John B. Whitehead

Bio: John B. Whitehead is an academic researcher from Johns Hopkins University. The author has contributed to research in topics: Dielectric & Dielectric loss. The author has an hindex of 10, co-authored 36 publications receiving 249 citations.

The Conductivity of Insulating Oils

Abstract: The paper describes experiments on the charging current and other associated phenomena in high grade transformer oil. The charging currents remaining after the elimination of the initial transient were studied to within a few hundredths second of application of continuous voltage. Two samples of the same oil obtained at different times differed radically. In one case the charging current fell off from the start, while in the other it remained constant for an appreciable time or even temporarily increased. On reversal of polarity after a long charge the initial current with both specimens was the same as before reversal, but with one a large momentary increase occurred a few seconds later. Evidence is given on the time of recovery of the initial condition. The existence of space charges in the charged oil is shown, and the time of formation of these charges together with the resulting non-uniform distribution of voltage is measured. The importance of the charging current in its early stages is emphasized, since it is the initial conductivity which determines the a-c. loss. In an appendix a theoretical discussion is given on the influence of the space charges. It is shown that at least qualitatively these account for the observed phenomena.

The Dielectric Losses in Impregnated Paper

Abstract: Accurate measurements have been made of the electrical and other physical properties of 10 insulating oils, of a single grade of wood pulp paper, and of the paper when impregnated with each of the oils. Short time charge and discharge curves under continuous potential have thrown further light on the anomalous conduction as found in oils, and permit the separation of the total dielectric loss into two components. Of the total increase of the loss found in impregnated paper, over the separate losses in oil and in dry paper, one component is proportional to the effective conductivity of the oil. The other component, due to reversible absorption, is, for a given paper, a definite function of the free ion content of the oil as indicated by the product of the conductivity by the viscosity. These relations hold over the entire ranges of type of oil and temperature under study. The product of conductivity by viscosity is proposed as a measure of the ``electrical purity'' of an oil used for impregnation. The simplicity of the relations shown suggests that an extension of these studies to other grades of paper should make it possible to predict accurately, from the separate properties of paper and oil, the electrical behavior of any grade as impregnated with any type of oil.

The Electric Strength of Air.-II

Abstract: In a former paper 1 with the title of the present one the author described a series of investigations of the conditions under which the air breaks down in the neighborhood of clean, round wires subjected to high voltage. A principal feature of that paper was the description of a method for observing with a close degree of accuracy the critical or corona voltage for various sizes of wire when centred in cylinders forming the opposite side of the source of voltage. There has been a great diversity in the values of critical voltage as given by other observers, who for the most part have used the appearance of the visible corona and the readings of instruments in the primary circuits of transformers as indications of the voltage at which the air breaks down. The method referred to was developed as the result of a conviction that the laws governing the loss between high-tension lines could not be satisfactorily determined without a study and knowledge of the fundamental phenomena. So far therefore these investigations have been concerned only with the conditions under which the air actually breaks down causing a large increase in conductivity and power loss. The results of the former paper show among other things that when corrected for wave form, temperature and pressure the electric intensity at the surface of a clean, round conductor, corresponding to the voltage at which corona starts and loss sets in is a constant for each size of wire. This value of surface intensity varies with the temperature and pressure and is that corresponding to the maximum value of the voltage wave. It is different for different sizes of wire but is independent of the material of the wire, of the moisture content, and of the amount of free ionization in the air. In the present paper some further facts bearing on the fundamental relation between diameter and critical surface intensity are given, and a series of investigations of the influence of stranding a conductor, of variations of atmospheric pressure, and of frequency on the critical electric intensity are also described.

The Influence of Residual Air and Moisture In Impregnated Paper Insulation

Abstract: The paper describes experiments in study of the separate influence of residual air and moisture in impregnated paper as used for the insulation of high voltage cables. Some sixty similar samples are prepared, dried, evacuated, and impregnated under the same program, except as regards the pressure of evacuation and impregnation. In groups of three, the samples were evacuated at various absolute pressures between 2 mm. and 76 cm. Hg. The samples were brass tubes 2.54 cm. in diameter, 122 cm. long with 25 layers of wood pulp paper applied in the usual lapping spirals. Each sample was equipped with outside test and guard electrodes. Throughout their entire history, i. e., before and after impregnation, observations were made on the samples of their dielectric absorption and their final conductivity. These studies have led to the conclusions as to the influence of moisture. After impregnation, the samples are studied as to power-factor and dielectric loss over the range of electric stress 20 to 300 volts per mil, and of temperature 20 deg. cent. to 80 deg. cent. These studies have led, principally, to the conclusions of the influence of residual air. They show clearly the importance of thorough impregnation, and the conditions under which it may be accomplished. The causes of rising power-factor?voltage curves and methods of avoidance are clearly indicated. The principal results and conclusions are given at the end of the paper.

The Distribution of Relaxation Times in Typical Dielectrics

Abstract: K W Wagner's treatment of the distribution of relaxation times in dielectrics is reviewed; the effect of the density of distribution upon the frequency variation of the dielectric constant and dielectric loss factor is discussed; and a graphical method of evaluating the constants of Wagner's equation from experimental data is described These constants have been evaluated for a number of typical dielectrics, and the frequency variations of the dielectric constant and dielectric loss factor as computed from Wagner's equations and from the simple equations for a single relaxation time are compared with the observed behavior The quantitative correlation obtained between Wagner's equations and the experimental data investigated is viewed as a confirmation of Wagner's theory of a statistical distribution of relaxation times in dielectrics

Physics and chemistry of partial discharge and corona. Recent advances and future challenges

Abstract: Results of recent research on physical and chemical processes in partial discharge (PD) phenomena are reviewed. The terminology used to specify different types or modes of PD are discussed in light of a general theory of electrical discharges. The limitations and assumptions inherent to present theoretical models are examined. The influence of memory propagation effects in controlling the stochastic behavior of PD is shown. Examples of experimental results are presented that demonstrate the nonstationary characteristics of PD which can be related to permanent or quasi-permanent discharge-induced modifications (aging) of the site where the PD occur. Recommendations for future research are proposed. >

The Law of Corona and the Dielectric Strength of Air-II

Abstract: For several years an extensive investigation on the dielectric strength of air, and more particularly, on the law of corona, has been carried on by the Consulting Department of the General Electric Company, under the general supervision of Dr. C. P. Steinmetz. The facilities were practically unlimited in regard to power, apparatus, instruments and their standardization, and available engineering skill. Thanks for their active assistance in this investigation are due to Dr. E. J. Berg, Messrs. C. M. Davis, J. L. R. Hayden, A. B. Hendricks, W. K. Page, L. T. Robinson, L. A. Schloss, W. I. Schlicter, C. W. Stone and J. B. Taylor.

Limitations of the Newtonian Time Scale in Relation to Non-Equilibrium Rheological States and a Theory of Quasi-Properties

Abstract: The behaviour of complex materials under stress is described in terms of entities which are not strictly 'physical properties'. These so-called 'quasi-properties' range from entities hardly distinguishable from dimensionally true physical properties to concepts which are much less clearly defined. Quasi-properties measure an ordered process towards equilibrium rather than a state of equilibrium. The Newtonian definition for equality of time intervals which leads to the concepts velocity, acceleration, momentum and force having whole-number dimensional exponents, does not apply to 'quasi-equilibrium states'. In order to keep the Newtonian time scale, fractional differential equations are introduced. The simplest fractional differential equation relating stress, strain and time integrates to a series equation whose first term is a simple power law (Nutting's equation) already known to describe the behaviour of many complex materials under constant stress. The physical meaning of the fractional differential is considered. An apparatus is described for loading test-pieces of plastics and the like under tension or compression at constant stress to a preselected strain, and for following the subsequent stress dissipation; and the results of tests on thirty-eight materials are studied statistically. Introducing a second term from the series equation (and, very rarely, a third term) greatly improves the fit for materials for which Nutting's equation is inadequate and explains hitherto unaccountable anomalies when the Nutting plot is otherwise satisfactory. Constants derived by the equation from constant-stress and constant-strain conditions are compared. The form of the series equation suggests that the relative importance of the second term may sometimes disclose the presence of undissipated stresses in the materials. The accuracy of tests on individual test-pieces is high, but, on account of frequent lack of homogeneity in the samples available, the use of the unmodified Nutting equation is often adequate even when the addition of a second term would significantly improve individual curves. Some alternative treatments are discussed, but, both theoretically and practically, the fractional differential approach is preferred for most of the materials tested.