US1669494A

US1669494A - Lightning arrester

Info

Publication number: US1669494A
Application number: US709717A
Authority: US
Inventors: Slepian Joseph
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1924-04-29
Filing date: 1924-04-29
Publication date: 1928-05-15
Anticipated expiration: 1945-05-15

Description

May 15, 192s.

J. SLEPIAN LIGHTNING ARRESTER Filed April 29, 1924 ffy.

wlTNEssEs:

ATTORNEY Patented May 15, 1928.

UNITED STATES PATENT FFlCE.

JOSEPH'. SLEPIAN, OF SWISSVALE, PENNSYLVANIA., ASSIGNOR TO WESTINGHOUSE ELECTRIC zMANUFACTURING COMPANY, A. CORPORATION OF PENNSYLVANIA.

LIGHTNING ARRESTER.

Application filed April 29, 1924. Serial No. 709,717.

lily invention relates to lightning arresters and it has particular relation to lightning arresters for direct-current circuits.

In protecting electrical apparatus from the effects of excessive voltages caused by lightning, switching surges and the like, it has been found that a maximum degree of protection is obtained by the use of the so-called valve lightning arrester. Such arresters discharge current freely when the voltage across the arrester is above a certain definite value, called the relief voltage, and prevent thev passage of current when the voltage across the arrester is substantially below the relief value. The relief voltage of the valve arresters is usually so chosen that the voltage of the line to which the arrester is connected is below the value required for causing discharge through the arrester. If the electric circuit which is to be protected is at a potential considerably above the line voltage, current passes freely through the valve arrester until the potential of the line is lowered to the value corresponding to the relief voltage. Among the types of arresters which have the above-described valve characteristics, I may mention the electrolytie arrester, the oxide film arrester and the auto-valve arrester, the latter being described in my copending application, Serial No. 535,810, filed February, 11, 1922, patented September 23, 1924, No. 1,509,493.

One of the characteristic features of valve arresters developed at present' is the fact that they permit the passage of relatively large leakage currents when directly connected across the line operating at normal voltages. In the case of electrolytic arresters, such leakage current is to a certain degree desirable since it promotes the maintenance of an active film, upon which the valve action of the arrester depends. However, when using valve arresters of the other types mentioned above, it is found desirable to eliminate the passage of leakage current through the arrester during the normal or non-discharging period, and to this end there is added a series insulating gap or open gap which is so balanced with respect to the relief voltage of the arrester that the break-down voltage for surges remains substantially unchanged, while the open gap insulates the valve-aggregate from the line during the ordinary operating period.

When operating on an alternating current system a series insulating gap of any type, such as a gap having brass or iron electrodes, may be employed.

In an arrester having such electrodes the small leakage current may occasionally keep the gap broken down after the passage of a lightning discharge on account of the relatively low potential, about 400 volts, suflicient to maintain the current through such gap. The small current is, however, positively interrupted at the next alternation, when the current passes through Zero, and the insulating character of the gap is thus quickly restored. I have found, however, that when an arrester of the above-described character is connected across a direct-current circuit, an ordinary insulating gap having brass, copper or iron electrodes does not fully perform the functions of insulating the valve aggregate from the line under normal operating conditions, since the series gap, once it is broken down by the passage of the discharge current, remains ionized and permits the passage of the small leakage current at ordinary operating voltages, on account of the unidirectional character of the impressed voltage.

One object of my invention is to provide a lightning arrester comprising, in combination, a protective unit and an insulating gap of such character as to render currents of the order of the leakage current of the protective aggregate unstable and to interrupt the same.

In practicing my invention I employ aluminum electrodes in a series gap instead of the brass or iron electrodes ordinarily used, or in general, I employ electrodes requiring, for minute currents of the order of the leakage current, a voltage of the order of the relief voltage of the arrester.

I have found that gap devices having electrodes made of carbon, graphite, carbonaceous resistor material and the like, giveta performance similar to that obtained with aluminum electrodes requiring a much larger voltage for maintaining small currents than brass and iron electrodes. The performance described above is quite dierent from the behaviour of suoli electrodes when carrying ordinary low voltage arcs. Thus.v while carbon electrodes, for instance, maintain with difficulty minute currents of the order of the above-mentioned leakage currents,it is well known that carbon holds an arc of ordinary strength with great ease.

With the foregoing and other objects in view, my invention comprises the arrangements and details of construction described and claimed hereinafter and illustrated in the accompanying drawing wherein Figure 1 is a sectional View of a lightning arrester made in accordance with my invention,

Fig. 2 is a circuit diagram showing a direct-current railway distribution system protected by means of arresters made according to my invention, and

F ig. 8 is a diagram showing characteristic curves of discharges between gaps of various materials, explained more fully hereinafter.

Referring to Fig. 2, there is shown a railway distribution system comprising a trolley conductor land a return conductor 2 supplied from a direct-current generator 3. A load device, such as a vehicle a, derives power from the trolley and return conductor and supplies it by means of a control-device 5 to adriving motor `6.

In order to protect the circuit and apparatus from excessive voltages, the generator 3 as Well as the electric apparatus of the vehicle 4f `are shunted by

lightning arresters

7 and 8, respectively. I have found that arresters of the type' described in the above mentioned application, Serial No. 535,810, are particularly well adapted for service on railway vehicles on account of their simplicity and ruggedness.

vAn arrester of this character is shown in Fig. 1 and comprises a valve aggregate 10 and a series gap 11. The valve aggregate may consist of a plurality of stacks 412 to 16, inclusive, of resistance disks 'held between terminal plates 18 to 28, inclusive of conducting material, such as brass, constituting terminals of the valve aggregate. Each stack is composed of four resistance plates 26: constituting Vthree gaps spaced lby thin mica rings 27. The thickness of the mica rings and of the gaps is very much exaggerated in the drawing in order that they may appear. The resistivity of the disks 26 is of the order of 10 ohms per centimeter 'cube or more and the thickness of the mica spacers is approximately 3 to 5 thousandths ofan inch.

The reverse sides of the end plates 28 of each stack are coated with a conducting layer 29, such as sprayed copper, for making con-tact with the terminal plates 18 to 23, inclusive. The terminal plates 18 to 23 are so connected to two

'terminal conductors

31 and 32 that the several stacks 12 to 16 constitute parallel-connected gap units in order to increase the vdischarge current capacity of the aggregate. One ofthe terminalconductors 32 rleads to the return conductor 2 of the line and the other terminal conductor 31 leads to one aluminum-electrode 33 of the series gap 11, the other aluminum electrode 34C being connected to the terminal leading to the trolley wire.

Assuming that the line operates `at a voltage of approximately 750 volts direct-current, the relief voltage of the valve aggregate having three gaps is about 3 350= 105() volts, which is a proper relief voltage. The series gap 11 is so adjusted that its break-down voltage is approximately 1100 volts, but the combined break-down voltage of the valve aggregate and series gap on alternating potential is not much higher thany the breakdown voltage ofthe se-r1es gap alone on account of vthe relatively large electrostatic capacity of the valve aggregate and the relatively low leakage resista-nce ofv the same.

. The principal novel feature of my invention is the fact that the series gap 11 has an electrode of such material as to cause the minute leakage-current discharges to become unstable at thel normal operating voltage of the line. `When operatingy on .alternating current, no special provisions need to be made to Ainterrupt the leakage Current since it is automaticallyV interrupted upon the reversal of the alternation. However, in direct-current. applications, the directcurrent potential is continuously applied across the arrester and theseriesgap 11,. if once broken down by a discharge, remains broken down and carries al small leakage current rendering the insulatinggap practically inedective. In order to interrupt that current and secure the full insulating eect of the series gap, I use electrodes of aluminum, or of a material having a similar effect, in the series gap.

In Fig. 3 are shown curves illustrating the very marked difference in the performance of aluminum gaps as comparedwith gaps of brass and iron when passingfcurrents ofthe order of one milliampere. VlVhen carrying currents of the order of one ampere or more, the voltage between the electrodes' is approximately the same for all materials, thispart of the curve being omitted in the drawing. lhen the current through the gap is reduced, the voltage across the same increases to a value of about 350 to 450 volts for iron and brass electrodes respectively. The voltage which is necessary to maintain such minute currents between aluminum electrodes rises sharply and is more than twice the value of the voltage between brass electrodes for currents of the order of two milliamperes, and is many times more for still lower currents. l

Of the three gap devices Vused in obtaining the above-described curves the aluminumelectrode gap had the lowest break-down voltage, 1350 volts, while the breakdown voltage of the Airon-electrode gap was 1750 i IUD nasales volts and that of the brass-electrode gap was 2910 volts. Using gaps having the same breakdown voltage, the diiierence in the perormance of the gap having aluminum electrodes would be still more marked.

A performance similar to that obtained with aluminum electrodes, may be secured byusing carbon, graphite or similar electrodes.

Vithout restricting myself to my present understanding of the reasons for the ditlerences in the performance' ot gaps having electrodes of different materials, I believe that the reason for the above-described opera'tion'ot an alu'minum gap is that the oxide film upon the surface ot the electrode, which is broken down by heavy current discharges, iscapable of maintaining itself when passing very minute currents and thus insulates the kelectrodes andv interrupts the discharge. In the' case of o'pengap arresters having electrodes of carbon, graphite or resistor materials, I believe that the gases occluded in the electrodes are treed, causing the cathode spot to Wander around and be unstable. In general, I have noted that in gaps made of materials requiring a high voltage for maintaining minute discharges, the cathode spot of the discharge has a tendency to Wander over the surface ot the electrode thus rendering it unstable. This renders it feasible to utilize in any gap, a negative electrode of one of the materials set forth above and a positive electrode of any suitable maf terial.

By providing a series insulating gap requiring a large voltage when passing small currents, in combination with the valve arrester, the leakage current passing through the series gap is quickly interrupted since the line voltage is not sufficient to maintain such minute currents in the series gap. By using the combination, as described above, I thus fully utilize the valve properties of the main protective aggregate, and, at the same time, protect the same from any detrin mental eiiect that the continuous passage of leakage current therethrough may have.

Although I have described my invent-ion, giving a specific construction thereof, I do not Wish to be limited thereto, but desire that the appended claims shall be construed to cover all modifications `falling in the spirit of my invention.

I claim as my invention:

l. The combination With a direct-current line, ot an excess-voltage protective device connected thereto and adapted to pass a small leakage current of the order of milliamperes and including a series-gap device of which the negative electrode is aluminum, whereby said small leakage current is quickly interrupted after the passage of an excessvoltage discharge.

2. A discharge-gap protective device adapted for use on direct-'current circuits, the negative electrode, at least, of said gap device being 'ot aluminum, and means connected in series With said gap device for causing any discharge current which flows during normal line-voltage 'conditions 'to vbe ot the order ot milli-amperes.

3. A discharge-gap protectivedevice adaptr` ed for use on d'irectecurrent circuits, the negative electrode, at least, of said gap de vice being of aluminum, and means connected in series with said gap device for causing any discharge' current which iovvs during normal line-voltage conditions to be so small that the discharge across the gap is unstable and cannot llong endure after the' termination of excess-voltage conditions on the fline to be protected.

1. The combination With a direct-'current line, of an excess-voltage' protective device connected thereto and adapted to pass a small leakage current ot the order of milliamperes during normal voltage conditions on the line but passing current With comparative freedom during excess-voltage conditions on the line, the circuit connecting said device to the line comprising a seriesgap device of which at least the negative electrode is aluminum.

5. The combination with a direct-current line, ot an excess-voltage protective device connected thereto and adapted to ass a small leakage current of the order o milliamperes and including a series-gap device ot which the negative electrode is of a material causing the cathode spot to quickly Wander in an unstable manner, during current discharges of the order of milliamperes.

6. A discharge-gap protective device adapted for use on direct-current circuits, the negative electrode, at least, of said gap device being ot a material causing the cathode spot to quickly Wander, in an unstable manner, during current discharges of the order of milliamperes, and means connected in series With said gap device for causing any discharge current which tlows during normal line-voltage conditions to be of the order of milliamperes.

7. A discharge-gap protective device adapted for use on direct-current circuits, the negative electrode, at least, ot said gap device being ot a material having a discharge voltage considerably in excess of that for a similar gap device With brass electrodes, for discharge currents of the order of milli` amperes, and means connected in series with said gap device for causing any discharge current which flows during normal linevoltage conditions to be so small that the discharge across the gap is unstable and can not long endure after the termination of excess-voltage conditions on the line to be protected.

8. The combination with a direct-current line, of an excess-voltage protective gap therefor having electrodes of a material having a dischargevoltage considerably in excess of thatfor a similar gap device with brass electrodes, for discharge currents of the order. of kmilliamperes, and means connected in series with said gap device for causing any discharge current which flows during normal line-voltage conditions to be of the order of milliamperes. n

. 9. The combination yWith a direct-current line, of an excess-voltage protective device connected thereto and vadapted to pass a small leakage current `of the order of milliamperes during normal voltage conditions on the line but passing current With comparative freedom 'during excess voltageconditions on the line, the circuit connecting said device to the lineccomprising a series excess-voltage protective gap having electrodes at least one of Which is of a. material having a discharge voltage considerably in excess of that for a similar gap device with brass electrodes, for discharge currents of the order of milliamperes. y

10. A lightning arrester comprising van air-gap device having an electrode of a material having a discharge Voltage considerably in excess of that for a gap device with similarly spaced brass electrodes, for discharge currents less than about six milliamperes, and means for limiting the discharge at normal-voltage conditions to a leakage current so small that the arc is unstable under direct-current operating conditions, said means permitting the relatively free HOW of discharge currents during eX- cess-voltage conditions.

In testimony whereof, I have Ahereunto subscribed my name this 16th day .of April,

1924. y y c JOSEPH SLEPiAN.