G. D. McCann

The Measurement of Lightning Currents in Direct Strokes

E. Beck (Westinghouse Electric and Manufacturing Company, East Pittsburgh, Pa.): The growth of statistical data, as accumulated in the widespread investigation described by Mr. McCann, provides more and more reliable information on what we may expect of lightning strokes. In fact, it is now possible to make valid estimates of the probability of lighting damage. The interpretation of these stroke data in terms of the performance of protective devices such as lightning arresters will require some further clarification and relation to similar data on the lightning currents discharged by arresters in service. At the University of Pittsburgh, one of the stroke-current measuring stations, types of lightning arresters have been connected in the stroke circuit during the several years covered by the study.

Induced voltages on Transmission Lines

ABOUT the year 1929 it first became appreciated that the lightning discharge is not instantaneous. The recognition of this fact had a profound influence upon the theories of lightning protection, the emphasis being diverted from induced to direct strokes1 as the agent to be guarded against. With the meager information then available the calculations indicated that induced voltages were unimportant when the finite time of discharge was taken into account.2 The more accurate information available today calls for a critical analysis of this subject, which is the purpose of the present paper.

Shielding of Transmission Lines

MODERN theories of direct-stroke protection premise that the ground wires are so located as to intercept the stroke and provide perfect shielding. In spite of the fundamental importance of this question there still exists considerable doubt as to the correct position of the ground wires relative to the transmission conductors. Two avenues of approach are suggested for the attack of this question; first, the collection of statistical information regarding actual line performance, and second, the use of laboratory models. Line performance is, after all, the final criterion. However, it is difficult to isolate the shielding effect from other factors which may produce outage due to lightning. Studies with models eliminate this difficulty but always contain the element of doubt as to whether the laboratory conditions are sufficiently representative of actuality as to justify general conclusions. Perhaps the best course, that which is attempted in this paper, is to try both and co-ordinate the results so obtained.

Shielding of Substations

AS compared to transmission lines, it is more important that overhead ground wires or vertical masts over substations be correctly located so as to provide shielding of the structure against direct strokes of lightning. In a previously published paper1 the authors discussed the shielding characteristics required for transmission lines. The present paper extends these investigations to the shielding of substations.

The Effect of Corona on Coupling Factors Between Ground Wires and Phase Conductors

Measurements of coupling factors between ground wires and phase conductors, as a function of ground-wire voltage, have been made for a sufficient number of cases to determine them for any of the standard transmission-line configurations. It was found that the effect of corona can be expressed in terms of an effective corona ground-wire radius to be used in the conventional coupling-factor equations. Curves are also presented for the quick determination of coupling factors for all standard line configurations. A photographic study of the visual characteristics of surge corona was made for a comparison with the effective radius and with 60-cycle corona. Actual measurements were made for potentials up to about 2,000 kv, but the data were extrapolated to 6,000 kv with what is considered to be reasonably good accuracy. Coupling factors for positive polarity were found to be considerably higher than those for negative polarity and both higher than values previously calculated from purely theoretical considerations. For a potential of 1,000 kv the actual coupling is 1.2 to 1.5 and 1.4 to 1.8 times as great, for negative and positive polarity respectively, as the values obtained by assuming no corona. At 2,000 kv the corresponding figures are 1.4 to 2 and 1.6 to 2.7 respectively, and at 4,000 kv 1.7 to 2.9 and 2 to 4. Ground-wire size was found to be unimportant above about 200 kv and, therefore, need not be considered.

Lightning Protection for Rotating Machines

FIGURE 1 lists the valve-type machine-arrester and capacitor ratings recommended for each voltage class of machine together with their method of application. It is important that the arresters at the machine be valve type. They always should be applied with capacitors that are located either reasonably close to the arresters or between the arresters and machine. This combination prevents the occurrence of steep-front surges resulting from a sudden voltage drop when the arrester discharges.

Electrical Analogy Methods Applied to Servomechanism Problems

This paper describes the application of the transient analyzer to the solution of servomechanism problems by the electrical-mechanical analogy method. The analogies for angular position servomechanisms are developed in detail together with the fundamental concepts by which the systems can be defined in terms of dimensionless parameters. This permits the results to be given in dimensionless form suitable for general application. Typical solutions of representative systems are given in the form of transient response curves. These show the response characteristics for suddenly applied constant velocity and sinusoidal motion. Also shown are the effects of varying the controlling parameters of the system including the stiffness constant, one, two, and three time delays, and methods of producing system stability including negative RC feedback and anticipatory control. A method is developed for setting up the analogy for a specific system on the transient analyzer by a suitable change in time and impedance bases.

Determination of Transient Shaft Torques in Turbine Generators by means of the Electrical-Mechanical Analogy

Discussion and authors closure of paper 45–14 by H. S. Kirschbaum and discussion of paper 45–15 by G. D. McCann, G E. Warren, and H. E. Giner, presented at the AIEE winter technical meeting. New York, N. Y., January 22–26, 1945, and published in AIEE TRANSACTIONS, 1945, February section, pages 65–70 and pages 51–56.

Field Investigation of the Characteristics of Lightning Currents Discharged by Arresters

Data have been obtained during the past three years on the magnitude and wave shape of lightning currents discharged by arresters in service on several solidly grounded neutral circuits of the American Gas and Electric Company system. Correlated measurements have been obtained with the cathode-ray oscillograph, the fulchronograph, and the surge-front recorder. The maximum arrester-phase leg current recorded in this investigation was 9,600 amperes with 70 per cent of the currents less than 1,000 amperes. The wave fronts of the low-magnitude currents were, in general, abrupt. For crest magnitudes of over 1,000 amperes they ranged from two to over 25 microseconds to crest. The maximum rate of rise recorded was 2,500 amperes per microsecond.

Dimensionless Analysis of Servomechanisms by ElectricaI Analogy

Discussion and authors closure of paper 46-150 by S. W. Herwald and G. D. McCann, presented at the AIEE summer convention, Detrolts Michn June 24-28, 1946, and published in AIEE TRANSACTIONS, 1946, October section, pages 636-9.