A. R. Hileman

The Lightning Stroke

IN RECENT YEARS renewed interest has developed in the effect of lightning on electrical transmission lines. The most vital characteristic of lightning in most methods of calculating its effect is the current measured at the ground terminal. A large amount of data is available about the stroke current crest magnitude and a considerable amount of data is available which covers the time-to-crest of the stroke current. Despite these data there is still controversy over the time-to-crest. Measurements in the region of a microsecond are admittedly difficult. The industry is still seeking new data. In approaching this problem, it was felt that it might be conducive of results to analyze the mechanism of the return stroke and then, from the factors governing the mechanism, try to synthesize the stroke. Perhaps certain limits to the rate of rise could be ascertained. Some of the characteristics of the component factors could be obtained in the laboratory, others by computation. It was with this in mind that the present investigation was undertaken.

Voltage divider for measuring impulse voltages on transmission lines

Impulse tests were performed on the experimental transmission lines of the American Gas and Electric Company. When these tests were being planned, it was necessary to select voltage dividers capable of reducing the high surge voltages to a level suitable for measurement by a cathode-ray oscillograph. Selecting dividers for measuring transmission line surges is complicated by the necessity of obtaining a high divider impedance to prevent excessive reflections on the line while maintaining an acceptable divider response. An investigation was undertaken to determine the suitability of the resistance- type divider for this application. Particular attention was given to the sources of error which influence divider response and to methods of overcoming principal errors for physically long, highresistance dividers.

The Influence of the Prestrike on Transmission-Line Lightning Performance

1. The paper presents an analysis of the voltage-time and current-time relations to be expected on transmission lines based on the prestrike theory of the companion paper.1 2. The prestrike current is correlated with the AIEE2 return stroke current. The amplitudes of the two currents are found to be of the same order of magnitude. 3. The prestrike current waveform is influenced by the electric circuit characteristics of the stricken structure, and is found to have very high time rates of change as compared to the AIEE2 stroke current. 4. Calculated data on tower-top crest voltages and their durations are presented for a range of transmission-line design parameters. 5. The severity of the prestrike varies approximately as the square of the tower height, over the usual range of tower heights. 6. The prestrike theory predicts much higher tower-top potentials than the AIEE method, for equal stroke intensities and zero tower footing resistance. 7. Consideration of conclusions 5 and 6 in combination suggests that the poor lightning performance of some lines is due to the prestrike phenomena. 8. The presented data on prestrike waveforms are suggestive of the response required of recording devices to confirm this theory. 9. A calculation method was developed which permits the long-time analysis of numerous cases by digital computer procedures.

Surge Transfer Through 3-Phase Transformers

ALTHOUGH a low-voltage circuit, connected to the system by a transformer, has no direct exposure to lightning, lightning surge voltages can be produced in this circuit as a result of surge voltage impact on the high-voltage side of the transformer. Even though a lightning arrester is connected on the high-voltage side of the transformer, surge voltages transferred to the low-voltage side can be dangerous especially to lower BIL equipments such as rotating machines and dry-type transformers.

Analysis of switching surges on a 220-kv line

IN RECENT YEARS, improvements in lightning arrester characteristics have permitted reductions in transformer insulation levels and have led to extensive analysis and investigation of switching surges. Before the common use of reduced BIL transformers, switching surges were of no concern in most cases, since their magnitudes were well below the insulation strength of the windings. As transformer BIL is reduced, however, normal switching surges become a factor and arresters may be universally required for complete protection.

Lightning performance of transmission lines — III

THE purpose herein is to present a simplified method of calculating the voltage across the line insulation for a stroke terminating on the tower top and zero tower footing resistance based on the theory and equations derived in two previous papers. For purposes of analysis, the stroke has been resolved into two components: (1) a linear-front flat-top wave of positive charge and associated current that flows upward along the axis of the downward leader at a constant velocity less than that of light, and (2) a linear-front flat-top wave of negative charge and current which is fed into the tower and ground-wire combination.

Overvoltages caused by disconnect-switch operation

RECENTLY field tests were performed at the Tanners Creek Station of the Indiana and Michigan Electric Company to investigate overvoltages which were produced when a 1,360-foot section of 345-kv bus was de-energized by an air-break disconnecting switch.1 The high transient overvoltages not only caused severe duty on connected lightning arresters, but also induced surge voltages in the station low-voltage power and control circuits. In conjunction with the field investigations, analytical studies were initiated in an attempt to determine the causes of the overvoltages and to develop remedial measures which could be used to eliminate the overvoltages.

Lightning protection of turbine generators

At present, it is the general practice to apply lightning protective equipment at the terminals of large unit-connected generators. There is a hesitation on the part of industry to omit protective equipment, even though analysis of the problem indicates that no protection is needed. This is primarily because the cost of the added insurance or protective devices is trivial when compared with the investment in a large machine, the expense of damage to the machine, and the expense of the loss of a large unit of generating capacity.

Lightning protection in a 24-kv station

THE RESULTS of synchronograph field tests and Anacom laboratory studies on a 24-kv multiline bus are presented. Previous studies on higher voltage stations indicated the influence of multiple lines on the surge voltage at the bus and the connected transformer. Extrapolation of that data to lower voltage systems seemed questionable, however, because of differences in the station circuit constants. This study has extended the station surge-performance d a t a down to the 24-kv level and, also, has provided additional verification of the miniature-system method by means of synchronograph field tests.

Measuring transmission-line impulse voltages

THREE companion papers describe impulse tests performed on the experimental transmission lines of the American Gas and Electric Company.1–3 When these tests were being planned, it was necessary to select voltage dividers capable of reducing the high-surge voltages to a level suitable for measurement by a cathode-ray oscillograph. Selecting dividers for measuring transmission-line surges is complicated by the necessity of obtaining a high divider impedance to prevent excessive reflections on the line while maintaining an acceptable divider response.