indeterminate by other variables which cannot be separately measured.

The design of transmitting aerials for broadcasting stations

Abstract: The paper is divided into two sections and a conclusion. The first section gives the theory of the aerial as a radiator, with special reference to its ability to radiate rays parallel to the surface of the earth at its base. An account is given of experiments with different types of aerial designed to achieve this result. The reduction of the indirect ray, that is the ray producing a component of radiation at angles less than 90° to the vertical, should produce less fading of signals, and theoretical predictions are made as to the extent of such reduction. An account is given of practical tests on the fading of signals from different forms of aerial. There is some discussion as to how the desired results can be achieved in practice, having special reference both to the use of T aerials and very high masts. The second section deals with the theory of attenuation of waves having frequencies between 500 and 1500 kilocycles per second, and gives a complete set of curves actually taken from a transmitting aerial near London. The conclusion sums up the results and gives data to enable the broadcasting engineer to specify, in general terms, the extent of service area and the effectiveness of the service to be expected, using a given power, frequency of emission of carrier wave, height of aerial and earth conductivity.

The action of a reflecting antenna

Abstract: A historical survey of the literature dealing with the reflection of wireless waves reveals a difference of opinion concerning the phase-change produced when reflection takes place from Hertzian rod oscillators. In this paper the currents produced in two tuned vertical aerials both acting as receivers in a radiation field are discussed theoretically. Two special cases are considered in detail, viz. (1) when the aerials are less than one wave-length apart and are in line with the transmitter (?end-on? position), and (2) when they are less than one wave-length apart and at right angles to the transmitter (?broadside-on? position). In the first case, the current in the aerial nearer to the transmitter attains a maximum value when the distance D between the aerials is approximately 0.33? or 0.85?, and is minimum when D is approximately 0.60?, where ? is the wave-length. The current in the aerial remote from the transmitter reaches a maximum value when D is 0.16?, and then steadily decreases as the distance between the aerials increases to one wave-length. For the ?broadside-on? position, the current in each aerial attains a maximum when the distance between the aerials is 0.71?. The details of the design of the short-wave oscillator and receiving aerials are outlined, and the method of procedure in order to obtain the current measurements is described. The experimental results obtained are compared with the theoretical calculations. The solutions of the general equation tan ?(1 + cos s) = (?2 ? 1)/? gives the critical values of the ratio of the distance between two tuned antennae to the length of the wave, for which the antenna currents are a maximum or a minimum, the plane of the antennae being any angle s to the direction of propagation.

Sheath and armour losses in single-core cables for single-phase and three-phase transmission

Abstract: The accurate practical determination of inductive losses in an a.c. system of single-core lead-covered cables is considered in detail, and test-results which are correlated with calculations are quoted by way of illustration. Bridge methods have been developed for the measurement of both the total and individual losses in single-phase and 3-phase single-core systems. These methods are shown to be capable of great accuracy. The feasibility of armouring single-core cables has been discussed by various authorities, whose opinions are widely divergent. A series of tests has been carried out, the main results of which are given herein. These show clearly the practical limitations imposed by the inductive effects from armouring each cable separately with wire armour. They also provide a sound empirical basis from which the losses in any given system may be estimated.

Load-levelling relays and their application in connection with future metering problems

Abstract: In this paper it is the author's endeavour to show not only that metering methods have played an important part in load-levelling, but that the load-levelling relay in connection with simple metering is a solution of the load-levelling problem. A considerable amount of attention and energy has been devoted to the ways and means of bringing about an improvement in the load factors of the various electricity supply undertakings; and as the load-levelling of the future will concern the distributing mains and consumers' supply services, the author suggests that it will be a problem for the mains and meter engineers rather than for the power station engineers. In order not to delve too deeply into the very complex metering problems, the paper has been limited to the following six considerations:- (1) Load-levelling by metering, with various tariff methods. (2) Load-levelling control at the generating station and substations. (3) Some attractive off-peak loads. (4) Load-levelling relays on consumers' premises. (5) General description of load-levelling relays. (6) Where load-levelling relays can be usefully employed.

Contact effects between electrodes and dielectrics

Abstract: This report covers a research on the phenomena of so-called ?contact resistance?. The research was carried out, on the suggestion of the E.R.A., by the Metropolitan-Vickers Electrical Co., Ltd., and the results show the nature and importance of contact phenomena, also their true bearing on resistivity and dielectric-loss measurements, and that certain conclusions reached by a previous investigator were exaggerated. The authors give a detailed account of a series of experimental investigations and describe the development of methods for the determination of the actual resistance and capacitance of the interface between electrodes and dielectrics. Section (I) of the report deals with two methods which demonstrate the existence of a potential drop across the contact face but which do not lend themselves to accurate quantitative work. A third method is described in which the potential distribution along a cylindrical specimen, subjected to a known d.c. voltage, is ascertained by means of exploring electrodes, and the contact voltage-drops deduced. From these, and the known resistance of the specimen, the contact resistances at each electrode are obtained in ohms per cm2. In the case of slate this was found to be 0.8 to 0.9 ? 106, and fibre 5 to 9 ? 106. Since the contact voltage-drop with direct current is not affected by direct-contact capacitance effect, a fourth method was developed in which the current and watt-loss were measured for a given a.c. stress of known frequency. From these measurements the equivalent series resistance and capacitance of the specimen can be calculated. The specimen was then machined down to a different thickness and similar calculations were made for a range of thicknesses. These factors plotted against thickness and extrapolated to zero thickness revealed the residual quantities attributable to the interface. In Table 6 it is shown that, over a wide range of power factors, the contact resistance does not vary greatly, it being of the same order as already determined. Ultimately the potential distribution method, previously adopted for direct current was modified for a.c. determinations and a large number of tests were carried out on slate, fibre and bakelite. Table 9 gives a complete summary of the results of 75 tests, including a number of check tests to ascertain the effect of frequency, voltage stress, humidity, temperature, surface leakage, roughness of surface and dirty mercury. Tests 53 to 69 were carried out with electrodes other than mercury. The results are important in that they reveal methods of obtaining better contact than can be obtained by the use of a simple mercury electrode. For comparative purposes the authors give in Table 11 the maximum, minimum and mean values of contact resistance and capacitance for normal conditions at 50 cycles and show that the agreement obtained between the two a.c. methods mentioned above is as good as the agreement obtained in different tests by any one method. The significance of the contact effect in relation to measurements on dielectrics is discussed, certain typical conditions being postulated and the percentage error in power factor and permittivity shown to be negligible except in the case of thin specimens of low-grade material. In the extreme case of a hypothetical material 1 mm thick, having a power factor of 80 per cent, the error attributable to contact impedance is shown to be 18 per cent, while in a more normal case where the power factor is 5 per cent and the thickness 1 cm, the error is appreciably less than 1 per cent.

Phase advancing, with particular reference to the scherbius shunt-wound advancer

Abstract: An aggregate consisting of an induction motor and a Scherbius series advancer will operate with a good, and nearly constant, power factor from about half full load to its maximum overload As the load decreases, however, the advancing emf also decreases, so that on light loads the power-factor improvement is slightIn the Scherbius shunt-wound advancer the exciting winding is shunted across the rotor of the induction motor The advancer field, and therefore the advancing emf, is proportional to the current in this winding Within the working range of slip frequencies this current, and therefore the advancing emf, is nearly constantFurther, the relationship between the emf's is such that if the advancer and slip emf's are at right angles for any particular value of slip, they will remain so for all slips Consequently the motor operates with a constant power factor and with its normal slip at all loads

The construction of the “grid” transmission system in Great Britain

Abstract: This paper gives an outline of the chief constructional features of the system of high-voltage transmission lines which are being constructed under the provisions of the 1926 Electricity Act. It shows generally how the British system compares and contrasts with existing transmission systems in other countries. The factors influencing the choice of system, types of conductors, towers, substations, transformers and other constructional parts are explained. The chief difficulties which have been encountered, and the steps which have been taken to reduce these to a minimum, are indicated. Individual pieces of apparatus are described from the standpoint of the user and operator. Technical details of the standard units which make up the transmission lines, switchgear, and transformers are tabulated to enable readers to determine overall efficiencies, voltage-changes and other quantities in which they may be specially interested. The diagrams and drawings provide further details as to location of lines and supply points and constructional features of the scheme in general. Concrete examples of work which is in process of construction are given in Part 2 of the paper. Definite engineering costs are included and are reduced to a kVA basis to provide a guide to the present economic position of high-voltage transmission systems in Great Britain

A measurement of the sound pressures on an obstacle

Abstract: An investigation of the performance of a small condenser transmitter under different conditions of test is described in Part 1. The results are reduced to terms of the augmentation of the sound pressures at the surface of the transmitter, considered as an obstacle in a sound field; and a comparison is made with the calculated pressures developed on a spherical obstacle. In Part 2, which is of the nature of an Appendix, the accuracies of the methods of measurement are discussed

Phase advancers of the Scherbius type

Abstract: The Scherbius phase advancer has recently been adopted as a permanent addition to certain types of 3-phase motor. The author gives a simple theory of the advancer, and applies the equations to an analysis of an “All-Watt” motor. It is shown that quantities usually regarded as constants are not constant in actual practice. In this connection the effect of brush resistance and saturation of the advancer iron are dealt with. When these variations are taken into consideration the agreement between prediction and test is very close, but the locus diagram of primary current is no longer the arc of a circle, although it can be arrived at from a series of such arcs.

The modern use of pulverized fuel in power stations

Abstract: As a result of the experimental work carried out in the Birmingham electric power station, it has been demonstrated that the use of pulverized fuel gives a higher combustion heat efficiency in the boilers than is obtained with mechanical stokers. Boilers having a capacity far greater than those equipped in the past with mechanical stokers can be used, with a consequent reduction in the capital cost of boilers and boiler house; this is a set-off against the higher capital cost of the pulverized-fuel equipment compared with mechanical stokers. The evolution of pulverized-fuel installations is tending towards reduced equipment and greater simplicity in operation; this is effected by eliminating the coal driers and the conveying and storage of the pulverized fuel.

The anticipation of demand, and the economic selection, provision and lay-out of plant (telephone systems)

Abstract: The object of this paper is to give a description of the methods employed by the British Post Office in making telephone development studies, framing lay-out schemes and determining the economic period for which to provide plant. As the paper is to be read in conjunction with another paper * dealing with the methods employed in power systems, detailed descriptions which would only be of interest to telephone engineers have been avoided. It has thus been possible to cover a great deal of ground; but certain matters, each of which would require much space if it were dealt with adequately, have had to be dealt with in a few paragraphs. Nevertheless, it is thought that, with the information contained herein, a fair idea can be obtained of the methods adopted in the Post Office in making the studies described in the title of this paper

Recent developments in electricity meters, with particular reference to those for special purposes

Abstract: The paper gives a general survey of metering work in this country.Some particulars are given regarding the more usual faults found in service, and the requirements for maintained accuracy are briefly discussed.Various standard types of meters are examined and recent modifications detailed.Methods used for obtaining summated demands and consumption are discussed, together with various types of equipment for special purposes.The development of reactive kVA metering methods is traced and apparatus described. The testing of such apparatus is difficult and calls for a high degree of skill.Finally, a few remarks are made regarding special tariffs and testing procedure.

Voice-frequency telegraphs

Abstract: After defining the title of the paper, the author proceeds to discuss the position of telegraphs in this country, comparing it with those of Germany and the United States. Multiplex a.c. transmission is considered and the limits to its use are determined. A description of the various systems in use follows, and proposals are advanced for its wider adoption in this country. The main arguments against its adoption are economic, but some technical criticism is included.

The prevention of ionization in impregnated paper dielectrics

Abstract: This paper is an account of work which was undertaken in the first place to study the performance under various conditions of paper-dielectric condensers such as are commonly used for power-factor correction. It shows that breakdown of dielectrics in service is due to ionization of entrapped air; and that, removal of the air being impossible, the design of cables and condensers should be such as to prevent ionization. The principle of this design is to subdivide the dielectric. The effect of surges in causing breakdown is indicated, and a new test for finished products is suggested.

The reversibility of radio direction-finding and local error at rotating-loop beacons

Abstract: In this paper consideration is given to the applicability of the reciprocal theorem to the practice of radio direction-finding with either a rotating loop transmitter or a loop receiver. A description is given of some experiments carried out with the object of demonstrating that the permanent errors to which a rotating-loop beacon transmitter may be subject as a result of conditions local to its site are similar to those observed on a direction-finder erected on the same site. This similarity has materially assisted in locating the cause of the permanent error associated with the rotating beacon erected at Fort Monckton, near Gosport. From other experiments carried out under suitable conditions, it is shown that the occurrence of night errors is experienced on both systems of direction-finding to approximately the same extent.

Precision permeability measurements on straight bars and strips in the region of high permeability

Abstract: The construction of a yoke apparatus to give magnetizing forces up to H =3000 on straight samples 20 to 25 cm long, and up to H =5000 on samples 8 to 10 cm long, is described. The method of measuring the magnetizing force by means of search coils is used, and an examination is made of its applicability to strip and bar samples mounted in the yoke apparatus. It is found that, below H = 15, large errors result from the leakage taking place along the length of specimen under test. The conditions for satisfactory compensation for this leakage are detemined, and the method is them shown to give good agreement with tests on rings of similar magnetic properties.In Part 2 the effect of stress in the specimen is considered. Stress may be caused by clamping in the case of bars, or by windings on rings or strip samples is found to be the reluctance between the strips. Conditions of testing to obviate both these sources of error are indicated