« SebelumnyaLanjutkan »
RECENT DEVELOPMENTS IN PROTECTIVE
Lightning arresters, if perfect, would be able to suppress all kinds of disturbances occurring on a transmission line, both those of abnormal voltage and those of abnormal frequency. Lightning, in this connection, has therefore taken a broader meaning than is common, and includes the disturbances set up in the line from internal causes.
Lightning can be divided into (1) external and (2) internal lightning, the external caused by atmospheric conditions, the internal having its origin in the generated power.
(a) Direct Strokes. The line at the point of striking is suddenly raised to an enormously high voltage, so high that it is probable that no insulation could prevent arcing to ground. Such a wave peak of voltage would set up a wave train traveling in both directions away from the point struck, the same as a pebble dropped into a pond sends off traveling waves, and in both cases these waves would become lower as they traveled and gradually die out. The more concentrated and steep the peak, the higher the frequency of the wave train and the less distance it can travel before becoming insignificant. A direct stroke, therefore, will arc to ground near the point of striking, no matter whether distant stations are protected by lightning arresters or not. Its effect will be local and, unless occurring near a station, will not be the primary cause of danger to the apparatus; it may, of course, start dangerous short-circuits and arcing grounds.
(b) Electromagnetic Secondary Strokes. A direct stroke
) passing near a line will magnetically set up an oscillation, severe only when the stroke is parallel to the line.
(c) Electrostatic Secondary Stroke. If a cloud is charged with electricity, an equal charge of opposite sign is attracted and held beneath it on the earth. A transmission line beneath the cloud gradually—by leakage, "static"-assumes the potential
of the earth. If, then, lightning strikes from cloud to earth, and the potentials are neutralized, the line, being insulated, is left at high potential to earth. This potential is at first concentrated as a single wave beneath the cloud, but, as in case of the direct stroke, at once spreads out with the speed of light, in both directions, forming a traveling wave train. In this case the initial wave is not as high as under the direct stroke and may not arc over the insulators, but it covers a much longer distance and the wave sent out will be of lower frequency. The wave train in this case will not die out so quickly; it may, however, contain considerable more energy than the direct stroke. Such a wave will travel into the station and there will either pass to ground through the lightning arresters or will be reflected by the inductance of the apparatus and, as when an ocean wave is reflected by a cliff, may pile up to twice its original crest; then something arcs over, usually the transformer bushings.
Lightning strokes do not always consist of a single discharge, lasting only for an instant; the same path may serve for a great number of discharges closely following each other. Mr. Alex. Larsen has photographed lightning with a turning camera and in this way has been able to separate on the plate and count the individual discharges, and also to record the total time of the flash. In one case he finds 48 separate discharges, lasting in all for 0.624 second. This, by the way, at once brings us to the conclusion that no arrester should depend on moving parts to interrupt the current of discharge. Such an arrester, for perhaps a very brief interval, will leave the line unprotected, with disastrous results in case of a stroke such as is shown in the photograph.
(d) Static. As explained above, when a charged cloud moves over a line a static charge accumulates by leakage on the line; when it moves away the static must leak off. On a line not connected to ground by grounded neutral some provision must be made to supply this leakage, otherwise the potential will rise until the insulation punctures. Static is easily handled by proper protective apparatus. Static on a line is the first indication of an approaching storm. A static charge may also accumulate on a line from driving wind, fog, snow, or rain, or from difference in altitude.
(a) Interrupted Arc. An arc suddenly extinguished gives rise to high voltage, the energy stored in magnetism going into
12 L ER energy stored in the dielectric
If I is the shortcircuit current of the system interrupted at its maximum value, E may become so large that the surge set up will require almost short-circuit current through the lightning arrester to release it.
(b) Arcing Ground. In a system with no ground connection, if an arc takes place to ground a high-frequency oscillation is set up. This is due to the capacity of the line in parallel with the spark to ground. This arcing ground will set the whole system oscillating in an extremely dangerous manner and may give practically continuous arc-over voltage across the arrester. This is one of the troubles that can be eliminated by running the line Y-connected with grounded neutral.
(c) There are numerous other causes that will produce internal lightning, such for instance as poor synchronizing, connecting-in dead transformers and lines, sudden changes of load, and so forth. A line has a certain definite fundamental frequency of vibration depending upon its inductance and capacity, but it may vibrate at any harmonic of that fundamental, or combination of harmonics. The low-frequency oscillations usually contain the most power and are therefore the most dangerous and difficult to handle. High-frequency oscillations are dangerous locally, but do not travel far. Oscillations on the line, due, for instance, to spark discharges, may have frequencies running into the millions of cycles.
GENERAL RECOMMENDATIONS CONCERNING PLACING OF LIGHTNING
ARRESTERS ON SYSTEMS
Constant-Voltage Alternating-Current Overhead Systems
1. The graded-resistance multigap arrester should be used in stations and substations on each of the outgoing and incoming lines. Arresters are to be as near as possible to the points of entrance and should be connected by disconnecting switches outside of all station apparatus. Choke-coils must be in the circuit just back of the arresters.
2. Graded-resistance multigap arresters should also be installed in special lightning-arrester houses at a few points on
a long high-voltage line. These lightning-arrester houses should be located at points where the line is most exposed to lightning, such as at the crossing of a river or the crest of an exposed hill. Care must be taken to have such arresters inspected regularly.
3. For voltages above 60,000 volts, special arresters of the aluminum cell or of the liquid-electrode types are designed to meet the required conditions. While aluminum cell and liquic! electrode arresters are developed from an engineering standpoint, they are not available for immediate installation in quantities.
4. On secondary distribution systems graded-resistance
4 multigap arresters should be placed in the station on the outgoing lines and on poles to protect transformers.
On long feeders, pole arresters should be placed one to the mile.
5. In the generating station it is at times advisable to install arresters on the generator 'buses to protect against inter
6. Arresters for single-phase locomotives are of the gradedresistance multigap type of a more compact form than the station arrester. These are usually designed to meet the required conditions of space.
7. On single-phase railway circuits, in addition to multigap arresters in cars and stations, sometimes it is desirable to use horn arresters with no resistance, set at a gap very near the arc-over voltage of insulators along the line.
8. In order to protect the insulators of a high-voltage line against puncture due to direct stroke it is often advisable to have horn arresters set along the line. These horn arresters should be adjusted to arc over at a voltage slightly below the wet arcover voltage of the insulators. They should be installed about every half mile and connected to earth direct. Only one phase should be protected on a pole, three horns on three adjacent poles forming one group to protect the three phases. In this way the resistance of the ground is used to limit the current in case of two horns operating at the same time. If a ground wire is used it should not be connected to earth at poles carrying horns. Figures i to 5 show in detail wood pole lines protected by horns in this manner.
9. Transmission systems should be protected by ground wires; one substantial wire well grounded is recommended. Two