SELF-COMPRESSIVE SURGE ARRESTER MODULE AND METHOD OF MAKING SAME
BACKGROUND OF THE INVENTION The present invention relates generally to electrical power distribution equipment More particularly, the invention relates to sub-assemblies or modules that contain discrete electrical components and that are employed in protective devices such as surge arresters Still more 10 particularly, the invention relates to apparatus and methods for applying an axially-compressive force to an array of electrical components and retaining those components under compression in end-to-end relationship within the module
Under noittftl operating conditions, electrical transmission and distnbutiqn is subject to voltages within a fairly narrow range Due to bghtning stnkes, switching surges or 15 other system disturbances, portions of the electrical network may experience momentary or transient voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions Left unprotected, critical and costly equipment such as transformers, switching apparatus, computer equipment, and electrical machinery may be damaged or destroyed by such over-voltages and the resultant current surges Accordingly, it is 20 routine practice with'n the electrical industry to protect such apparatus from dangerous over-voltages through the use of surge arresters
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert the over-voltage-mduced cuirent surges safely around the equipment, thereby protecting the equipment and its 25 internal circuitry from damage When caused to operate, a surge arrester forms a current path lo ground having a>ery low impedance relative to the impedance of the equipment that it is
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protecting In ti.*s way, current surges which would otherwise be conducted through the equipment are instead diveitsd through the arrester to ground. Once the" transient condition has passed, the arrester operates to open the recently-formed current path to ground, thereby again 30 isolating the distribution or transmission circuit in order to prevent the non-transient current of the system frequency from "following" the surge current to ground, such system frequency current being known as "power follow current"
Conventional surge arresters typically include an elongate outer enclosure or housing made of an electrically insulating matenal, a pair of electrical terminals at opposite ends of the
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enclosure for connecting the arrester between a line-potential conductor and ground, and an array of other electrical components forming a series path between the terminals These components typically include a stack of voltage-dependent, nonlinear resistive elements These nonlinear resistors or "vanstors" are characterized by having a relatively high resistance at the normal steady-state voltage and a much lower resistance when the arrester is subjected to transient over-voltages Depending on the type of arrester, it may also include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the vanstors Some present-day arresters also include electrically conducting spacer elements coaxially aligned with the vanstors and gap assemblies Electrodes of a vanety of types and configurations may also be included in the component array in conventional arresters
For an arrester to function properly, it is important that contact be maintained between the ends of the vanous surge arrester components in the array To accomplish this, an axial load is placed on the elements m the array Such loading is typically applied by employing springs within the housing to urge the stacked elements into engagement with one another Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the components, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the arrester elements in the array and the end terminals
Another conventional means for supplying the required axial force is to wrap the stack of arrester elements with glass fibers so as to axially-compress the elements within the stack Examples of such pnor art surge arresters include US Patent Nos 5,043,838, 5,138,517, 4,656,555 and 5,003,689 These patents generally descnbs rather elaborate techniques for winding the fibers about the ends of a stack of arrester components to apply the appropnate axial force to the components within the stack Employing certain of these techniques requires the inclusion of specially-configured components within the stack, such as special end terminations for maintaining specific separations between the fibers (for example, U S Patent No 5,043,838) or for creating a.shoulder against which the fibers can be wound (for example, U S Patent No 5,138,517)
In addition to maintaining an axial compression, these stacked arrester components must be retained in such a manner that will permit gases evolved during arrester failure to be safely vented from the arrester Occasionally, a transient ovcrvoltage condition may cause some degree of damage to one or more of the resistive elements Damage of sufficient seventy can result in arcing within the arrester housing, leading to extreme heat generation and gas evolution as the internal components in contact with the arc are vaponzed This gas evolution causes the pressure within the arrester to increase rapidly until it is relieved by either a pressure relief
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means or by the rupture of the arrester housing The failure mode of arresters under such conditions may mclude the expulsion of components or component fragments at high velocities and in all directions Such failures pose potential risks to personnel and equipment in the vicinity
Attempts have been made to design and construct arresters that will not catastrophically fail with the expulsion of components or component fragments One such arrester is described m U.S Patent No 4,404,614 which discloses an arrester having a non-fragmenting liner and outer housing, and a pressure relief diaphragm located at its lower end. A shatterproof arrester is also disclosed m US Patent Nos 4,656,555, 4,930,039 and 5,113,306 Arresters having 10^^pressure relief means formed m their ends are described in U.S Patent Nos 3,727,108, |£4,001,651, and 4,240,124 US Patent No 5.043,838 discloses a filament wrapped arrester module that mcludes openings between the criss-cross pattern of windings These openings are filled with an epoxy or similar insulating matenal that is permitted to rupture to allow the expulsion of gasses
Despite such advances, however, state of the art arresters may still occasionally fail with the expulsion of components or fragments of components This may, m part, be due to the fact that once the internal components in these arresters fail, the resulting arc vaporizes the components and generates gas at a rate that cannot be vented quickly enough to prevent rupture of the arrester enclosure Accordmgly, there remains a need in the art for an arrester which, 20 upon failure, will fail in a non-fragmentmg and safe manner A need also exists for an arrester
» whose components are axially compressed without the use of a spring
There further remains a need m the art for a means to compress axially an array of arrester components that may be applied simply and easily, without elaborate and costly manufacturing procedures or the addition mto the component stack of specialized components 25 Preferably, the means would be easily applied to the external surfaces of the stacked components It would be further advantageous if the compression means were to include features enhancing the tensile and cantilever strengths of the arrester assembly Further, the device should provide a venting means for relieving gas pressure and preventing the electrical assembly from failing in a dangerous fashion, and should provide good bonding at each interface 30 from the MOV stack outward without requiring complicated assembly procedures or costly
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waste It is an obiect of the present invention to address the needs set out above, or to at least pro vide the public with a useful choice
SUMMARY OF THE INVENTION
The present invention comprises a surge arrester subassembly that mcludes a plurality of electrical components stacked in an axial array and an insulanve coating disposed over the outer
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surface nf the axial array The coating is preferably bonded to the outer surface of the array and applies both axially- and radially directed forces to said array to maintain the components of the array in good electrical contact According to the present invention, the coating has a coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the electrical 5 components and is cured at a temperature in the range of the operating temperature of the components, so that when the coated array is cooled below the cu?e temperature, the coating will tend to shrink more than the electric* i components, thereby exerting compressive forces on the array. The present invention also may mclude both longitudinal and circuniferential fibrous reinforcement withm the coating, which reinforcement preferably comprises glass fibers Those 10 skilled m the art will understand that the present coating can be applied over the desired portions of the array so as to result m a predetermined coatmg thickness
BRIEF DESCRIPTION OF THE DRAWINGS For an introduction to the detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings, wherein 15 Figure 1 is a cross-sectional view of an electrical subassembly module made m accordance with the present invention,
Figure 2 is a top view of a grooved electrode of the subassembly module shown in F:gure 1,
Figure 3 is an enlarged view of a portion of the subassembly module shown m Figure 1, 20 Figure 4 is an elevational view of the module shown in Figure 1 shown with layers of the msulanve coatmg partially cut away,
Figure 5 is a top view of the subassembly module shown in Figure 1,
Figure 6 is an elevational view of 'he module of Figure 1 shown at an intermediate stage of assembly,
Figure 7 is an end view of the module of Figure 1 shown at another intermediate stage of assembly,
Figure 8 is an elevation view of a surge arrester employing the subassembly module of Figure 1,
Figure 9 is an elevational view of an alternative embodiment of the present invention, 30 with portions of the insulative coatmg partially cut-away,
Figure 10 is a top view of another alternative embodiment of the present invention, Figure 11 is a cross-sectional view of an alternative electncal subassembly made in accordance with the present invention, and
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Figure 12 shows alternative arrays of components that can be used in modules constricted in accordance with the present invention
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring initially to Figures 1 and 8, there is shown a modular subassembly 10 of 5 electrical components made in accordance with the present invention. Module 10 has particular utility when employed in a distribution class surge arrester such as arrester 60 (Figure 8) Accordingly, to best descnbe the features and advantages of the present invention, module 10 will be desenbed with reference to a lOkA heavy duty lOkV (8 4 kV MCOV) distribution class surge arrester 60 It should be understood, however, that the invention is not limited to use in a 10 distribution class surge arrester, or in any size or rating of surge arrester, the invention instead having utility and advantages in any apparatus where it is necessary or desirable to retain an array or stack of electncal components under axial load.
Refemng once again to Figure 1, module 10 generally comprises an array 20 of electncal components stacked tn end-to-end arrangement and retained ui that arrangement by an 15 axially applied force supplied by an insulative coatmg 16 The present invention relates to the coating 16, and is not limited to any particular type, number or size of electncal components within array 20, for purposes of explanation, however, array 20 is depicted m Figure 1 as including three metal oxide vanstors 17 ("MOV's"), a pau of terminal blocks 14 and a pair of contact plates 18
Each MOV 12 is made of metal oxide that preferably is formed into a short cylindrical disk having an upper face 30, a lower face 32 and an outer cylindncal surface 31 The metal oxide for MOV 12 may be of the same matenal used for any high energy, high voltage MOV disk, and is preferably made of a formulation of zinc oxide See, for example, U S Patent 3,778,743 of the Matsushita Electric Industrial Co, Inc, Osaka, Japan, incorporated herein by 25 reference In the preferred embodiment, MOV 12 will have a uniform micro structure throughout the MOV disk and the exponent n for the zinc oxide formulation of MOV 12 will be in the range of about 10-25 at the steady state system voltage An exponent n of approximately 20 is most preferred It is preferred that the circular cross-section of MOV 12 have a diameter between approximately 1 to 3 inches to insure that there is sufficient surface area of between 30 about 0 785 and 7 07 square inches to maintain the desired durability and recoverabihty of the MOV's At the same time, it is also desirable that MOV 12 have as small a cross-sectional area as possible in order to reduce the size, weight and cost of the arrester As size is reduced, however, the durability and recoverabihty of the disk is lessened Given these competing considerations, a diameter of approximately 1 6 inches is the most preferred The thickness of
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MOV 12 as measured between faces 30 and 32 is preferably about 0 75 incheo. As understood by those skilled in the ait, given a particular metal oxide formulation and a uniform or consistent microstnicture throughout the MOV disk, the thickness of the MOV disk determines the operating voltage level
In the preferred embodiment, upper and lower faces 30, 32 of MOVs 12 are coated with sprayed-on metallized coatings of molten aluminum having a thickness approximately equal to 0.002 to 0010 inches MOVs 12 tn the present invention are preferably formed without insulative collars on outer surface 31 as are typically employed in conventional arresters
Contact plates 18 are disposed between the upper and lower faces 30 32 of adjacent 10 MOV's 12 As best shown in Figures 2 and 3, contact plates 18 generally comprise a metallic disk having outer edge 34 It is preferred that contact plates 18 include upper and lower ndged surfaces 38,40 which generally take the form of concentnc grooves such thai an outermost ridge 42 is formed on each of the upper and lower surfaces 38, 40 Electrode 18 is preferably produced from annealed aluminum, but may also be made from brass or other conducting 15 metals Contact plates 18 have an outside diameter approximately equal to that ofMOV's 12
As shown in Figures 1 and 5, terminal 14 is disposed at each end of array 20 and is a relatively short, cylindncal block machined or cast from any conducting matenal, preferably aluminum Terminals 14 have a diameter substantially equal to that of the collarless MOV's 12 and contact plates 18, and mclude a threaded bore 44 for receiving a threaded conducting stud 20 46 The outer cylindncal surface 48 of the blocks may be knurled or nbbed or otherwise textured to facilitate the physical connection between the biocks and coating 16 as descnbed more fully below
Coaling 16 retains MOVs 12, terminals 14 and contact plates 18 of array 20 m stackcd, end-to-end relationship, and provides an axially compressive force as desired for insuring low 25 contact resistance between the vanous electncal components and a uniform current distnbutton through (he components As descnbed in detail below, coating 16 is bonded to the internal components and further seals the electncal components m array 20 preventing the undesired entry of moisture or other contaminants, and provides increased tensile and mechanical strength to the stacked array 20, and provides controlled venting of gases during an arrester failure 30 Referring now to Figures 4 and 5, in its preferred form, coating 16 generally includes a matnx 21 of resinous layers and a plurality of axially aligned fibrous tape segments 24 and a spiral wrapped fibrous tape segment 28, segments 24 and 28 being embedded within matnx 21 As descnbed in more detail below, matnx 21 preferably includes a base resinous layer 22 and three outer resinous layers 25-27 (Figure 4) Resinous layers 22 and 25-27 are thermosetting
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resins selected from among the following polyester resins, phenolic resins and epoxy resins
1 '! preferred resin further includes a flameout ingredient and particle fillers to control consistency, aid in modifying thermal expansion coefficient, and increase tensile strength, as known to those skilled in the art 5 Resin layers 22, 25-27 may comprise a single resrn formulation, or they may comprise two to four different rcnns The resins used for layers 22, 25-27 are selected so as to have similar cure temperatures and so as to be mutually compatible with the other resin layers making up matnx 21 Further, the resin of matnx 21 must be stable at high temperatures and high voltages, meaning that the cured resins in matnx 21 must not depolymenze or lose bonding 10 strength at the temperatures and voltages to which the components in array 20 will be subjected during operation Normal operating temperatures are typically between -60 and +60°C Failure mode temperatures can be as high as 350°C The matenal selected for layers 22, 25-27 undergoes no thermal degradation at or below the failure temperature of the electncal equipment
According to the preferred embodiment, it is important that insulative coatmg 16, when cured, have a coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the electncal components in array 20 This will ensure that, at any temperature below its cure temperature, coating 16 will exert axially and radially compressive forces on array 20 The components in array 20 typically have an average coefficient of thermal 20 expansion in the range of 5 x 106 to 25 x 106 in/in/°C, so it is desired that the matenal(s) of which coatmg 16 is formed have an coefficient of thermal expansion of at least 50 x 106 to 250 x I06 in/in/°C
Each of layers 22, 25-27 may be applied by conventional spraying, dipping, rolling, powder failing, or fluidized bed methods, whichever is appropnate or convenient, depending 25 upon the particular consistency of the resinous matenal and the equipment available In the preferred embodiment of the invention, layers 22, 25-27 of coatmg 16 are applied using a conventional fluidized bed process
As best shown in Figure 4, base layer 22 is applied to the outer cylindncal surfaces 31 of MOV's 12, outer surfaces 48 of terminals 14, and outer edge 34 of contact plates 18 and is 30 applied so as to have a substantially uniform thickness of approximately 0 001 to 0 015 inches Base layer 22 is chosen to have a high bonding strength to MOVs 12 Because of its ability to strongly adhere to the components of array 20, base layer 22 forms a secure base for the other constituents of coating 16, specifically tapes 24, 28 and outer layers 25-27 It is also preferred that, relative to layers 25-27, the resin of base layer 22 be relatively quick to achieve a first level
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of hardness so that tape segments 24, descnbed below, are not placed m direct contact with the elements of array 20
Refemng now to Figures 4 and 5, it is preferred that axially aligned fibrous tape segments 24 are resin impregnated fiberglass tape comprised of multiple fiberglass strands or 5 bundles of strands that are arranged side by side in parallel rows and retained in that parallel relationship by the B-stage thermosetting resin that is preimpregnated cr embedded within and surrounding the bundles Preferably, for the array shown m Figures 1 and 4, fiberglass tape 24 is B-stage resin impregnated tape that's approximately 0 10 inches thick by 0 750 inches wide and has a length substantially equal to the length of anay 20 Four segments of tape 24 are applied 10 over inner base 22 in spaced-apart configuration in respective quadrants about the penphery of array 20 so as to provide untaped, longitudmally aligned gaps 50, which in the embodiment descnbed herein, are approximately 125 to 625 inches wide
Refemng still to Figures 4 and 5, insulative coatmg 16 preferably further mcludes spiral wrapped tape 28 that is disposed about array 20 Tape 28 is preferably also a B-stage resin 15 impregnated fiberglass tape substantially idenUcal to tape 24 previously descnbed, except that tape 28 may be narrower Tape 28 again mcludes fiberglass strands or bundles of strands arranged in parallel rows that are held in position by embedded thermosetting epoxy resin In this embodiment, coating 16 preferably mcludes four turns of tape 28 disposed about the outer surface 48 of upper terminal 14 and lower terminal 14, and a plurality of spaced apart turns 20 disposed about the central portion of array 20 Tape 28 is wrapped about the central portion of array 20 at a pitch of approximately 2 wraps per linear inch In this configuration, coating 16 is formed with polygonal regions 29 that are comprised entirely of resin layers 22, 25-27 and are free from fibrous tapes 24 or 28 One or more tape segments 28 can be used to wrap the array 20 in this manner
Resinous layers 25-27 are layers of resin that are applied separately as descnbed below
Layers 25-27 are preferably, but not necessanly, are fomied of the same resin as layer 22 Layers 25-27 must adhere securely to base layer 22 and are applied, in part, to ensure that the glass fibers and bundles m tapes 24,28 are completely and adequately wetted pnor to module 10 being cured It may be desirable to use different resins for one or more of layers 25-27, such as, 30 for example to enhance the ability to wet, resins of lower viscosity or slower cure rate may be desired In any event, each resin should be mutually compatible with the other resins selected Additionally, it is preferred that resins for layers 25-27 be relatively slower to cure as compared to base layer 22 so that tape segments 24,28 may be pressed and embedded within the preceding resinous layer pnor to the resin setting up or hardening to an extent that would prevent the tape
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from being pressed into the preceding layer Upon final curing, the thickness of coating 16 is preferably approximately 005 to 050 inch
The method for manufacturing module 10 of the present invention generally comprises the following steps First, the components of array 20 are heated to a temperature of between 5 about 150 to 275°C, the final temperature of this preheating step being dependent upon the type and charactenstics of the resin(s) employed m coating 16 More specifically, the final preheat temperature is selected in the lower temperature range of 150 lo 200°C so as to reduce gel rates, while final cure temperature is set in the range of 225 to 275°C Once heated, the components are then arranged in a conventional V-block type fixture in the desired axial relationship An 10 axially directed clamping force of between approximately 0 to 1500 psi is applied to the end terminals 14 of array 20 For convenience of manufacture, the component array is held in a horizontal plane In order to maintain good contact during the coating process, a force sufficient to maintain component-to-component contact is required To facilitate defonnauon of the nbs on contact plates 18, the preferred clamping force is approximately 50 to 150 psi The clamping 15 force should be sufficient to ensure that MOV's 12, contact plates 18 and terminals 14 are in complete contact over substantially their entire areas of abutment. Good contact between the adjacent components in array 20 is important for uniform current distribution, low resistance and optimal heat dissipation through the stacked array 20
When the axial force is applied in the predetermined magnitude, the ndges m contact 20 plates 18, to varying degrees, bite or embed themselves into the adjacent faces 30,32 of MOVs 12 to compensate for irregularities in MOV surfaces 30, 32 Additionally, contact plates 18 compensate for a degree of nonuniformity with respcct to the thermal expansion of MOV's 12 during operation of the surge arrester, the ndges on contact plates 18 flex somewhat and allow continuous elcctncal contact. Contact plates 18 further serve to prevent the resinous layers 22, 25 25-27 of coating 16 from seeping between the opposing faces 30,32 of adjacent MOVs or other components in array 20 that are not geomctncally true or that have physical irregularities Essentially, the outermost ndges 42 of contact plates 18 forms a seal around the penphery of each MOV-electrode-MOV interface
With the array's components axially loaded, base layer 22 is uniformly applied to the 30 outer surfaces of the components in array 20 A thin coating (003 to 010 inches) of first outer layer 25 is immediately applied before the fast gelling layer 22 has started to gel First outer layer 25 has a relaUvely slower rate of hardening than base layer 22 so as to permit fibrous tape segments 24 to be partially embedded within layer 25 Layers 22 and 25 serve to prevent fibrous tape segments 24 from contacting the outer radial surfaces of MOV's 12, terminals 14
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and contact plates 18 It is important to avoid such contact because even though the fibrous tape has been impregnated with resin, it is still likely that minor levels of porosity or voids exist It is important to minimize the level of porosity present in any dielectric coatmg, but this is especially important when in close proximity to the active electncal components in order to achieve good 5 high current impulse durability After layer 25 has been applied, tape stnps 24 are pressed into first outer layer 25 so as to be partially embedded Tape segments 24 are axially aligned and circumferenbally spaced apart about the outer surfaces of components in array 20 At this point, module 10 has the configuration shown in Figure 6
After tape segments 24 have been embedded within first outer layer 25, the partially 10 assembled module 10 coated with second outer layer 26 An important function of layer 26 is to ensure that the fiberglass strands or bundles within tape segments 24 are well wetted (resin saturated) and to ensure that no voids are created within coating 16
After layer 26, tape 28 is applied Beginning at one end of array 20, tape 28 is wrapped approximately four times around the knurled outer surface 48 of upper terminal 14 and then 15 wound about the central portion of array 20 in a spiral fashion The wrapping step is preferably completed with four final turns of tape 28 about lower terminal 14 Tape 28 is wrapped about array 20 at a tune when layer 26 is still relatively soft such that tape 28 is at least partially embedded in layer 26 Figure 7 shows module 10 at this stage of assembly After tape 28 has oeen applied, module 10 is coated with a final outer layer 27 20 Although layers 25-27 may compnse different resins, it is presently preferred that layers
-27 consist of the same resinous matenal Further, although coating 16 has been descnbed as have three discretely-applied outer layers 25-27 of resinous matenal, in practice, any desired number and combination of outer layers may be applied While three such layers are presently preferred in the preferred embodiment, the important function served by the outer layers 25-27 is 25 to thoroughly wet the fibers in tapes 24, 28 and depending on numerous factors, such as the charactcnstics of the resinous materials and of tapes 24,28, this may be accomplished with more or fewer number of layers
After final outer layer 27 has been applied, array 20, still held in compression by a clamping mechanism (not shown), and coatmg 16 are subjected to curing temperature so that 30 layers 22 and 25-27 will cross-link and harden Matnx 21, compnsing resin layers 22 and 25-27 are cured at a temperature which is well above the normal steady state operating temperature of the module, which is typically about 60°C It is preferred that the final curing take place at a temperature above the maximum temperature that will be experienced by module 10 during operation In instances when module 10 is employed in a surge arrester, the matnx 21 should
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cure at a temperature above the temperature that the module is likely to experience during a transient overvoltage Such temperatures may be, for example, 250°C or more Accordingly, the resins chosen for use m matnx 21 are preferably those that cure at a temperature of 250°C or more During the final cure, module 10 shown in Figures 1 and 4 will typically remain m an 5 oven for approximately 10 to 30 minutes at the predetermined cure temperature before being removed from the oven and allowed to cool to room temperature Because the resin layers 22, 25-27 are not completely cured until the final curmg process, layers 22, 25-27 become integral with each adjacent layer, rather than forming discrete, discemable strata
In some cases, the shrinkage due to cure is enough to result m adequate compressive 10 force such that the assembly would not have to be cured at the elevated temperature It is preferred, however, that insulative coatmg 16, after curmg, have a coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the electncal components in array 20 As a result, upon coolmg of the module 10, insulative coating 16 shrinks more than array 20 and thus imposes axially- and radially-compressive forces on array 20 to ensure that the 15 components in array 20 remain in stacked relationship and to ensure that good electncal connection is maintained between the components in anay 20 If a coating having a higher coefficient of thermal expansion is used and shrinkage during cure is not considered, then the cure temperature would have to be higher than the temperatrre expenenced by the components at designed operating temperature, so as to ensure compressive forces at operating temperatures 20 The most severe temperatures expenenced by state of the art arresters is m the range of
250 to 300°C If a resin having a lower coefficient of thermal expansion were used, then the effects of low temperature operation would have to be considered. In this case, shnnkage dunng cure would be minimized in order to prevent cracking of the coating In each case, the forces would be highest at the lowest temperatures In any case, an object of this invention is to 25 coordinate shnnkage dunng reaction (cure) and thermal expansion properties so as to maintain axial compression on the coatcd parts as well as to maintain a good dielectnc interface to the component penpheiy The art of coordinating thermal expansion mismatch is well understood by those skilled in the art The novel aspect of the present invention is to use these coating parameters to control contact pressure in the stacked array of coatcd electncal components 30 The degree of expansion mismatch is limited by the hardness and tensile strength of the coatmg Generally, some degree of flexibility is desirable in order to control compressive forces over a narrower range If materials are too hard or bnttle, the force exerted on the MOV components will rise dramatically with falling temperature, while if the coatings are somewhat soft or elastic, the coating will begin to yield as forces increase
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With the presently preferred resins, particles of rubber filler such as ethylene vmyl acetate (EVA) or ethylene propylene rubbers (EPR) are used to enhance the flexibility of the cured resin These systems can withstand large mismatches without cracking or debondmg The actual limits of mismatch and/or shnnkage have not been measured Instead, a trial and 5 error approach has been used to determine acceptable matenal parameters A processed arrester module was subjected to 50 thermal shock cycles of fast heating to 120°C followed by quenching at two high current impulses such as those required by ANSI C62 11-1991 The sample was then inspected for damage as well as change in operating characteristics A longer term multistress test (ENEL DY1009) was used to assure that dielectnc interfaces remained 10 intact Matenal systems meeting these test cntena were then subjected to a complete set of design tests per ANSI C62 11-1991 and IEC 99 4-1993
Hardened matnx 21, in conjunction with longitudinally aligned fiberglass tape segments 24 and spiral wrapped tape segment 28, provides sufficient cantilever strength to module 10 to permit the module to tolerate the external forces that may be applied to the array when in use, 15 such as in surge arrester 60 where the arrester and module will be subjected to wind forces and other unintentional, but occasionally-occurnng, forces such as those that might be applied to the arrester dunng shipment or installation by utility personnel
In addition to providing the required strength and ngidity to module 10, insulative coating 16 further includes a venting means permitting the module 10 to vent gas that may 20 evolve dunng arrester component failure In particular, polygonal regions 29 serve as weakened wall regions through which venting may occur dunng component failure More specifically, when an MOV 12 or other internal component in array 20 fails, the pressure within module 10 will build as the internal arc burns adjacent matenals As the arc bums, the pressure within module 10 will increase until it reaches a magnitude that will cause weakened wall regions 29 to 25 burst, so as to relieve the internal pressure and vent the evolved gas
Refemng bnefly to Figure 8, there is shown a distnbution class surge arrester 60 that employs module 10 previously descnbed Arrester 60 generally includes module 10, polymenc housing 62, and arrester hanger 64 Module 10 is disposed within polymenc housing 62 with an RTV silicone compound (not shown) filling any voids between module 10 and the inner surface 30 of housing 62 A threaded conducting stud 46 is disposed in bore 44 of each terminal 14 Upper stud 46 extends through housing 62 far threadedly engaging a terminal assembly (not shown) Lower stud 46 extends through an aperture (not shown) m hanger 62 for connection to ground lead disconnector 65 Threaded stud 67 extends from disconnector 65 for engaging a ground
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lead ten^aial assembly (not shown) Housing 12 is sealed about module 10 at its upper and lower ends.
Refemng now to Figure 9, there is shown an alternative embodiment of the present invention that includes module 100 containing an array 120 of electncal components that 5 mclude MOV's 12, contact plates 18 and terminals 14, all as previously descnbed In this embodiment, module 100 incli'des an insulative coating 116 compnsmg a matnx 121 Matnx 121 includes a base layer of resinous material 122, substantially the same as resinous layer 22 previously descnbed with reference to Figures 1 -7 Matnx 21 further includes one or more outer layers 125 of resinous matenal that has included therein relatively short fiber strands 126 10 intermixed with the resin matenal Base layer 122 and outer layer or layers 125 are applied by means of a fluidized bed or other known technique and cured as previously descnbed with reference to the curing of insulative coating 16 After cunng, insulative coating 116 applies an axially compressive force to the arrester components in array 120 Coating 116 has a coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the components 15 in array 120 Additionally, the fiberglass strands 126 randomly disposed within layers 125 provide strength and ngidity to module 100
Refemng now to Figure 10, module 210 is shown in top view to best disclose another embodiment of the invention According to the invention, module 210 includes an axial array of MOV's 12 and contact plates 18 and terminals 14, all as previously descnbed, that are coated 20 end held in axial compression by insulative coating 211 Coating 211 includes resinous layers 22, 25-27 all as previously descnbed Coating 211 further includes a plurality of axially aligned preimpregnated tape segments 224, 226 that are identified to tape segments 24 previously descnbed In this embodiment, however, the lateral edges of the innermost tape segments 224 are overlapped so that the entire circumferencc of the array of electncal components is covered 25 by a layer 225 of axially-ahgned tape segments 224 The module 210 further mcludes axially-aligned tape segments 226 that arc disposed at predetermined locations about layer 225 to provide arcuate regions 227 having multiple thicknesses of tape 224, 226 and other arcuate regions 229 having single thickness of tape 224 A resinous layer that may be ident'eal to any one of the previously-descnbed outer layers 25-27 is applied between the taped layer 225 and 30 tape segments 226 and another layer applied over the module 210 after tape segments 226 have been applied to thoroughly wet all tape segments 224 and 226 Thereafter, spiral wrapped tape segments 228 is applied outside tape segments 224 and 226 and a final outer resinous layer is applied After the module 210 is cured, module 210 will include relatively weaker wall regions 230 corresponding to regions 229 that have relatively thin regions of glass fiber reinforcement as
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compared to regions 227 As will be recognized by those skilled in the art, relatively v/eaker wall regions 230 and regions 227 may have any number of thicknesses of tape segments 224, 226 provided that the relatively weaker wall regions 230 have fewer thicknesses of tape 224, 226 than regions 227 The embodiment thus descnbed has particular application in surge 5 arresters having a relatively large number of components in array 220 or where the MOV's are larger than MOV's 12 previously descnbed, as may be the case with surge arresters having higher voltage or duty rating" than arrester 60 shown m Figure 8
Referring now to Figure 11, there is shown an alternative embodiment of the present invention that mcludes module 300 containing an array 120 of electncal components that 10 include MOVs 12, contact plates 18 and terminal 14, all as previously descnbed, and spark gap assemblies 315 In this embodiment, module 300 mcludes an insulative coating 316 As descnbed above, coatmg 316 retains MOVs 12, terminals 14, contact plates 18 and spark gap assemblies 315 in stacked, end-to-end relationship, and provides an axially compressive force as desired for insuring low contact resistance between the vanous electncal components and a 15 uniform current distnbution through the components As descnbed m detail above, a preferred embodiment of coating 316 includes a matrix of resinous layers, a plurality of axially aligned fibrous tape segments and a spiral wrapped fibrous tape segment, with the tape segments being embedded in the matnx Coating 316 is bonded to the internal components and further seals the electncal components, preventing the undesired entry of moisture or other contaminants 20 Coating 316 applies axial and radial compressive forces and provides increased tensile and mechanical strength to the stackcd components, and provides controlled venting of gases dunng an arrester failure
Because spark gap assemblies 315 contain air, it has been found preferable to position spark gap assemblies 315 adjacent one end of module 300 and to mclude in module 300 a 25 vented terminal 320 that includes a borehole 322 adjacent spark gap assemblies 315 Borehole 322 allows air contained in spark gap assemblies 315 to escape as it expands dunng the heating process, and allows the re-entry of air mto spark gap assemblies 315 when the module 300 returns to room temperature following cure Venting the module in this manner dunng heating and cooling prevents the final product from havmg an internal pressure that is different from 30 ambient If no borehole 322 were provided and module 300 were sealed at the elevated coatmg temperature, the pressure of the gas surrounding spark gap assemblies 315 would be well below one atmosphere when the sealed module cooled to ambient temperature
Once module 300 is assembled, cured and cooled, and before it is inserted into a housing or similar device, a stopper 324, preferably of rubber or a similar resilient sealuip matenal, is
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inserted into vented terrainal 320 so as to close borehole 322 Vented terminal 322 is preferably constructed with a reccptacle 323 for receiving stopper 324
In constructing the module 300, it was found that the epoxy coating 316 did not stick as readily to spark gap assemblies 315 as it did to MOV's 12 In order to improve adhesion of 5 coating 316 to spark gap assemblies 315, it is thus preferred to heat the stacked components to a higher temperature pnor to applying the coating 316 Specifically, it is preferred to preheat the stacked components to at least 275°C Similarly, because the spark gap assemblies 315 do not retain heat as well as MOV's, it is preferred that the time between the preheating step and the coating step be minimized so as to minimize the cooling that occurs 10 In order to facilitate manufacturing and assembly of modules 300, it is preferred to provide spark gap assemblies 315 in groups of three having a unit height equal to the unit height of the other components in module 300, namely MOV's 12 and terminals 14 In a most preferred embodiment the unit height of each type of component is 1 1 inches, which corresponds to the height of a smgle shed Thus, a 9 kV surge arrestor having two MOV's 12 15 and three spade gap assemblies 315 would be the same height and would fit in the same size housing as a 9 kV surge arrestor having three MOV's and no spark gaps This allows surge arrestors with and without spark gaps to be built interchangeably The number and arrangement of spark gap assemblies 315 within the module 300 can be varied as needed It is preferred that, if the number of spark gap assemblies 315 is large, they be divided between the two ends of 20 module 300, so as to reduce electncal stress Examples of arrays of electncal components that include vanous combinations of MOV's and spark gaps are shown in Figure 12
The rigid epoxy skin, together with end plug 320 and stopper 324, completely enclose and seal the components of the surge arrestor, making it suitable for use in a vanety of environments, including under oil 25 While the preferred embodiments of this invention have been shown and descnbed,
modifications thereof can be made by one skilled in the art without departing from the spmt of the invention As an example, rather than employing preimpregnated fiberglass tapes 24, 28, unimpregnatcd fiberglass stranded tape may be employed to provide the desired strength and ngidity to module 10, provided that the strands or bundles are sufficiently wetted with each 30 preceding and succeeding layer of rean Furthermore, the invention does not require the use of tapes such as tapes 24, 28 Instead, parallel strands or bundles of strands of fiberglass, not in tape form, may be thoroughly wetted and embedded within successive resinous layers Thus, the embodiments desenbed herein are exemplary only and are not limiting Many variations and modifications of the invention are possible and are within the scope of the claims that follow
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