The magnitude and wave shape of the discharge currents in 46 direct strokes have been measured during a five-year period at 25 direct-stroke stations on structures ranging in height from 75 to 585 feet. Measurements have been made with instruments capable of recording the wave front and tail of the high-current peaks and the low-magnitude continuing currents to as low as 0.1 ampere. These data are correlated with 138 special camera photographs of strokes to open ground obtained in this investigation, with similar photographs taken in South Africa and with stroke current measurements made on the Empire State Building in New York City. Most strokes to structures, less than 600 feet high, appear to have discharge characteristics essentially the same as those to open ground. Their probability of being struck is about proportional to height, being in the order of two strokes per year for 500-foot structures and 0.3 per year for 100-foot structures. Whereas practically all strokes to the 1,250-foot Empire State Building are initiated by an upward streamer propagating from the building to the cloud, only four such strokes have been recorded in this investigation: three out of 13 strokes to a 585-foot stucture and one out of 13 to a 360-foot structure. Strokes of this type, as recorded here and to the Empire State Building, had discharge currents consisting principally of a continuous flow of low-magnitude current upon which the high-magnitude short-duration peaks are superimposed.

The Measurement of Lightning Currents in Direct Strokes

Surge Protection of Cable-Connected Equipment

A New Approach to the Calculation of the Lightning Performance of Transmission Lines [includes discussion]

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.

Induced voltages on Transmission Lines

This paper describes a method of calculating voltage distribution in a stator winding exposed to impulse voltage. In this method, the voltage distribution in an equivalent-circuit representation of the stator winding is calculated. The winding is treated as an infinite number of identical coils connected in series, with each coil represented by an equivalent circuit including inductance, turn-to-ground capacitance and conductance, and with mutual inductance, capacitance, and conductance between turns. The impulse voltage is approximated by a Fourier series of components. The voltage distribution for each component is calculated, and the complete voltage distribution is obtained by summing the voltages due to each component. The complete calculating procedure, including the calculation of the equivalent circuit parameters has bee programmed in Basic language for computer calculation. Examples of the coil and turn voltages distributions calculated by this program and a comparison of calculated data with test data for a particular stator winding are included.

Voltage Distribution among the Coils and Turns of a Form Wound AC Rotating Machine Exposed to Impulse Voltage

In the investigation described in this paper, measurements have been made and statistical data obtained on the multiple character and wave shape of the currents in direct strokes and the currents discharged by lightning arresters. These data, in conjunction with other data on the field performance of arresters, have revealed definite differences between the nature of direct strokes and the currents they produce in arresters. 1. Arrester discharge currents have fewer components than found in direct strokes. Only about 30 per cent were found to be multiple and not over 8 components were recorded, while from 50 to 70 per cent of direct strokes were multiple with as many as 32 components recorded. 2. Components of arrester discharges are of lower crest magnitude. Only 30 per cent of those measured exceeded 1,000 amperes, while for direct strokes 50 per cent exceeded 5,000 amperes. 3. The initial high current portions of the components of arrester discharges do not vary greatly in wave shape and are similar to those of direct strokes, having for both types of discharges times to half value which, in general, lie between 25 and 100 microseconds and average about 50 microseconds. 4. The long duration low magnitude portions of direct strokes vary over wide limits, both in magnitude and duration. However, for arrester discharges they are seldom present to any appreciable degree. This portion of the discharge may have current magnitudes as high as 1,000 amperes and measurable durations from 50 to 20,000 microseconds in direct strokes, while lightning discharges through arresters seldom last more than 300 microseconds. 5. There is definite evidence that system neutral ground conditions influence the currents discharged by arresters on systems, and that the most severe duty is imposed on arresters on ungrounded systems, while the least severe service conditions appear on four-wire grounded neutral circuits. This is shown by the fact that no long duration surge components have been recorded on four-wire circuits, while about 50 per cent of the records obtained on ungrounded circuits had components of long duration. It is also shown by the performance record of the new Autovalve lightning arrester. With over 40,000 arrester years service this arrester has experienced only 18 lightning failures or a lightning failure rate of 0.045 of one per cent. Of 27,000 arrester years experience on four-wire systems there have been no failures, while of 4,500 arrester years of service on ungrounded circuits there have been 11 lightning failures. 6. There is indication that one of the factors influencing the nature of a lightning stroke is earth resistivity or geological structure. Observations lead to the inference that lightning discharges of long duration are more prevalent in regions of high earth resistivity and ancient geological structure than elsewhere. Thirteen of the 14 long duration arrester current records and 16 of the 18 arrester failures due to lightning occurred on four systems in two regions of high earth resistivity for which there was only 1,078 arrester years experience. 7. These data indicate that it is only with a combination of high earth resistivity and type of system for which the transformers cannot absorb the long duration portion, that appreciable long duration arrester discharges and accompanying severe duty on arresters is obtained. The data on which this is based are not yet conclusive, but the indications are sufficiently definite that this probability should not be overlooked.

Field Investigations of Lightning

Rotating machines directly connected to overhead lines are subject to damage from lightning surges. Many methods of protection against such damage are possible, several of which are discussed in this paper. Laboratory experiments have been made to show the normal distribution under various steepnesses of waves, and the improvement of unsatisfactory distributions by means of condensers or lightning arresters connected to various parts of the windings has been studied.

Effects of Lightning Voltages on Rotating Machines and Methods of Protecting Against Them

Transformers and Lightning Arresters-Tidd 500-Kv Test Lines

The high monetary value and low insulation levels of rotating machines make it necessary to employ special protection. A rotating machine can be conceived as a transmission line with distributed constants, the essential difference being that the machine winding is wound back on itself in the form of turns, which may permit high voltage across the turn-to-turn insulation. The installation of special arresters with low and consistent spark-over characteristics will limit satisfactorily the maximum voltage that can appear across the terminals. However, capacitors also are required which, in conjunction with the inductance in the line, slope the front of the wave and thus prevent pilin up of voltage across the turns. Present methodsl1-7 of protection, on the whole, have proved quite satisfactory. However, in view of the statistical data that have been accumulated in extensive lightning investigations,8-12 the recognition of the importance of providing good grounding for the line arresters, and the appreciation of the value of the inherently low discharge voltage characteristics of the deion arrester, it has been considered desirable to study this problem further. The more important results of this study are summarized here with the viewpoint of providing a simple and straightforward basis for machine protection which affords the increased protection and saving in installation cost and material that it was found possible to achieve. Although the protection required at the machine and in the line are mutually interdependent, it is possible to resolve the suggestions for protection into the four following categories: A.

Edward Beck

Papers

Field Investigations of Lightning

Effects of Lightning Voltages on Rotating Machines and Methods of Protecting Against Them

Transformers and Lightning Arresters-Tidd 500-Kv Test Lines

Lightning Protection for Rotating Machines

The Duty on Lightning Arresters for A-C Systems