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1887

Proceedings of the Institution of Electrical Engineers

Volume 118, Issue 1, January 1971

Volumes & issues:

Volume 118, Issue 1

January 1971

Inaugural address. The impact of advanced technology on the electrical industry
  • Author(s): Lord Nelson of Stafford
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 1 –8
  • DOI:  10.1049/piee.1971.0001
  • Type: Article
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  • p. 1 –8 (8)
    In the past 100 years, developments in electrical engineering and technology have brought about profound changes in human living standards. The increasing momentum of change presents serious challenges to man's ability to adapt himself to them. In the last 15 years the rate of technological advances in communications, power generation and utilisation, and in the application of electronic techniques, has been accelerating, and this is examined.The growth and development of the electrical manufacturing industries are discussed, and the effects on the industry of the rapid technical advances of recent years are considered. The changes which have already taken place are outlined, and the challenges of the future are analysed. Finally, the role of the engineer in management is discussed and the IEE is urged to help its members to acquire expertise in the managerial field.
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IEE Electronics Division: Chairman's address. Universities and technology
  • Author(s): John Brown
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 9 –18
  • DOI:  10.1049/piee.1971.0002
  • Type: Article
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  • p. 9 –18 (10)
    The functions of universities in respect of undergraduate courses, postgraduate courses and research are reviewed, and it is concluded that there should be greater variety of treatment in each case. Existing differences in the content and objectives of undergraduate and postgraduate courses should be stated more clearly to enable students to make sensible choices. Since the majority of future engineers are likely to be graduates, there is a danger that the advantages of the present interaction between graduate and industrially trained entrants to the profession will be lost. This could be avoided by reviving the Diploma-in-technology type of course with increased emphasis on design and production. Further expansion of student numbers should be used to create institutes of technology, concentrating on such courses and of strengthening the links with industry, for example by joint industry-institute staff appointments. Such staff should continue as effective members of industrial-design and production teams. The Lighthill Committee has advocated a concentration of research within large departments. Such a move would radically alter the nature of small departments. It is suggested that a swing to institutes of technology with a design and production emphasis would lead to a better overall solution.
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Qualitative analysis of 1st- and 2nd-order nonlinear networks
  • Author(s): L.O. Chua
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 19 –28
  • DOI:  10.1049/piee.1971.0003
  • Type: Article
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  • p. 19 –28 (10)
    The paper presents several circuit-theory approaches for analysing the qualitative behaviour of nonlinear networks containing one or two energy-storage elements. The main concept consists in the transformation of a dynamic-network problem into an equivalent resistive-network problem. In particular, it is shown that any 1st-order nonlinear network can be analysed by obtaining the solution waveforms of an associated resistive constant-slope network. For 2nd-order autonomous nonlinear networks, it is shown that their isoclines are identical to the transfer-characteristic plots of an associated resistive isocline network. Many qualitative properties are shown to be derivable from this network. The classical approach for finding singular points, separatrices, and limit cycles is shown to have simple circuit-theory interpretations. The paper concludes with the presentation of three symmetry theorems useful for analysing 2nd-order nonlinear networks.
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Review of high-speed addition techniques
  • Author(s): J.B. Gosling
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 29 –35
  • DOI:  10.1049/piee.1971.0004
  • Type: Article
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  • p. 29 –35 (7)
    A new large computing machine, the MU5, is under construction at Manchester University. As one aspect of the design of this machine, a review of available addition techniques was undertaken. This included both the use of special circuits for improving carry-propagation speeds, and the effects of restricted fan out and fan in on logical adders. The fastest adder uses a combination of block-carry and conditional-sum approaches and may be further improved by use of the sequential-state technique; emitter-coupled logic being used. Where economy is more important than speed, transistor-transistor medium-scale-integrated logic had advantages. Other techniques discussed include the saturated-transistor carry-path and serial-parallel adders.
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Radiation from a spherical antenna covered by a layer of lossy hot plasma. Theory and experiment
Small holes in a waveguide wall
Computer field-matching solution of waveguide transverse discontinuities
  • Author(s): P.H. Masterman  and  P.J.B. Clarricoats
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 51 –63
  • DOI:  10.1049/piee.1971.0007
  • Type: Article
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  • p. 51 –63 (13)
    A computational method for solving a wide range of transverse waveguide discontinuity problems is described. Results are obtained by the simultaneous solution of matrix equations, generated by Fourier analysis, which relate the complex amplitudes of orthogonal electric and magnetic field components. In some cases, the solution is found to be sensitive to the way in which infinite series of field functions are truncated, and it is shown how the optimum form of truncation can be determined for many configurations of practical importance. Several examples showing the application of the method are given, and comparison of results with those obtained by experiment, and by other analytical techniques, confirms its accuracy.
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Measurements of transmission loss at high frequencies over 960 km temperate-latitude path
  • Author(s): P.A. Bradley
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 64 –78
  • DOI:  10.1049/piee.1971.0008
  • Type: Article
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  • p. 64 –78 (15)
    Signal strengths measured at a number of frequencies over a short-range (960 km) path between Lerwick, Shetland Islands, and Slough, England, are compared with those given by two well known prediction methods. Some of the assumptions inherent in the predictions, and their limitations when applied to the path under review, are examined. Suggestions are made for amendments to the methods. The measured signal-strength data are used to test the validity of wave-polarisation calculations and to give estimates of the ionospheric absorption for comparison with the absorptions indicated by the prediction methods.
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Erratum: Radiation patterns of circular apertures with structural shadows
New development in lightning-counter design
  • Author(s): K.S. Kreielsheimer  and  D. Lodge-Osborn
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 79 –87
  • DOI:  10.1049/piee.1971.0010
  • Type: Article
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  • p. 79 –87 (9)
    Counters developed in Auckland use a combination of the electrostatic and radiative components of the electromagnetic field created by a lightning discharge. By combining the output of a CIGRÉ-type extra-low-frequency circuit with the lightning noise radiated in the 10 MHz region, counters have been designed which will respond to ground flashes only, or discriminate between ground and cloud flashes, and at the same time resolve a multiple ground flash into its individual strokes. In all these applications, the gap in the high-frequency noise occurring after a ground stroke has been utilised in the discrimination between the different types of lightning discharge.
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Propagation in corrugated waveguides
IEE Power Division: Chairman's address. Let there be light
  • Author(s): A.G. Milne
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 89 –98
  • DOI:  10.1049/piee.1971.0012
  • Type: Article
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  • p. 89 –98 (10)
    Appropriately to the centenary year of the IEE, the paper reviews the past 100 years of the distribution and utilisation activities of the electrical-power industry. With their origins back in Faraday's momentous discovery of electromagnetic induction in 1831, the enoumous electrical-power industrieds of today have been built on the brilliant inventions of the past. The latter half of the 19th century saw the exploitation of the dynamo, the arc and incandescent lamp and the electric motor. At the end of that period, the new supply utilities had a reliable and economic means of production, namely the dynamo, and a diversified load with motors added to lighting. The ‘battle of the systems’ at this time resulted in the predominance of a.c., given an imperishable lead by the young Ferranti's concept of larger-scale central generation at high voltage. Combined with Parsons's inverntion of the steam turbine, generation by turboalternators in central stations holds gool to this day. Many of the fundamental principles and basic materials for transmitting, controllong and measuring date from this time, although steady advances in design have brought them to an efficiency undreamed of in those days. The continuity of development is traced and projected into the future, in the scope for research, for management, for the profession, and in the exercise of social responsibility.
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Artificial- and natural-pollution tests on outdoor 400 kV substation insulators
Optimum hydroelectric-power-generation scheduling by analogue computer
IEE East Midland Centre: Chairman's address. An area board today
IEE North-Western Supply Section: Chairman's address. Core movement in 33 kV assisted-type cables
Transient-recovery voltage characteristics of a 3-phase power system
  • Author(s): V.N. Sujeer  and  R. Vaidyanathan
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 119 –124
  • DOI:  10.1049/piee.1971.0017
  • Type: Article
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  • p. 119 –124 (6)
    The application of R, S, T components to solve faults in power systems and transient unbalances in power networks has been attempted, to simplify the problems to a greater extent than other components, such as Clarke's or Concordia's components. However, these attempts were not completely successful, because the equivalent circuits developed to represent various types of faults did not completely eliminate the use of isolating transformers, which was an objection to a study of the above problems in a network analyser. The introduction of isolating transformers placed certain limitations in representating power systems under changing fault conditions in a transient analysis. It is the purpose of the paper to introduce the modified equivalent circuits in such a way that they completely eliminate the use of isolating transformers, without any need to modify the original equivalent circuits. A systematic analytical approach is also developed for the determination of the transient-recovery voltage characteristics, by illustrating the various steps with a typical fault.
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Solution of large power-system networks by ordered elimination: a comparison of ordering schemes
  • Author(s): B. Stott  and  E. Hobson
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 125 –134
  • DOI:  10.1049/piee.1971.0018
  • Type: Article
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  • p. 125 –134 (10)
    A variety of different ordering schemes are used with sparsity-programmed Gaussian elimination to solve a range of practical power-system networks, with a view to assessing their relative merits for general-purpose applications. It is shown that, for networks of the sizes and types studied, the choice between the more successful schemes is dependent mainly on the relative amounts of work required to establish the orderings. One simple and popular scheme, involving pivot selection at each stage of the elimination process on a row-sparsity basis, emerges as the most attractive.
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Triac phase control with a line-frequency control signal
Effect of load power factor on current-transformer performance
  • Author(s): S.V.N. Sarma
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 139 –142
  • DOI:  10.1049/piee.1971.0020
  • Type: Article
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  • p. 139 –142 (4)
    The current transformer, for metering as well as protection, may not, in practice, have a purely resistive load. In certain special applications, the load is inductive, e.g. oscillograph coils etc., and, when the control cables are very short, it is possible that the load may have a power factor of less than unity. The steady-state and transient fluxes under these conditions are analysed. The effect of the load power factor of the current transformer on the errors is presented. The equations developed in the paper are directly helpful in forecasting the performance of a current transformer when connected to a nonresistive load.
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Corrigendum: Starting methods for generator/motor units employed in pumped-storage stations
Transient performance of doubly excited synchronous machine
  • Author(s): M. Rama Murthi  and  B.W. Hogg
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 143 –147
  • DOI:  10.1049/piee.1971.0022
  • Type: Article
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  • p. 143 –147 (5)
    The paper describes an investigation of the transient behaviour of a doubly excited synchronous machine, both with and without excitation controls. The analysis shows how the positional change of the resultant rotor m.m.f., due to the induced currents, gives an inherent advantage to the doubly excited machine over a conventional synchronous machine, for the improvement of transient stability. When excitation controls are employed, there is a further improvement in the transient-stability limit, with a greater contribution coming from the voltage regulator.
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Effects of subharmonics and winding asymmetry on the performance of slip-ring machines
  • Author(s): B.M. Bird ; A.R.W. Broadway ; A.C.A. Colvile
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 148 –152
  • DOI:  10.1049/piee.1971.0023
  • Type: Article
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  • p. 148 –152 (5)
    The acceptable level of subharmonics in the m.m.f. waveform of primary windings for slip-ring induction motors has been investigated, and certain maximum tolerable levels are here determined which, if exceeded, seriously affect the performance of slip-ring machines. It has already been established that surprisingly large subharmonics in the m.m.f. waveform of a squirrel-cage induction motor can be accepted without perceptible effects on the machine performance. In large single-speed machines, with relatively large numbers of slots per pole, there is little diffculty in minimising subharmonic m.m.f.s. The design of change-speed pole-amplitude-modulated (p.a.m.) windings is more difficult in this respect; but, with the aid of recently developed techniques, remarkably harmonic-fee windings can be obtained, given a sufficient number of slots per pole. It is in smaller units that the design problem is most severe, and that large subharmonics might inhibit the development of change-speed slip-ring motors. There are, however, certain unorthodox windings, two of which are described, which may be used successfully as secondary windings, in conjunction with primary windings which might otherwise be unacceptable.
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Digital simulation of a synchronous generator in direct-phase quantities
  • Author(s): P. Subramaniam  and  O.P. Malik
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 153 –160
  • DOI:  10.1049/piee.1971.0024
  • Type: Article
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  • p. 153 –160 (8)
    The paper describes a mathematical model for the simulation of a 3-phase synchronous machine using direct-phase quantities, thus obviating the need for any transformation. Numerical solution using a digital computer has also been described, and compared with digital simulation in transformed d-q-0- and α-β-0-axes models of a synchronous machine. The proposed model in direct-phase quantities enables a unified approach to be adopted in the study of both symmetrical and asymmetrical conditions. Since the constraints to be imposed are direct operating conditions, asymmetrical operating conditions can be studied very easily. Modifications required in the model to simulate various types of faults are described. Versatility of the proposed model is illustrated by the study of a single-line-earth fault with single-phase opening and automatic reclosure. It is shown that this type of fault can be studied as simply as, say, a 3-phase fault.
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Line-line short circuit of synchronous machine: illustration of computer-aided machine analysis
  • Author(s): I. Gopal Reddy  and  C.V. Jones
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 161 –168
  • DOI:  10.1049/piee.1971.0025
  • Type: Article
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  • p. 161 –168 (8)
    The sudden unbalanced short circuit of the 3-phase synchronous machine constitutes one of the most difficult problems of conventional machine analysis, whether traditional or unified. It is, however, readily susceptible to numerical integration using digital computers. The general approach, which is believed to have a wide application, is illustrated by the simple case of the 2-winding single-phase machine, and is then extended to the practical problem of the salient-pole machine with damper windings. Predicted and test results are found to be in good agreement.
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Permeance of closed-slot bridges and its effect on induction-motor-current computation
  • Author(s): T.S. Birch  and  O.I. Butler
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 169 –172
  • DOI:  10.1049/piee.1971.0026
  • Type: Article
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  • p. 169 –172 (4)
    A procedure is described for the calculation of the specific permeance of the bridge of a closed slot as a function of the slot m.m.f. It is an improvement on other published procedures as it includes the changing saturation in each section of the bridge, using the actual magnetisation curve of the material to calculate the m.m.f. drop. The influence of the slot-top and airgap leakage fluxes on the saturation of the bridge is also included. For cast-aluminium rotors, where the rotor slot is completely filled by the conductor, account is taken of the changes in m.m.f. producing the slot-top leakage fluxes. The theory is applied particularly to slots having semicircular tops, but it is sufficiently general to be capable of application to any shape of slot. Further, the method employed can readily be adapted to similar problems involving parallel magnetic circuits having changing cross-sectional areas and varying degrees of saturation. Computed and experimental results of the specific permeance of slot bridges, and the variation of the input currents with the rotor skew of a cage induction motor, confirm the validity of the theory.
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IEE East Anglia Centre: Chairman's address. Thyristors and motor control
Effects of rotor eccentricity on acoustic noise from induction machines
  • Author(s): A.J. Ellison  and  S.J. Yang
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 174 –184
  • DOI:  10.1049/piee.1971.0028
  • Type: Article
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  • p. 174 –184 (11)
    The variations of acoustic-noise power radiated from electric machines, having varying degrees of eccentricity of the rotor in the stator bore, were studied using a specially constructed rig in an anechoic chamber. A series of noise components, which were found in experiments to vary considerably with rotor eccentricity, were identified analytically from the expressions for the mode numbers and frequencies of the magnetic-force waves, taking into account the stator and rotor slotting, eccentricity and iron saturation. A method is presented for calculating the variation of important noise components with eccentricity. The effects on the noise power of different numbers of stator parallel paths and of different ways of connecting them were investigated using specially wound machines. Information is given concerning the choice of parallel paths and connections for noise reduction.
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Natural frequencies of stators of small electric machines
  • Author(s): A.J. Ellison  and  S.J. Yang
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 185 –190
  • DOI:  10.1049/piee.1971.0029
  • Type: Article
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  • p. 185 –190 (6)
    In designing electric machines for low vibration and noise levels, the accurate calculation of the natural frequencies of the stator is of great importance, since it is essential to avoid coincidence between these frequencies and those of the exciting forces. Existing methods give unsatisfactory results for small and medium-sized 2-pole machines, owing to their high ratio of core thickness to mean radius of the stator core. As an extension of earlier work, the paper presents a method of calculating the natural frequencies of a stator having a thick core loaded with teeth and windings coupled solidly through ribs (key bars) to an outer thin frame, taking into account bending, shear, extension and rotary inertia. Analytical results are compared with experimental results, and good agreement is obtained.
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Field distribution and performance of copper-filled slotted solid iron rotors
  • Author(s): P.K. Rajagopalan ; V. Balarama Murty ; P.S. Sarma
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 191 –200
  • DOI:  10.1049/piee.1971.0030
  • Type: Article
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  • p. 191 –200 (10)
    The paper presents an extensive set of experimental results of the flux measurements on slotted solid iron rotors, in which the slots were filled with copper to establish the actual field distribution along the tooth and slot axes under sinusoidal-excitation conditions. These results, when compared with those for machines without copper, show first, that the tooth-axis flux distributions are identical in both cases, and secondly, that the leakage flux is much larger in machines with copper, and lead to the conclusion that this large leakage flux has little influence on the tooth-axis flux distribution and confines itself to a region very close to the tooth profile. An analysis for obtaining the field distribution along the tooth and slot axes has been developed, and performance equations for the machine have been obtained. The analysis gives the current-density distribution in the copper, and yields simplified expressions and equivalent circuits when the deep-bar effect in the copper conductor is negligible. Results of this analysis are in good agreement with the test results.
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Calculation of system-fault currents due to induction motors
  • Author(s): S.S. Kalsi ; B. Adkins ; D.D. Stephen
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 201 –215
  • DOI:  10.1049/piee.1971.0031
  • Type: Article
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  • p. 201 –215 (15)
    The paper makes recommendations for a standard method of determining the contribution made by induction motors to the fault currents in a power system. The need for such a method arises from the increasing number of large induction motors now being installed, and from the more rapid operating times of present-day switchgear. The basis of the proposals is that, for this purpose, an induction motor can be treated in the same way as a synchronous machine, using mainly the ‘subtransient reactance’ of the induction motor. The recommendations are supported by a detailed theory of induction-motor transients, in which accurate formulas are derived for the current following a sudden short circuit. The ‘deep-bar effect’ usually present in large squirrel-cage induction motors is dealt with in a similar manner to that adopted internationally for the ‘solid-pole effect’ in synchronous machines, where the effect is theoretically simulated by two corresponding time constants appear in the formulas. An operational impedance function, similar to that of a synchronous machine, is used to obtain the transient parameters. The method is a simple one, and is particularly useful for determining switchgear ratings in a mixed system containing both synchronous and induction machines.
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Trends in the design of high-voltage high-rupturing-capacity fuses
  • Author(s): J. Feenan
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 216 –221
  • DOI:  10.1049/piee.1971.0032
  • Type: Article
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  • p. 216 –221 (6)
    High-voltage high-rupturing-capacity fuses are now the most popular form of short-circuit protection for distribution-transformer circuits up to 1000kVA, and are increasing in popularity as the best form of short-circuit protection for high-voltage motor circuits up to, for example 3000hp at 3.3kV. The paper describes the progress made in recent years in the standardisation of dimensions and characteristics of h.v. fuses suitable for the protection of distribution-transformer circuits, and suggests that, with the increasing demand for suitable fuses for motor-circuit protection, particularly from overseas, there is justification for the existence of fuses with time/current characteristics which permit maximum utilisation of the motor-starting capabilities of contactors. The paper illustrates the problems facing the fuse manufacturer in producing suitable motor-circuit fuses, by describing some of the design concepts employed in a typical fuse, and discusses the advantages gained in its application. In addition to describing recent trends in the design of voltage-transformer fuses, the paper discusses the difference between the ‘backup’ fuse and ‘general-purpose’ fuse as defined in the recently published. IEC Recommendation 282-1 ‘High voltage fuses’, and the problems which have had to be overcome in designing and testing general-purpose fuses. Finally, a basis for selecting the appropriate type for each of the popular applications is suggested.
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Composite instantaneous comparators: basis for design and their transient performance
Overhead-line oscillations and the effect of aerodynamic dampers
IEE Control & Automation Division: Chairman's address. Control's limitations
  • Author(s): John C. West
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 225 –231
  • DOI:  10.1049/piee.1971.0035
  • Type: Article
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  • p. 225 –231 (7)
    The history of the development of control theory, as distinct from a historical catalogue of the inventions of control devices, is given, showing the high level of sophistication attained, even before the Second World War. The 1939-45 war saw the application of the theories take shape in manners which were certainly not envisaged by the theories.Since 1945, tremendous attention has been paid to furthering control theory by mathematicians and engineers, but the address emphasises the great impact which control concepts have made on other sciences. Examples are described in many fields. The attempts to describe sociological phenomena are shown in some recent work on colour segregation in the USA. This is followed by a discussion on propaganda and advertising and a parallel is drawn with the spread of disease in epidemics. In the economics field, a trivial example of inflation is modified to take account of capital investment. It is shown how a hierarchial social structure has an effect on salary distributions with particular reference to engineers.After dealing with several examples taken from the medical field, the paper concludes with the dangers of using mathematical models and the use of abstract theory without experiment in each field of application.
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Boolean-functional representation of a thyristor
Calculation of the transient response of lumped linear systems from their frequency response
Comment: Calculation of the transient response of lumped linear systems from their frequency response
Reply: Calculation of the transient response of lumped linear systems from their frequency response
Corrigendum: Calculation of the transient response of lumped linear systems from their frequency response
IEE Science, Education & Management Division: Chairman's address. Science in heavy electrical engineering
  • Author(s): R.W. Sillars
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 237 –243
  • DOI:  10.1049/piee.1971.0041
  • Type: Article
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  • p. 237 –243 (7)
    The contributions made by scientists (chemists, metallurgists, physicists etc.) to the heavy electrical in dustry over the past 50 years are reviewed in four categories: development of diversified products, introduction of new basic principles into the industry, efforts to overcome the limitations of materials in use in electric plant and the incorporation of materials appearing on the market into new designs of electric plant. The first category no longer finds much opportunity in a compartmentalised industry; the second has been vigorously pursued, but the heavy outlay needed in the absence of clear signs of economic gain has produced a lull; the third has had some moderate successes; but the fourth has been the principal contribution, and an essential part of the improvements in equipment over the past 20 years. The diffuse nature of this last contribution, and the difficulty of identifying the benefit it gives to any particular organisation, have obscured its value, but there seems to be every prospect that the industry will need both the personal and professional contributions of scientists more in the future than it has in the past.
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Relationship between permittivity and loss tangent
Conduction in polythene with strong electric fields and the effect of prestressing on the electric strength
  • Author(s): A. Bradwell ; R. Cooper ; B. Varlow
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 247 –254
  • DOI:  10.1049/piee.1971.0043
  • Type: Article
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  • p. 247 –254 (8)
    There has been speculation in the past about the extent to which space charge formed by the flow of prebreakdown current affects the electric strength of polythene. In the present work, conduction and absorption-current measurements up to about half the breakdown field have given clear evidence of space-charge effects. Further evidence was obtained from measurements of the apparent ‘intrinsic’ electric strength under various modes of stressing. It was found that the electric strength of polythene measured with direct voltage below about 30°C exceeded the corresponding value measured with impulse voltages by up to about 25% The impulse electric strength could be increased to equal the direct voltage by prestressing each specimen for 4min with direct voltage of the same polarity as the impulse voltage. However, when the impulse was of opposite polarity to the direct prestressing voltage, the electric strength decreased with increase in the prestressing field. The effect was dramatic, for the impulse electric strength changed from the normal value of about 6MV cm−1 to less than 2MV cm−1 as the prestressing field was increased to about 75% of the electric stength measured with direct voltage. Experiments in which the breakdown impulse was applied some time after removal of the prestressing field showed that the impulse electric strength at 20°C recovered in about 10−2s to the value obtained with impulses without prestressing. Thus the prestressing caused no permanent damage. The observed effects are attributed to space-charge injection from the electrodes. The observations are not consistent with the concept that breakdown occurs when the field at a point within the specimen exceeds a critical value, the intrinsic electric strength of polythene.
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Precise measurement of dielectric properties at radio frequencies
  • Author(s): W. Reddish ; A. Bishop ; K.A. Buckingham ; P.J. Hyde
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 255 –265
  • DOI:  10.1049/piee.1971.0044
  • Type: Article
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  • p. 255 –265 (11)
    A new and improved apparatus is described for the measurement of dielectric loss and permittivity at radio frequencies using a resonance technique. Although designed primarily for use with low-loss polymers, such as polythene (tanδ = 80μrad at 6MHz), the instruments developed are capable of covering the properties of most practical insulating materials. The equipment may be used either with the Hartshorn and Ward air-substitution method or with a liquid-immersion technique. The former is preferred for accurate loss-angle measurements on low-loss materials; repeatability using the same 127 mm-diameter test specimen is about 0.4μrad throughout the 0.1−30 MHz frequency range. By using a 53mm-diameter specimen, together with suitable electrodes, the upper frequency limit may be extended to 100MHz, with only a small sacrifice of resolution. The accuracy of measurements by this methods is advanced by the use of a conductance calibration procedure. Permittivity can be measured by the air method to within about 1% which is adepquate for some purposes; otherwise a liquid-immersion technique with an error of 0.03% is available. A full describtion of calibration procedures required to support these measurements is given, and the accuracy of the equipment is assessed in detail for the specific case of a low-density polythene.
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Influence of air density on electrical strength of air gaps at 50Hz
Flashover voltage of artificially contaminated surfaces
  • Author(s): B. Macchiaroli  and  M. Rea
  • Source: Proceedings of the Institution of Electrical Engineers, Volume 118, Issue 1, p. 271 –274
  • DOI:  10.1049/piee.1971.0046
  • Type: Article
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  • p. 271 –274 (4)
    Tests were carried out to analyse the influence of geometrical and physical parameters of a contaminating layer on the flashover of small flat surfaces. The wet-contaminant method was used, and the alternating and direct flashover voltages were related to different values and distributions of thickness, width and resistivity of the contaminant layer. The results show that the flashover voltage of a polluted strip depends strongly on the surface conductivity and slightly on the layer width and thickness. The nonuniformity of the parameters giving rise to the surface conductivity over the length of the polluting layer does not affect the flashover voltage.
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Erratum: Influence of inclusions on domain-wall motion and power loss in oriented electrical steel
Universal loss chart for the calculation of eddy-current losses in thick steel plates

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