US2277912A - Resistor - Google Patents

Description

v.' E. JOHNSON ET AL March 31, .1942.

REsIsToR Filed Oct. 29, 19:58

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)QM/M41 V. E. JOHNSON ET AL RESISTOR Marclfl 31, 1942.

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WITNEssEs; 64 e '64 INVENToRs .e Franlfl Farmez; Perry I( Stewart, Vietorl Johnsonay AJ 1n P6011 ATTORNE Patented Mar. 31, 1942 UNITED STATES PATENT OFFICE RESISTOR Sylvania Application October 29, 1938, Serial No. 237,786

6 Claims.

Our invention relates generally to resistors, and it has reference, in particular, to an improved construction for neutral grounding resistors.

In large polyphase power distribution systems, it is usually desirable to provide a grounding resistor between the neutral wire of the system and the ground, for limiting the flow of fault current and assisting in stabilizing the system when a ground fault occurs therein. The duty imposed on neutral resistors is of a radically different nature than that imposed on ordinary control resistors, and While called upon to function at relatively rare intervals, they must at those timesl be able to handle unfailingly and Without distress large currents for indefinite periods. Under normal conditions, the grounding resistor may carry current for only a very short period of time, while under abnormal conditions, exceedingly heavy currents must be carried by the grounding resistor for relatively long periods of time of from one minute to ten minutes in extreme cases before the system is disconnected from the source of power, or before the faulty section of the system is isolated. Since these grounding resistors are necessarily relatively large, owing to the peak currents and the extended periods of fault duration for which they must be designed, it is desir'- able to locate the resistors out of doors adjacent to the point of ground connection of the system, so as not to use up valuable space within the station, or require separate housings therefor. It may, therefore, be seen that in general it is d'esirable to provide a resistor which is not only capable of withstanding exceedingly heavy surges of current for extended periods without failure, but which is also unaffected by exposure to varied weather conditions.

Heretofore, it has been customary to utilize indoor mounted resistors comprising a plurality of cast iron grid sections rigidly mounted on supporting rods in 'a frame, for providing the necessary amount of resistance and the desired current carrying capacity. Resistors of this type were required to be thus protected as such construction necessarily involves a large number of joints between adjacent grids, which, if the grids are exposed to Weather, often become a source of trouble, since corrosion of the grid surfaces tends to increase the contact resistance between the 'adjacent grids. This results in excessive heating at the joints, during the flow of fault current, so that the cast iron grids may become sufficiently overheated to melt. Failure of a neutral grounding resistor under such conditions,

during the existence of a fault on the system, may result in serious system disturbances, disrupting the service at points quite remote from the source of the fault. Furthermore, resistor elements of this kind are inherently weak structurally and may become broken at any time, depriving the system of the ground protection the resistor is intended to afford, thus requiring constant vigilance and testing to eliminate the hazards of open circuits resulting from broken grids. A cast resistor is inherently a low-temperature device, and when rated at ve or ten minutes, becomes abnormally large and expensive.

Therefore, the object of our invention, generally stated, is to provide an improved resistor of simple and rugged construction, which may be economically constructed and maintained, which is reliable in operation, and which will withstand continuous exposure to the weather and extreme heating and cooling cycles without deterioration or breakage.

A more specific object of our invention is to provide for utilizing, in a resistor, a non-corrosive continuous resistance element of a material which will resist deformation at high temperatures, and for yieldingly supporting the element in a frame to allow expansion and contraction of the element.

Another object of our invention is to provide for supporting or suspending a fiexible strip resistance element in a rigid frame structure in such manner as to eliminate distortion thereof due to expansion and contraction.

A further object of our invention is to provide for resiliently suspending a continuous strip resistance element in a support frame Without impairing the current carrying characteristics, or the ventilation thereof.

Another object of our invention is to provide for supporting or suspending a flexible strip resistance element formed in a zigzag shape in a supporting frame by utilizing insulating hangers connected to the reversely curved or bent portions of the element.

A further object of our invention isv to provide for forming a resistance element of the abovedescribed character in such manner that it may be suspended in a frame by supporting it at the reversely curved or bent portions only.

A still further object of our invention is to utilize a grounding resistor comprisingv a continuous stainless steel stripelement in the neutral ground connection of a power system.

A further object of our invention is to utilize a plurality of yieldingly supported stainless steel resistor elements in a resistor unit to provide a compact and durable neutral grounding resistor.

Other objects will, in part, be obvious, and will, in part, appear hereinafter.

In a preferred embodiment of our invention, the resistor may comprise, generally, a continuous strip resistance element which is disposed to be supported in a rigid frame so as to allow for expansion and contraction of the element without subjecting the parts thereof to undue stress. In order to safely withstand the heavy current surges to which the resistor may be subjected when utilized, for example, for grounding the neutral point of a power system or the like, the resistance element comprises preferably a strip of one of the nickel chromium alloy steels, commonly known as stainless steels, which is not only highly resistant to corrosion, but which also retains a relatively high degree of mechanical strength even at relatively high temperatures, so that the resistor may be safely operated at much higher temperatures than can cast iron grid resistors, thus permitting a marked reduction in the mass of the resistance element, and at the same time utilizing the greatly increased ohmic value of the resistance element at these higher temperatures for restricting the flow of fault current.

For a further understanding of the nature and scope of our invention, reference may be had to the following detailed description, taken in connection with the accompanying drawings, in which:

Figure l is a plan view of a portion of a resistor embodying a preferred form of our invention;

Fig. 2 is a side view of a spacer member used with the resistor of Fig. 1;

Fig. 3 is an enlarged perspective view of a portion of the resistor' of Fig. l;

Fig. 4 is a side elevation view of a resistor construction comprising a plurality of the resistors of Fig. 1;

Fig. 5 is an enlarged perspective View of a portion of an alternate form of resistor construction;

Fig. 6 is a sectional view of the resistor of Fig. 5 along the line VI--VI;

Fig. 7 is a sectional view of the resistor element of Fig. 6 along the line VII-VII;

Fig. 8 is a side elevation view of a different embodiment of our invention; and

Fig. 9 is an end elevation view in part of the I embodiment of Fig. 8.

Referring particularly to Figs. 1 and 2 of the drawings, the reference numeral II) denotes generally a resistor comprising a frame I2 having a strip or ribbon resistance element I4 supported therein, so as to allow for expansion and contraction of the element without subjecting parts of the resistor to undue stress.

The frame I2 may preferably be fabricated from structural steel shapes, in any suitable manner, so as to provide a substantially rigid support for the resistance element I4. As it is preferable to install neutral grounding resistors outdoors on account of their size, without any housing or other protection from the weather, the frame I2 may be heavily galvanized or otherwise weatherproofed, so as to prevent corrosion.

In order to provide a resistance element for grounding the neutral point of a system, having a high degree of resistance to corrosion, and a high degree of mechanical strength at relatively high temperatures, so as to resist deformation and prevent sagging of the element at the relatively high temperatures under which it may operate, we prefer to form the resistance element I4 from a strip of one of the nickel chromium alloy steels, commonly known as stainless steels. At temperatures ranging as high as 800 C. and 100 C., the short time tensile strength of stainless steel is of the order of 15,000 and 3,000 pounds per square inch, respectively, thus permitting the safe operation of the resistor at temperatures far above the safe operating temperature of cast iron grids, and materially reducing the mass of resistance element necessary to dissipate a surge of energy in limiting a fault disturbance in the system.

In our preferred form of construction, a flat strip of stainless steel may preferably be formed flatwise into a plurality of substantially parallel back and forth turns, having rounded U-shaped ends I6 therebetween, which is commonly referred to as a zigzag shape. If desired, the straight portions of the back and forth or zigzag turns may be stiff'ened, by providing them with one or more longitudinal indentations It through a suitable pressing operation, thus giving the resistance element a dished cross-section, as shown in Figs. 6 and '7, for the purpose of preventing undue vibration of the element and increasing its resistance to deformation. Insulating spacers I9, which are clearly illustrated in Figs. l and 2, may be provided, comprising a plurality of spaced apart fingers 2D which may be disposed between the adjacent straight portions of the resistance element, near the central portion thereof, to further increase the rigidity thereof. Connection may be made to the resistance element I4 in any suitable manner, such as by connecting a bus bar 22 directly to the resistance element by suitable means, such as the bolts 23, which may be reinforced by welding if desired.

Support members 20 may be provided `for supporting the resistance element Iii in the frame I2 so as to provide for a yielding or lost-motion connection therebetween. In a preferred form, as is clearly shown in Fig. 3 of the drawings, the support member 28 may comprise a substantially flat strip of stainless steel, or other suitable material, having a reasonable degree ol flexibility and of sufficient width to prevent undue side sway, one end of which is preferably bent ilatwise at a slight angle to vthe main body, and subjected to a forming operation to provide upstanding edges 29 by which it may be connected to the resistance element III at the rounded U-shaped end portion Iii, being, for example, welded thereto. The other end of the support member 28 may be connected by any suitable means, such as the bolts 29, to an insulator support 30 which comprises, i'or example, a substantially hollow body member oi a refractory material, having a flanged base portion 32. In this manner, We provide a flexible hinge or support for allowing expansion and contraction ol' the resistance element I4 without subjecting it to undue stress.

Bracket members may be provided for securing the insulator supports to the frame I2, comprising, for example, channel members having inturned longitudinal edges 30 and S'I which are disposed to form a longitudinal pocket in Which the flanged base portions 32 of the insulator supports 30 may positioned. By utilizing such a form of construction, the insulator supports 3B may be readily slid into position through the open end ol the bra ret members 35. If desired, the edge portion 3l of the bracket member may comprise a separate member removably secured to the base thereof, and upon removal thereof, any or all of the insulator supports 30 may be removed, and cleaning or repairing thereof may be easily effected when desired. In order to provide further insulation between the resistance element I4 and the frame I2 of the resistor, the bracket members 35 may be positioned in the frame I2 by being mounted in any suitable manner on insulators 4u, which are secured to the frame.

Referring particularly to Fig. 4, it may be seen that a compact and efficient resistor may be provided by combining a number of resistors, such as illustrated in Fig. l, in stacked relation, utilizing either separate frame structures or a single frame having a plurality of pairs oi support brackets 35 secured thereto in the manner hereinbefore disclosed in connection with Figs. 1 and 2. Suitable connection may be made between the ditferent tiers o f resistance elements I4 by the conductor 22, which may be easily connected to the resistance elements by any suitable means, such as the bolts 23, and welded, to provide an integral connection with no loose connections to corrode or become open.

Referring to Figs. 5, 6 and '7, which illustrate details of an alternate form of resistor construction, the reference number 135 designates a modied form of support member which is disposed to be secured to the rounded end portion I6 of the resistance element i4 for supporting the resistance element in the frame I2 and providing a yielding or lost-motion connection therebetween, so as to permit expansion and contraction of the element without subjecting it to undue stress.

The support member 45 may, as indicated particularly in Figs. and (i of the drawings, comprise a C-shaped body, preferably formed by bending a length of stainless steel strap flatwise, and providing a suitable wear bushing 41, located in the central portion of the body for reducing friction and eliminating localized wear. Parallel slots may be cut in the lip portions of the support member 45, and the centrally disposed tongue portion 4B dened thereby may be bent outwardly to occupy the position shown in dotted lines in Fig. 6. By then placing the lip portions of the support member 45 against the resistance element I4 and bending the tongue portions 43 inwardly about the resistance element, the support member 45 may be secured to the resistance element without impairing the current carrying capacity of the element, and without providing too ready a path for the flow of heat from the resistance element to the means of support, thus facilitating high temperature operation of the resistance element.

In order to yieldingly support the resistance element in the frame I2 and provide for relative motion therebetween, support insulators 49 may be provided, having an extending support pin 5I] which is disposed to effect a sliding t in the bushing 4T of the support member 45. In this manner, the resistance element I4 may be properly supported in the frame I2, and yet free to expand and contract without setting up undue stresses. I'f desired, suitable baffle members or cooling ns 52 may be provided in connection with the support pins 5D, for additionally reducing the flow of heat between the resistance member and the insulators 49, and permitting the resistance element I4 to operate at a higher temperature, thus further reducing the required amount of mass of the element.

In Figs. 8 and 9 a different embodiment of our invention is illustrated wherein a flexible stainless steel resistance element I4 is disposed to be suspended in looped relation in a rigid frame structure 60, which may be of any suitable form, comprising, for example, structural steel members or pipe members.

In a preferred form of construction, horizontal side members 62 may be secured to the frame 60 and provided with suitable insulator members 64 secured thereto in any desirable manner, such as by means of the clamp type brackets 65 for positioning upper and lower support members 66 and 68. These support members may, for example, comprise suitable lengths or" pipe having a plurality of oppositely disposed laterally extending fingers B9. Grooved insulating spools 'i2 of a rerractory material may be positioned on the ngers G9 for supporting the resistance element I4. If desired, these spools may be secured to the fingers 39 so as to be rotatable thereon, for a reason which will be hereinafter explained in detail.

Suitable saddle members "I4 and It may be provided for securing the upper and lower support members @Si and 63, respectively, to their respective insulator members Gi. The support members may be provided with suitable means, such as the projecting pins i8 for engagement with openings in the top or" the saddle members so as to prevent rotation oi' the support members therein. By making the throat portion of the lower saddle member 'It sufciently deep, provision may be made for relative vertical movement of the support member t2 therein.

The resistance element I4 may then be suspended from the upper support member 66 by looping the element I4 alternately about successive spools l2 of the upper and lower support members to provide a plurality of zig-zag back and forth turns. Ey looping the element I4 about the insulator spools 2 of the upper and lower support members with the lower support member G8 in a raised position in the saddle member 1G, a moderate loading may be placed upon the loops of the resistance element I4 because of the downward force exerted by the lower spacing member 55, so that the back and forth portions thereof may be maintained in tension, unaffected by expansion of the element when under load, or the subsequent contraction thereof when cooling after the load is removed. Since the spools 72 arev rotatable upon the fingers 69, uneven expansion or" the resistance element may be compensated for, as the element may move relative to the points of support. The turns of the resistance element may thus be maintained under a constant tension and the spacing between successive turns may be maintained without the necessity of intermediate spacers or support members.

It may be seen from the above detailed description, taken in connection with the accompanying drawings, that we have by our invention provided an improved construction for resistors whereby a non-corrosive continuous strip resistance element may be supported in a rigid frame in such a manner as to allow for expansion and contraction of the element without impairing the current carrying capacity thereof. With the resistance element secured to the support members in the manner hereinbefore disclosed, the transfer of heat from the element to the supporting insulators is reduced to a minimum, so that the element may safely operate at relatively high temperatures. By utilizing commercial stainless steel, which may be easily secured in any desirable length, thickness and Width, a resistance element may be provided which has a high cooling factor, a high degree of resistance to corrosion, and is highly resistant to deformation, even at relatively high temperatures. Accordingly, We are able to provide a neutrai grounding resistor of rugged construction, having a non-corrosive high strength resistance element wherein the number of joints is held to a minimum, which will not fail when subjected to a heavysurge of current, and Which may comprise a reduced mass because of the permissible higher operating temperatures, thus insuring the reliability of the resistor in service, and a low cost of maintenance.

While our invention has been found of particular value in connection with applications of neutral grounding resistors, Where sudden and excessive surges of fault current must be carried, it is to be understood that the invention is not intended to be limited to such applications, and may be readily and advantageously used Wherever an economical, reliable resistor is desired.

Since certain changes may be made in the above construction, and different embodiments of the invention may be made Without departing from the spirit thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be considered as illustrative, and not in a limiting sense.

We claim as our invention:

1. A resistor comprising, a frame structure, a resistor element formed of a relatively wide strip of flexible material bent ilatvvise to provide a plurality of substantially horizontal back and forth turns on edge and having a Width sufficient to prevent undue sagging of the center portions oi the turns when supported only at the ends, a plurality of insulating members supported by the frame structure in spaced relation with the said ends of the back and forth turns, and connecting means interposed between the insulating members and the ends of the back and forth turns for supporting said resistor element from said insulating members, said connecting means comprising relatively wide vertically rigid metal support members rigidly secured to said ends of the back and forth turns and having a limited area of contact therewith to reduce the heat flow to the insulating members, and means connecting the support members to the insulating inembers to provide a substantially rigid vertical support for supporting the ends of the said turns in spaced relation to the insulating members, said connecting means functioning to provide a support yieldable horizontally in the longitudinal direction of the back and forth turns to permit longitudinal expansion and contraction of the back and forth turns and support the bach and forth turns rigidly in the vertical direction.

2. A neutral grounding resistor comprising, a frame, insulating support members removably supported thereby, a continuous strip resistance element comprising a plurality of substantially horizontal back and forth turns on edge, means comprising metallic support members having the ends rigidly secured to the resistance element adjacent the opposite edges thereof at the ends of the said turns and having openings intermediate the ends, and support means projecting from the insulating support members positioned in said openings for connecting the resistance element to the insulating support members and rigidly supporting the resistance element vertlcally in the frame in spaced relation to the insulating support members without appreciably stressing the turns longitudinally to allow longitudinal movement of the ends of the turns of the element relative to the frame.

3. A resistor comprising, a frame, a horizontally disposed resistance element having a plurality of back and forth turns of stainless steel ribbon on edge with a width sufficient to render the turns self-supporting when supported at the end thereof, a plurality of oppositely disposed insulator supports removably secured to the frame, and means comprising a plurality of flexible hanger members for resiliently supporting the resistance element at the said ends from the insulator supports Without appreciably stressing the turns longitudinally for movement in a horizontal direction only to provide for expansion and contraction of the resistance element.

Ll. A resistor comprising, a frame, insulating means disposed to be removably secured to the frame, a plurality of substantially horizontal back and forth turns of flexible resistance ribbon formed from a non-corrosive nickel chromium alloy steel having a high temperature of deformation, and resilient support means comprising a plurality of hangers dependent from the insulating means disposed to support the resistance ribbon in yielding relation with the insulating means Without appreciably stressing the resistance ribbon longitudinally.

5. A resistor comprising, a frame, a plurality of insulating members supported by the frame, a resistance element comprising a plurality of substantially horizontal back and forth turns formed from a continuous strip of resistance material, and a plurality of substantially fiat hanger members dependent from the insulating members connected to the resistance element at fixed points so as to permit movement of said points only in the longitudinal direction of expansion and contraction of the spaced turns of the resistance element.

6. A resistor comprising, a frame, a continucus ribbon resistance element comprising a plurality of substantially horizontal back` and forth turns, a plurality of insulating members supported by the frame having metallic pins projecting substantially horizontally therefrom, and means comprising substantially C-shaped support members rigidly connected to the resistance element at the ends of the back and forth turns having openings, for receiving the metallic pins, said pins and support members functioning to slidably support the resistance element on the pins in spaced relation to the insulating mem bers.

VICTOR E. JOHNSON. FRAITK H. FARMER. RAY A.. SIMPSON. PERRY V, STEWART.

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