US2414308A - Protective gap device - Google Patents

Description

Jan. 14, 1947. W, KALB PROTECTIVE GAP DEVICE Filed Oct. 4, 1943 gigi.

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His Attorney lj'aienec Jan. 14, 1947 PROTECTIVE GAP DEVICE JohnW. Kalb, Pittsfield, Mass., assigner to General Electric Company, a corporation of New York Application October 4, 1943, Serial No. 504,819

(Cl. F75-30) 6 Claims. l

My invention relates to a protective device and to an improved gap structure therefor.

It has been customary to provide gap structures in overvoltage protective devices which include a pair of spaced electrodes to provide a gap across which most of the volta-ge during normal conditions will be impressed. Thus, when a protective device includes an expulsion tube arrangement, the series gap will take most of the voltage and substantially no voltage will be impressed across the electrodes of the expulsion tube. This minimizes the power frequency voltage to which the liber or gas evolving material of the protective tube is continuously subjected. If this were not done the liber shunting the internal gap would carbonize causing rapid failure.

The series gap for overvoltage protective devices of the arc expulsion type has customarily been located external to the housing which is used to contain the expulsion element. This has been necessary since these devices permit the ow of the follow currents of the order ofmagnitude oi system short circuit current which is larger than can be carried by gaps of prior design which are contained in a housing, such as those internal gaps used in lightning arresters employing series resistors usually of the non-linear type which limit the follow current to low values so as not seriously to damage the electrodes of gaps separated by a small spacing. However, external gaps are undesirable since they may be bridged by snow, sleet, birds or animals, and because of this the settings have been made as large as possible with consequent increase in impulse spark-over voltage and because the exposed metal parts at high voltage are dangerous to linemen.

It is therefore an object of my invention to provide a protective device with an improved gap construction so that it may be placed internally within the same housing in which the expulsion element is also placed.

It is a further object of my invention to provide an improved gap structure which not only has a relatively low impulse ratio but which may pass power frequency current until the arc is extinguished without substantially changing the voltage at which gap sparkover occurs for impulses or power frequency voltages.

I It is a further object of my invention to provide a gap assembly which is simple in construction, eiliclent in operation, and which will have a. relatively long life.

A further object of my invention is to provide an improved protective devicel combination hav- '2 ing a valve or expulsion clement and a series gap assembly enclosed in a single housing.

Further objects and advantages of my invention will become apparent from the following description referring to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particularity inI the claims annexed to and forming a part of this specication.

In the drawing Fig. 1 is a sectional side elevation of a protective device which is provided with an embodiment of my invention, and Fig. 2 illustrates another type of characteristic element from the type illustrated in Fig. 1 which may be employed with the gap assembly, as is illustrated in Fig. 1.

In Fig. 1 of the drawing I have illustrated a protective device including a gap element indicated generally by the numera1 I0 and a characteristic or current interrupting element indicated generally by the numeral I I, both of which are surrounded or enclosed by a suitable housing I2, such as a tubular porcelain member. The characteristic or current interrupting element indicated generally by the numeral II may be of any suitable construction or type and includes in the illustrated construction of Fig. 1 conventional electrode members I3 and I4 which are spaced in a. tubular member I5 having an internal bore surface of material such as hard fiber which has the properties of giving off gases when heated. Thus when the are produced by the power follow current passes through zero the arc is extinguished between the electrodes I3 and I4 by blowing it out due to the gases formed by the heat of the arc. The tubular member I 5 is enclosed in a suitable insulating member I6 as is shown.

In order to minimize the amount of power frequency voltage to which the valve element device II is subjected, it is conventional to provide any mum spacing, This is accomplished by provid lng the electrode Il of a rod shape and surrounding the electrode II by a surface of revolution atrasos which may have any suitable shape so as to cbtain arcing regions of progressively diierent dimensions. As will be seen in Fig. l the electrode i8 has a shape similar to a Venturi tube or is provided by a convex surface of revolution, or it may be considered as having a substantially hyperbcloidal shape. In order to support the rst electrode I8 as well as protect the inside surface of the housing I2 from the heat and metallic contamination due to the arcing, I provide a tubular or cylindrical member I9 from one end of which is supported the ilrst electrode. Thus the electrode I8 has a relatively short tubular shaped portion which may ber attached in any suitable manner to the adjacent inside surface of the cylindrical member I9 suchV as by spot welding, as indicated by the numeral 2I. 'I'he cylindrical member I9 may be supported in any suitable manner within the housing I2, and as will be seen in Fig. 1 the cylindrical member I9 is suspended from the upper end of the housing I2 through a metal end cap 22. Thus the cylindrical member I9 has a peripheral ilange portion 23 which may be supported in any suitable manner on the under surface of the cap member 22, such as by spot welding. The cylindrical member I9 may be suitably spaced on the cap member 22 by providing a concavo-convex portion 24 which iits into the tubular member I9. The cap member is provided with a peripheral iiange portion 25 which is resilient so that it may pass around the upper edge of the housing I2 and fit into a suitable peripheral groove 26. A ring shaped gasket 21 of suitable material such as rubber may be placed between the cooperating surfaces of the cap member 22 and the top surface of the housing I2. In order to connect the ilrst electrode or bell shaped electrode I8 to a suitable line, a line lead 28 is provided which may be electrically connected to the metallic cap 22, such as by spot welding, as is indicated by the nuineral 29. A weatherproof construction may be provided with a porcelain cap 30 which passes over the top of the housing I2 and is supported in place by a plastic material such as pitch, indicated by the numeral 3l.

In order to support the second electrode I1 in proper spaced relation with the surrounding electrode I8 there is provided a diaphragm 32 which extends from the inner surface of the housing I2 and is supported on a shoulder 33. So as to electrically connect the electrode I1 withA the upper electrode I3 of the expulsion member a coupling member 34 is provided having a sleeve portion 35 into which the lower end 36 of the electrode I1 relatively tightly fits. accomplished by forming an end which is suitably scored as is indicated. The member 34 is also provided with an extending screw portion 31 which is adapted to be connected to the upper electrode of any suitable current interrupting element which may be of the expulsion tube design, and as shown in the drawing it includes a metallic member 38 having a bore in which the screw portion 31 is placed.. Also the electrode I3 is supported in the bore of the member 38 thus making electrical connection with the member 31. In order to hold rigidly the lower end of the valve element Il in the lower end of the porcelain housing I2, I provide a washer 40 which is forced against the lower end ofthe housing in the groove 4I by providing an integral sleeve member which is in threaded engagement with the outer surface of the insulating members. The electrode Il may be connected to a This may be .electrode I of the expulsion tube connected to ground through the screw d4, voltages in excess of the arrester sparkover voltage will cause an arc to strike from I8 to I1 in the region A of minimum spacing in the gap structure lll. This will in turn cause a breakdown of the valve or A expulsion tube between the electrodes I3 and I4. completing the circuit to ground forthe dissipation of the excess voltage. Since protective devices with expulsion tubes can pass currents oi short circuit magnitude, a relatively large current may have to be carried by the arc in the gap Io. However, due to the particular construction of the gap very little melting of the electrodes occurs in the region A because electrode I8 has been shaped so that the magnetic field produced by the current exerts a positive force that rapidly moves the arc from region A of minimum spacing into region B where it remains, often going straight upward to cap 22 from the end of I1. Since the electrodes are more separated in region B, electrode melting in this region will not affect the gap sparkover voltage or impulse ratio which are determined by the electrode spacing and condition in region A. Furthermore, since the gap region around B and beyond the end of the electrodes in B is surrounded by the tubular member I9 and because the arc` is held in this region by the electromagnetic forces, the electrode supporting member I9 acts as a baille to prevent possible damage to the porcelain housing I2 due to the heat of the arc or loss of insulating quality due to deposition of vaporized metal. In order to provide a positive force to move the arc from A to B, it is essential that the current enter the lower portion of electrode I8 which is made certain by attaching the tube I9 to the bottom of electrode I8. Interruption of the current by arrester characteristic element II extinguishes the arc in the gap II).

It is to be understood that my improved protective device may employ my improved gap assembly within a housing in combination with any suitable type of current interrupting element, and I have illustrated in Fig. 2 a modied form of an expulsion device which includes a pair of spaced auxiliary electrodes 50 and 5I. The electrode 50 it will be seen may t into a plug 38 which also makes electrical connection through the coupling 34 to the electrode I1 of the gap assembly II). The lower electrode 5I may be connected to ground. The assembly includes a tubular casing 52 of any suitable insulating material and the space between the electrodes which provides the gap is lled with pellets of suitable arc extinguishing material. A member 53 is placed between the electrodes so as to narrow the gap space. and upon an arc occurring between the electrodes 50 and 5I through the interstices of the pellets, the heat of the arc will cause gases to be formed by the pellets which in turn extinguishes the arc. A further detailed description of the device illustrated in Fig. 2 will be found in my copending application S. N. 489,149. led May 31, 1943, which application is assigned to the same assignee as this present invention.

YJI have found that when a protective device includes my improved gap assembly the impulse breakdown of the arrester employing this series gap is appreciably lower than that with a conventional external gap having the same 60 cycle breakdown. As an example, when tested with impulse voltage rising at about 100 kv. per microsecond the impulse breakdown of an arrester including my improved gap assembly in series with a fiber pellet expulsion arrangement, as is illustrated in Fig. 2, has a value of about 47 kv. crest as compared with 65 kv. crest for an arrester with the same pellet expulsion device when connected in series with a conventional gap having a washer or cap shaped member for one electrode and the vend of a strap or rod for the other, even though my improved gap and the conventional gap both had the same sparkover voltage at 60 cycle frequency. Furthermore, by moving the arc from the region A to region B any burning due to the power arc will not affect the smooth contour in region A where breakdown initially occurs. This results in greater uniformity of the dielectric field and reduction of corona that occurs with the conventional series gap at voltages substantially lower than required for sparkover. This corona lowers the 60 cycle frequency breakdown voltage of a gap of a given setting, so its elimination allows a much closer gap setting for the same 60 cycle spark potential. The reduction in sparkover voltage due to corona is much less in the extremely short intervals of time in which impulse breakdown occurs. Consequently my improved gap with its shorter spacing has lower impulse breakdown than the conventional series gap of equal 60 cycle spark potential. Since spacing of this gap is relatively short the surfaces should be kept clean and smooth and also protected from rain, dirt, etc. This is accomplished in my improved construction by placing the gap assembly inside the porcelain housing of the protective device, Of course, the internal gap offers the additional advantages of absence of external flame which could cause arcover of apparatus and elimination of exposed high voltage parts with resulting danger to linemen and possibility of outages due to accidental grounding by birds, squirrels, etc.

To differentiate between the characteristics of different types of gaps, tests were made on my improved gap and the conventional gap described above. Although both gaps had a 60 cycle breakdown of 16 kv. R. M. S. when dry, my improved gap had an impulse breakdown (100 kv. per microsecond) of 30 kv. crest while the conventional gap had a breakdown of 48 kv. crest with asimilar impressed wave front. Since horn gaps do not produce a uniform field they also would be susceptible to corona and so would have a high impulse ratio as does the other conventional gap described above,

In order to determine the 60 cycle current discharge capacity of gaps made according to my invention, tests were made on a gap as illustrated in Fig. 1 having an 18 kv. R. M. S. 60 cyce breakdown and with a minimum spacing in the region A between the electrodes of the order of 300 mils. The maximum spacing in the region B from the center line of the rod electrode I1 to the edge of the surface of revolution was about 3A inch, and the distance from the end of the rod electrode I1 straight up to the cap member at 24 was about 11A inches. Such a design would be appropriate for a gap for use on 7.2 kv. circuits. This gap was given ve operations with a 60 cycle current of 2000 amperes crest, each operation of one-half cycle duration. After test no roughening of the electrode surfaces could be observed in 6 the region A of minimum breakdown and the rod electrode retained its shape fairly well even at the top where burning was concentrated due to the moving of the arc into the region B. The gap was then given an even more severe test of one operation at 3400 amperes crest followed by five more at 6000 amperes crest, each operation consisting of one-half cycle of 60 cycle current.

The electrodes remained smooth in the region A and there was no metallic deposit on the walls of the porcelain housing. The 60 cycle spark potential of the gap after this test was still 18 kv. R. M. S. which was the value before test. It will be seen therefore that with applicants improved construction an enclosed gap is provided which will withstand very largel 60 cycle current with the enclosed gap only occupying a total space of about 2% inch internal diameter and about 4 inches long for the particular design described above. This may be compared. with the conventional enclosed type gaps used in valve type lightning arresters which gaps usually weld shut and become permanently short circuited if subjected to 60 cycle current of one-half cycle duration and only a few hundred amperes in magnitude.

In order to determine the ability of my improved gap to withstand continuous arcing the gap having the dimensions given above was given one shot at 1700 crest amperes with a duration of 18 half cycles. The arc was interrupted at the end of 9 cycles by a circuit breaker. Even after such a severe test substantially no burning of the electrodes in the region A could be detected, and there was little burning even at the upper end of the rod electrode, although the copper cap at 24 had a small hole burned in it. Such severe tests indicate that a protective device having my improved gap would not readily become short circuited even in the event of failure of the interrupting or expulsion element.

Although I have shown and described particular embodiments of my invention, I do not desire to be limited to the particular embodiments described, and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States ls:

1. A gap structure including a rst electrode, a second electrode provided by a surface of revolution surrounding said first electrode providing an axially extending gap space, a metallic tubular member surrounding said electrodes and extending axially beyond ends of said electrodes to provide an enclosed space, means for electrically connecting said tubular member in a circuit with said electrodes so that the now of current during arcing will tend to force the arc toward the enclosed space at the end of said electrodes.

2. An enclosed gap assembly including, a tubular housing of insulating material, a first electrode having a rod shape, means extending from the inner walls of said housing for supporting said first electrode, a second electrode spaced from said first electrode'to provide a gap, a metallic cylindrical member within said housing and completely surrounding said electrodes and extending axially from one end of said second electrode to provide an enclosure for the arc space between said electrodes so as to prevent injury to said housing due to the heat of arcs between said electrodes, the other end of said second electrode being supported by said tubular member, means for electrically connecting said metallic member to an associated electric circuit, and

i means for supporting said tubular member from said housing.

3. A gap structure including a ilrst electrode having a rod shape. a second electrode surrounding said first electrode and having a substantially hyperboloidal shape so as to provide a gap with smooth electrode surfaces and a low impulse ra tio determined by the narrow gap between said electrodes defined at the midpoint of the hyperboloid shaped electrode, and a metallic member completely surrounding said second electrode and providing an enclosure for the arc space between said electrodes, said metallic member extending axially beyond one end of said second electrode and being connected only to the other end portion of said second electrode for mechanically supporting said second electrode and electrically connecting it to an associated circuit.

4. A gap structure including a iirst electrode having substantially a rod shape, a second electrode surrounding said rst electrode and having the shape of a' convex surface of revolution so as to provide a gap with smooth electrode surfaces with the spacing between said electrodes being a minimum at one point and progressively increasing in spacing from this minimum, and a metallic member completely surrounding said second electrode and providing an enclosure for the arc space between said electrodes, said metallic member extending axially beyond one end of said second electrode and being connected only to the other end portion of said second electrode for mechanically supporting said second electrode and electrically connecting it to an associated circuit.

5. A gap structure including a rs't electrode having substantially a rod shape. a second electrede surrounding said rst electrode and having the shape of a convex surface of revolution so as to provide a gap with smooth electrode surfaces with the spacing between said electrodes being a minimum at one point and progressively increasing in spacing from this minimum along the 1ongitudinal axis of said iirst electrode, and a metallic member completely surrounding said second electrode and providing an enclosure for the arc space between said electrodes, said metallic member extending axially beyond one end of said second electrode and being connected only to the other end portion of said second electrode for mechanically supporting said second electrode and electrically connecting it to an associated circuit.

6. An enclosed gap structure contained in an insulating housing including a flrst electrode having a rod shape, a second electrode surrounding said first electrode and having a substantially hyperboloidal shape so as to provide a gap with smooth electrode surfaces. and a low impulse ratio determined by the narrow gap between said electrodes dened at the midpoint of the hyperboloid shaped electrode, and a metallic member completely surrounding said second electrode and extending axially beyond one end of said second electrode to provide an enclosure for the arc space between said electrodes so as to prevent injury to said housing due to the heat of an arc formed between said electrodes, said metallic member being connected only to the other end of said second electrode for mechanically supporting said secondelectrode and electrically connecting it to an associated circuit.

JOHN W. KALB.

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