An accurate analysis of the lightning performance of a HV transmission line using the EMTP program is presented. By means of simulation using a digital model developed within the ATP-EMTP program with its MODELS-feature, the performance of a 110 kV unshielded overhead line under the influence of a direct lightning stroke is analysed. The simulation includes an adequate model of the transmission line, while also taking account of other factors such as the frequency dependence of the line parameters, models of wooden-porcelain insulation, wooden, and metallic towers and grounding systems including their impulse resistance characteristics. An impulse corona model (described elsewhere) is used, which, together with an adequate presentation of the flashover mechanism appropriate to air/porcelain insulation, allows one not only to predict the crest value of the overvoltages throughout the line, but also to assess accurately the real impact on the line fittings and substation equipment of the lightning surge travelling over the line. All the models are thoroughly verified by an extensive comparison of the simulated results with experiments by previous authors. Practical recommendations based on the simulations are made for an improvement in the lightning performance of a typical 110 kV transmission line.

EMTP analysis of the lightning performance of a HV transmission line

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.

The Lightning Stroke

Impulse Flashover Characteristics of Long Strings of Suspension Insulators

The considerable difference between the predicted flashover rate as calculated by the AIEE method and the actual flashover rate on the OVEC system over a 2-year period has forced a re-evaluation of present procedures for estimating lightning flashovers on transmission lines. Data and preliminary conclusions are set forth based on the first year of the field investigation.

1956 Lightning Field Investigation on the Ovec 345-Kv System

Factors Affecting the Lightning Performance of Transmission Lines

1. There are differences among the lightning current data obtained by various investigators. These differences appear to be a function of the height and the terminating ground impedance of the structure on which the measurements were made. 2. There is agreement among the data obtained by magnetic links when the results were obtained from direct readings on tower or diverter rods or by the summation of readings made at each end of the ground wire of a stricken span. 3. Comparison of data obtained at sea level or at a high altitude (14,000 feet) based on measurements made of strokes to tower rods or ground wires as in conclusion 2, show no appreciable difference due to altitude. This appears to be contrary to the previously drawn conclusion based on a comparison of the high-altitude diverterrod data to sea-level lightning-current data based on the summation of tower currents. 4. There is disagreement among the lightning current magnitudes obtained as in conclusion 2 and those obtained by the summation of tower currents. 5. Lightning stroke data measured at the ESB are in disagreement with those obtained as in conclusion 2 by a factor of about two to one, particularly at the higher currents. 6. Examination of oscillograms obtained at the ESB show variations in the current wave shape that may be attributed to reflections occurring from the base of the building and other reflection zones around Manhattan Island. Thus, there are indications that the lightning stroke is not a constant current source.

I. B. Johnson

Papers

A Hypothesis Concerning Lightning Phenomena and Transmission-Line Flashover