ELEVENTH LECTURE LIGHTNING PROTECTION W"~l HEN the first telegraph circuits were strung across the country, lightning protection became necessary, and ■^ was given to these circuits at the station by connecting spark gaps between the circuit conductors and the ground. When, however, electric light and power circuits made their appearance, this protection against lightning by a simple small spark gap to ground became insufficient, and this addi- tional problem arose : to open the short circuit of the machine current, which resulted from and followed the lightning dis- charge. This problem of opening the circuit after the discharge was solved by the magnetic blow-out, which is still used to a large extent on 500 volt railway circuits; by the horn gap arrester — a gap between two horn-shaped terminals, between which the arc rises, and so lengthens itself until it blows out ; and later on, for alternating current, the multi-gap between non-arcing metal cylinders, a number of small spark gaps in series with each other, between line and ground, over which the lightning discharges to ground — the machine cur- rent following as arc, but stopped at the end of the half wave of alternating current; but not starting at the next half wave, due to the property of these "non-arcing" metals (usually zinc-copper alloys), to carry an arc in one direction, but requir- ing an extremely high voltage to start a reverse arc. These lightning arresters operated satisfactorily with the smaller machines and circuits of limited power used in the earlier days, but when large machines of close regulation, and therefore of very large momentary overload capacity were in- 138 GENERAL LECTURES troduced, and a number of such machines operated in multiple, these lightning arresters became insufificient : the machine cur- rent following the lightning discharge frequently was so enor- mous that the circuit did not open at the end of the half wave, but the arrester held an arc and burned up. Furthermore, the introduction of synchronous motors, and of parallel operation of generators, made it essential that the lightning arrester should open again instantly after dis- charge. For, if the short circuit current over the arrester lasted for any appreciable time: a few seconds, synchronous motors and converters dropped out of step, the generators broke their S3mchronism, and the system in this way would be shut down. The horn gap arrester, in which the arc rises between horn-shaped terminals, and by lengthening, blows itself out, therefore became unsuitable for general service ; since without series resistance, the short circuiting arc lasted too long for synchronous apparatus to remain in step, and with series resist- ance reducing the current so as not to affect synchronous ma- chines, it failed to protect under severe conditions. Thus it has been relegated for use as an emergency arrester on some over- head lines, to operate only when a shutdown is unavoidable. To limit the machine current which followed the light- ning discharge, and so enable the lightning arrester to open the discharge circuit, series resistance was introduced in the arrester. Series resistance, however, also limited the discharge current, and with very heavy discharges, such lightning arresters with series resistance failed to protect the circuits, that is, failed to discharge the abnormal voltage without destructive pressure rise. This difficulty was solved by the introduction of shunted resistances, that is, resistances shunt- ing a part of the spark gaps. All the minor discharges then pass over the resistances and the unshunted spark gaps, the LIGHTNING PROTECTION 139 resistance assisting in opening the machine circuit after the discharge. Very heavy discharges pass over all the spark gaps, as a path without resistance, but those spark gaps which are shunted by the resistance, open after the discharge; the machine current, after the first discharge, therefore is deflected over the resistances, limited thereby ; and the circuit so finally opened by the unshunted spark gaps. With the change in the character, size and power of electric circuits, the problem of their protection against light- ning thus also changed and became far more serious and difficult. Other forms of lightning, which did not exist in the small electric circuits of early days, also made their appear- ance, and protection now is required not only against the damage threatened by atmospheric lightning, but also against "lightning" originating in the circuits : so called "internal light- ning," which is frequently far more dangerous than the dis- turbances caused by thunder storms. Under lightning in its broadest sense we now understand all the phenomena of electric power when beyond control. Electric power, when getting beyond control may mean excessive currents, or excessive voltages. Excessive currents are rarely of serious moment : since the damage done by exces- sive currents is mainly due to heating, and even very excessive currents require an appreciable time before producing danger- ous temperatures. Usually circuit breakers, automatic cut- outs, etc., can take care of excessive currents, and such currents produce damage only in those instances where they occur at the moment of opening or closing a switch, by burning con- tacts, or where the mechanical forces exerted by them are dangerously large, as with the short circuit currents of the modern huge turbo-generators. 140 GENERAL LECTURES Excessive voltage, however, is practically instantaneous in its action, and the problem of lightning protection therefore is essentially that of protecting against excessive voltages. The performance of the lightning arrester on an electric circuit is analogous to that of the safety valve on the steam boiler, that is, to protect against dangerous pressures — whether steam pressure or electric pressure — ^by opening a discharge path as soon as the pressure approaches the danger limit. Therefore absolute reliability is required in its operation, and discharge with as little shock as possible, but over a path amply large to discharge practically unlimited power without danger- ous pressure rise. However, the causes of excessive pressures, and the forms which such pressures may assume, are so much more varied in electric circuits than with steam pressures, that the design of perfectly saitisfactory lightning arresters has been a far more difficult problem than the design of the steam safety valve. Such excessive pressures may enter the electric circuit from the outside by atmospheric disturbances as lightning, or may originate in the circuit. Excessive pressures in electric circuits may be single peaks of pressure, or "strokes" or discharges, or multiple strokes; that is, several strokes following each other in rapid succession, with intervals from a small fraction of a second to a few seconds, or such excessive pressures may be prac- tically continuous, the strokes following each other in rapid suc- cession, thousands per second, sometimes for hours. Atmospheric disturbances, as cloud lightning, usually give single strokes, but quite frequently multiple strokes, as has been shown by the oscillograms secured of such lightning discharges from transmission lines. Any lightning arrester LIGHTNING PROTECTION 141 to protect the system must therefore be operative again im- mediately after the discharge, since very often a second and a third discharge follows immediately after the discharge within a second or less. 6 O o o o o o- o o o o o o o o G o- o o o o ^ Fig. 27 142 - GENERAL LECTURES Continuous discharges, or recurrent surges, (lightning lasting continuously for long periods of time with thousands of high voltage peaks per second), mainly originate in the circuits: by an arcing ground, spark discharge over broken insulators, faults in cables, etc. These phenomena, which have made their appearance only with the development of the modern high power high voltage electric systems, become of increasing severity and danger with the increase in size and power of electric systems. Single strokes and multiple strokes, that is, all the dis- turbances due to atmospheric electricity, as cloud lightning, are safely taken care of by the modern multi-gap lightning arrester. In its usual form for high alternating voltages, it comprises a large number of spark gaps, connected between line and ground, and shunted by resistances of different sizes, as shown in Fig. 27, in such manner that a high pressure dis- charge of very low quantity, as the gradual accumulation of static charge on the system, discharges over a path of very high resistance R^, and so discharges inappreciably and even frequently invisibly. A disturbance of somewhat higher power finds a discharge path of moderate resistance R2, and so dis- charges with moderate current, that is; without shock on the system ; while a high power disturbance finds a discharge path over a low resistance Ra, and, if of very great power, even over a path of zero resistance, Z. On lower voltage, commonly only two resistances are used, one high and one moderately low, as shown by the diagram of a 2000 volt multi-gap arrester. Fig. 28. The resistance of the discharge path of the present multi- gap arrester therefore is approximately inversely proportional to the volume of the discharge. This is an essential and im- portant feature. Occasionally discharges of such large volume LIGHTNING PROTECTION H3 occur, as to require a discharge path of no resistance, as any resistance would not allow a sufficient discharge to keep the voltage within safe limits. At the same time the discharge should not occur over a path without a resistance or of very low resistance, except when necessary, since the momentary short Fl$. 28 circuit — that is, the short circuit for a part of the half wave — of a resistanceless discharge is a severe shock on the system, which must be avoided wherever permissible. This type of lightning arrester takes care of single dis- charges and of multiple discharges, no matter how frequently 144 GENERAL LECTURES they occur or how rapidly they follow each other, with the mini- mum possible shock on the system. It cannot take care, how- ever, of continuous lightning — those disturbances, mainly originating in the system, where the voltage remains exces- sive continuously (or rather rises thousands of times per second to excessive values), and for long times. With such a recurring surge, the multi-gap arrester would discharge con- tinuously in protecting the system, until it destroys itself by the excessive power of the continuously succeeding discharges. Where such continuous lightning may occur frequently, as in large high power systems, and the system requires pro- tection against them, a type of lightning arrester which can discharge continuously, at least for a considerable time, with- out self-destruction, is necessary. The only lightning arrester which is capable of doing this, is the electrolytic, or aluminum arrester. In its usual form (cone or disc type) it comprises a series of cone-shaped aluminum cells, connected between line and ground through a spark gap. As soon as the voltage of the system rises above normal, by the value for which the spark gap is set, a discharge takes place through the aluminum cells, over a path of practically no resistance; but the volume of the discharge which passes, is not that given by the voltage on*the system, but is merely that due to the excess voltage over the normal, since the normal voltage is held back by the counter e. m. f. of the aluminum cells. As a result — with strokes following each other, thousands per second, that is, with a recurrent surge — the aluminum arrester discharges continu- ously; but it can stand the continuous discharge for half an hour or more without damage, since it does not carry the short circuit current of the system, but merely the short circuit current of the excess voltage, and so protects the circuit LIGHTNING PROTECTION 145 against continuous lightning for a sufficiently long time, until the cause of the high voltage can be found and eliminated. Even the cone type of aluminum arrester discharges with a slight shock on the system, as the voltage must rise to the value of the spark gap, before the discharge begins, and in systems, in which even a small voltage shock is objectionable, as mainly in large underground cable systems, and also in cases where it is necessary to take care of recurrent surges for an indefinite time, the no-gap aluminum arrester becomes necessary. In principle, this type is the same as the cone type, but the aluminum cells are connected between the conductors and the ground without any spark gap, that is, are continu- ously in circuit, taking a small current. For this reason, the cells are made larger, and of different construction, so as to radiate the heat of the current which, while small, would still give a harmful temperature rise when allowed to accumulate. Being continuously in circuit, a no-gap aluminum arrester allows no sudden voltage rise whatever, however small it may be, that is, it acts just like a flywheel on the engine : while it allows gradual changes of voltages, any sudden change of voltage is anticipated and cut off, just as any sudden change of speed by the flywheel. The no-gap aluminum cell so can hardly be called a lightning arrester, but rather fulfills the duty of a shock absorber, an electrical flywheel on the voltage of the system, and as such finds its proper place on the bus bars of the station or substation, as "surge protector." The three types of apparatus : the no-gap aluminum cell, the aluminum cone arrester, and the multi-gap lightning arrester, then are not different types of apparatus intended for the same purpose, but their operation and proper field of use- fulness is different : the multi-gap arrester protects the system against atmospheric lightning and similar phenomena; the 146 GENERAL LECTURES aluminum cone arrester adds hereto protection against recur- rent surges, where such surges may occur and the system re- quires protection against them, and thus finds its field, but at the same time requiring somewhat more attention than the multi- gap arrester; and the no-gap aluminum cell should be installed as electrical flywheel at the bus bars of the station, and in cable systems, usually in addition to other protection on lines and feeders; it requires, however, occasional attention, and continuously consumes a small amount of power. Of other forms of lightning arresters, the magnetic blow- out 500 volt railway arrester is still in use to a large extent, but is beginning to be superseded by the aluminum cell. The multi-gap, being based on the non-arcing or rectifying prop- erty of the metal cylinders which exists only with alternating current, is not suitable for direct current circuits. In arc light circuits, that is, constant current circuits, horn gap arresters with series resistance are generally used, especially on direct current arc circuits, in which the multi-gap is not permissible. In such circuits of limited current, and very high inductance, the series resistance is not objectionable. Other- wise the horn gap arrester is still occasionally used outdoors as emergency arrester on transmission lines, set for a much higher discharge voltage than the station arrester, and then preferably without series resistance.