CA2063096A1 - Bushing for electrical equipment - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An insulating bushing according to the present invention comprises an electrical conductor, a primary bushing surrounding the conductor, a protective shield surrounding a portion of the primary bushing, and an insulating elastomeric filler which occupies the interstitial space between the primary bushing and the protective shield. The electrical conductor is a metal rod which extends through an aperture in an electrically conductive surface such as a containment wall for gas-insulated electrical equipment. The primary bushing is a conically tapered casting of an insulating plastic material which extends much of the length of the conductor. The protective shield is a cylindrically-shaped insulating tube which extends outward from the conductive surface to protect the primary bushing from exposure to weather, sunlight, and airborne contaminants. The shield is constructed from a material which is resistant to these hazards, such as glazed porcelain. The insulating elastomeric filler is a resilient, cast-in-place material having a dielectric constant which approximates that of the other insulating materials used in the bushing. The filler eliminates air-filled voids within the bushing and thereby minimizes variation in the dielectric constant among the insulating portions of the bushing. This results in smaller electric field gradients and inhibits electrical breakdown.
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Description

2~09~

1 BUsHING FOR ~EÇT~IC~_EQUIPMENT

2 BA~KGROUND QF THE I~VENTIQ~

3 This invention relates to electrical tran~ ion and 4 distribution equipment, and ~orQ particularly to in3ulating bushings for permitting an electrical co~ductor to pa~s through a 6 conductive surface, such as a wall of an electrical equipm~nt 7 enclosure, while maintaining electrical insul~tion between th~
8 conductor and the surface.
9 Certain equipment used ~or transmitting and dis~ributing elQCtriCity i5 ~requently housed in en~l~sures havin~ wall~ and 11 othar surfaces constructed of metal. Because the metal sur~aces 12 conduct electricity, a common problem with design o~ such 13 equipment is permitti~g electrical conductors to pass through the 14 surfaces while remaining electrically insulated ~rom the surf~ces. A variety o~ insulating bushing~ have ~s~n developed 1~ in the past to solve this problem. Such bushing3 ~re typically 17 constructed of an insulating material which surround~ th~
18 conductor in the region where the conductsr penetrates an 19 aperture in the ~urface. ~he bushing mechanically and electrically separates the con~uctor Pro~ the conductive sur~a~e~
21 T~e insulating portion~ o~ prior art bushings have be~n ~2 cons~ruct~ from glazed porcelain, ceramic~, pl~tic re6in~, and ~3 other suitable insulating mat~rials. For equipment W~ich i~

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24 loc~ted o~tdoors or is otherwi~e exp~s~d to sunlight, weather, and airborne contaminan~s, glazed porcelain has been a preferred 26 material for the insulating portion of the bushing, becau~e of 27 its resistance to each of these envir~mental hazard~. Al~hough 28 plastics could be used, they tend to be mor2 ~u~ceptible to 2g damage fro~ the afor~mentioned environmental hazard~.
certain el~ctrical eqUipmQnt i~ d~igned to contain a 31 quantity of a special insulating gas, such as sul~ur hexa~luoride 32 (SF6), which provid0s substantially more ef~ective in~ulation 33 than air. The design of bushings for u~e with such gas-~illed 34 equipment presents several additional di~ficulti~s beyond those encountered in the design of bushinqs for ordinary equipment. One 36 such di~ficulty is th~ need to prevent significant leakage of the 37 insulatiny gas from the equipment. Typically, equipment i8 38 specially designed to exploit the enhanced insulating qualitie~
39 of the insulating gas. In addition, the insulation per~ormance of the typically-used insulating gases i~ improved a~ pressures 41 somewhat elevated above normal atmospheric pres~ure. Thus, the 42 equipment may not operate properly if the insulating ga~ i8 43 allowsd ~o escape through the bushing or associated apertures 44 such that tho pressure is reduced or the gas is r~plac~d by air or another less-eff~c~ive insulator. In addition, th~ insulating 46 gas i5 relatively sxpsnsive. Accordingly, bushing~ for use with 47 gas-fill~d equipment must provid~ a sQal ~ean~ to pr~v~nt 28~3~

48 significant ga~ leakage bstw~en the conductiv2 sur~ace and the 49 electrical conductor.
An additional dif~iculty encount~r~d in the de~lgn of 51 bu~hing~ for use with insulatinq ga~-~illed equipment i8 that 52 when an electrical arc occurs in the insulating gas environment, 53 byproducts are produced which attack certain ~aterials, In 54 particular, arc byprcduct~ produced in an SF6 ~nvironment can attack glazed porcelain materials, making the~ incom~atible with 56 bushing applications i~ which the glazed porcelain would be 57 directly ~xposed to that insulating gas. Certain pla8tic resin 58 materials, however, are compatible with the SF6 insulating gas.
59 ~ccordingly, bushings have been produced whic~ ¢o~pris~ a primary bushing portion constructed of an SF6-co~patibl~ pla~tic 61 resin and a protective outer sh~11 constructed of g~azed 62 porcelain. The primary bushing surrounds the electrical 63 conductor as i~ passes through an ap~rture in th~ conductive 64 surface and ex~ends ou~wardly on both sid~s o ~he sur~ac~. A
seal between the perimeter of th~ apertur~ and the primary 66 bushing substantially preve~ts esc~pe of thQ SF6 insulating gas 67 at this interfac. The protective outer shell surround~ th~
68 primary bushing on the exterlor side o~ the conductiv~ sur~ac~ to 69 prevent exposure o~ the primary bushing to sunlight, weathQr, and airborn~ contaminants. ~h~ outar sh~ll Qx~ends fro~ ths sur~ac~
71 for a ~uitabl~ distance ~o provide the ~sired amount Or 2~V96 72 electrica~ in~ulation. Only the primary bushing extends on the 73 interior side of the conductive sur~acs, and therePore, only the 74 primary bushing i~ exposed to the in~ula~ing ga~ and arc byproducts.
76 A proble~ with such existing bushing~ is that the outer 77 diameter o~ the primary bushing i5 ~ub~tantially small~r than the 78 inner diameter of the protectiv~ sholl, th~reby producing a 79 cone-like air-filled void between these compon~nts. Because the dielectric constant of air i8 significantly lower th~n thQ
81 dielectric constants of the materials from which thQ primary 82 bushing and protective shell are constructed, a sharp variation 83 in the electric field is produced at the boundaries of the void.
84 At high voltages, the sharp variation in the electri¢ fi~ld promot~s Qlectrical breakdown in the void. In addition, th~
86 dielectric streng~h of air is significan~ly s~aller than t~at o~
87 ths primary bushing and protective shell material~. TherePore, 88 in order for the air-filled void to provide enough insulation to 8~ withstand a desired voltage, the bushing must sxtend a substantially greater distance from thQ conductive surfacQ than 91 would be required if the ~oid were not pre~entO

` 20~09~

92 o~J~Ç~ ~ SUMM~Y OF ~HE I~VE~TIQ~
93 It is therefore an object Or the present invention ~o 94 provide a bushing for use with insulating-gas-~illed electrical 9~ equipment which provides improved re~istance to electrical 96 breakdown in regions within and around the bushing.
97 It is another object of the present inv~ntion to provide a 98 bus~ing for use with insulating-gas-fill~d electrical equipment 9~ which achieves a desired resistance to el~c~rical breakdown while 100 requiring a shorter bu~hlng length than prior art dQsigns.
101 It is a ~urther object of the pressn~ invention to provide a 102 bushin~ for use with insulating~gas-filled electrical equipment 103 which provides a relatively consistent dielectric constant 104 throughout the insulating portion~ of the structure.
105 It i8 another object of the invention to providQ a bu hing, 106 and construction method therefor, for use with electrical 107 equipment which prevents the formation o~ or eliminate~
108 air-filled voids within the bushing~
109 An in~ulating bushing according to the pre~ent in~ention 110 compri e~ an electrical conductor, a primary bushing surroun~ing 111 th~ conductor, a prot2ctive shield surrounding a portion o~ the 112 pri~ary bu~hing, and an insulating elastomPric ~iller which 113 occupie3 the interstitial space between the primary buRhing and 114 th~ pro~ ctive shield~ The el~c~rical conduc~or i9 a mQtal rod 115 which extQnd~ ~hrough an aper~ur~ in an el~ctrically aonductive 2~3~9~

116 surface such a~ a containment wall ~or ga~-in3ulat~d electrical 117 equipment. The primary bushing i3 a conically tapered cas~ing of 118 an insula~ing plastic material which ext~nd~ much o~ the length 119 of the conductor. The protective ~hleld i~ a cylindrically-120 shaped i~sulating tube which exte~d~ out~rd fro~ the conductiv~
121 surfa~e to protect the primary bushing ~rom expoaur~ to weather, 122 sunlight, and airborne contaminant~. The shiald ~ con~tructed 123 from a material which is resistant to these hazards, such as 124 glazed porcelain. The insulating elastomeric filler is a 125 re~ilient, cast-in-place material having a dielectric constant 126 which approximates that of the other insulating ~aterial~ u-cad in 127 the bushing. The ~iller eliminates air-filled void~ within the 128 bushing and thereby minimizes variation in the diele~tric 129 constant among the insulating portions of the bu~hing. This 130 results in smaller electric field gr~dients and inhibit~
131 electrical breakdown.

132 BRIEF DESCRIPTION OF THE DR~WINGS
133 These and other features of thi~ inv2ntion will be b~st 134 understood by reference to the following detailed description of 135 a preferred embodiment of the invention, taken in conjunction 136 with the acco~panying drawing~, in which:
137 Fig. 1 is a sidc perspective ViQW of an in~ula~ing bushing 138 100 con ~ructed according to th~ pre~snt invention;

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139 Fig. 2 iR a side cross-section view o~ bushin~ 100 taken 140 along the view line3 2-2 of FigO l;
141 Fig. 3 is a top cross-section ViQW 0~ bushing 100 taken 142 along the view lines 3-3 of Fig~ 2;
143 Fig. 4 is a sid~ cross-section view o~ thQ inve~tive bushing 144 shown in a first intermediate ~tage of manufacture according to a 145 first embod~ment o~ the method o~ th~ preaent invention;
146 Fig. 5 is a side cross-section view of the inventivQ bushing 147 shown in a second intermediate stage of manufacture according to 148 a first embodiment the method of the pre~ent inven~ion;
149 Fig. 6 is a side cross-section viaw of the inventiv2 bushing 150 shown in a third intermediate stage o~ manufacture ac~ording to 151 the mekhod of the present invention;
152 Fig~ 7 i~ a flow char~ illustrating a first embodiment of a 153 method of manufacturing the bushing 100 according to the present 154 invention;
155 ~ig. 8 is a flow chart illustrating a secon~ embodiment o~ a 156 method o~ manufacturing the bu~hing 100 according to ~he present 157 invention;
158 Fig. 9 is a si~e cross-section view of the inventive bushing 159 shown in a ~irst inter~ediate stage of manufacture according to a 160 second embodiment o~ the method o~ the present inYention; and 161 Fig. lO is a side cro~s-section view o~ th~ inv~ntive 162 bushing ~hown in a second intermediate stage o~ manu~aoture `- ` 2~3~

163 according to a sacond ~mbodi~ent the ~ethod o~ th~ pres~nt 164 invention.

165 ~
166 Fig. 1 depicts an insulating bushing 100 construct~d 167 according to the pre~ent invention. The bu~hing 100 allow~ a 168 conductor 110 to pass through an ap~rtur~ in a co~uctive sur~ace 169 120 while remaining insulated from that surface. The conductive 170 -~urface 120 preferably forms a wall of an enclosurQ for 171 electrical equipment containing an appropriate insulating gas, 172 ~uch as sulfur hexafluoride ~SF6)~ The conductive sur~ace 120 173 accordingly separates an interior region 122 in which th¢
174 insulating ga~ i5 pre3ent from an exterior region 124 in which 175 the insulating gas is not pres~nt. Usually, the ¢xterior r~gion 176 124 is outdoors or i~ vented to the atmosphere and therefore 177 contains air. However, the bushing could also bs used in 178 applications in which an insulating gas i~ not used.
179 In addition to the electrical conductor 110, a~ shown in 180 Fig. Zl the bushing 100 also comprises a primary bushing 130, a 181 protec~iv~ cover 140, and a retaining cap 150. The p~i~ary 182 bu~hing 130 and electrical conductor 110 extend throu~h th~
183 conductiva sur~ace 120 into th~ in~erior region 122. One or more 1~ cla~ps 132 pre~erably secure th~ bushing 100 in place on 185 conductive sur~ace 120.

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186 Con~tructiQn of bushing 100 i~ shown in greater detail in 187 Figs. 2-3. Electrical conductor llO i~ pre~erably a 188 cylindrically shaped rod o~ a suitable conductive metal such as 189 sopper or aluminu~. The conductor 110 pr~ferably ha~ thread~ 112 190 at an outside end 114 thereo~.
191 The conductor 110 extend through an ap~rtur~ 160 in ~hQ
192 conductive sur~ace 120. A primary bushing 130 surround~ the 193 conductor 110 along most of it5 lenqth. The primary bushing 130 194 insulate~ the conductor llO ~rom the conductiv~ sur~ace 120 and 195 also serves to locate the conduc~or llO in a prede~ined po~ition 196 in aperture 160. The primary bushin~ 130 ha~ a lower portion 197 136, an intermediate ledge 180, and an upper portion 13~. ThQ
198 lower portion 136 extends from ~he intermediate ledge 180 toward 199 the interior end of conductor ~lO and i~ preferably conically 200 tapered so that its diameter is reduced toward that end. The 201 upper portion 138 extends from the intermediat~ ledg~ 180 toward 202 the exterior end 114 o~ conductor 110 and i~ pre~erably ~onically 203 tapered so that its diameter i~ reduced toward that end 114 204 The intermediate ledge 180 ~xtend~ radially ou~ward ~rom th~
205 conductor 1~0 and ~ixes ~h~ prim~ry bu~hing 130 in po~ition along 206 th~ long ~xi~ Or the bushing 100. ~ shork cylindrical lip 182 207 preferably extends ~ro~ ledg~ 180 ~owar~ ~he in~rior end of 208 conduckor 110. The lip 182, ledg~ l~0, ~nd conduc~v~ ~urf~
209 120 cooperate to form a circular chann~l 184. An appropriat~

2~63~6 21a seal mean~ 162, which may be a circular gask2t or ~onwring i~
211 preferably disposed in channel 18~ to pr~vent leak~g~ of 212 insulating gas ~rom the interior r~gion 122 through the apexture 213 160 at its inter~ace with the primary bushin~ 130. Preferably, 214 the seal mean~ 162 is constructed ~ro~ a re~lient material and 215 is slightly thicker than the chann~l 184 ~o that it ~ay be 216 compressed due to pressuxe fro~ clamps 132 into sealing 217 engagem~nt with surface 120 and ledge 180.
218 Primary bushing 130 is pre~erably con~tructed by molding lt 219 around conductor 110 from a suitable moldable, in~ulating, 220 thermo setting material. Because the primaxy bushing 130 ex~ends 221 into the interior region 12~ of the equipment enclosure, ths 222 material selected must be resistant to attack ~rom byproduct~
223 produced when arcing occurs in the SF6 insulating ga~
224 environm~t. In addition, in order to providQ suitabl~
225 electrical insulating characteristics, the primary bushing 226 material should have a dielectric constant similar to tho ~ o~
227 the other insulating material~ ~ro~ which the bushing i8 228 constructed. It is believed that ma~erials having di~lectric 229 constants in the range ~rom 3 to 7 are suitable for U52 in this ~30 application. Appropriate materlals includ~ selected poly~st~r 231 and epoxy resins.
23~ For example, one suitab~e commercially availabl~ ~aterial is 233 "Bi~phenol ~ epoxy resin which is a product o~ ~ywax 2~g30~6 234 Corporation, De~ Plaine~, Illinoi~ 6001~ Thi~ ~aterial is a 235 moldable, 2-part epoxy which i~ pre~rably poured cold but i8 236 subjected to elevated curing temperatures. I~prov~d re~ult~ may 237 be obtained when the mold i~ partially pr~-heated. Othar 238 material3 having suitable charactari~tic~ could al~o b~ u~edO
239 Dif~erent molding technigue , ~uch a3 injection ~olding and 240 pressure gelation may be appropriatR ~or other materials.
241 Protective shield 140 pre~erably comprise~ a hollow 242 cylindrical insulating member 148 which extends fro~ ~h~
243 intermediate ledge 180 toward the exterior end 114 o~ conductor 244 110. ~ plurality of saucer-shaped insulating member~ 1~2 extend 245 ~ubstantially radially outward from tho cylindrical m~ber 148.
246 The diameter of the inner wall 144 of cylindrical memb~r ~48 i9 247 larger than the diameter o~ the outer wall 174 o~ the upper 248 portion 138 of the primary bushing. Accordingly, the prot~ctive 249 sh~eld 140 entirely surrounds the upp~r portion 13~ o~ th~
250 primary bu~hing 130 and iQ substantially di~posed in ~paced 251 concentric rela~ion with the bushin~ 130. The corner ~78 ~FigO
252 4~ fo~med by the int~rmediata ledge 180 and the cuter wall 174 o~
253 pri~ary bu~hing 130 is preferably chamfared. Thi~ cha~f~r ~as~
254 asse~bly by helping to prop~rly centQr th~ protQctiv~ shiold 140 255 in po~ition about the pri~ary bushing 130 and th~ conduc~or 110.
256 A re~aining cap 150 i~ pre~era~ly provided to secure the 257 protac~ive shield 140 to the pri~ary bushing~ The retaining cap 1~

2~630(3~

258 150 preferably ha~ ~uita~l~ means 152 for ~astening the c~p to 259 the exterior end 114 of electrical conductor 110. For exa~ple, 260 r~taininy cap 150 may hav~ threads for Qng~ging mating thread~
261 112 on conductor 110. Retaining cap 150 pr~er~bly ha~ a 262 peripheral lip 154 which aids in concentrically locating the 263 ext~rior end of the protective shield 140 with re~p~ct to the 264 conductor 110 and the primary ~ushing 130. Th~ retaining cap 150 265 is preferably constructed of a sturdy, conductive material, such 266 as copper or aluminum, which i~ compatibl~ with th~ con~uctor 267 110. A suitable resilient seal means 164, which ~ay be a 268 circular gasket, is preferably interposed between the retaining 269 cap 150 and the protective shield 140 to prevent infiltration of 270 foreign matter therebetween.
271 Preferably, the securem~nt means 112, 15~ o~ the conductor 272 110 and retaining cap 150 are kept free of insulati~g coatings or 273 contaminants so that when the retaining cap 150 is installed on 274 conductor 110, a good electrical connect~on i5 formed 275 therebetween. In a typical application of the bushing 100, the 276 bushing is ins~alled ~o tha~ th~ r~taining cap 150 is loca~ed on 277 the outside of an electrical eguipmen~ enclo ure, and an interior 278 end 208 o~ th~ conductor is locat~d within the electrical 279 equip~ent enclosure. Thus, the conductor 110 and retainin~ cap 280 coop~rate to permit the flow of electricity through th~ ap~rture 281 in ~he enclosure wallsO Typic~lly~ anot~er elec~rical co~duc~or 20~09~

282 (not shown) i8 electri~a~ly conn~ct~d by appropriat~ known m~an 283 to the r~taining cap 150 to connect tho equipm~nt to other 284 electrical devices.
285 Protective shield 140 i~ prefera~ly constructed ro~ a 286 suitable insulating porcelain or cera~i~ ma~erial. O~her 287 materi~l~ having suitable insulating char~ct~ri~ttc~ and which 288 resi~t attack by sunlight, weather, and airborne ~onta~inants 289 could also b2 used. The shield material smployed pxeferably ha~
290 a di~lectric constant which is sim~lar to those o~ thQ other 291 insulating materials from which the bushing loo i~ ~onstructed.
292 Xt is believed that materials having dielectric constant~ in tha 293 rang~ from 3 to 7 are suitable ~or u~e in thi3 application.
294 glaze layer 176 is preferably applied to the inner and ou~er 295 surfac~s 1~4, 146 o~ the protective shield. The glaze laysr 176 296 protect~ the ~emi-porous interior material 206 of the shield from 297 contamination by water and other fluids. Th0 upper and low~r 298 surfaces lB6 of prot~ctive shield 140 are preferably laft 299 unglazed.
300 ~ecause the diameter of the inner wall 144 o~ aylindr~cal 301 member 148 i~ larger than the diameter of the outer wall 17~ o~
302 ~he upper por~on 138 of the pri~ary bushing, the~e co~pon~nt~
303 form a tapered inter~titial region. According to thæ pr~nt 30~ invantion, thQ intersti~ial region i5 pre~erably ~ d wi~h an 305 appropriat~ ela~tomeric insulating ~a~erial 170 having a 2~3~

306 dielectric cons~ant si~ilar to ~ho~a o~ ~h~ o~h~r insulating 307 materials from which the bushing i8 constructed. It is believed 308 that material~ having dielectric con~tant~ in ~he range ~rom 3 to 309 7 are 3uitable for use in thls application~
310 In addition to possessing a di~l~ctric con tant which i3 311 compatibl~ with the other insulat~ng bush~n~ ~at~r~ , th~
312 inter~titial ~iller material sho~ld also have appropriate 313 physical characteri ticR to facilitat~ application of th~ filler 314 to the interior ~ur~ace of the protectiv~ shell 140 and 315 subsequent a~embly of the bushing 100. Pr~ferably, th~ ~iller 316 material 170 is a low-viscosity casting compound whic~ may b~
317 easily molded and which is cured at a relatively lcw te~peratur~.
318 Once cured, a suitable filler ~aterial i3 re~ nt and ha~
319 a very low "compression set" characteristic. For e~a~ple, a 320 material having a hardnes~ of 15 to 40 on the 5hor~ A duxometer 321 scale i beli~ved to be ~uitable. The~2 physical character~st~c~
3z2 are important in inhibiting the creation of air-~illed void~ at 323 th~ inter~ace between ~he ela~tomeric filler 170 and ~he pri~ry 324 bushing 130. One suitable in~er titial ~iller ~t~rial i~ a 325 two-par~ polyure~hane available fro~ Ca~tall, Xncr ~ E~st 326 Wey~ou~h, ~as~ashu3ett~, 02~, a~ typ~ UX-~5~4 polyur2thane.
327 Selectio~ of compatibl~ ~at~rials ~or pri~ary bushiny 130 328 and ~iller ma~erial 170 i~ i~portan~ in ordsr to inhi~it th~
32S creation o~ air-filled voidR in th~ bu~hing~ As th~ bu3hing 100 ~309~

330 i ~ub~ected to te~perature variation~, tha pximary bushing 130 331 expands and contracts. When the pri~ary bu~hing 130 expand~, it 332 co~presse~ the filler material 170 again~t the lnterlor wall o~
333 the protective shell 1~0. Accordingly, thermo~tting matexial~
334 and materials which are subject to a aubstantial de~ree of 335 compression setting are unsuitable for us~ as int~rs~itial ~iller 336 materials because such materials would ~ail to roturn to their 337 original shape when the primary bushing 130 contract~, th~reby 338 creating a void.
339 Tha insulating int~rstitial fill~r 170 of the prQsen~
340 invention avoids ~everal problems characteri~tic o~ prior-art 341 bushings. The air-filled void betwean the primary bu~hing and 342 the protective shield in prior-art bushings produced signl~icant 343 variations in th~ dielQctric constant within the bushing. The 344 resulting large electric field ~radients promoted ~l~ctrical 345 breakdown within the bushing, The interstitial filler o~ the 346 present invention displac~3 the lo~-dielectric-constant air with 347 a material having a dielactric con tant approxi~a~ing that o~ th~
348 other insulating portions of the bushingO The inter~titial 349 filler th~reby mini~izes variation in the diel~ctric onstant 350 among ~h~ insula~ing port on~ o~ the bushing, resulting in 351 smaller electric field gradi~nts and inhibiting el~ctrical 352 br~akdown.

2~fi3Q~

353 In addition, because th~ di~lectric strength o~ a~r i~ small 354 compared to ~oma other materials, a rQlatively large phy~ical 355 separation between conductors i~ requir~d to achieve a desired 356 level o~ insulation when air 18 the ih~ulat~ng mat~rial. The 357 filler material has a relatively high di~lectr~c str~ngth 358 (compared to air). A~ a rasult, t~ lntQr~titial ~iller i~ a 359 more spac~-~fficient insulator, and bushlng~ constructed 360 according to the present invention can achievQ a particular 361 voltage rating using a shoxter primary bu~hing 170 and protective 362 sh~ll 148.
363 The inter~titial filler 170 is preferably appll~d to the 364 int~rior wall 1~4 of the protec~ive shield 140 to producQ a 365 contour corxesponding generally to the shape of the exterior wall 366 174 of the primary bushing 1300 once the fillex material 170 has 367 been applied to the interior o~ the protective ~hi~ld, ths 368 bu~hing 100 may be assembled by in~erting khe pri~ary bushing 130 369 and electrical conductor llO into the prot~ctlve ~hield 140 and 370 urging the primary bushing 130 into secure engagsment with the 371 ~iller material 170. The r~spectiv~ ~ecurement m~ans 112, 158 o~
372 th~ conductor 110 and the retaining cap 150 ~ay be u~d to 373 ad~u t~bly control the posi~ion o~ the re~aining cap Y50 374 longitudinally on ~he conductor 10~. ~ecau~ s~al ~ean~ 1~4 i8 375 re~ilient, the re~aining cap 150 may be compre~d against the 376 seal means ~4 and protectiva ~hield 140 w~thout da~aging t~

- 2~0~6 7 shield. In this configuraticn, the retalnlng cap 150 and seal 8 mean~ 164 cooperate to urge tho prot~ctive ~h~ld 140 ~oward the ~9 interior end 208 of the conductor 110, thereby forcing the ,0 protective shield 140 and filler mat~riai 170 into 3~cure :1 engaqement with the primary bushing 130.
:2 In a pre~erred e~bodi~ent of the invsntion, th~ interstitial ,3 filler 170 may be ~olded in place by ins~xting a mandr~l having ~4 the de~ired shape into the protective shield 140 and introducing ~5 a ~uitable moldable filler material into the void between the 36 mandrel and the protective shield. Onc~ the moldabl~ r 37 material has been cured, the ~andrel may be re~oved to creatQ a 38 central channel through the fill~r. The mandrel ~y then be 39 replaced with a primary bushing 130 and electrical ~onductor 1~0.
~0 In a first embodiment of the invention, the mandrel 188 may be a 31 bushing having a shape similar to the primary bu~hing 130 but 32 having a shaft of slightly reduced width. In a second 33 embodiment, the mandrel may si~ply be a spare p~imary bu~hing 34 130.
In order to maintain the dielectric con~tant rPlatively 36 invariable throughout the insulating por~ions of ~he bu~hing 100, 37 it is desir2ble to minimize ~he crea~ion o~ air bubble~ or void~
3~ in the inters~itial filler 170, parti~ularly a~ th~ inter~ac~
~9 between the filler 170 and ~he primary bushing 130. ~ccordingly, the int~r~titial ~iller 170 i~ pre~erably ~old~d so that the ~3~6 ~01 central c~annQl is slightly s~aller than the primary bushing 130.
402 In addition, a small quantity o~ a ~uit ble in~ulating lubricant 403 is applied to primary bushing 130 nr th~ filler 170. When th~
404 primary b~shing 130 is completaly insérted into the ~entral 405 channel, th~ bushing 130 compres3e~ th~ ~$11er material and 406 displace~ any siqnificant air bubbls~ or void~.
407 In ord~r to en~ure that the ~iller material 170 alway~
408 remains in tight co~pressive engage~ent between the primaxy 409 hushing 1~0 and the prot~ctive shi~ld 140 r~gardle~s of 410 temperatura changes and other mQchanical variation3, it i~
411 desirabl~ ~o mold the filler material 170 in a layer which is 412 slightly thicker than the final d~ired distanc~ betw~en th~
413 inner wall of ~he shield 140 and thQ outer surface o~ the primary 414 bushing 130. Becau~e the upper portion of the pri~ary bushing 415 130 is conic~lly shap~d, a slightly larger interstltial void may 416 be created by constructing a mandrel of slightly r~duced diam~ter 417 compared to ~he primary bushing 130. The larger void create a 418 thicker filler material layer when the material, in its liquid, 419 uncured state, i~ poured into th~ void at mold timeO ~hen~ when 420 the protective shield ~40 is in~talle~ in its final po~ition, the 421 ~ r materl~l 170 is sub~ec~ed to co~preæsion bQ~wa~n the 422 primary bushing 130 and the shield~
423 Fig. 7 is a flow chart depicting a ~ir~ me~ho~ 400 o~
424 con~tructing a bushing according ~o the present invention --` 2~309~

425 including an elastomeric interstitial ~ r. Fig~. 4-6 depict 426 the inventivQ bushing in interm~diatQ stag~ o~ con~truction. In 427 step 412, a protective sh~eld is a~se~bled onto a ~uitable 428 mandrel. Flg. 4 shows a partially con~truct~d ~u~hing 200 a~ it 429 would appear after completion of step 412. Th~ prot~ctive shield 430 140 cooperates with the mandrel 188 to for~ an interstitial void 431 192 for receiving an interstitial filler. Th~ mandrel 1~8 i~
432 preferably constructed having an outer diameter which i~ reduced 433 compared to that of the primary bushing 130~ T~iC provides a 434 larger inter5titial void 192, which, in turn, creat~ a thic~r 435 interstitial layer 170. Al~hough th~ exact reduction ~n dia~eter 436 is not critical, the interstitial layer 170 is subject to 437 compression wh~n the larger primary bushing 130 is finally 438 installed, and the amount of compres~ion permissible establishes 439 a practical limit to the diameter reduction. Accordingly, it is 440 believed that a diameter reduction resulting in a 5 to 40 percent 441 compression of intersitial lay~r 170 is suitable.
442 In step 414, the interstitial .~iller 170 i~ introduc~d into 443 void 192. In step 416, the ~iller is cured. Th~ cur~ may occur 444 at an elevated temperature i~ ~he ~iller ~a~erial so requires~
445 Fig. 5 how a partially constructad bu~hing 202 as it would 446 appear a~ter completion of st~p 416.
447 In st2p 418, ~he mandrel ~8B i~ removed. In step 420, a 448 lubriaant is applied ~o the ou~er sur~ace o~ the pri~ary bu~hin~

20~3096 449 130, and an asse~bly consisting of ~he primary bushing 130 and 450 thQ electrical conductor 110 i8 in~rt~d into ~he pro~ec~ive 451 shell ~40. In step 422, the a~sQmbly is compr~ssed against the 452 filler ma~erial. Fig 6 show~ a partially constructed bu~hing 204 ~53 as it would appear after completion o~ step 422. ~he protective 454 shQll has been inserted into position, thereby compre~slng the 455 filler material 170. As best ~aen ~n F~g. 2, the amount of 456 filler material introduced into the interstitial space is limited 457 so that a Yoid 172 remains in the exterior end o~ th~.prot~c~ive 458 shell. The void 172 provides room for displacemen~ of the filler 459 material du~ to the aforementioned ~omprQssion or to thermal 460 expansion.
461 In a second method 300 of con~tructing a bushing according to 462 th~ present invention, a spare primary bushing is u~ed as a 463 mandrel. Because the upper portion of the primary bushing 130 i~
464 conically ~haped, a slightly larger interstitial void may b~
465 created for molding purposes by vertically displacing the 466 protective shield 140 a small distance A (Figs.-9-10) with 467 respect to the primary bushing 130. The larger void create~ a 468 thi~ker ~ller ~aterial layer when the material, in it~ uid~
469 uncured ~tat~, is poured into the void at mold time. Then, when 470 th~ protect~ve shield 140 is in~talled in it~ Xinal po~ition, the 471 ~iller mat~rial 170 is subjected to compres~ion between the 472 pri~ary bushing 130 and the shield. The d~sir~d displaee~nt may - ` 2 ~ 9 6 473 be achiev~d by in~erting a spacær 190 (Figs. 4 5) between the 474 primary bushing 130 and th~ prot$ctive shield 140~ The pr~ferred 475 displacement A i5 approximately 3/8 inch.
476 Fig. 8 is a flow c~art dapictlng the s~cond m~thod 300 o~
477 constructing ~e bushing. Figs. 9-10 and 6 dapict th¢ inventive 478 bushing in intermediate stag~ o~ constructionO In step 312, a ~79 spacer is a~sembled onto a ~andrel, which ~ay bQ a ~pare primary ~80 bushing or other suitable co~ponent. In s~ep 314, a protective 481 shield is assQmbled onto the mandrel OVQr the ~pacer. F~g. 9 482 shows a partially constructad bushin~ 210 a~ it would appsar 483 after co~pletion o~ step 314. Th~ prot~ctive ~hi~ld 140 484 cooperate~ with primary bushing 130 to ~or~ an inter~titial void 485 192 for raceiving an interstitial ~iller. In step 316, khe 486 interstitial filler 170 is introduced into void lg2. In ~tep 4~7 318, the fill~r is cured. The cure may occur a~ an elevated 488 temperature if the filler material 50 require~. Fig. 10 show~ a 489 partially constructed bushing 212 as it would appear a~ter 490 completion o~ ~tep 318.
49~ In step 320, the primary bushing 130 serving as a mandr~l 492 and the spac~r ar~ removed. In step 322, a lubricant i~ applied 493 to t~ out~r sur~ace of ~he prim~ry bu~hing 130, and an a68~mbly 494 con3i~tin~ oP the primary bushing 310 and the ~lectric~l 495 conductor 110 i~ inserted into the protective sh~ll 140. In ~tep 496 324, thQ a~embly is compressed again~t t~e fill~r ~ats~ial.

2~30~6 497 Fig.6 ~ho~ a partially construct~d bush~ng 204 a~ i~ would 498 appear after completion of step 324. ~he protective shell 140 499 has been displaced along arrow 194 by distance A, thereby 500 compressing the filler matexial 170.
501 The above-described embodiment of the invention i8 merely 502 one example of a way in which th~ in~ention may ba carried out.
503 Other ways may also be poq3ible, and ar~ wlthin th~ ~cope of the 504 following claims defining the invention.

Claims (17)

1. An insulating bushing for use in electrical equipment comprising:
an electrical conductor;
an insulating primary bushing surrounding said conductor;
an insulating protective shield having a cylindrical wall;
said cylindrical wall disposed in spaced concentric relation to said primary bushing;
said cylindrical wall and said primary bushing cooperating to define an interstitial region therebetween; and an elastomeric material substantially filling said interstitial region.

2. The bushing of claim 1 wherein said insulating primary bushing has a first dielectric constant and said elastomeric material has a dielectric constant approximating said first dielectric constant.

3. The bushing of claim 1 wherein said insulating protective shield has a second dielectric constant and said elastomeric material has a dielectric constant approximating said second dielectric constant.

4. The bushing of claim 1 wherein said insulating primary bushing comprises a moldable, thermo-setting epoxy resin.

5. The bushing of claim 4 wherein said insulating primary bushing is molded about said electrical conductor.

6. The bushing of claim 1 wherein said insulating protective shield is constructed of glazed ceramic.

7. The bushing of claim 1 wherein said insulating protective shield is constructed of glazed porcelain.

8. The bushing of claim 1 further comprising means removably attached to said conductor for retaining said insulating protective shield in a predefined position in relation to said primary bushing.

9. The bushing of claim 1 wherein said elastomeric filler comprises a moldable, low-viscosity polyurethane resin.

10. An insulating bushing for use in electrical equipment including at least one conductive surface and an aperture in said surface, said bushing comprising:

an electrical conductor extending through said aperture an insulating primary bushing surrounding said conductor and extending through said aperture;

an insulating protective shield disposed on a first side of said conductive surface;

said shield having a substantially cylindrical wall disposed in spaced concentric relation to said primary bushing;

said cylindrical wall and said primary bushing cooperating to define an interstitial region therebetween; and an elastomeric material substantially filling said interstitial region.

11. An insulating bushing for use in electrical equipment including an enclosure defining an interior region and an exterior region; the enclosure containing a quantity of electrically-insulating gas; the enclosure including at least one conductive surface and an aperture in said surface; said bushing comprising:

an electrical conductor extending through said aperture;

an insulating primary bushing surrounding said conductor and extending through said aperture;

a protective shield disposed in said exterior region and extending outward from said conductive surface, said shield having a substantially cylindrical wall disposed in spaced concentric relation to said primary bushing;

said cylindrical wall and said primary bushing cooperating to define an interstitial region therebetween; and an elastomeric material substantially filling said interstitial region.

12. An insulating bushing for use in electrical equipment including an enclosure defining an interior region and an exterior region; the enclosure containing a quantity of electrically-insulating gas; the enclosure including at least one conductive surface and an aperture in said surface; said bushing comprising:

an electrical conductor extending through said aperture;

an insulating primary bushing surrounding said conductor and extending through said aperture;

an protective shield disposed in said exterior region and extending outward from said conductive surface;

said shield having a substantially cylindrical wall disposed in spaced concentric relation to said primary bushing;

said cylindrical wall and said primary bushing cooperating to define an interstitial region therebetween; and means for modifying the dielectric constant filling substantially all of said interstitial region.

13. A method of constructing an insulating bushing for electrical equipment comprising the steps of:

inserting a mandrel into an interior channel of a protective shield to form an interstitial space between said mandrel and said shield;

introducing an uncured elastomeric filler into said interstitial space;

curing said elastomeric filler;

removing said mandrel to expose a central channel in said elastomeric filler;

inserting a primary bushing and conductor assembly into said central channel of said filler ; and urging said primary bushing and conductor assembly into secure engagement with said elastomeric filler to displace any voids which may have formed between said primary bushing assembly and said elastomeric filler.

14. The method of claim 13 further comprising the step of molding a partially-conically-tapered primary bushing around a rod-shaped electrical conductor to form a primary bushing and conductor assembly.

15. The method of claim 13 further comprising the step of inserting a spacing member between a portion of said mandrel and said protective shield to displace said shield with respect to said mandrel.

16. The method of claim 13 further comprising the step of applying an insulating lubricant to said primary bushing and conductor assembly.

17. A method of constructing an insulating bushing for electrical equipment comprising the steps of:

inserting a mandrel into an interior channel of a protective shield to form an interstitial space between said mandrel and said shield;

introducing a dielectric-constant-modifying material into said interstitial space;

removing said mandrel to expose a central channel in said dielectric-constant-modifying material;

inserting a primary bushing and conductor assembly into said central channel of said dielectric-constant modifying material;
and urging said primary bushing and conductor assembly into secure engagement with said dielectric-constant-modifying material to displace any voids which may have formed between said primary bushing assembly and said dielectric-constant-modifying material.