Protection of long distance transmission lines

PROTECTION OF LONG DISTANCE TRANSMISSION LINES. (**)

By Charles H. Baker. (Snoqualmie Falls Power Co., Seattle, Wash.)

The chief weakness in a transmission project is the transmission line itself. Although every precaution has been taken, and no expense spared in constructing well this artery between the generating point and the consumer, yet nearly all our troubles have centered here. We do not think a transmission system can be entirely reliable without a double circuit. Our present system transmits from the generating point at 30,000 volts by two circuits, each to Seattle, and two circuits each to Tacoma. These circuits parallel each other for 20 miles, and at the 20-mile point they are interchangeable through a high-tension plug-board system. For this distance we therefore have more assurance of continuity of service than from this point to the terminals where only two circuits cover.

It is our purpose later on, and we think it a good plan, to put in a plug board at one or two other intermediate points, in order to be able further to divide or inter change the circuits. If there is a break on any circuit, it can, by means of these plug boards be located on parallel short division and the circuit in trouble can be cut out and repaired.

It has been our practice to repair a circuit in trouble while current was running on the other circuit on the same sets of poles. An operation of this sort, however, must be undertaken by only skillful and cool-headed men. Our wires are spaced 30 íns. apart in the triangle.

About every fifth pole has a spare cross arm tacked upon it, and every half mile there is a spare pole resting on skids. The location of these spare cross arms and poles is known by the number of the pole at which they are located, and the patrolmen have a record of these, so that if there is any trouble he knows at once where to go for relief material.

Every three miles on our transmission we have a tele phone booth located, which is commodious enough to contain also a few insulators, pins, wire, etc. The patrolman in making his rounds enters the booth and reports by tele phone his presence there to the head works office at Snoqualmie Falls, and he also registers his call in the booth, so that his visits may be tallied at any time by the general superintendent or inspector. He reports the line "O. K." when he gets to the booth, if such be the case.

The length of the section in charge of each of our patrol men varies with the character of the country and the difficulty of traveling it, but in general one man has about 20 miles in charge and patrols it twice a week. After having made a tour of inspection the patrolman mails a report card to the Seattle office, certifying the condition of the line.

Built as our line is, and inspected as rigidly as it is, we have eliminated entirely all troubles due to falling trees, running fires or wind storms. In fact, our troubles narrow down entirely to those due to interference evidenced either by malicious persons or boys shooting off insulators or throwing wires over the transmission lines. The only remedy for this is to make an example of a guilty party, and the experience of other transmission concerns has shown that such an example puts a quietus on the interference. We have had a law enacted in the legislature of this state, making any such interference a penitentiary offense, but thus far we have been unable to convict anyone.

As soon as a short circuit occurs, the lines are tested out immediately, to find out on what section the trouble lies. The trouble is invariably on one circuit only, so that the entire load is immediately placed upon the other circuit pending the correction of the trouble. As soon as the trouble is located on a division, the patrolmen from each end of that division start towards each other in great haste to reach it and repair it.

Our apparatus is protected by lightning arresters installed on the main circuits at the primary station and each of the sub-stations. We have very little lightning in this country and only once, I believe, in three years, have these arresters actually been called to duty, and on this occasion they worked to perfection. All our circuits are fused at the different stations for 100% overload, so that in case of a dangerous short circuit coming in, the fuses open up and the apparatus is cut out.

As an improvement upon our own practice, and to make the protection of any long-distance system further as sured, I would suggest that the triangle of wires be constructed so that the apex insulator will be below instead of above, for the reason that a wire or anything thrown over the lines-will be less likely, under such an arrangement, to cross two wires. In our system the apex is above I would further suggest that the spacing in the triangle be greater for the same purpose, even though the inductive drop is thereby increased somewhat. Spacing should be at least 36 ins.

The insulator is the strong or weak point in a transmission system, and I am glad to say that we have been very successful, using the porcelain Imperial insulators, of the best quality. I would suggest, however, that the insulators receive a brown glaze instead of the white (as in our case) as they will thus present a poorer target for anyone aiming at them. We have had a great many insulators broken to the extent of the petticoats being shattered off, but the wire almost invariably remains upon the core of the insulator. This would not be true in the case of a glass insulator, which would be shattered to pieces, allowing the wire to come down upon the cross arm.

I would suggest, further, that the telephone system be maintained upon a separate set of poles, as trouble on the transmission line is very sure to put the telephone out of service if both are on the same poles. These suggestions we would certainly inaugurate in any new lines that we are to build. As it is, any interruptions we have, and they have now been reduced to a minimum, seldom last over three to six minutes, which is just long enough to test out the line and switch the load on to the circuit which is intact. The location of and repair of the break, of course, takes longer, depending upon the distance the patrolmen have to go to reach it, but the repair of this, as stated above, does not interfere with the service.

By L. Denis. (Quebec, Canada)

ad little experience with lightning trouble. Our transmission line consists of two three- phase 23,000 - volt lines, some 18 miles in length, and in several places passing in It is protected at both the neighborhood of tall trees. ends, at the very points where the wires enter buildings, with sets of Wurts lightning arresters arranged in the standard Westinghouse fashion. These arresters are our only protection against lightning. Although they have been noticed to spark occasionally, we have never experienced more than the result of a partial short circuit from the discharge. We seem to have had more trouble from gradual discharges, and from induced charges that take effect on the low- tension side of our station trans formers. On one occasion, one of these charges caused quite an amount of damage on the 2,000-volt switch board, arcing to the ground from terminals leading to current transformers. In this case, as in others of the same nature, but less serious, apparently no damage was done to our station transformers. This locality has quite a reputation for the severity of its lightning storms.

By P. N. Nunn. (Provo, Utah)

We have, at our several stations in Colorado, Utah and Montana, a number of lightning-arrester installations. These installations are all of the Wurts type of non-arcing arrester, and consist of a number of paths to ground, with a number of choke- coils between the line and apparatus to be protected. The exact arrangements employed are various and for voltages ranging from 4,000 to 60,000 volts, the higher voltage being, in general, from star-connected transformers with neutral point grounded.

In all of these various installations an attempt has been made to improve upon the previous practice, and, in general, this has been successful. We have never had a serious disaster caused by lightning, or by the violent oscillations caused by the opening and closing of long power lines of large capacity and at a very high potential. We have, however, had numerous small annoyances caused by lightning and the oscillations above referred to, and for this reason do not regard our present lightning protection as entirely satisfactory. We are now experimenting with the more recent type of arrester installation, including shunted and series resistances and fewer paths to ground.

By Henry J. Gille. (St. Paul, Minn.)

The St. Croix Power Co. is operating a 25,000-volt transmission line approximately 30 miles long, from Apple River Falls, Wis., to St. Paul, Minn. The transmission line consists of two three- wire circuits on one line of poles. The poles are spaced at a maximum distance of 110 ft. The size of wire is No. 2, B. & S. medium drawn copper, one circuit being supported on each side of the poles on two cross arms in the form of an equilateral triangle with 24-in. sides. One of these circuits runs straight through without transposition, while the other is spiraled twice at equal intervals.

At each end of the line is located a lightning - arrester house. The house at the St. Paul end is within two miles of the sub- station and is also used as a cable- terminal house. At each end of the line is a choke coil on each wire and 12 2,000-volt lightning arresters. Each arrester has four air gaps of about 1/8-in. each; cylinders being 1 x 1-1/4 ins. in series with two graphite resistances, each having a cold resistance approximately of 450 ohms.

From the cable-terminal house the current is carried underground through cables at the line voltage to the sub-station. The cables are two in number; one insulated with rubber and the other with prepared paper, both being lead covered, and each of sufficient capacity to carry the entire load. In both cables there are three conductors, each conductor consisting of seven strands of copper wire. At the sub- station in St. Paul there are cable terminals, switching devices and static arresters. The static arresters are lightning arresters, the same above, and are connected the same as lightning arresters except that the center of the Y is not grounded.

During electrical storms the line is almost always affected in some way, sometimes very slightly and sometimes shutting the system down entirely. When the storm is severe we usually observe a discharge on one leg of the arresters. The discharge affects the generators at the power house practically the same as does a heavy short circuit and there seems to be a strong tendency for part of the generators to motorize, and always slow down. In some cases they have come practically to a standstill. We have had several cases where the lightning has apparently disregarded the choke coils and the lightning arresters, and damaged the static transformers.

At the sub-station the synchronous converters slow down, but apparently remain in step until the fields become practically dead. This occurs in about 10 seconds. When the generators begin to pick up again at the power house the rotaries also come up with considerable flashing at the brushes. Sometimes they come up with the polarity part of the rotaries reversed. If they all come up right side up they pick up the load and carry it along as if nothing had happened, but if part of them come up wrong side up it is necessary to shut down and change the polarity.

The alternating-current system is not so easily affected as is the direct- current system. When the transmission line is affected by lightning discharge and the current comes back in the sub-station, the circuit breakers on the alternating-current feeders usually open up, especially on feeders where there are induction motors. This is without doubt due to the fact that the induction motors will take considerably more current to bring them to speed again than is required when they are up to speed; that being practically the only time when all the induction motors on the circuit are started at the same time, these circuit breakers therefore open up, due to a heavy overload.

The effect on the line is entirely dependent upon the severity of the electrical storm. If the electrical disturbance of the atmosphere is light, the lights in the city merely wink. Under these conditions the apparatus does not seem to be affected, but when the electrical disturbances in the atmosphere are very severe it is very difficult to keep the rotary converters in service, due to the fact, as explained above, that when they slow down some of them come up reversed, thus shutting down the whole system; or if they come up right side up, they have to pick up the load at zero, which puts a very heavy overload on them for a short time. This is liable to blow the alternating-current fuses and motorize the rotaries.

There is evidence on a great many poles on the transmission line of discharges jumping from the wire to the poles. We have not had poles so badly damaged that they have had to be replaced, but some of them were badly split. At the time of these discharges the insula tors on the line were apparently not affected; as a matter of fact, we have had no insulators broken through lightning discharge.

The lightning arresters and static arresters are so adjusted that they discharge constantly, the discharge being very light, being visible only on two or three air gaps next to the line wire. The lightning arresters at one end of the line were very badly damaged during one storm last summer, when the discharge apparently jumped from the line to the ground, arcing across the cylinders without regard to the resistances, forming a straight path to the ground. How long this arc was held we do not know; however, it held long enough to do considerable damage to the cylinders and porcelain.

The trouble where no synchronous apparatus is used is not regarded as so serious, but all the companies that I have been referred to by manufacturers, who are operating synchronous apparatus, state that they have considerable trouble during electrical storms.

By P. M. Lincoln. (Niagara Falls, N. Y.)

The effects of lightning on the Niagara Falls & Buffalo transmission line may, for the purposes of discussion, be divided into two classes: 1, Those lightning discharges which cause destruction of apparatus attached to the transmission line; and 2, those discharges which cause a general short circuit, but without the destruction of any apparatus.

So far as destruction of apparatus is concerned, the Niagara & Buffalo line has been peculiarly fortunate. During the 5½ years of its operation, practically the only apparatus that has ever been destroyed by lightning has been lightning arresters. The exception of any importance to this statement was the short-circuiting of a 500 K.-W. transformer on one occasion at Tonawanda (about halfway between Niagara Falls and Buffalo). The only other exceptions consist of one ammeter transformer and a few ground detectors.

In the matter of lightning arresters, however, the line has not been so fortunate. Several different styles and makes have been installed and tried with the result of ultimate disaster to the arrester. Failure has been due to the inability of the arrester to interrupt properly the dynamo current that follows the discharge, and the resulting short circuit at the arrester would usually put it out of business, at least temporarily, and occasionally, permanently. The conditions at Niagara are, of course, unusually severe. The low frequency and the large amount of power involved make it difficult to interrupt a short circuit when once started. Devices that work perfectly on higher frequencies and with less power back of them, have failed utterly on the Niagara & Buffalo line. One of the things which I consider that the experiences of this line have proven is that it is absolutely necessary to limit, by means of resistance, the dynamo current which flows on the discharge of the arrester. No matter what the number of gaps through which the discharge takes place, an arc once started seems to be maintained unless the dynamo current is thus limited.

The device that comes nearest answering out lightning-arrester problem is the one now in operation; that is, the so-called "low- equivalent" lightning arrester of the Westinghouse Electric & Mfg. Co. In that device the gaps have an ohmic resistance in series with them and another larger resistance in shunt to a portion of the gaps. This arrangement gives the line-discharging effect of the small series resistance and the current-interrupting effect of the combined series and shunt resistances. In their original form these were put into operation in the summer of 1899, and are still in use in a somewhat modified form. Our former experience, however, has led to the adoption of fuses in the discharge circuit, so that in the case of a short circuit taking place in the arresters, both line and arresters are protected; the line from a continuation of the short circuit, and the arresters from the damage that would necessarily follow that continuation. In all our lightning arresters, choke coils are put in the line and the discharge circuits tapped on the line side of the choke coils.

In the invitation to present these notes, the question is asked, "How frequently are your lines affected by lightning that your devices have failed to arrest?" Those occasions when the arrester conducts off the flash, but becomes short-circuited, can hardly be called failures to arrest. It is possible and perhaps even probable-that other apparatus might have been destroyed had not the flash been taken care of by the arrester. In most of the failures to arrest, as observed on the Niagara & Buffalo line, the lightning has caused short circuits somewhere along the overhead line, sometimes with a simultaneous discharge across the gaps of the arrester and sometimes with no sign of such discharge. Our observations would indicate that the voltage necessary to jump over a wet jump across the gaps of the lightning arrester. In case such a short circuit is once established anywhere along the line, it almost always continues until the current is pulled off the lines, in spite of the fact that the distance between our conductors is 36 ins. Most of our lightning interruptions are of this nature. No permanent damage is done. All that is necessary is to take off the current for an instant; supply of power can be resumed immediately. An examination of the records for the past three summers-those of 1899, 1900 and 1901-shows that during that time 18 interruptions occurred due to lightning, five of which occurred within a space of about three hours, during a storm of unprecedented severity. The average duration of the interruptions has been about four minutes. Considering the prevalence of lighting on the Niagara frontier, I consider this record a very good one.

(**) Abstract of a joint paper by Charles H. Baker, L. Denis, P. N. Nunn, Henry J. Gille and P. M. Lincoln, presented at the annual convention of the National Electric Light Association at Cincinnati, in May.