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Static discharges in electric circuits
Dec., i9o3.]
Static Discharges ipl Electric Circuits.
ELECTRICAL
433
SECTION.
5tated Meeting, held 3Iay 28, I9o3.
Static Discharges in Electric Circuits. BY PERCY I-I. THOMAS.
( Concluded from page 398. )
Partial Reflection.--There are other i m p o r t a n t cases of reflection in a d d i t i o n to those already treated. Suppose the m e d i u m or wire, i n s t e a d of t e r m i n a t i n g absolutely as a s s u m e d above, c h a n g e s to a n o t h e r " s t i f f e r " m e d i u m or smaller wire in w h i c h the wave can continue. By stiffer m e d i u m is m e a n t one of c o m p a r a t i v e l y larger i n e r t i a and smaller elasticity. It is e v i d e n t t h a t on account of its smaller elasticity or capacity the whole e n e r g y of the wave c a n n o t pass into the smaller section w i t h o u t an increase of intensity, b u t any increase of i n t e n s i t y m u s t cause a portion of the wave to be reflected b a c k w a r d t o w a r d the source ; t h a t is, the original wave will divide into two parts, a portion g o i n g forward into the new m e d i u m , a portion g o i n g backward, and there will be a rise of potential at the reflecting point less t h a n the double potential which is produced by complete reflection. T h e more rigid, t h a t is, the smaller the capacity of the second portion of the line, the larger the portion of the original wave w h i c h will be reflected. I t m u s t be noticed, however, t h a t the portion of the wave passing into the more rigid m e d i u m has as its initial voltage the voltage produced at the reflecting point by the original wave w h i c h is h i g h e r t h a n the latter. As a consesequence, if the wave in the second portion of the line be itself a f t e r w a r d reflected, a rise of p o t e n t i a l m a y occur g r e a t e r t h a n double the original wave. Similarly a still g r e a t e r rise of i n t e n s i t y will occur with the passage of the second wave into a t h i r d m e d i u m more rigid t h a n the second. On the o t h e r hand, if the m e d i u m b r a n c h into two parts, VOL. CLVI.
NO. 936.
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Thomas :
[J. F. I.,
each similar to the original, the e n e r g y of the wave will be divided into two parts a n d the i n t e n s i t y of each reduced. A t the same time there will u s u a l l y be a portion of t h e wave reflected b a c k w a r d w i t h c h a n g e of sign toward t h e souree as well as portions g o i n g forward into the two b r a n c h lines. In o t h e r words, partial refleetion by e h a n g i n g to either a more or less rigid m e d i u m divides the e n e r g y of the wave, r e d u c i n g t h e e n e r g y at a n y one point. If a wave divide into two parts to follow two similar lines of equal length, these partial waves m a y be reflected at the ends of the b r a n c h lines, a n d since t h e two lines are the s a m e length, will r e t u r n at the same time a n d u n i t e t h e i r e n e r g y a t the j u n e t i o n point and send a reinforced wave t o w a r d the generator. Electric W a v e s . - - S o far the waves t r e a t e d h a v e ' been linear waves, such as are confined to t u b e s or wires, b u t have not been l i m i t e d to a n y p a r t i c u l a r form of wave, as light, w a t e r or e l e c t r i c i t y . However, examples have been given more p a r t i c u l a r l y w i t h referenee to eleetrie waves. P r o m this p o i n t on, eleetrie waves only will be eonsidered and electrical terms only will be used. In the electric circuit " e l a s t i c i t y in the m e d i u m " is repr e s e n t e d by capacity. T h e stiffer the m e d i u m the s m a l l e r the capacity. W e i g h t or inertia is r e p r e s e n t e d by i n d u e t a n e e , t h e g r e a t e r the i n d u c t a n c e the g r e a t e r the inertia. T h e q u a n t i t y of static electricity corresponds to the int e n s i t y or d i s p l a c e m e n t in the wave. T h e velocity of m o t i o n of the wave along the line is inversely proportional to the U C L. In air lines this app r o x i m a t e s r o u g h l y the velocity of light, viz., i86,ooo miles per second. On account of the g r e a t velocity of the electrie wave t h e o r d i n a r y g e n e r a t o r frequencies of 25 to Ioo cycles per seeond are not sufficiently rapid to produce a perceptible diff e r e n e e i n time b e t w e e n the action at the g e n e r a t o r and t h e m o t o r end of the line; there is, however, actually, a v e r y small lag. T h a t is, a commercial circuit acts for all praetical power t r a n s m i s s i o n as a rigid t r a n s m i s s i o n system, e.g.,
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a pair of gears. W h e n , however, as b y l i g h t n i n g disturbances, short.circuiting of line wires, or g r o u n d i n g of t h e circuit, a v e r y s u d d e n e h a n g e of potential is produced, complete w a v e s are p r o d u c e d in the line on a c c o u n t of t h e e n o r m o u s f r e q u e n c y of such disturbances, and there is a g r e a t phase lag b e t w e e n the source and the end of the line. A n y of these causes of a b r u p t c h a n g e of potential are likely to be oscillatory, t h a t is, cause an oscillation of potential at the discharge point b a c k w a r d s and forwards for a few periods. T h e n u m b e r of such oscillations would, however, in commercial circuits, p r o b a b l y rarely exceed one-half dozen. T h e s e few, however, will send edrresponding w a v e s into the line, w h i c h m a y be reflected and m a y b u i l d up resonance or nodes and loops as conditions already described m a y determine. It is t h e s e a b r u p t c h a n g e s of potential not directly prod u e e d b y the g e n e r a t o r w h i c h for eommereial circuits are properly d e s i g n a t e d b y the term " S t a t i c Discharges." T h e i r d i s t i n g u i s h i n g characteristics is their e x t r e m e freq u e n c y of alternation, which is so high as to cause several c o m p l e t e w a v e s to be f o r m e d in transmission lines of moderate length. All the laws and properties of linear w a v e s w h i c h h a v e been here considered apply, of course, to these static electrical waves. For, example, static w a v e s have their form and i n t e n s i t y d e t e r m i n e d b y the n a t u r e of their source, t h a t is, b y the sort of c h a n g e of potential at the point of the line where the exciting discharge occurs ; and once formed each w a v e passes d o w n the line w i t h o u t reference to w h a t m a y s u b s e q u e n t l y occur at the source. T h i s w a v e will p r e s e r v e its form and i n t e n s i t y e x c e p t for losses in the medium, h e r e r e p r e s e n t e d b y the wire, or for 6omplete or partial reflection. As before, c o m p l e t e reflection occurs either at an open-ended wire c o r r e s p o n d i n g to an obstacle w h i c h p r o d u c e s as a maxim u m double potential at reflecting points and r e t u r n s the w a v e t o w a r d s the source w i t h o u t c h a n g i n g its sign ; or, b y a short circuit or v e r y large c o n d e n s e r w h i c h p r e v e n t s any rise of potential at the reflecting point, b u t r e t u r n s t h e w a v e t o w a r d s the source with its sign reversed. P a r t i a l
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."
[J. F. I.,
reflection occurs w i t h every c h a n g e in the q u a l i t y of the m e d i u m . T h i s q u a l i t y is m e a s u r e d by the capacity and inductance. If at the reflecting point the c a p a c i t y of the lines be increased or the i n d u c t a n c e be diminished, the r i g i d i t y is decreased and "the wave is p a r t i a l l y reflected; and the portion e n t e r i n g the new part of the line has a less voltage t h a n the original wave, i f the c a p a c i t y be decreased, or t h e i n d u c t a n c e increased, the r i g i d i t y of the m e d i u m is increased, partial reflection will a g a i n occur and the portion of the wave e n t e r i n g the new line will have a h i g h e r potential t h a n the original wave. S t a t i o n a r y nodes and lobps or resonance m a y occur in an electric line as in a n y o t h e r case of linear wave motion, the critical condition to cause t h e i r occurrence b e i n g a n u m b e r of similar waves following one a n o t h e r at u n i f o r m intervals in case of e i t h e r resonance or loops, and in a d d i t i o n exact a d j u s t m e n t of the l e n g t h of the line to the f r e q u e n c y of the a l t e r n a t i n g source in the case of resonance. Effect of Static Waves in Commercial Circuits.--The net results as regards the operation of commercial circuits m a y be indicated quite briefly. W h e n e v e r a line is connected to a live circuit, w h e n e v e r a line is accidentally grounded, w h e n e v e r a short circuit occurs b e t w e e n two lines, or a l i g h t n i n g arrester discharges, or in case of a n y similar disturbanee, there occurs at the point a very a b r u p t c h a n g e of potential e i t h e r up or down. T h e exact a b r u p t n e s s of such a c h a n g e depends upon the local conditions and c o n s t a n t s of t h a t portion of the circuit at which the discharge occurs. In actual cases it p r o b a b l y varies b e t w e e n 1--0-,~1-0-V part and l p a r t of a second for each alternation. It is evi. d e n t t h a t this d i s c h a r g e is e n o r m o u s l y rapid in comparison w i t h 2 5 to ioo cycles per seeo~ad of the g e n e r a t o r frequencies. T h e f r e q u e n c y of I,OOO,OOOperiods per second would give a wave l e n g t h to the static wave in a t r a n s m i s s i o n line of ,approximately I,OOO feet. U s u a l l y the discharges r e s u l t i n g from switching, g r o u n d i n g , etc., are oscillatory, b u t n o t very p e r s i s t e n t ; t h a t is, each oscillation of the d i s c h a r g e is considerably less intense t h a n the previous one, and after perh a p s half a dozen a l t e r n a t i o n s t h e y become too weak to be
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serious. T h i s m e a n s that no very high rises of potential will occur from r e s o n a n c e at these high frequencies, t h o u g h a rise of two or three times t h e original potential m i g h t perhaps s o m e t i m e s occur if conditions were v e r y favorable. Usually, also, t h e r e will be a considerable diminution in t h e intensity of traveling waves, d u e to the fact t h a t in their passage along t h e line t h e y will suffer loss of e n e r g y from resistance and o t h e r losses, so t h a t n e i t h e r will double t h e original potential be p r o d u c e d by reflection, nor will t h e full values of the original w a v e s be a d d e d t o g e t h e r in ease of resonance. T h i s loss of e n e r g y in transmission will b e especially effective in p r e v e n t i n g a rise due to r e s o n a n c e "since any increase in i n t e n s i t y of the w a v e g r e a t l y increases the losses in transmission. T o s u m up, in commercial circuits, as a result of s t a t i c disturbances, m o m e n t a r y rises of potential m a y be e x p e c t e d . more often at the ends of lines or other reflecting points~ usually n o t over 5o per cent. g r e a t e r than the a b r u p t c h a n g e of potential w h i c h caused the w a v e ; b u t i n - s o m e e a s e s where the l e n g t h Of line involved and o t h e r conditions a r e favorable for resonance a rise to two or three times t i l e original v o l t a g e of the source m a y perhaps be found. T h e r e is, however, an additional p h e n o m e n a which, apparently, s o m e w h a t increases the severity o f these disturbances. This results from the h e a v y m o m e n t a r y c h a r g i n g c u r r e n t taken b y a transmission line or o t h e r large capacity, w h e n c h a r g e d or discharged a b r u p t l y ; this c h a r g i n g current t e n d s to b u i l d up m o m e n t a r i l y the v o l t a g e from which the line is b e i n g charged. Its effect is s o m e w h a t the same as w o u l d be experienced if the field s t r e n g t h of the g e n e r a t o r were m o m e n t a r i l y increased. T h i s p h e n o m e n a m a y add perhaps another 5o per cent. to the i n t e n s i t y of the static disturbanee. T h e s e conclusions are b a s e d partly u p o n theoretical considerations, as indicated in the earlier part of the paper, anti, at least as to the e x t e n t to which a b n o r m a l potentials m a y be expected in commercial circuits, partly u p o n a considerable n u m b e r of actual tests w h i c h have b e e n m a d e b y the w r i t e r in c o n j u n c t i o n with the engineers of a n u m b e r of large installations for the purpose of d e t e r m i n i n g t h e
438
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[J. F. I.,
practical effect of such disturbances. T h e s e tests h a v e ext e n d e d over a considerable range of time and d i v e r s i t y of l o c a t i o n and h a v e included widely different v o l t a g e s and different l e n g t h s of line r u n n i n g to over too miles. In all cases the results o b t a i n e d have been in accordance with the conclusions above. L i g h t n h t g . - - T h e d i s t u r b a n c e s p r o d u c e d b y l i g h t n i n g have their m a x i m u m i n t e n s i t y d e t e r m i n e d b y different conditions from the w a v e s p r o d u c e d b y internal d i s t u r b a n c e s in the transmission s y s t e m itself. T h e former are e v i d e n t l y determ i n e d b y the i n t e n s i t y and p r o x i m i t y of the l i g h t n i n g flash and the protective apparatus, while the latter are d e t e r m i n e d directly b y the g e n e r a t o r voltage. Ordinarily the actual line d i s t u r b a n c e s p r o d u c e d b y l i g h t n i n g are not e x t r e m e l y severe, and with efficient arresters on high tension circuits m a y be considered as not e x c e e d i n g those due to the other c a u s e s of static disturbance. In exceptional cases, however, w h e r e lines are directly struck b y l i g h t n i n g discharges of g r e a t e r or less m a g n i t u d e , there is an e x t r e m e rise of potential locally on the transmission line which is b e y o n d the p o w e r of a n y commercial arrester to prevent. Fortunately, however, this d i s c h a r g e seeks the earth b y the s h o r t e s t path a n d will u s u a l l y choose one or m o r e line poles in the immed i a t e vicinity of the discharge. Such poles will s o m e t i m e s be chipped or split and s o m e t i m e s c o m p l e t e l y splintered. T h i s discharge m a y or m a y n o t cause a short circuit on the l i n e ; f r e q u e n t l y will not. A l t h o u g h the main discharge r e a c h e s g r o u n d in all p r o b a b i l i t y close b y the point struck, t h e r e will, of necessity, as already explained, be w a v e s sent in b o t h directions d o w n the line wires w h i c h will h a v e an e x t r e m e m a x i m u m intensity at the start and which m a y do serious d a m a g e at the stations ; b u t which are of a magnit u d e that m a y properly be handled b y protective apparatus. T h u s , t h o u g h the direct violence of the stroke cannot be g u a r d e d a g a i n s t or resisted, yet, except in the v e r y rare case w h e r e such discharge is in the i m m e d i a t e n e i g h b o r h o o d of t h e station, very often no h a r m will result, p r o v i d e d the station a p p a r a t u s be p r o t e c t e d b y efficient arresters. Sources o f Static I4Zaves.--The c o m m o n sources of static
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d i s t u r b a n c e with the effects produced are s u m m a r i z e d below. (i) Charging a dead line from a live t r a n s f o r m e r not alr e a d y s u p p l y i n g other lines, m o m e n t a r i l y drops the potential of the t r a n s f o r m e r lead to zero ; the g e n e r a t o r then supplies c h a r g i n g c u r r e n t at a c o m p a r a t i v e l y slow rate to b r i n g the line to normal potential. In this ease usually no wave will be produced, as t h e f r e q u e n c y is c o m p a r a t i v e l y low. T h e r e may, however, result a certain m o m e n t a r y rise of p o t e n t i a l (2) W h e n a dead transmission line is c h a r g e d from a syst e m h a v i n g a large q u a n t i t y of electricity already stored in o t h e r lines c o n n e c t e d to the same high tension b u s bars, the potential of the n e w line is very s u d d e n l y raised to the potential of the system, and this s e n d s an a b r u p t w a v e of positive sign along the line. (3) G r o u n d i n g or short-eircuiting a e h a r g e d line causes a s u d d e n fall of potential, which sends a w a v e of n e g a t i v e sign along the line in b o t h directions. T h i s w a v e e v i d e n t l y c a u s e s no rise of potential at any point and is h a r m l e s s exc e p t for a p h e n o m e n a to be discussed later, u n d e r the head of Local Concentration. (4) O p e n i n g a loaded line causes no rapid c h a n g e of potential, since the load eurrent produces a large v o l u m e of h e a t e d air, g r a d u a l l y e u t t i n g d o w n the potential of the line. (5) O p e n i n g an u n l o a d e d line has much the same effect as c h a r g i n g a line, since in o p e n i n g the switch there will u s u a l l y be a rapid charge and discharge of the line for a few alternations. T h a t is, the i n s t a n t after the line is free from the switch for the first time, the next alternation (being of opposite sign) will f r e q u e n t l y be able to j u m p the gap and recharge the line momentarily, t h u s c a u s i n g a static spark ; t h a t is, positive w a v e s will b e sent d o w n t h e line. F o r t u n a t e l y t h e s e potential rises are m o m e n t a r y and do n o t p r o d u c e as severe a strain u p o n solid i n s u l a t i n g material as similar potentials of long-continued duration. T h i s res u l t s p r o b a b l y from the h e a t and mechanical inertia of the i n s u l a t i n g material. Therefore, solid i n s u l a t i n g material w h i c h is sufficiently strong to w i t h s t a n d the strains of nor-
440
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[J. F. 1.,
mal operation will often not be punctured, even t h o u g h it m a y be s u b j e c t e d to double normal v o l t a g e b y t h e s e static strains ; on the other hand, air distances s u b j e c t e d to such strains, including " s u r f a c e d i s t a n c e s " so called, can readily break down i n s t a n t l y from static strains, and, indeed, especially surface distances, m a y be and are more sensitive to static than o t h e r strains. It is necessary, therefore, in plann i n g a high tension s y s t e m to provide a v e r y wide margin for air spaces at all exposed points and long surface distances b e t w e e n live parts to insure s a f e t y from static disturbances. On the o t h e r hand, v e r y little increase is r e q u i r e d in the actual thickness of solid material over that r e q u i r e d to stand normal strains. This conclusion m u s t be taken in conjunction with the discussion b e l o w u n d e r the head of " L o c a l Concentration of Potential." Most engineers do not appreciate the i m p o r t a n c e of this caution. Once a plant is installed with air distances too small, these will be an occasional cause of s h u t - d o w n and loss until remedied. In planning insulation distances it must be r e m e m b e r e d that the striking distance of high v o l t a g e s from 20,00o to i0o,0oo volts increases m u c h more rapidly than the voltage, and that c o n s e q u e n t l y for the h i g h e r v o l t a g e s m u c h m o r e than p r o p o r t i o n a t e spaces m u s t be allowed. T h i s phenomenon is one of the reasons w h y insulators for v e r y high v o l t a g e s have to be excessively large in c o m p a r i s o n with insulators for perhaps lo,ooo to I5,ooo volts. T h e disadvantage to the insulator m a n u f a c t u r e is f u r t h e r increased b y the fact that the w e i g h t and material of the insulator increase s o m e w h e r e nearly according to the cube of the linear dimensions, while the insulation s t r e n g t h increases m o r e slowly than the first power. It is of course c u s t o m a r y and desirable to p r e v e n t as far as possible an a b n o r m a l rise of t e m p e r a t u r e a b o v e the e a r t h b y m e a n s of suitable l i g h t n i n g arresters. H o w e v e r , it is evid e n t that l i g h t n i n g arresters cannot be placed at all p o i n t s of the s y s t e m w h e r e a rise of potential m a y be p r o d u c e d b y static disturbances. T h e y should usually be placed, however, where i m p o r t a n t a p p a r a t u s is to be protected. A t
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other points ample insulation distances, as outlined in t h e p a r a g r a p h above, m u s t be provided. Local Concentration of PotentiaL--More serious than t h e m o m e n t a r y rise of potential j u s t discussed is the strain produced upon coils c o n n e c t e d to the high.tension circuits by static waves. It is well k n o w n that v e r y a b r u p t w a v e s or discharges pass only with g r e a t difficulty and slowly t h r o u g h choke coils, and that a static discharge will j u m p t h r o u g h considerable insulation r a t h e r t h a n follow around the turns of a po,~erful choke eoil. T h e h i g h tension coils of t r a n s f o r m e r s form e x t r e m e l y p o w e r f u l choke coils and are s u b j e c t e d to v e r y severe strains b e t w e e n the t u r n s or layers when static w a v e s reach their terminals. T h e r e is a n o t h e r w a y of looking at this p h e n o m e n o n : W h e n a s t a t i e w a v e reaches a t r a n s f o r m e r coil, on account of the e n o r m o u s l y increased i n d u c t a n c e and eapacity of this w i n d i n g over the line, its speed is g r e a t l y reduced. T h i s m e a n s that w h e n the b e g i n n i n g of the w a v e has r e a c h e d a point, perhaps one-third of the w a y to the end of the coil, that the crest of the w a v e has reached the end of the terminal of the coil. Therefore, a difference of potential exists b e t w e e n the terminal and a point one-third the w a y t h r o u g h equal to the full p o t e n t i a l of the s t a t i c wave. T h i s w a v e m a y be even g r e a t e r t h a n full-line potential, and the high-tension coil referred to m a y be one of a n u m b e r w h i c h normally divide the full-line p o t e n t i a l between them. C o n s e q u e n t l y the m o m e n t a r y strains upor~ the o u t e r parts of the coil m a y be a g r e a t m a n y times t h e normal v o l t a g e i m p r e s s e d b y the generator. If the insulation b e t w e e n turns of the coil be not e x t r e m e l y good a spark will pass b e t w e e n them. T h i s spark tends directly only to pass the e n e r g y of the static wave, w h i c h is v e r y small in actual a m o u n t and almost harmless. U n f o r t u nately, however, there m a y exist at this time a v o l t a g e b e t w e e n the turns b e t w e e n w h i c h the spark passes, maintained b y the generator, which, t h o u g h m u c h smaller than the static voltage, is still able w h e n once the insulation is broken down b y the latter, to start and m a i n t a i n an arc. A l t h o u g h the static spark of itself was nearly harmless, the
442
T/wmas :
[J. F. I.,
arc m a i n t a i n e d by the generator, if once started, will very likely d e s t r o y the coil. In ease, however, t h a t the n o r m a l voltage b e t w e e n layers or t u r n s is very small, or in ease t h e static spark occurs at an i n s t a n t of time when the voltage of the g e n e r a t o r is n e a r l y zero, or, perhaps, u n d e r o t h e r conditions, the g e n e r a t o r electromotive force will n o t be able to s t a r t a local arc. F o r t u n a t e l y , this will u s u a l l y be the case. However, a l t h o u g h each static spark w h i c h m a y pass b e t w e e n t u r n s is in itself not p a r t i c u l a r l y h a r m f u l , y e t if for a n y reason a large n u m b e r of such sl~arks pass in the same spot, a final deterioration of the i n s u l a t i o n will result and a short circuit ensue at a later time, which m a y finally occur a p p a r e n t l y w i t h o u t a n y connection w i t h a n y static disturbance. As a m a t t e r of fact, static d i s t u r b a n c e s are very difficult to trace out in their effects w i t h i n transformers.
It is e v i d e n t t h a t the same s t r a i n b e t w e e n turns in a coil will be produced by a n e g a t i v e wave as by a p o s i t i v e - if it be as abrupt. T h a t is, a s u d d e n d r o p p i n g to zero of the potential of the t r a n s f o r m e r t e r m i n a l produces the same s t r a i n as s u d d e n l y c h a r g i n g it to the same a m o u n t . T h i s local c o n c e n t r a t i o n of potential is n e i t h e r an unc o m m o n nor an u n c e r t a i n p h e n o m e n o n . It m a y be r e p r o d u e e d at a n y t i m e in a l a b o r a t o r y or factory, or at a powerhouse, by a simple experiment, and u n d o u b t e d l y occurs in c o m m e r c i a l operation of plants. It is necessary only to cause a static d i s t u r b a n c e near the t e r m i n a l of a coil a n d m e a s u r e by a spark gap the m o m e n t a r y voltage between the lead and points a portion of the w a y t h r o u g h the coil. Static waves are of g r e a t i m p o r t a n c e to the o p e r a t i n g man, however, only on very h i g h voltages. It is e v i d e n t t h a t the condition w h i c h causes the local c o n c e n t r a t i o n of strain t o be e x t r e m e l y severe is the abruptness of the static wave. T h e more a b r u p t the wave, t h e less distance will it have p e n e t r a t e d a t r a n s f o r m e r coil before the t e r m i n a l has a t t a i n e d the full p o t e n t i a l of the wave. T h i s sort of strain is a n a l o g o u s to the b r e a k i n g of a plastic b o d y by a very s u d d e n blow. Protection o f Circuits and A p p a r a t u s . - - P r o t e c t i o n of two
Dec., 19o3.]
Static Discharges in Electric Circuits.
443
t y p e s is r e q u i r e d : (I) L i g h t n i n g arresters to protect a g a i n s t a b n o r m a l rise of potential a b o v e the earth and the low tension circuits; (2) protection against local concentration of potential. Grounds.--There are a n u m b e r of ways, theoretically, in which static w a v e s can be destroyed, or r a t h e r diffused, and p r e v e n t e d from d o i n g serious injury. (I) T h e introduction of a choke coil in the line will divide a static w a v e in two parts, one portion p a s s i n g on, the o t h e r b e i n g reflected b a c k ; the proportions of these parts for any given w a v e b e i n g d e t e r m i n e d b y the size of the choke coil. If a number of these choke coils were placed at a n u m b e r of points in the line so that reflection w o u l d occur several times, the w a v e w o u l d be d i s t r i b u t e d in a n u m b e r of parts, each of which w o u l d act alone and be unable to do damage. However, this m e t h o d is i m p r a c t i c a b l e b e c a u s e each different static w a v e l e n g t h w o u l d require a different choke coil to divide the static w a v e properly, and b e c a u s e such choke coils w o u l d seriously interfere w i t h the normal operation of the system. (2) S h o r t b r a n c h lines w o u l d be excellent, as each w o u l d s u b t r a c t a portion of the e n e r g y of the original wave, which w o u l d soon be r e d u c e d to a harmless m a g n i t u d e . T h i s m e t h o d is of d o u b t f u l effectiveness, however, since the l e n g t h of line necessary to actually w i t h d r a w e n e r g y from the original w a v e m u s t be g r e a t e r than the w a v e l e n g t h of the static discharge. T h i s length will v a r y greatly and m i g h t be so g r e a t as to render the cost of such branch lines prohibitory. S u c h lines m u s t e v i d e n t l y be taken from some point of the main line, other than a reflecting point, or, as a l r e a d y explained, t h e y will t h e m s e l v e s experience an abn o r m a l rise of potential. It is p r o b a b l e t h a t branch lines a n d c h a n g e s in the size or n u m b e r of line wires (which has m u c h the s a m e effect) do help in m a n y instances to r e d u c e the severity of waves, b u t it is the opinion of the a u t h o r that it is hardly feasible to devise such an a r r a n g e m e n t with reasonable expense w h i c h can be relied u p o n to dissip a t e w a v e s in all cases. Both of the a b o v e m e t h o d s fail also in cases where static
444
T h o m a s ."
EJ. F. I.,
w a v e s originate near a p p a r a t u s and d.o n o t pass t h r o u g h any considerable l e n g t h of line before h a v i n g an Opportunity to do d a m a g e . (3) L i g h t n i n g Arresters. T h e only m e t h o d in practical u s e at the present time is the e m p l o y m e n t 0f l i g h t n i n g arresters which are merely o p p o r t u n i t i e s for the line to d i s c h a r g e to g r o u n d w h e n r e a c h i n g a p o t e n t i a l sufficiently high a b o v e normal. T h e y m a y be m a d e q u i t e s a t i s f a c t o r y for a c c o m p l i s h i n g this p u r p o s e at t h e particular p o i n t of the s y s t e m to w h i c h t h e y are connected, provided two s e c o n d a r y effects are neglected. (i) T h e arcing over the arrester following a discharge, which has been f r e q u e n t l y discussed, and (2) t h e local concentration of potential r e s u l t i n g from the e x t r e m e l y a b r u p t n e g a t i v e w a v e caused b y the r e d u c t i o n to e a r t h potential of the circuit a t the arrester. V a r i o u s w a y s are proposed and used for the i n t e r r u p t i o n of the are in t h e arrester, all of w h i c h hinder the original function of the a t t e s t e r to some e x t e n t ; that is, the free d i s c h a r g e of the • line. In high tension work the m o s t successful m e a n s is series resistance in the discharge p a t h of the arrester. By p r e v e n t i n g any considerable flow of c u r r e n t from the generator this series resistance m a y be m a d e to render a l m o s t a n y l i g h t n i n g a t t e s t e r non-arcing. E v e r y available m e a n s , however, should be u s e d to r e d u c e the a m o u n t of s e r i e s resistance to a m i n i m u m , as it m a y seriously i m p e d e t h e discharge of the line. T h e use of non-arcing m e t a l and a large n u m b e r of small gaps in place of one large gap, as is done in nearly all commercial arresters, m a k e s a v e r y considerable gain in this direction. A n o t h e r m e a n s for still f u r t h e r r e d u c i n g the series resistance will be briefly described u n d e r the head of the low e q u i v a l e n t arrester. T h e local concentration of potential p r o d u c e d b y t h e l i g h t n i n g a t t e s t e r discharge is not different from a similar effect d u e to o t h e r causes except t h a t it is p e r h a p s m o r e severe since closer to the apparatus. It needs no s e p a r a t e consideration. S h o r t C i r c u i t s . - - T h e p r e v e n t i o n of d a n g e r o u s local concentration of potential m a y be a c c o m p l i s h e d b y several
Dec., I9o3.]
Static Discharges in Electric £Trcuits.
445
methods. (I) By i n s u l a t i n g windings, especially near the terminals, sufficiently h e a v y to w i t h s t a n d the strains produeed b y static w a v e s in the partieular eireuits in whieh this a p p a r a t u s is to be used. T h i s m e t h o d is v e r y satisfaetory in its simplicity, and is entirely feasible for m o d e r a t e l y high voltages. It is the m e t h o d u s u a l l y e m p l o y e d for this purpose. (2) W i n d i n g s m a y be so arranged, for example, b y winding one turn per layer, as to v e r y g r e a t l y r e d u c e the statie p o t e n t i a l b e t w e e n layers and also the normal v o l t a g e bet w e e n layers, a n d t h u s v e r y m u c h r e d u c e the p r o b a b i l i t y of the f o r m a t i o n of a local are in ease of a static spark. T h i s a r r a n g e m e n t of w i n d i n g s extends t h e range of v o l t a g e for w h i c h insulation can be used commercially sufficient to stand static strains, b u t is i n a d e q u a t e in the h i g h e s t voltages for the protection of the insulation b e t w e e n the coils or section of coils which are t h u s wound. A more c o m p l e t e method, a m e t h o d of p r e v e n t i o n r a t h e r t h a n resistance, w h i c h is available for h i g h e r v o l t a g e s than those already mentioned, is t h e use of the static interrupter, which is s u b s t a n t i a l l y a magnified choke coil. Its function is to so delay the static w a v e in its e n t r y into the transformer coil that a considerable portion of the l a t t e r will become c h a r g e d ' b e f o r e the terminal will have reached full potential. P r o t e c t i o n b y a choke coil will first be discussed. If a v e r y heavily i n s u l a t e d p o w e r f u l choke coil be placed in the lead of the transformer, w h e n a static w a v e approaches electricity will b e g i n to pass in small q u a n t i t y and will pass in g r a d u a l l y increasing q u a n t i t y at later instants of time, so t h a t the coil will be, c o m p a r a t i v e l y speaking, g r a d u a l l y b r o u g h t to full potential; m e a n w h i l e the v o l u m e of the static wave is b e i n g reflected and choked back and p e r h a p s b e i n g discharged to the g r o u n d if there be a l i g h t n i n g atrester near. It is e v i d e n t that this choke coil, to be effective, m u s t be so p r o p o r t i o n e d as to delay the i n c o m i n g w a v e e n o u g h so t h a t the portion of the w i n d i n g which has b e c o m e c h a r g e d w h e n full potential is reached at the terminal shall be sufficient to w i t h s t a n d the strain of the full voltage of the wave. It is e v i d e n t that such a d j u s t m e n t
446
T/zomas
[J. F. I.,
."
does not d e p e n d directly on t h e f r e q e n e y or a b r u p t n e s s of the static wave, since b o t h the t r a n s f o r m e r and the ehoke coil are similarly affected b y t h e frequency. But a choke coil sufficiently powerful to accomplish this
Transl. c C C O C c C 0 0 C 0
[
Choke Coil
Line
b
I C3 0 0 0 0
qTG'5~ ~ Lt.Arr. ~ Choke Coil
Lt. Arr.
t Line
FIG. 3.--Static interrupter protecting transformer.
result satisfactorily is found to be i m p r a c t i c a b l e on v e r y high potential circuits on a c c o u n t of t h e size, cost and interference with the operation of t h e system. H o w e v e r , if t h e a r r a n g e m e n t of the static i n t e r r u p t e r be used, t h a t is, if a Choke Coil =
r
Cond.
Line
~ Lt. Arr.
T Cond. Choke Coil
I1"'
~Lt. Arr.
Line
FIG, 4.--Static interrupter protecting high-tension generator.
c o n d e n s e r be c o n n e c t e d b e t w e e n line and g r o u n d b e h i n d the choke coil nearer t h e a p p a r a t u s to be protected, this choke coil will a b s o r b a considerable portion of the c u r r e n t actually passed b y the choke coil, and the time required to pass
Dec., 19o3.]
Static Dischargex in Electric Circuits.
sufficient electricity to charge the terminal will be increase. W i t h this a r r a n g e m e n t a c o m p a r a t i v e l y choke coil m a y be used. T h e c o n d e n s e r has a v e r y electro-static capacity, and has no appreciable effect normal operation, and y e t has a v e r y "o powerful effect on the static w a v e on acc o u n t of its e x t r e m e l y high frequency. ot A s in the case of the choke coil, the static O i n t e r r u p t e r m u s t be r o u g h l y proportioned to the t r a n s f o r m e r winding to be proteeted. T h e condenser m u s t also be 0¢ suitable for the v o l t a g e b e t w e e n line and ground. If static i n t e r r u p t e r s be placed in each lead of high tension a p p a r a t u s which m a y be i n j u r e d b y local concentration of potential, its w i n d i n g s will be a m p l y p r o t e c t e d a g a i n s t d a n g e r of short circuits from static w a v e either positive 0 or negative. Such an a r r a n g e m e n t is ~ 0 s h o w n d i a g r a m m a t i c a l l y in connection with a t r a n s f o r m e r and a high tension g e n e r a t o r in Etgs. 3 and 4. A large n u m b e r of these interrupters ..c have b e e n m a n u f a c t u r e d and are in ac- c o O tual service. Static Interrupters and Low Equivalent Lightning Arresters.--A short description of the salient features of some actual l i g h t n i n g arresters and static interrupters d e s i g n e d b y the a u t h o r for the purpose of p r o t e c t i n g a p p a r a t u s from static ~ 0 d i s t u r b a n c e s will be given. .o_ 0 The low equivalent A.C. lightning attester ~o 0 consists of a n u m b e r of ~7-inch air gaps i 0 b e t w e e n non-arcing m e t a l cylinders in series with non-inductive resistance. A portion of the resistance, ealled s h u n t resistance, is s h u n t e d b y a second set of air
447 much small small upon
i
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?5 u~
T/wmas :
448
[J. F. 1.,
:gaps called s h u n t e d gaps. T h e o b j e c t of this a r r a n g e m e n t is to r e d u c e the a m o u n t of t he series r e s i s t a n c e t h r o u g h wh ich th e d i s c h a r g e m u s t pass to g r o u n d . T h i s a r r e s t e r is d i a g r a m m a t i c a l l y i l l u s t r a t e d in F i g . 5. T h e a p p a r a t u s is s h o w n in Fig. 6. T h e series ga ps w i t h h o l d t he line v o l t a g e and are chosen so as to b r e a k dow n at s o m e t h i n g b e t w e e n 50 per cent. and Ioo p er cent. rise of v o l t a g e a b o v e t h e e a r t h . A p o r t i o n of th e series r e s i s t a n c e is s h u n t e d b y gaps so t h a t the static d i s c h a r g e can pass a r o u n d this p o r t i o n , t h u s a v o i d i n g i t s resistance. It e v i d e n t l y is t h e n n e c e s s a r y to s u p p r e s s the are f r o m th e g e n e r a t o r w h i c h t e n d s to follow t h r o u g h t he s h u n t e d gaps. It is f o u n d t h a t w i t h a n u m b e r of s h u n t e d g ap s eq u al to t he series gaps, t he arc will be w i t h d r a w n from th e s h u n t e d .gaps b y t he s h u n t r e s i s t a n c e w h e n the
FIG.
6.--Low equivalent A. C. lightning arrester.
s h u n t r e s i s t a n c e does n o t e x c e e d a p r o p e r value, which is a c o n s i d e r a b l e p o r t i o n of t h e t ot a l r e s i s t a n c e in t h e arrester. A v e r y m a r k e d ga i n in t he r e d u c t i o n of t h e r e s i s t a n c e offered to t h e d i s c h a r g e is t h e r e f o r e m a d e b y m e a n s of t he use of s h u n t e d g ap s and s h u n t r e s i s t a n c e . It m u s t be n o t e d as well t h a t no m o r e v o l t a g e is r e q u i r e d to cause a rise of pot e n t i a l to j u m p ove r all t h e gaps series and s h u n t than w o u l d be r e q u i r e d to j u m p t he series gaps alone, since on a c c o u n t of th e s h u n t r e s i s t a n c e t he series and s h u n t e d gaps a r e b r o k e n d o w n s e p a r a t e l y one a f t e r t he other. A static interrupter, see Fig. 7, consists of a choke coil in series w i t h the line and a c o n d e n s e r c o n n e c t e d bet w e e n line a n d g r o u n d on t he a p p a r a t u s side of t h e choke coil. Since i n t e r r u p t e r s are u s e d only on h i g h v o l t a g e plants, coil and c o n d e n s e r are pl a c e d in oil tanks w h i c h are m a d e self-cooling. T h e coils are e x t r e m e l y h e a v i l y insu-
Dec., I9O3.]
Static Disc/targes hi Electric Circuits.
449
lated. T h e condensers are built up of specially prepared i n s u l a t i n g paper and h a v e r e c t a n g u l a r copper plates w i t h r o u n d e d edges and corners. T h e r e is a considerable a m o u n t of h e a t g e n e r a t e d in condensers which is a difficult m a t t e r to dissipate on account of the poor h e a t - c o n d u c t i n g q u a l i t y of the i n s u l a t i n g material. T h e s e condensers are all provided with v e n t i l a t i n g ducts w h i c h allow a passage of oil from the b o t t o m u p w a r d s at f r e q u e n t intervals to the interior.
FiG. 7.--Static intrrrupter, showiug coil and condenser oat of case.
T h i s condenser loss is q u i t e an i n t e r e s t i n g p h e n o m e n o n . Its a m o u n t increases as the square of the voltage upon the condenser, roughly, and is very rapidly increased w i t h increase of t e m p e r a t u r e . T h e loss increases several ttmes in a m o u n t between 5°0 C. and Ico ° C. T h i s loss also depends u p o n the f r e q u e n c y of the circuit and for the kind of material used ; for the dielectric in these condensers it increases a p p r o x i m a t e l y as the square root of the frequency. Cables.--The h i g h tension electric cable is in principle no different from the electric air line, b u t has a different insul a t i n g material, paper or r u b b e r in place of air. T h i s has VOL. C L V I . . N o . 936.
29
450
7homas :
[J. F. I.,
the double effect of increasing its electrostatic c a p a c i t y and c h a n g i n g t h e velocity at which w a v e s progress. T h e increased electrostatic c a p a c i t y t e n d s to decrease the speed, b u t as the i n d u c t a n c e of the cable is small this partially c o m p e n s a t e s for the increased capacity. T h e differences from the air line are differences in degree only and do not affect the p a s s a g e of waves, reflection, resonance, etc., as already discussed. Consequently, no p h e n o m e n a different from the air lines m a y be expected as a result of static dis turbances. Since the cable contains no coils of wire, no local con. eentration of potential will be f o u n d like that in transformer coils, and there is no occasion for the use of a static interrupter. C o n c l u s i o n . - - T h e r e is in all probabilit'y n o t h i n g m y s t e r i o u s in static w a v e s in electric circuits and n o t h i n g r e q u i r i n g for its explanation a n y n e w theories, and y e t t h e r e is no s u b j e c t c o n n e c t e d with commercial p o w e r transmission which is so little u n d e r s t o o d b y those w h o h a v e n o t given it special attention, and in which it is so difficult to explain results. T h e difficulty arises not so m u c h f r o m intrinsic blindness in the s u b j e c t as from the g r e a t difficulty of m e a s u r i n g and t r a c i n g these e x t r e m e l y rapid and invisible waves. In a general w a y it m u s t be expected t h a t the s u r g i n g a b o u t of the e n e r g y which is stored w h e n e v e r a line is c h a r g e d will cause increased potential at certain points. T h i s should be p r o v i d e d for b y placing suitable l i g h t n i n g arresters at all points w h e r e i m p o r t a n t and v u l n e r a b l e appar a t u s is located. T h e r e will also be local c o n c e n t r a t i o n of potential in w i n d i n g s c o n n e c t e d to the circuits as the result of all static disturbances. Such t r a n s f o r m e r or o t h e r coils should always be either sufficiently i n s u l a t e d or p r o t e c t e d b y choke coils or static i n t e r r u p t e r s or b y some o t h e r suitable method. T h e a p p a r a t u s s h o w n in connection with this lecture s h o w s one a r r a n g e m e n t designed b y the a u t h o r for protection of circuits and a p p a r a t u s from static disturbances. It is h i g h l y desirable that this s u b j e c t should receive as
Dec., 19o 3. I
Notes and Comments.
45I
much discussion as possible, and that further experiments should be tried and observations made by engineers and superintendents of power plants, and that all work together to determine the best methods.of protection against this troublesome and elusive source of interruption of service and loss of apparatus.
PRODUCTION OF L E A D AND L E A D OXIDt~S. It is important to note that while there has been a large increase in the consumption of lead in the United States during the last three years, the production o[ domestic mines remains at a standstill at about 28o,ooo tons. The additional quantity required by the home demand has not been, as might have been supposed, aulyplied by lead from Mexico and other foreign ores smelted and refined in this country. This lead, which is refined here in bond, has beet, almost entirely re-exported, the entries for consumption at.home having been very small. The fact is that three years ago production had considerably outrun the demand then current, and the stocks carried by producers were very heavy. The increased demand has enabled producers to work off these stocks, and to reduce their holdings to normal amount,-so that at the present time they are really low. The increase in consumption during the last three years has come partly from the great activity in building operations, which has required the use of large quantities of lead, partly f r o m the increase in the demand for sheet lead for insulating cables ancl similar electric work, and partly from the demand for lead for use in various alloys. The production of red lead, litharge and orange mineral has shown no very important increase during the last three years; nor has the production:of white lead, although there was a considerable increase in the make in 19o2. White lead has certainly suffered from the competition of other white pigmeuts. If the improved Dutch processes that are now attracting so much attention., result in such a decrease in the cost of manufacture asis anticipated, white lead may be able to regain its lost g r o u n d . - - E n g , and M'ining./ournal.
P R I Z E S OF T H E SOCII~TE I N D U S T R I E L L E D E MULHOUSE. We have received a copy of the program of prizes to be awarded in 19o4 by the Soci6t6 Industrielle de Mulhouse. There are no fewer than fiftysix competitions concerned with chemical technology, more than twenty dealing with the mechanical arts, and twelve with natural history and agriculture. Several prizes are offered with the object of improving and stimulating local industries. The program also contains full particulars of several large prizes of $,oeo francs, which are awarded for scientific work at intervals of in some cases ten, and in others five years. The present publication takes the place of all previous issues, and copies of the program, in which certain changes have been made, can be obtained on application to the secretary of the society.
Static Discharges ipl Electric Circuits.
ELECTRICAL
433
SECTION.
5tated Meeting, held 3Iay 28, I9o3.
Static Discharges in Electric Circuits. BY PERCY I-I. THOMAS.
( Concluded from page 398. )
Partial Reflection.--There are other i m p o r t a n t cases of reflection in a d d i t i o n to those already treated. Suppose the m e d i u m or wire, i n s t e a d of t e r m i n a t i n g absolutely as a s s u m e d above, c h a n g e s to a n o t h e r " s t i f f e r " m e d i u m or smaller wire in w h i c h the wave can continue. By stiffer m e d i u m is m e a n t one of c o m p a r a t i v e l y larger i n e r t i a and smaller elasticity. It is e v i d e n t t h a t on account of its smaller elasticity or capacity the whole e n e r g y of the wave c a n n o t pass into the smaller section w i t h o u t an increase of intensity, b u t any increase of i n t e n s i t y m u s t cause a portion of the wave to be reflected b a c k w a r d t o w a r d the source ; t h a t is, the original wave will divide into two parts, a portion g o i n g forward into the new m e d i u m , a portion g o i n g backward, and there will be a rise of potential at the reflecting point less t h a n the double potential which is produced by complete reflection. T h e more rigid, t h a t is, the smaller the capacity of the second portion of the line, the larger the portion of the original wave w h i c h will be reflected. I t m u s t be noticed, however, t h a t the portion of the wave passing into the more rigid m e d i u m has as its initial voltage the voltage produced at the reflecting point by the original wave w h i c h is h i g h e r t h a n the latter. As a consesequence, if the wave in the second portion of the line be itself a f t e r w a r d reflected, a rise of p o t e n t i a l m a y occur g r e a t e r t h a n double the original wave. Similarly a still g r e a t e r rise of i n t e n s i t y will occur with the passage of the second wave into a t h i r d m e d i u m more rigid t h a n the second. On the o t h e r hand, if the m e d i u m b r a n c h into two parts, VOL. CLVI.
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each similar to the original, the e n e r g y of the wave will be divided into two parts a n d the i n t e n s i t y of each reduced. A t the same time there will u s u a l l y be a portion of t h e wave reflected b a c k w a r d w i t h c h a n g e of sign toward t h e souree as well as portions g o i n g forward into the two b r a n c h lines. In o t h e r words, partial refleetion by e h a n g i n g to either a more or less rigid m e d i u m divides the e n e r g y of the wave, r e d u c i n g t h e e n e r g y at a n y one point. If a wave divide into two parts to follow two similar lines of equal length, these partial waves m a y be reflected at the ends of the b r a n c h lines, a n d since t h e two lines are the s a m e length, will r e t u r n at the same time a n d u n i t e t h e i r e n e r g y a t the j u n e t i o n point and send a reinforced wave t o w a r d the generator. Electric W a v e s . - - S o far the waves t r e a t e d h a v e ' been linear waves, such as are confined to t u b e s or wires, b u t have not been l i m i t e d to a n y p a r t i c u l a r form of wave, as light, w a t e r or e l e c t r i c i t y . However, examples have been given more p a r t i c u l a r l y w i t h referenee to eleetrie waves. P r o m this p o i n t on, eleetrie waves only will be eonsidered and electrical terms only will be used. In the electric circuit " e l a s t i c i t y in the m e d i u m " is repr e s e n t e d by capacity. T h e stiffer the m e d i u m the s m a l l e r the capacity. W e i g h t or inertia is r e p r e s e n t e d by i n d u e t a n e e , t h e g r e a t e r the i n d u c t a n c e the g r e a t e r the inertia. T h e q u a n t i t y of static electricity corresponds to the int e n s i t y or d i s p l a c e m e n t in the wave. T h e velocity of m o t i o n of the wave along the line is inversely proportional to the U C L. In air lines this app r o x i m a t e s r o u g h l y the velocity of light, viz., i86,ooo miles per second. On account of the g r e a t velocity of the electrie wave t h e o r d i n a r y g e n e r a t o r frequencies of 25 to Ioo cycles per seeond are not sufficiently rapid to produce a perceptible diff e r e n e e i n time b e t w e e n the action at the g e n e r a t o r and t h e m o t o r end of the line; there is, however, actually, a v e r y small lag. T h a t is, a commercial circuit acts for all praetical power t r a n s m i s s i o n as a rigid t r a n s m i s s i o n system, e.g.,
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a pair of gears. W h e n , however, as b y l i g h t n i n g disturbances, short.circuiting of line wires, or g r o u n d i n g of t h e circuit, a v e r y s u d d e n e h a n g e of potential is produced, complete w a v e s are p r o d u c e d in the line on a c c o u n t of t h e e n o r m o u s f r e q u e n c y of such disturbances, and there is a g r e a t phase lag b e t w e e n the source and the end of the line. A n y of these causes of a b r u p t c h a n g e of potential are likely to be oscillatory, t h a t is, cause an oscillation of potential at the discharge point b a c k w a r d s and forwards for a few periods. T h e n u m b e r of such oscillations would, however, in commercial circuits, p r o b a b l y rarely exceed one-half dozen. T h e s e few, however, will send edrresponding w a v e s into the line, w h i c h m a y be reflected and m a y b u i l d up resonance or nodes and loops as conditions already described m a y determine. It is t h e s e a b r u p t c h a n g e s of potential not directly prod u e e d b y the g e n e r a t o r w h i c h for eommereial circuits are properly d e s i g n a t e d b y the term " S t a t i c Discharges." T h e i r d i s t i n g u i s h i n g characteristics is their e x t r e m e freq u e n c y of alternation, which is so high as to cause several c o m p l e t e w a v e s to be f o r m e d in transmission lines of moderate length. All the laws and properties of linear w a v e s w h i c h h a v e been here considered apply, of course, to these static electrical waves. For, example, static w a v e s have their form and i n t e n s i t y d e t e r m i n e d b y the n a t u r e of their source, t h a t is, b y the sort of c h a n g e of potential at the point of the line where the exciting discharge occurs ; and once formed each w a v e passes d o w n the line w i t h o u t reference to w h a t m a y s u b s e q u e n t l y occur at the source. T h i s w a v e will p r e s e r v e its form and i n t e n s i t y e x c e p t for losses in the medium, h e r e r e p r e s e n t e d b y the wire, or for 6omplete or partial reflection. As before, c o m p l e t e reflection occurs either at an open-ended wire c o r r e s p o n d i n g to an obstacle w h i c h p r o d u c e s as a maxim u m double potential at reflecting points and r e t u r n s the w a v e t o w a r d s the source w i t h o u t c h a n g i n g its sign ; or, b y a short circuit or v e r y large c o n d e n s e r w h i c h p r e v e n t s any rise of potential at the reflecting point, b u t r e t u r n s t h e w a v e t o w a r d s the source with its sign reversed. P a r t i a l
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reflection occurs w i t h every c h a n g e in the q u a l i t y of the m e d i u m . T h i s q u a l i t y is m e a s u r e d by the capacity and inductance. If at the reflecting point the c a p a c i t y of the lines be increased or the i n d u c t a n c e be diminished, the r i g i d i t y is decreased and "the wave is p a r t i a l l y reflected; and the portion e n t e r i n g the new part of the line has a less voltage t h a n the original wave, i f the c a p a c i t y be decreased, or t h e i n d u c t a n c e increased, the r i g i d i t y of the m e d i u m is increased, partial reflection will a g a i n occur and the portion of the wave e n t e r i n g the new line will have a h i g h e r potential t h a n the original wave. S t a t i o n a r y nodes and lobps or resonance m a y occur in an electric line as in a n y o t h e r case of linear wave motion, the critical condition to cause t h e i r occurrence b e i n g a n u m b e r of similar waves following one a n o t h e r at u n i f o r m intervals in case of e i t h e r resonance or loops, and in a d d i t i o n exact a d j u s t m e n t of the l e n g t h of the line to the f r e q u e n c y of the a l t e r n a t i n g source in the case of resonance. Effect of Static Waves in Commercial Circuits.--The net results as regards the operation of commercial circuits m a y be indicated quite briefly. W h e n e v e r a line is connected to a live circuit, w h e n e v e r a line is accidentally grounded, w h e n e v e r a short circuit occurs b e t w e e n two lines, or a l i g h t n i n g arrester discharges, or in case of a n y similar disturbanee, there occurs at the point a very a b r u p t c h a n g e of potential e i t h e r up or down. T h e exact a b r u p t n e s s of such a c h a n g e depends upon the local conditions and c o n s t a n t s of t h a t portion of the circuit at which the discharge occurs. In actual cases it p r o b a b l y varies b e t w e e n 1--0-,~1-0-V part and l p a r t of a second for each alternation. It is evi. d e n t t h a t this d i s c h a r g e is e n o r m o u s l y rapid in comparison w i t h 2 5 to ioo cycles per seeo~ad of the g e n e r a t o r frequencies. T h e f r e q u e n c y of I,OOO,OOOperiods per second would give a wave l e n g t h to the static wave in a t r a n s m i s s i o n line of ,approximately I,OOO feet. U s u a l l y the discharges r e s u l t i n g from switching, g r o u n d i n g , etc., are oscillatory, b u t n o t very p e r s i s t e n t ; t h a t is, each oscillation of the d i s c h a r g e is considerably less intense t h a n the previous one, and after perh a p s half a dozen a l t e r n a t i o n s t h e y become too weak to be
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serious. T h i s m e a n s that no very high rises of potential will occur from r e s o n a n c e at these high frequencies, t h o u g h a rise of two or three times t h e original potential m i g h t perhaps s o m e t i m e s occur if conditions were v e r y favorable. Usually, also, t h e r e will be a considerable diminution in t h e intensity of traveling waves, d u e to the fact t h a t in their passage along t h e line t h e y will suffer loss of e n e r g y from resistance and o t h e r losses, so t h a t n e i t h e r will double t h e original potential be p r o d u c e d by reflection, nor will t h e full values of the original w a v e s be a d d e d t o g e t h e r in ease of resonance. T h i s loss of e n e r g y in transmission will b e especially effective in p r e v e n t i n g a rise due to r e s o n a n c e "since any increase in i n t e n s i t y of the w a v e g r e a t l y increases the losses in transmission. T o s u m up, in commercial circuits, as a result of s t a t i c disturbances, m o m e n t a r y rises of potential m a y be e x p e c t e d . more often at the ends of lines or other reflecting points~ usually n o t over 5o per cent. g r e a t e r than the a b r u p t c h a n g e of potential w h i c h caused the w a v e ; b u t i n - s o m e e a s e s where the l e n g t h Of line involved and o t h e r conditions a r e favorable for resonance a rise to two or three times t i l e original v o l t a g e of the source m a y perhaps be found. T h e r e is, however, an additional p h e n o m e n a which, apparently, s o m e w h a t increases the severity o f these disturbances. This results from the h e a v y m o m e n t a r y c h a r g i n g c u r r e n t taken b y a transmission line or o t h e r large capacity, w h e n c h a r g e d or discharged a b r u p t l y ; this c h a r g i n g current t e n d s to b u i l d up m o m e n t a r i l y the v o l t a g e from which the line is b e i n g charged. Its effect is s o m e w h a t the same as w o u l d be experienced if the field s t r e n g t h of the g e n e r a t o r were m o m e n t a r i l y increased. T h i s p h e n o m e n a m a y add perhaps another 5o per cent. to the i n t e n s i t y of the static disturbanee. T h e s e conclusions are b a s e d partly u p o n theoretical considerations, as indicated in the earlier part of the paper, anti, at least as to the e x t e n t to which a b n o r m a l potentials m a y be expected in commercial circuits, partly u p o n a considerable n u m b e r of actual tests w h i c h have b e e n m a d e b y the w r i t e r in c o n j u n c t i o n with the engineers of a n u m b e r of large installations for the purpose of d e t e r m i n i n g t h e
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practical effect of such disturbances. T h e s e tests h a v e ext e n d e d over a considerable range of time and d i v e r s i t y of l o c a t i o n and h a v e included widely different v o l t a g e s and different l e n g t h s of line r u n n i n g to over too miles. In all cases the results o b t a i n e d have been in accordance with the conclusions above. L i g h t n h t g . - - T h e d i s t u r b a n c e s p r o d u c e d b y l i g h t n i n g have their m a x i m u m i n t e n s i t y d e t e r m i n e d b y different conditions from the w a v e s p r o d u c e d b y internal d i s t u r b a n c e s in the transmission s y s t e m itself. T h e former are e v i d e n t l y determ i n e d b y the i n t e n s i t y and p r o x i m i t y of the l i g h t n i n g flash and the protective apparatus, while the latter are d e t e r m i n e d directly b y the g e n e r a t o r voltage. Ordinarily the actual line d i s t u r b a n c e s p r o d u c e d b y l i g h t n i n g are not e x t r e m e l y severe, and with efficient arresters on high tension circuits m a y be considered as not e x c e e d i n g those due to the other c a u s e s of static disturbance. In exceptional cases, however, w h e r e lines are directly struck b y l i g h t n i n g discharges of g r e a t e r or less m a g n i t u d e , there is an e x t r e m e rise of potential locally on the transmission line which is b e y o n d the p o w e r of a n y commercial arrester to prevent. Fortunately, however, this d i s c h a r g e seeks the earth b y the s h o r t e s t path a n d will u s u a l l y choose one or m o r e line poles in the immed i a t e vicinity of the discharge. Such poles will s o m e t i m e s be chipped or split and s o m e t i m e s c o m p l e t e l y splintered. T h i s discharge m a y or m a y n o t cause a short circuit on the l i n e ; f r e q u e n t l y will not. A l t h o u g h the main discharge r e a c h e s g r o u n d in all p r o b a b i l i t y close b y the point struck, t h e r e will, of necessity, as already explained, be w a v e s sent in b o t h directions d o w n the line wires w h i c h will h a v e an e x t r e m e m a x i m u m intensity at the start and which m a y do serious d a m a g e at the stations ; b u t which are of a magnit u d e that m a y properly be handled b y protective apparatus. T h u s , t h o u g h the direct violence of the stroke cannot be g u a r d e d a g a i n s t or resisted, yet, except in the v e r y rare case w h e r e such discharge is in the i m m e d i a t e n e i g h b o r h o o d of t h e station, very often no h a r m will result, p r o v i d e d the station a p p a r a t u s be p r o t e c t e d b y efficient arresters. Sources o f Static I4Zaves.--The c o m m o n sources of static
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d i s t u r b a n c e with the effects produced are s u m m a r i z e d below. (i) Charging a dead line from a live t r a n s f o r m e r not alr e a d y s u p p l y i n g other lines, m o m e n t a r i l y drops the potential of the t r a n s f o r m e r lead to zero ; the g e n e r a t o r then supplies c h a r g i n g c u r r e n t at a c o m p a r a t i v e l y slow rate to b r i n g the line to normal potential. In this ease usually no wave will be produced, as t h e f r e q u e n c y is c o m p a r a t i v e l y low. T h e r e may, however, result a certain m o m e n t a r y rise of p o t e n t i a l (2) W h e n a dead transmission line is c h a r g e d from a syst e m h a v i n g a large q u a n t i t y of electricity already stored in o t h e r lines c o n n e c t e d to the same high tension b u s bars, the potential of the n e w line is very s u d d e n l y raised to the potential of the system, and this s e n d s an a b r u p t w a v e of positive sign along the line. (3) G r o u n d i n g or short-eircuiting a e h a r g e d line causes a s u d d e n fall of potential, which sends a w a v e of n e g a t i v e sign along the line in b o t h directions. T h i s w a v e e v i d e n t l y c a u s e s no rise of potential at any point and is h a r m l e s s exc e p t for a p h e n o m e n a to be discussed later, u n d e r the head of Local Concentration. (4) O p e n i n g a loaded line causes no rapid c h a n g e of potential, since the load eurrent produces a large v o l u m e of h e a t e d air, g r a d u a l l y e u t t i n g d o w n the potential of the line. (5) O p e n i n g an u n l o a d e d line has much the same effect as c h a r g i n g a line, since in o p e n i n g the switch there will u s u a l l y be a rapid charge and discharge of the line for a few alternations. T h a t is, the i n s t a n t after the line is free from the switch for the first time, the next alternation (being of opposite sign) will f r e q u e n t l y be able to j u m p the gap and recharge the line momentarily, t h u s c a u s i n g a static spark ; t h a t is, positive w a v e s will b e sent d o w n t h e line. F o r t u n a t e l y t h e s e potential rises are m o m e n t a r y and do n o t p r o d u c e as severe a strain u p o n solid i n s u l a t i n g material as similar potentials of long-continued duration. T h i s res u l t s p r o b a b l y from the h e a t and mechanical inertia of the i n s u l a t i n g material. Therefore, solid i n s u l a t i n g material w h i c h is sufficiently strong to w i t h s t a n d the strains of nor-
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mal operation will often not be punctured, even t h o u g h it m a y be s u b j e c t e d to double normal v o l t a g e b y t h e s e static strains ; on the other hand, air distances s u b j e c t e d to such strains, including " s u r f a c e d i s t a n c e s " so called, can readily break down i n s t a n t l y from static strains, and, indeed, especially surface distances, m a y be and are more sensitive to static than o t h e r strains. It is necessary, therefore, in plann i n g a high tension s y s t e m to provide a v e r y wide margin for air spaces at all exposed points and long surface distances b e t w e e n live parts to insure s a f e t y from static disturbances. On the o t h e r hand, v e r y little increase is r e q u i r e d in the actual thickness of solid material over that r e q u i r e d to stand normal strains. This conclusion m u s t be taken in conjunction with the discussion b e l o w u n d e r the head of " L o c a l Concentration of Potential." Most engineers do not appreciate the i m p o r t a n c e of this caution. Once a plant is installed with air distances too small, these will be an occasional cause of s h u t - d o w n and loss until remedied. In planning insulation distances it must be r e m e m b e r e d that the striking distance of high v o l t a g e s from 20,00o to i0o,0oo volts increases m u c h more rapidly than the voltage, and that c o n s e q u e n t l y for the h i g h e r v o l t a g e s m u c h m o r e than p r o p o r t i o n a t e spaces m u s t be allowed. T h i s phenomenon is one of the reasons w h y insulators for v e r y high v o l t a g e s have to be excessively large in c o m p a r i s o n with insulators for perhaps lo,ooo to I5,ooo volts. T h e disadvantage to the insulator m a n u f a c t u r e is f u r t h e r increased b y the fact that the w e i g h t and material of the insulator increase s o m e w h e r e nearly according to the cube of the linear dimensions, while the insulation s t r e n g t h increases m o r e slowly than the first power. It is of course c u s t o m a r y and desirable to p r e v e n t as far as possible an a b n o r m a l rise of t e m p e r a t u r e a b o v e the e a r t h b y m e a n s of suitable l i g h t n i n g arresters. H o w e v e r , it is evid e n t that l i g h t n i n g arresters cannot be placed at all p o i n t s of the s y s t e m w h e r e a rise of potential m a y be p r o d u c e d b y static disturbances. T h e y should usually be placed, however, where i m p o r t a n t a p p a r a t u s is to be protected. A t
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other points ample insulation distances, as outlined in t h e p a r a g r a p h above, m u s t be provided. Local Concentration of PotentiaL--More serious than t h e m o m e n t a r y rise of potential j u s t discussed is the strain produced upon coils c o n n e c t e d to the high.tension circuits by static waves. It is well k n o w n that v e r y a b r u p t w a v e s or discharges pass only with g r e a t difficulty and slowly t h r o u g h choke coils, and that a static discharge will j u m p t h r o u g h considerable insulation r a t h e r t h a n follow around the turns of a po,~erful choke eoil. T h e h i g h tension coils of t r a n s f o r m e r s form e x t r e m e l y p o w e r f u l choke coils and are s u b j e c t e d to v e r y severe strains b e t w e e n the t u r n s or layers when static w a v e s reach their terminals. T h e r e is a n o t h e r w a y of looking at this p h e n o m e n o n : W h e n a s t a t i e w a v e reaches a t r a n s f o r m e r coil, on account of the e n o r m o u s l y increased i n d u c t a n c e and eapacity of this w i n d i n g over the line, its speed is g r e a t l y reduced. T h i s m e a n s that w h e n the b e g i n n i n g of the w a v e has r e a c h e d a point, perhaps one-third of the w a y to the end of the coil, that the crest of the w a v e has reached the end of the terminal of the coil. Therefore, a difference of potential exists b e t w e e n the terminal and a point one-third the w a y t h r o u g h equal to the full p o t e n t i a l of the s t a t i c wave. T h i s w a v e m a y be even g r e a t e r t h a n full-line potential, and the high-tension coil referred to m a y be one of a n u m b e r w h i c h normally divide the full-line p o t e n t i a l between them. C o n s e q u e n t l y the m o m e n t a r y strains upor~ the o u t e r parts of the coil m a y be a g r e a t m a n y times t h e normal v o l t a g e i m p r e s s e d b y the generator. If the insulation b e t w e e n turns of the coil be not e x t r e m e l y good a spark will pass b e t w e e n them. T h i s spark tends directly only to pass the e n e r g y of the static wave, w h i c h is v e r y small in actual a m o u n t and almost harmless. U n f o r t u nately, however, there m a y exist at this time a v o l t a g e b e t w e e n the turns b e t w e e n w h i c h the spark passes, maintained b y the generator, which, t h o u g h m u c h smaller than the static voltage, is still able w h e n once the insulation is broken down b y the latter, to start and m a i n t a i n an arc. A l t h o u g h the static spark of itself was nearly harmless, the
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[J. F. I.,
arc m a i n t a i n e d by the generator, if once started, will very likely d e s t r o y the coil. In ease, however, t h a t the n o r m a l voltage b e t w e e n layers or t u r n s is very small, or in ease t h e static spark occurs at an i n s t a n t of time when the voltage of the g e n e r a t o r is n e a r l y zero, or, perhaps, u n d e r o t h e r conditions, the g e n e r a t o r electromotive force will n o t be able to s t a r t a local arc. F o r t u n a t e l y , this will u s u a l l y be the case. However, a l t h o u g h each static spark w h i c h m a y pass b e t w e e n t u r n s is in itself not p a r t i c u l a r l y h a r m f u l , y e t if for a n y reason a large n u m b e r of such sl~arks pass in the same spot, a final deterioration of the i n s u l a t i o n will result and a short circuit ensue at a later time, which m a y finally occur a p p a r e n t l y w i t h o u t a n y connection w i t h a n y static disturbance. As a m a t t e r of fact, static d i s t u r b a n c e s are very difficult to trace out in their effects w i t h i n transformers.
It is e v i d e n t t h a t the same s t r a i n b e t w e e n turns in a coil will be produced by a n e g a t i v e wave as by a p o s i t i v e - if it be as abrupt. T h a t is, a s u d d e n d r o p p i n g to zero of the potential of the t r a n s f o r m e r t e r m i n a l produces the same s t r a i n as s u d d e n l y c h a r g i n g it to the same a m o u n t . T h i s local c o n c e n t r a t i o n of potential is n e i t h e r an unc o m m o n nor an u n c e r t a i n p h e n o m e n o n . It m a y be r e p r o d u e e d at a n y t i m e in a l a b o r a t o r y or factory, or at a powerhouse, by a simple experiment, and u n d o u b t e d l y occurs in c o m m e r c i a l operation of plants. It is necessary only to cause a static d i s t u r b a n c e near the t e r m i n a l of a coil a n d m e a s u r e by a spark gap the m o m e n t a r y voltage between the lead and points a portion of the w a y t h r o u g h the coil. Static waves are of g r e a t i m p o r t a n c e to the o p e r a t i n g man, however, only on very h i g h voltages. It is e v i d e n t t h a t the condition w h i c h causes the local c o n c e n t r a t i o n of strain t o be e x t r e m e l y severe is the abruptness of the static wave. T h e more a b r u p t the wave, t h e less distance will it have p e n e t r a t e d a t r a n s f o r m e r coil before the t e r m i n a l has a t t a i n e d the full p o t e n t i a l of the wave. T h i s sort of strain is a n a l o g o u s to the b r e a k i n g of a plastic b o d y by a very s u d d e n blow. Protection o f Circuits and A p p a r a t u s . - - P r o t e c t i o n of two
Dec., 19o3.]
Static Discharges in Electric Circuits.
443
t y p e s is r e q u i r e d : (I) L i g h t n i n g arresters to protect a g a i n s t a b n o r m a l rise of potential a b o v e the earth and the low tension circuits; (2) protection against local concentration of potential. Grounds.--There are a n u m b e r of ways, theoretically, in which static w a v e s can be destroyed, or r a t h e r diffused, and p r e v e n t e d from d o i n g serious injury. (I) T h e introduction of a choke coil in the line will divide a static w a v e in two parts, one portion p a s s i n g on, the o t h e r b e i n g reflected b a c k ; the proportions of these parts for any given w a v e b e i n g d e t e r m i n e d b y the size of the choke coil. If a number of these choke coils were placed at a n u m b e r of points in the line so that reflection w o u l d occur several times, the w a v e w o u l d be d i s t r i b u t e d in a n u m b e r of parts, each of which w o u l d act alone and be unable to do damage. However, this m e t h o d is i m p r a c t i c a b l e b e c a u s e each different static w a v e l e n g t h w o u l d require a different choke coil to divide the static w a v e properly, and b e c a u s e such choke coils w o u l d seriously interfere w i t h the normal operation of the system. (2) S h o r t b r a n c h lines w o u l d be excellent, as each w o u l d s u b t r a c t a portion of the e n e r g y of the original wave, which w o u l d soon be r e d u c e d to a harmless m a g n i t u d e . T h i s m e t h o d is of d o u b t f u l effectiveness, however, since the l e n g t h of line necessary to actually w i t h d r a w e n e r g y from the original w a v e m u s t be g r e a t e r than the w a v e l e n g t h of the static discharge. T h i s length will v a r y greatly and m i g h t be so g r e a t as to render the cost of such branch lines prohibitory. S u c h lines m u s t e v i d e n t l y be taken from some point of the main line, other than a reflecting point, or, as a l r e a d y explained, t h e y will t h e m s e l v e s experience an abn o r m a l rise of potential. It is p r o b a b l e t h a t branch lines a n d c h a n g e s in the size or n u m b e r of line wires (which has m u c h the s a m e effect) do help in m a n y instances to r e d u c e the severity of waves, b u t it is the opinion of the a u t h o r that it is hardly feasible to devise such an a r r a n g e m e n t with reasonable expense w h i c h can be relied u p o n to dissip a t e w a v e s in all cases. Both of the a b o v e m e t h o d s fail also in cases where static
444
T h o m a s ."
EJ. F. I.,
w a v e s originate near a p p a r a t u s and d.o n o t pass t h r o u g h any considerable l e n g t h of line before h a v i n g an Opportunity to do d a m a g e . (3) L i g h t n i n g Arresters. T h e only m e t h o d in practical u s e at the present time is the e m p l o y m e n t 0f l i g h t n i n g arresters which are merely o p p o r t u n i t i e s for the line to d i s c h a r g e to g r o u n d w h e n r e a c h i n g a p o t e n t i a l sufficiently high a b o v e normal. T h e y m a y be m a d e q u i t e s a t i s f a c t o r y for a c c o m p l i s h i n g this p u r p o s e at t h e particular p o i n t of the s y s t e m to w h i c h t h e y are connected, provided two s e c o n d a r y effects are neglected. (i) T h e arcing over the arrester following a discharge, which has been f r e q u e n t l y discussed, and (2) t h e local concentration of potential r e s u l t i n g from the e x t r e m e l y a b r u p t n e g a t i v e w a v e caused b y the r e d u c t i o n to e a r t h potential of the circuit a t the arrester. V a r i o u s w a y s are proposed and used for the i n t e r r u p t i o n of the are in t h e arrester, all of w h i c h hinder the original function of the a t t e s t e r to some e x t e n t ; that is, the free d i s c h a r g e of the • line. In high tension work the m o s t successful m e a n s is series resistance in the discharge p a t h of the arrester. By p r e v e n t i n g any considerable flow of c u r r e n t from the generator this series resistance m a y be m a d e to render a l m o s t a n y l i g h t n i n g a t t e s t e r non-arcing. E v e r y available m e a n s , however, should be u s e d to r e d u c e the a m o u n t of s e r i e s resistance to a m i n i m u m , as it m a y seriously i m p e d e t h e discharge of the line. T h e use of non-arcing m e t a l and a large n u m b e r of small gaps in place of one large gap, as is done in nearly all commercial arresters, m a k e s a v e r y considerable gain in this direction. A n o t h e r m e a n s for still f u r t h e r r e d u c i n g the series resistance will be briefly described u n d e r the head of the low e q u i v a l e n t arrester. T h e local concentration of potential p r o d u c e d b y t h e l i g h t n i n g a t t e s t e r discharge is not different from a similar effect d u e to o t h e r causes except t h a t it is p e r h a p s m o r e severe since closer to the apparatus. It needs no s e p a r a t e consideration. S h o r t C i r c u i t s . - - T h e p r e v e n t i o n of d a n g e r o u s local concentration of potential m a y be a c c o m p l i s h e d b y several
Dec., I9o3.]
Static Discharges in Electric £Trcuits.
445
methods. (I) By i n s u l a t i n g windings, especially near the terminals, sufficiently h e a v y to w i t h s t a n d the strains produeed b y static w a v e s in the partieular eireuits in whieh this a p p a r a t u s is to be used. T h i s m e t h o d is v e r y satisfaetory in its simplicity, and is entirely feasible for m o d e r a t e l y high voltages. It is the m e t h o d u s u a l l y e m p l o y e d for this purpose. (2) W i n d i n g s m a y be so arranged, for example, b y winding one turn per layer, as to v e r y g r e a t l y r e d u c e the statie p o t e n t i a l b e t w e e n layers and also the normal v o l t a g e bet w e e n layers, a n d t h u s v e r y m u c h r e d u c e the p r o b a b i l i t y of the f o r m a t i o n of a local are in ease of a static spark. T h i s a r r a n g e m e n t of w i n d i n g s extends t h e range of v o l t a g e for w h i c h insulation can be used commercially sufficient to stand static strains, b u t is i n a d e q u a t e in the h i g h e s t voltages for the protection of the insulation b e t w e e n the coils or section of coils which are t h u s wound. A more c o m p l e t e method, a m e t h o d of p r e v e n t i o n r a t h e r t h a n resistance, w h i c h is available for h i g h e r v o l t a g e s than those already mentioned, is t h e use of the static interrupter, which is s u b s t a n t i a l l y a magnified choke coil. Its function is to so delay the static w a v e in its e n t r y into the transformer coil that a considerable portion of the l a t t e r will become c h a r g e d ' b e f o r e the terminal will have reached full potential. P r o t e c t i o n b y a choke coil will first be discussed. If a v e r y heavily i n s u l a t e d p o w e r f u l choke coil be placed in the lead of the transformer, w h e n a static w a v e approaches electricity will b e g i n to pass in small q u a n t i t y and will pass in g r a d u a l l y increasing q u a n t i t y at later instants of time, so t h a t the coil will be, c o m p a r a t i v e l y speaking, g r a d u a l l y b r o u g h t to full potential; m e a n w h i l e the v o l u m e of the static wave is b e i n g reflected and choked back and p e r h a p s b e i n g discharged to the g r o u n d if there be a l i g h t n i n g atrester near. It is e v i d e n t that this choke coil, to be effective, m u s t be so p r o p o r t i o n e d as to delay the i n c o m i n g w a v e e n o u g h so t h a t the portion of the w i n d i n g which has b e c o m e c h a r g e d w h e n full potential is reached at the terminal shall be sufficient to w i t h s t a n d the strain of the full voltage of the wave. It is e v i d e n t that such a d j u s t m e n t
446
T/zomas
[J. F. I.,
."
does not d e p e n d directly on t h e f r e q e n e y or a b r u p t n e s s of the static wave, since b o t h the t r a n s f o r m e r and the ehoke coil are similarly affected b y t h e frequency. But a choke coil sufficiently powerful to accomplish this
Transl. c C C O C c C 0 0 C 0
[
Choke Coil
Line
b
I C3 0 0 0 0
qTG'5~ ~ Lt.Arr. ~ Choke Coil
Lt. Arr.
t Line
FIG. 3.--Static interrupter protecting transformer.
result satisfactorily is found to be i m p r a c t i c a b l e on v e r y high potential circuits on a c c o u n t of t h e size, cost and interference with the operation of t h e system. H o w e v e r , if t h e a r r a n g e m e n t of the static i n t e r r u p t e r be used, t h a t is, if a Choke Coil =
r
Cond.
Line
~ Lt. Arr.
T Cond. Choke Coil
I1"'
~Lt. Arr.
Line
FIG, 4.--Static interrupter protecting high-tension generator.
c o n d e n s e r be c o n n e c t e d b e t w e e n line and g r o u n d b e h i n d the choke coil nearer t h e a p p a r a t u s to be protected, this choke coil will a b s o r b a considerable portion of the c u r r e n t actually passed b y the choke coil, and the time required to pass
Dec., 19o3.]
Static Dischargex in Electric Circuits.
sufficient electricity to charge the terminal will be increase. W i t h this a r r a n g e m e n t a c o m p a r a t i v e l y choke coil m a y be used. T h e c o n d e n s e r has a v e r y electro-static capacity, and has no appreciable effect normal operation, and y e t has a v e r y "o powerful effect on the static w a v e on acc o u n t of its e x t r e m e l y high frequency. ot A s in the case of the choke coil, the static O i n t e r r u p t e r m u s t be r o u g h l y proportioned to the t r a n s f o r m e r winding to be proteeted. T h e condenser m u s t also be 0¢ suitable for the v o l t a g e b e t w e e n line and ground. If static i n t e r r u p t e r s be placed in each lead of high tension a p p a r a t u s which m a y be i n j u r e d b y local concentration of potential, its w i n d i n g s will be a m p l y p r o t e c t e d a g a i n s t d a n g e r of short circuits from static w a v e either positive 0 or negative. Such an a r r a n g e m e n t is ~ 0 s h o w n d i a g r a m m a t i c a l l y in connection with a t r a n s f o r m e r and a high tension g e n e r a t o r in Etgs. 3 and 4. A large n u m b e r of these interrupters ..c have b e e n m a n u f a c t u r e d and are in ac- c o O tual service. Static Interrupters and Low Equivalent Lightning Arresters.--A short description of the salient features of some actual l i g h t n i n g arresters and static interrupters d e s i g n e d b y the a u t h o r for the purpose of p r o t e c t i n g a p p a r a t u s from static ~ 0 d i s t u r b a n c e s will be given. .o_ 0 The low equivalent A.C. lightning attester ~o 0 consists of a n u m b e r of ~7-inch air gaps i 0 b e t w e e n non-arcing m e t a l cylinders in series with non-inductive resistance. A portion of the resistance, ealled s h u n t resistance, is s h u n t e d b y a second set of air
447 much small small upon
i
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?5 u~
T/wmas :
448
[J. F. 1.,
:gaps called s h u n t e d gaps. T h e o b j e c t of this a r r a n g e m e n t is to r e d u c e the a m o u n t of t he series r e s i s t a n c e t h r o u g h wh ich th e d i s c h a r g e m u s t pass to g r o u n d . T h i s a r r e s t e r is d i a g r a m m a t i c a l l y i l l u s t r a t e d in F i g . 5. T h e a p p a r a t u s is s h o w n in Fig. 6. T h e series ga ps w i t h h o l d t he line v o l t a g e and are chosen so as to b r e a k dow n at s o m e t h i n g b e t w e e n 50 per cent. and Ioo p er cent. rise of v o l t a g e a b o v e t h e e a r t h . A p o r t i o n of th e series r e s i s t a n c e is s h u n t e d b y gaps so t h a t the static d i s c h a r g e can pass a r o u n d this p o r t i o n , t h u s a v o i d i n g i t s resistance. It e v i d e n t l y is t h e n n e c e s s a r y to s u p p r e s s the are f r o m th e g e n e r a t o r w h i c h t e n d s to follow t h r o u g h t he s h u n t e d gaps. It is f o u n d t h a t w i t h a n u m b e r of s h u n t e d g ap s eq u al to t he series gaps, t he arc will be w i t h d r a w n from th e s h u n t e d .gaps b y t he s h u n t r e s i s t a n c e w h e n the
FIG.
6.--Low equivalent A. C. lightning arrester.
s h u n t r e s i s t a n c e does n o t e x c e e d a p r o p e r value, which is a c o n s i d e r a b l e p o r t i o n of t h e t ot a l r e s i s t a n c e in t h e arrester. A v e r y m a r k e d ga i n in t he r e d u c t i o n of t h e r e s i s t a n c e offered to t h e d i s c h a r g e is t h e r e f o r e m a d e b y m e a n s of t he use of s h u n t e d g ap s and s h u n t r e s i s t a n c e . It m u s t be n o t e d as well t h a t no m o r e v o l t a g e is r e q u i r e d to cause a rise of pot e n t i a l to j u m p ove r all t h e gaps series and s h u n t than w o u l d be r e q u i r e d to j u m p t he series gaps alone, since on a c c o u n t of th e s h u n t r e s i s t a n c e t he series and s h u n t e d gaps a r e b r o k e n d o w n s e p a r a t e l y one a f t e r t he other. A static interrupter, see Fig. 7, consists of a choke coil in series w i t h the line and a c o n d e n s e r c o n n e c t e d bet w e e n line a n d g r o u n d on t he a p p a r a t u s side of t h e choke coil. Since i n t e r r u p t e r s are u s e d only on h i g h v o l t a g e plants, coil and c o n d e n s e r are pl a c e d in oil tanks w h i c h are m a d e self-cooling. T h e coils are e x t r e m e l y h e a v i l y insu-
Dec., I9O3.]
Static Disc/targes hi Electric Circuits.
449
lated. T h e condensers are built up of specially prepared i n s u l a t i n g paper and h a v e r e c t a n g u l a r copper plates w i t h r o u n d e d edges and corners. T h e r e is a considerable a m o u n t of h e a t g e n e r a t e d in condensers which is a difficult m a t t e r to dissipate on account of the poor h e a t - c o n d u c t i n g q u a l i t y of the i n s u l a t i n g material. T h e s e condensers are all provided with v e n t i l a t i n g ducts w h i c h allow a passage of oil from the b o t t o m u p w a r d s at f r e q u e n t intervals to the interior.
FiG. 7.--Static intrrrupter, showiug coil and condenser oat of case.
T h i s condenser loss is q u i t e an i n t e r e s t i n g p h e n o m e n o n . Its a m o u n t increases as the square of the voltage upon the condenser, roughly, and is very rapidly increased w i t h increase of t e m p e r a t u r e . T h e loss increases several ttmes in a m o u n t between 5°0 C. and Ico ° C. T h i s loss also depends u p o n the f r e q u e n c y of the circuit and for the kind of material used ; for the dielectric in these condensers it increases a p p r o x i m a t e l y as the square root of the frequency. Cables.--The h i g h tension electric cable is in principle no different from the electric air line, b u t has a different insul a t i n g material, paper or r u b b e r in place of air. T h i s has VOL. C L V I . . N o . 936.
29
450
7homas :
[J. F. I.,
the double effect of increasing its electrostatic c a p a c i t y and c h a n g i n g t h e velocity at which w a v e s progress. T h e increased electrostatic c a p a c i t y t e n d s to decrease the speed, b u t as the i n d u c t a n c e of the cable is small this partially c o m p e n s a t e s for the increased capacity. T h e differences from the air line are differences in degree only and do not affect the p a s s a g e of waves, reflection, resonance, etc., as already discussed. Consequently, no p h e n o m e n a different from the air lines m a y be expected as a result of static dis turbances. Since the cable contains no coils of wire, no local con. eentration of potential will be f o u n d like that in transformer coils, and there is no occasion for the use of a static interrupter. C o n c l u s i o n . - - T h e r e is in all probabilit'y n o t h i n g m y s t e r i o u s in static w a v e s in electric circuits and n o t h i n g r e q u i r i n g for its explanation a n y n e w theories, and y e t t h e r e is no s u b j e c t c o n n e c t e d with commercial p o w e r transmission which is so little u n d e r s t o o d b y those w h o h a v e n o t given it special attention, and in which it is so difficult to explain results. T h e difficulty arises not so m u c h f r o m intrinsic blindness in the s u b j e c t as from the g r e a t difficulty of m e a s u r i n g and t r a c i n g these e x t r e m e l y rapid and invisible waves. In a general w a y it m u s t be expected t h a t the s u r g i n g a b o u t of the e n e r g y which is stored w h e n e v e r a line is c h a r g e d will cause increased potential at certain points. T h i s should be p r o v i d e d for b y placing suitable l i g h t n i n g arresters at all points w h e r e i m p o r t a n t and v u l n e r a b l e appar a t u s is located. T h e r e will also be local c o n c e n t r a t i o n of potential in w i n d i n g s c o n n e c t e d to the circuits as the result of all static disturbances. Such t r a n s f o r m e r or o t h e r coils should always be either sufficiently i n s u l a t e d or p r o t e c t e d b y choke coils or static i n t e r r u p t e r s or b y some o t h e r suitable method. T h e a p p a r a t u s s h o w n in connection with this lecture s h o w s one a r r a n g e m e n t designed b y the a u t h o r for protection of circuits and a p p a r a t u s from static disturbances. It is h i g h l y desirable that this s u b j e c t should receive as
Dec., 19o 3. I
Notes and Comments.
45I
much discussion as possible, and that further experiments should be tried and observations made by engineers and superintendents of power plants, and that all work together to determine the best methods.of protection against this troublesome and elusive source of interruption of service and loss of apparatus.
PRODUCTION OF L E A D AND L E A D OXIDt~S. It is important to note that while there has been a large increase in the consumption of lead in the United States during the last three years, the production o[ domestic mines remains at a standstill at about 28o,ooo tons. The additional quantity required by the home demand has not been, as might have been supposed, aulyplied by lead from Mexico and other foreign ores smelted and refined in this country. This lead, which is refined here in bond, has beet, almost entirely re-exported, the entries for consumption at.home having been very small. The fact is that three years ago production had considerably outrun the demand then current, and the stocks carried by producers were very heavy. The increased demand has enabled producers to work off these stocks, and to reduce their holdings to normal amount,-so that at the present time they are really low. The increase in consumption during the last three years has come partly from the great activity in building operations, which has required the use of large quantities of lead, partly f r o m the increase in the demand for sheet lead for insulating cables ancl similar electric work, and partly from the demand for lead for use in various alloys. The production of red lead, litharge and orange mineral has shown no very important increase during the last three years; nor has the production:of white lead, although there was a considerable increase in the make in 19o2. White lead has certainly suffered from the competition of other white pigmeuts. If the improved Dutch processes that are now attracting so much attention., result in such a decrease in the cost of manufacture asis anticipated, white lead may be able to regain its lost g r o u n d . - - E n g , and M'ining./ournal.
P R I Z E S OF T H E SOCII~TE I N D U S T R I E L L E D E MULHOUSE. We have received a copy of the program of prizes to be awarded in 19o4 by the Soci6t6 Industrielle de Mulhouse. There are no fewer than fiftysix competitions concerned with chemical technology, more than twenty dealing with the mechanical arts, and twelve with natural history and agriculture. Several prizes are offered with the object of improving and stimulating local industries. The program also contains full particulars of several large prizes of $,oeo francs, which are awarded for scientific work at intervals of in some cases ten, and in others five years. The present publication takes the place of all previous issues, and copies of the program, in which certain changes have been made, can be obtained on application to the secretary of the society.