Publication: Western Electrician
Chicago, IL, United States
Lightning Protection Discussed in the
Light of Practical Experience.
At the last meeting of the main body of the American Institute of Electrical Engineers in New York city on May 19th two papers of considerable value to engineers dealing with problems in lightning protection were presented. These were "Comparative Tests of Lightning protection Devices on the Taylor's Falls Transmission System," by J. F. Vaughan, and "Studies in Lightning Performance Season 1907," by N. J. Neall.
TAYLOR'S FALLS TESTS.
Mr. Vaughan said that on account of the wide divergence of opinion on the subject of lightning protection and the impossibility of reconciling the conflicting results of practice, it was decided to try out on the Taylor's Falls' system all existing devices of promise and such others as might be devised in order by comparing them in actual service to work out some effective scheme for future protection.
The line runs 40 miles from Taylor's Falls to Minneapolis. Near its center was found a zone of some nine miles that was especially subject to severe lightning, and the splintering of six poles in different parts of this zone before any wire was strung suggested the necessity for special protection at exposed points against direct stroke. Three No. 0000 copper cables were supported on 14-inch four-part porcelain insulators arranged on a six-foot equilateral triangle with the apex at the pole-top. The line voltage is 50,000. At the substation in the outskirts of Minneapolis the voltage is stepped down to 13,800. For protection of the power house and sub-station low-equivalent multi-gap arresters, and oil-insulated choke-coils were installed, supplemented at the sub-station by a set of experimental aluminum-cell-type arresters connected to the entering line through a small number of arrester gaps in zigzag arrangement, set so as to be normally active. The transformers at all stations were further protected on their low-tension sides by static discharge gaps.
For line protection three kinds of horn-type arresters were installed, one at each end and one in the middle of the line, primarily to pass off disturbances of unusual magnitude, and also to experiment with the different forms.
The sub-station arrester consisted of a single gap on each phase, arranged with a sheaf of water jets forming series resistance to ground. This required too much water and was replaced by tanks of water with terminals of carbon rods in, fiber tubes.
The power-house arrester had two gaps in series between each phase and ground, with the second gap shunted by carbon terminals placed in the river. The arrester at the middle of the line was built on the selective resistance principle, with three gaps in series on each leg, the second and third gaps being shunted by water-box resistances.
Four types of overhead grounded wires were erected in the nine-mile zone in one-half-mile lengths, alternating with one-half-mile lengths of unprotected line as follows: Type A, two wires mounted on a cross-arm five feet apart on either side of the top line wire and about 18 inches below it. Type B, two wires supported on standards of 1 1/4 - inch iron pipe six feet apart and 18 inches above the top line wire (Fig. 1). Type C, one wire on knobs attached to the pole near the center of the delta. Type D, two wires in the same position as in type B, but supported on pipe pins set in the ends of a cross-arm attached near the top of the pole.
In the above constructions the grounded wires were of No. 6 hard-drawn copper. The ground connections were of galvanized iron ribbon wire at every fourth pole and the ground made by 3/4-inch galvanized iron pipe driven to moist ground.
Four types of lightning rods were used, erected in the nine-mile zone in sections from one to two miles long, separated by unprotected sections as in the case of the grounded wire constructions, as follows: Type A, rods of 1 1/4 - inch galvanized iron pipe attached to the poles and extended by tridents of copper wire reaching six feet above the top line wire (Fig. 1). Type B, rods of 1 1/2 - inch galvanized iron pipe mounted on separate poles 20 feet to one side of the transmission line and topped by tridents of copper wire extending 25 feet above the top line wire. Rod poles were spaced at the centers of alternate spans (Fig. 1). Type C, same as type B, but spaced three rods to four spans. Type D, same as type B, but spaced 1,000 feet apart.
Ground connections were made as on grounded wire construction, but with two grounds for each rod pole, one at the base of the pole and the other 20 feet or more away to insure especially wet ground and to increase the discharge area.
For recording the phenomena, all station arresters were provided with tell-tale papers. To determine the character of disturbances, their extent, magnitude and effect on line and apparatus, ground connections of all protective devices were provided with gaps for the insertion of tell-tale papers. To study the stresses on the line insulation throughout its length and the behavior of the insulators, each insulator pin at every third pole was grounded through a separate ground wire and tell-tale box supplemented by choke-coils shunted with tell-tale gaps cut into the line wires at various points.
After a number of insulators on grounded pins had been damaged the pin grounds were removed from four sections of about one mile long each, separated by one-mile sections left grounded to prove whether the grounding was in any way responsible for insulator failures.
Full records were kept of the weather conditions, and particularly of storm data supplied by reports of the United States Weather Bureau and of local observers. Operating and special reports of interruptions to service and damage to system were obtained and the tell-tale papers collected after each storm. These papers were graphically analyzed and are treated of at length in Mr. Vaughan's paper.
The transmission system was started up in December, 1906, and lightning protection records were carried through the following summer. The lightning season opened late in March and lasted into October. Thirty-two storms occurred during this season within reach of the line.
The records show that in 17 out of the 32 storms the service was interrupted and eight of these storms caused damage to insulators, while only one storm affected the station apparatus; this was due probably to a defect in a transformer bushing, but was not sufficient to interrupt the service. Of the 5,500 insulators on the line 43 were damaged. Of these only three were punctured, the rest being shattered. Over one-half the insulators damaged were located at the tops of the poles and about two-thirds were in more or less exposed heights. Thirty-five insulators on grounded pins were damaged and only seven on ungrounded pins. Under overhead grounded wires two insulators near the ends of the line suffered damage. Under lightning rods on the line poles three insulators were damaged, and under lightning rods on separate poles none. One pole was split and two were burned. One very violent storm did the largest damage, 10 insulators being shattered, a pole split and the line completely short-circuited.
A careful analysis of the charts and records shows among other things that points where the line was especially liable to damage are, first, exposed heights, and next, wet bottom lands. Damage was usually concentrated within a distance of a mile or two, with the exception of a few storms which crossed the line at a small angle or traveled along it for some distance. Where grounded and ungrounded pins come on adjacent poles, damage to the insulators generally occurred at the grounded pin, but the damage of a group of insulators on a section of line where all grounds had been removed indicates that grounding the pins probably did not materially increase the danger of damage. Overhead grounded-wire construction materially protects insulators. The lightning rods do not seem to have much effect except in cases of direct stroke.
Further conclusions are: Storms follow no well-defined paths, nor are their effects confined to any particular part of the line. Stresses do not occur at any definite points. In general, the protected nine-mile zone showed an appreciable decrease of insulator stress and activity of devices for distant storms and a very decided shielding effect of, overhead grounded wires as well.
The principal trouble is from temporary or permanent breakdown of line insulation by static charges induced in the line by passing storms.
Direct strokes between cloud and ground may occur at any time. Although there were several cases of damage so caused during construction, the first season's operation gives evidence of only one case, and that without damage.
The induced charges are highly concentrated, and often of immense volume and intensity, discharging to ground over insulators with a disruptive effect that tends to shatter, but rarely to puncture them, often without line current following. Line current may or may not follow these discharges; if it follows it may only temporarily ground or short-circuit the line.
Arcs established by insulator spillovers or leakage of charging current through damaged insulators may burn pole structures or further damage insulators and even fuse the line wire. There is no evidence of surges other than the direct effects of grounds or short-circuits, nor is there of stress at any definite points on the line such as from reflected or standing waves.
Overhead grounded wires are of decided value in shielding the line from induced static charges and in preventing insulator breakdowns. A grounded conductor running down the pole is of decided value in preventing splintering of the pole. The selective resistance, multi-gap type of arrester is effective in disposing of ordinary disturbances. The aluminum cell type arrester is in general more sensitive and freer in discharge; it gives great promise for station protection. Horn arresters of the series gap and selective resistance type are fairly sensitive to static discharge as well as to disturbances of lower periodicity, and of special value as emergency devices to relieve the station arresters in case of abnormal discharge. They may be adjusted to be fairly sensitive without interrupting service or necessarily throwing out synchronous apparatus.
The results of these experiments have led to the recommendation that the overhead grounded wire construction (Fig. 2) be immediately extended about 15 miles in sections covering both ends and the more exposed parts of the line; that the use of horn arresters be continued and further adjustments studied; that the rest of the station and line protective apparatus be left as it is, and that the system of tell-tale paper and other records be continued during the conning season.
STUDIES IN LIGHTNING PERFORMANCE.
Mr. Neall discussed the general import to high-tension transmission of the data gained in 1907 as to lightning performance on the 50,000-volt Taylor's Falls line of the Minneapolis General Electric Company and on the Presumpscot Electric Company feeders supplying power at 11,000 volts to the Cumberland mills, near Portland, Me.
In the Taylor's Falls situation, line disturbances chiefly are being combated; whereas, in the case of the Cumberland Mills, station disturbances have demanded attention.
Lightning disturbances to a long-distance transmission line may be due to an induced (bound charge on the line) direct stroke, or both. The bound charge embodies the well-known principles of the operation of a static charge imparted to any insulated body. A modern high-voltage transmission line must be highly insulated from ground at every point; for 60,000-volt service, therefore, it is easily conceivable that, in general, a tremendous electric charge can accumulate on the lines and be held there with relatively small leakage. When in such a case as bound charge release takes place from the line, due to the discharge to ground, or to another cloud from the exciting cloud, an enormous rush of discharge undoubtedly follows the first point of breakdown to earth. It has been shown that all such charges, once they are free to move, tend to travel in waves, the progress and amplitude depending on the resistance in their path.
The tendency of such charges to travel in the transmission line is undoubtedly retarded very much by the skin-effect of the conductors; in fact, at the moment of release of bound charge we may consider that the charge has the choice of dissipating itself in three ways — along the line wires, over the insulators, or both. If the insulation is particularly good the charge endeavors to travel along the conductors. From the data presented by Mr. Vaughan it has been shown that such discharges can be very violent and yet not harm the insulator or produce a short-circuit.
Curiously enough, there is very little trouble reported from lines operating at lower potentials, even at potentials of 30,000 volts. The reason is probably partly this: No line can take a charge much in excess of its maximum insulator arcing-over strength. The result is, therefore, that the higher voltage lines must handle a proportionately larger and higher potential static disturbance.
Judging alone from the data at hand as to its behavior, an overhead grounded wire placed near the line conductors may be considered beneficial. The ideal static protection would be a metallic envelope for the line; but since this is not commercially feasible, it is thought possible to approximate it by supporting several grounded wires above the line. These wires are so placed with reference to the line wires that an imaginary plane extending at an angle of 45 degrees from the wire on either side of the center will just pass over the transmission wire. There are as yet no positive data to show the value of this consideration, but there are data of a definite nature showing the effectiveness of the double-wire overhead grounded-wire protection.
The power of absorption shown by such protection leads to the conviction that if a transmission line were so equipped throughout, very little static disturbance would aver reach the stations; in other words, the overhead grounded wire would not only shield against the line troubles, but reduce consequent station disturbances.
There is evidence to show the value of lightning rods to a transmission line. On a steel-tower construction (solely because of the usually long spans) lightning rods should be placed on each tower; on a wood pole line one of these could be placed at each grounded pole. These rods should perhaps extend four to six feet above the overhead grounded wire and end in spreading tips.
The importance of good grounds for the overhead grounded wire is patent. Too great care cannot be exercised in this direction. Even on the grounded wire the passage of the charge to earth is apparently impeded, even by the short distance it had to travel along the line. For this reason it is clear that grounding at every pole is beneficial to the discharge.
It is an open question of economy whether to equip a line completely or partially with overhead devices; but in consideration of the charges which they may be called upon to carry at any point, the whole line protected is theoretically and generally the safer policy.
So far as attempts have been made to protect against line distubances by larger insulators, or special features at insulators, such as horns to modify the effect, it is clear there is no reduced interruption to service itself when discharges actually occur. For this reason Mr. Neall prefers where feasible any protection scheme which aims to prevent the static disturbances getting on the line at all.
The lively operation of the station protective apparatus, as evidenced by the action of the series and shunted gaps and the heavy discharges over the electrolytics, indicates a considerable strain to ground, which, for the present must be considered effectively handled by the protective apparatus.
In consideration of what appeared to be pronounced line-to-line characteristics, each protective apparatus equipment on the Presumpscot system was supplied with extra gaps and fuses (until best adjustment should have been accomplished) and in one case low-equivalent arresters were placed across the line. In addition to this the regular low-equivalent arresters between line and ground were shunted with gaps and fuses, two each, around the shunt and series resistances in series. The whole system was carefully equipped with tell-tale papers so that nothing could happen without leaving a record.
The results on this system show improved service during lightning storms. When the arresters operated to ground there would often be a complete action over the low-equivalent arrester between line and ground as well as over the gaps and fuses in shunt thereto. In any given discharge not all the fuse paths to ground would operate. Fuses did not always blow even though a discharge had passed over them. The action over the fuse paths was apparently freer than over the arrester.
A record was established of the existence of line-to-line disturbances in addition to those disturbances usually considered in lightning-arrester operation. These disturbances were found to be taking place during "uneventful" periods of operation, with nothing to show from the general indications why they had happened.
Indications existed that line accidents, such as the wind blowing the wires together, will cause a wave of phase-to-phase disturbance to travel a long distance and operate the phase-to-phase protection; that on the closely adjusted gap-sets the fuses invariably blew and that some phase-to-phase action nearly always accompanied phase-to-ground operation.
From a summary of the results on the Presumpscot system the conclusion was reached that proper line-to-line protection is as essential as line-to-ground protection, and that the line-to-line operation is likely to take relatively considerable time, and therefore that in the long run high and stable resistance in the discharge path is necessary for uninterrupted operation. A shunted gap equipment, however, may be valuable.
Since phase-to-phase protection can be easily added to any multi-gap arresters not already so provided for, its adoption is not difficult. For this purpose Mr. Neall favors electrolytic arresters arranged in star with a common jar or jars to ground, since this does not impair the line-to-ground protection and adds to the phase-to-phase. Mr. Neall's conclusions are:
Lightning disturbances on a line are more than likely to be local, varying approximately from 1,000 feet or less, to several miles, the longer areas being less frequent.
They are likely to happen at any part of the line. A direct stroke of lightning is not necessarily harmful; it depends upon the quality of the stroke, etc.
A line may be affected by a bound charge and direct stroke simultaneously. There is as yet no evidence to enable these to be measured separately.
An overhead grounded system of one wire, two wires, or more, above the line wires is desirable.
An overhead grounded wire should be grounded at every pole near stations and important places, otherwise every few poles.
When an overhead grounded wire is used exclusively for station protection it should not be less than two miles in length and be grounded at every pole.
The higher the voltage design of the line the greater the possible disturbance from lightning to the line. High-voltage lines, therefore, need, more than low-voltage ones, overhead grounded wire or its equivalent.
Lightning rods added to the overheaded grounded system probably add to the protective power. Shattered insulators are liable to occur in every severe thunderstorm, but not in light ones. Puncture is more probable with power on the line at the time.
Insulators should be carefully selected for the service, to test not less than two times normal voltage between line and p'n, with pin grounded. The equivalent spark-gap should be higher than any arrester path to ground on the line.
Horn lightning arresters should be employed for extraordinary service only. The general arrangement known as the multi-gap selective path type promises to be of value.
Lightning-arrester stations to discharge line disturbances become increasingly expensive and of questionable assistance the higher the voltage transmitted. Their total number depends on the length of line.
Wood poles may be effectively protected against splintering from lightning discharge by providing them with a small metallic conductor to ground.
Electrolytic lightning arresters have so far behaved creditably, particularly in sensitive relief of grounds, but do not indicate a complete superiority over other types.
Line-to-line protection is desirable.
Apparently there cannot be too much protection on a plant. Every lightning arrester added plays a part, but practically there is a limit. For many reasons it is obviously desirable to keep the station protection as simple as possible. It is as yet impossible to define what this should be.
If a full measure of line protection is added, such as overhead grounded wires, the stations will, in turn, be much relieved.
Harold W. Buck of New York city opened the discussion by stating among other things that it has very conclusively been shown that these line troubles have been very much reduced by means of the overhead ground wire. It is not only shown in this particular system, but the experience all over the country with transmission lines where the overhead ground wire has been properly installed shows a similar corroborative evidence. The function of this ground wire is perhaps double. It allows the induced charge, so to speak, to accumulate on the ground wire instead of accumulating upon the transmission wire itself; so that when the cloud induces this charge, the corresponding charge on the ground wire can discharge itself through the ground connections on the pole instead of having to spill over the insulators. There is perhaps another important function of this overhead ground wire in shielding the wire from induced currents due to neighboring lightning discharges which are presumably of high frequency. The guard wire forms one side of the circuit and the around the other: so that we really have a short-circuited shield against induction on the line itself, the ground and the shield wire being connected electrically. If this is so, it would seem to be of decided advantage to have two overhead ground wires rather than one, having the connection made between those two shield wires at every pole. Then we would have a shielding effect on the vertical plane and also on the horizontal plane against induced charges in the transmission wire. These short-circuited loops in fairly close proximity to the transmission wires might represent some loss due to the normal frequency of the transmission wire; but it is doubtful whether that would be enough to be very serious; whereas with the frequency of the lightning discharge, the possible current might be very high, indeed, and have a decided shielding effect against the transmission. The evidence of the spilling over of accumulated charge on the insulators seems to show that the actual magnitude of the current in this ground wire is very large indeed. It is interesting to note in this paper that certain discharges have been noticed on the secondaries of the transformers which accords with the experience of Mr. Buck and that of other engineers. There have been a number of cases of overhead lines passing through transformers into underground cable systems where quite frequent cable puncturzs have resulted simultaneously with thunder storms on overhead lines. This seems to indicate the necessity of having lightning arresters of some form on the secondaries as well as the primaries.
Percy H. Thomas of New York city noticed that the punctures in the tell-tale papers were just as severe when there was no generator connected with the line as at any time. These punctures show on examination the fibers of the paper unburned, being therefore as purely static discharge as anything ever seen. The spillovers are cases where a static discharge has passed over the wire to the pin from the surface of the insulators and not an induced charge. In only a small percentage of these have there been insulators injured.
E. E. F. Creighton of Schenectady, N. Y., said the authors were fortunate in obtaining records on an idle line. The only data of great value in the study of' lightning potential should be taken on an idle l.ine, and furthermore should be collected on tapes moving by clockwork so as to separate the punctures due to each cloud discharge. A comparison of the aluminum arrester and the resistance horn-gap arrester will show the limitation of tne latter and the uselessness of its further development. At 60,000 volts every ampere of discharge represents roughly 60 kilowatts per phase, or 180 kilowatts. If an arrester will not discharge 100 amperes at least, it will not take the usual run of heavy discharges. With this rated discharge the generator would be called upon to furnish 18,000 kilowatts. On the other hand, if an aluminum arrester is used the energy taken from the generator will be less than 20 kilowatts and the rated discharge of lightning will be several hundred amperes at the same abnormal value of potential. The valve action of the aluminum arrester prevents almost entirely the flow of dynamic at normal potentials and has a free discharge only at normal potential. This information is sufficient to show the futility of using the resistance type of horn-gap arresters. Furthermore, the current taken by the aluminum arrester from one phase to ground is leading, the same as the capacity current of the line or cable, and consequently one phase may arc continuously between line and ground without setting up surges on the system. It should be noted that the aluminum arrester actually maintains the voltage and thus prevents grounding the line conductor.
W. N. Goodwin, Jr., declared that the only feasible possibility of entirely protecting an overhead power circuit lies in the development of an insulator which will not puncture, and while having a fairly low spillover will not shatter. This perfection in the petticoat type of insulator is probably out of the question, but he believed can be attained by a properly designed and proportioned insulator of the suspension type. Such an insulator would be the equivalent of a line arrester at each pole or tower on the line.
President H. G. Stott recalled an experience he had about n years ago when the Niagara transmission lines first started. It was the first, or one of the first lines equipped with the overhead ground wire. After a few months' experience with this protection it was decided to take it down, as the choice of two evils was that the lesser one was to take it down to avoid the troubles due to the ground wire rather than the benefits that it might give. That might have been due to poor construction work in the ground wires themselves, as they were continually breaking and short-circuiting the lines. Now, 11years afterward, it was shown tonight, that as the result of a most elaborate series of tests, apparently the ground wire gives us the most certain protection. If you think of it in a very practical way there is every reason why it should. No one ever heard anywhere of an underground cable being struck by lightning. That means that if you can get the earth potential, or the ground itself, above your conductor you are safe. We cannot possibly afford to bring our cables underground. The cost of burying a 60,000-volt line 100 or 200 miles long is absolutely absurd from a financial or commercial standpoint; but now we can do the other thing, and instead of putting the wire underground we can put the ground above the wire. It seems that the most perfect protection will be gotten where we have the most perfect network of ground wires above our conductors, and I think everything tonight points to that fact. Another point is somewhat emphasized, and that is, to avoid ordinary troubles you have got to have a high factor of safety in all apparatus. Troubles with underground cables are almost unknown outside of mechanical damage and damage to joints and that sort of thing; and the reason is that those cables are made with an extremely high factor of safety. For instantaneous voltage it is 10 to 1. Just as it is with human nature — if a man is in a broken-down condition every few germs that come along will give him a new disease; but if he is strong enough they won't affect him at all. We have got to make our system germproof, as it were, and have a very large factor of safety on the insulators. That and the ground wires are going to be the solution of our overhead work.