Note: Descriptions are shown in the official language in which they were submitted.
1339~8~
MODULAR ELECTRICAL ASSEMBLIES WITH PRESSURE RELIEF
Thls application is a division of Canadian patent
application Serial No. 587,516 filed January ~, 1989.
Field of the Invention
The present invention relates to polymer housed electrical
assemblies which are formed as modules and which can be
selectively coupled together to vary the overall electrical
rating of the device. Each electrical assembly is formed from
electrical components that are wrapped with a non-conductive
filament winding in a pattern with lateral openings for
relieving gas pressure. The components can be varistors,
resistors, capacitors, or any combination thereof.
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Background of the Invention
A surge protector or arrester is commonly connected
across a comparatively expensive piece of electrical
equipment to shunt over-current surges. Such over-current
surges occur, for example, when lightning strikes. When
this happens, the surge arrester shunts the surge to ground,
thereby protecting the piece of electrical equipment and the
circuit from damage or destruction.
Present day surge arresters commonly include an
elongated, hollow cylindrical housing made of porcelain or
the like, and a plurality of non-linear resistive blocks
within the housing. Some of these structures also include
spark gaps, the blocks and gaps being electrically
interconnected to handle voltage and current surge
conditions arising on a power line. The blocks commonly
contain silicone carbide (SIC) or metal oxide varistors
(MOV), and are usually in the shape of relatively short
cylinders stacked within the arrester housing. The number
of blocks employed is a function of the material (SIC or
MOV) and the voltage and current ratings of the assembly.
For a surge arrester to function properly, intimate
contact must be maintained between the MOV or SIC blocks.
This necessitates placing an axial load on the blocks within
the housing. Prior art arresters utilize bulky contact
springs within the housing to provide this axial load.
Typically, these springs can provide only relatively small
loads, for example, about sixty pounds. As a result, prior
art surge arresters experience one or more problems such as
poor heat transfer ~etween the MOV or SIC blocks and
arrester terminals; non-uniform current distribution; and
high contact resistances at joints. Furthermore, units
having low contact force sputter and the ionized metal which
is produced can cause axial flashover at high currents.
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An additional problem with surge arresters of the prior
art is that they, on rare occasions, fail in a dangerous
fashion. When these arresters fail and experience high
fault currents producing high internal gas pressures, the
bursting unit may throw parts and cause ~roperty damage.
In addition, some of the prior art devices are
difficult to asgemble, have poor dielectric design, are
susceptible to water invasion, and requlre totally different
devices to provide varied voltage ratings.
Examples of prior art surge arresters are disclosed in
the following V.S. patents: 2,587,587 to Bellezza et al;
2,947,903 to Westrom; 2,997,529 to Fink; 3,018,406 to Innis;
3,261,910 to Jacquier; 3,412,273 to Kennon et al; 3,524,107
to Reitz; 3,566,183 to Olsen; 3,567,541 to Raczerginski;
3,586,934 to Nakata; 3,706,009 to Reitz; 3,725,745 to Zisa;
3,850,722 to Kreft; 3,973,172 to Yost; 3,987,343 to
Cunningham et al; 4,029,380 to Yonkers; 4,092,694 to
Stetson; 4,100,588 to Rresge; 4,107,567 to Cunningham et al;
-~ 4,161,012 to Cunningham; 4,218,721 to Stetson; 4,404,614 to
~och et al; 4,467,387 to Bergh et al; 4,491,687 to
- ~aczerginski et al; and U.S. Defensive Publication T102,103,as well as U.K. patents 730,710; 1,109,151; and 1,505,875.
In the surge arresters of commonly assigned U.S. Patent
- No. 4,656,555 to Raudabaugh, copending Canadian patent
~- 25 application Serial No. 590,452 of Donald E. Raudabaugh
entitled Polymer Housed Electrical Assemblies Using Modular
Construction and filed February 8, 1989, and
Canadian patent application Serial No. 587,515
filed January 4, 1989 entitled Modular Electrical
Assemblies with Plastic Film Barriers of
Donale E. Raudabaugh, resin soaked glass fibers completely
surround and axially compress the varistor
blocks. This complete enclosure of the varistor blocks may
not permit the gases generated upon varistor block failure
to escape to the weathershed housing interior and then out
- of the weathershed housing before the gas pressure becomes
too great and causes the assembly to break apart. If~~the
- 1~39484
filament wrap is relatively thin, the wrap can be burned through
or can split before an extremely high pressure develops.
Summary of the Invention
Accordingly, this invention seeks to provide
electrical assemblies, particularly for surge arresters, which
can vent gases generated upon electrical component failure to
minimize damage, are relatively simple and inexpensive to
manufacture, have good dielectric design, resist water invasion
and have modular components and housings to simply vary voltage
ratings.
The invention in one broad aspect in this divisional
application pertains to a sealed envelope based on a filamentary
winding constituted by resin-impregnated fibers, for equipment
likely to be subjected to large thermal stresses and thus to be
the seat of internal excess pressures, the winding being on the
outside surface of the equipment, wherein the outside surface of
the equipment with the winding thereon has fiber-free zones
forming resin-filled gaps, with the surface area not covered
with the fibers being less than the total surface area covered
with the fibers.
Another aspect of the invention in this divisional
application provides a composite lightning arrestor comprising
a stack of pellets made of a varistor type material together
with end fittings and means for ensuring electrical connection
throughout the stack. An envelope is formed by a filamentary
winding of resin-impregnated fibers providing radial binding of
the stack, the envelope being coated with fins made of injected
elastomer, with the surface area not covered with the fibers
being less than the total surface area covered with the fibers.
More particularly.there is provided in a preferred aspect
a modular electrical assembly including a plurality of
conductive electrical components aligned in a row or column and
electrically connected through their axially directed ends and
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a non-conductive fiber filament winding wrapped about the
electrical components. The winding applies an axially directed
compressive force on the electrical components to maintain their
electrical connection and defines a pattern with lateral
openings therein for venting gases generated upon failure of one
of the electrical components.
Other aspects, advantages and salient features of the
invention will become apparent from the following detailed
description, which, taken in conjunction with the annexed
drawings, discloses a preferred embodiment of the invention.
Brief Description of the Drawinqs
Referring to the drawings which form a part of this
original disclosure:
FIGURE l is a side elevational view in partial section of
a modular electrical assembly in the form of a surge arrester,
in accordance with the present invention, illustrating the outer
surface of the filament winding.
FIGURE 2 is a side elevational view in longitudinal section
of the assembly illustrated in FIGURE 1.
FIGURE 3 is an enlarged end elevational view in section
taken along line 3 - 3 of FIGURE 1.
FIGURE 4 is an end elevational view of the end member of
FIGURES 1 and 2.
- FIGURE 5 is a side elevational view in section of the end member taken along line 5 - 5 of FIGURE 4.
FIGURE 6 is a side elevational view of the end member of
FIGURE 4.
FIGURES 7 - 9 are diagrammatic illustrations of the wrap
plan for forming the pattern of the filament winding of FIGURE
1.
Detailed Description of.Preferred Embodiment
Referring to FIGURES 1 - 3, an electrlcal device 50, in
the form of a surge arrester, according to the
present invention is formed of a modular electrical assembly
52, enclosed in a polymeric, elastomeric weathershed housing 58.
The illustrated electrical assembly can be advantageously
substantially identical to and interchangeable with the
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other electrical assemblies, and is in turn formed from one
or a plurality of cylindrical electrical components 60 and
62. These components are aligned in a row, and are in
electrical connection with one another throush their
axially-directed ends and under an axially-directed
compressive force developed by a non-conductive filament
winding 64, as disclosed in U.S. Patent ~o. 4,656,555 and
~anadian Serlal No. ~YO, 452. The electrical components can
be metal oxide varistors (e.g. zinc oxide varistor blocks),
resistors, capacitors or any combination thereof.
In the case of varistors used to form a surge arrester,
voltage ratings can be enlarged merely by serially and
selectively coupling the plurality of modular electrical
assemblies together mechanically and electrically.
15The elastomeric weathershed housing 58 receives the
electrical assemblies therein via a slight interference fit.
This facilitates construction and allows the practice of
good dielectric design by reducing radial gaps.
Electrical assembly 52 has a substantially cylindrical
overall outer surface and comprises first end member, or
terminal 72, spring washer 74, contact disc 76, electrical
component 60, contact disc 78, electrical component 62,
- contact disc 80, spring washer 82, and second end member or
terminal 84. Additional spring washers can be employed in
the electrical assembly against the contact discs at some or
all of the intermediate varistor joints, particularly for
base mounted assemblies, to maintain contact pressure when
the assembly bends under cantilever loading. The
~ non-conductive filament winding 64 is coupled to end members
-30 72 and 84, encloses the electrical components, and maintains
them under an axially-directed force, which is augmented by
the spring washers.
- A plastic film barrier 110 laterally surrounding
electrical components 60 and 62 is interposed coaxially
between the electrical components and filament winding 64.
Preferably~ the plastic ls polypropylene. ~he barrier 110 is
formed by wrapping a rectangular plastic sheet tightly about
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the electrical components and the adjacent portions of end
members 72 and 84 in preferably two layers before filament
winding 64 is added. The thickness of the plastic sheet and
of each layer is about 0.0005 inch.
Since the plastic film barrier extends along the entire
length of the electrical components and onto the end
members, the plastic film barrier seals the electrical
components from the epoxy or resin on the filament forming
the winding. For surge arresters, this prevents the wet
- 10 epoxy or resin on the filament from bonding to the fragile
ceramic insulating collars on the metal oxide varistor
blocks 60 and 62. Such bonding can be prevented by other
adhesion blockers, such as silicone oil or grease.
Advantageously, end members 72 and 84 are formed frPm
aluminum. They can also be formed of any other material
with suitable conductivity and mechanical strength.
End members 72 and 84 form internal terminals, have
cylindrical exposed outer surfaces, and have opposite, first
and second axially-directed planar ends with internally
threaded sockets or bores 86 and 88 formed respectively
therein. Socket 86 t-hreadedly receives threaded end stud 90
which can be connected to an electrical power source and is
in the form of a metallic, conductive bolt with an
internally threaded nut 91. End plate 92 is received on end
stud 90, tightly engages an end of the weathershed housins
as seen in Figs. 1 and 2 and is held in place via rigid nut
91 on the stud. For base mounting, a base plate with a bolt
circle can be attached. A second end plate 96 is similarly
positioned at the other end of the housing and is received
on end stud 98 which is connected to ground and maintained
thereon via internally threaded nut 99 on the stud. Studs
90 and 98 in essence form external terminals for the overall
device 50.
Weathershed housing 58 has a through passageway in the
form of a throughbore with an inwardly facing cylindrical
surface 100 which tightly receives therein the outer
cylindrical surface of the electrical assembly 52. ~he
reception of the assembly in the throughbore is preferably
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via an interference fit with the assembly having an outer
surface diameter that is about 2~ to about 9~ greater than
the throughbore diameter and is substantially constant along
its length. This reduces radial gaps and thus provides
advantageous dielectric design.
Since end members 72 and 84 are identical, only end
member 72 is described in detail. Referring particularly to
Figs. 4-6 end member 72 comprises an inner section 120 and
an outer section 122 separated by a radially extending
flange 124. Inner section 120 is oriented adjacent the
electrical components 60 and 62 and has a cylindrical
lateral surface with a transverse diameter substantially
equal to the electrical components. Inner section 120
defines that portion of the en2 member which receives film
barrier 110. Outer section 122 also has a cylindrical
- lateral surface, but has a transverse diameter substantially
less than inner section 120.
Flange 124 is generally circular in plan view and
extends radially outwardly from the interface between
sections 120 and 122. Radially inwardly extending and
radially outwardly opening notches 126 are formed in the
flange. Eight uniformally dimensioned notches are evenly
and circumferentially spaced about flange 124 in the
illustrated embodiment. The number of notches will vary
depending upon the component diameter. More notches will be
used with larger component diameters, and less notches will
be used with smaller component diameters.
The end members facilitate wrapping a non-conductive
filament, e.g., glass in a pattern with diamond shaped
lateral openings 128. Openings 128 are filled with a
fracturable insulating material 130 having suitable
insulating and mechanical characteristics, for example
epoxy. Other suitable insulating materials include
polyester, foam, rubber, silicone grease or gas, such as
air. If the housing is molded about the electrical assembly
wrap, the molded housing material can fill the openings.
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The crisscross winding pattern illustrated in Fig. 1 is
formed by wrapping one filament, or preferably a plurality
~ of filaments simultaneously (typically 9) according to the
- pattern diagrammatically illustrated in Figs. 7-9 wherein
the end member notches 126 are spaced at 45~ angles. ~he
wrap plan used for a particular arrester will depend on
component diameter, length and mechanical requirements. In
these figures, end members 72 and 84 are denoted by the
letters "L'' and "R" in Figs. 7 and 9, respectively. The
individual notches 126 in each end member are numbered 1
through 8, respectively. In passing from end member to the
other, the assembly is rotated through 180~ as a filament is
moved axially. Subsequently, the filament is rotated at the
end member through an angle of 315~ to the next notch
position. This specific pattern illustrated is as follows
~ and is illustrated in Fig. 8:
From To Rotation
lL 5R 180~
5R 4R 315~
4R 8L 180~
8L 7L 315~
7L 3R 180~
3R 2R 315~
2R 6L 180~
2 6L 5L 315~
5L lR 180~
lR 8R 315~
8R 4L 180~
- iL 3L 315~
3L 7R 180~
7R 6R 315~
6R 2L 180~
2L lL 315~
The pattern is repeated until the filament develops a
thickness equal to the lateral peripheral extent of flange
124. Additional fiber filament is wound about the outer
sections 122 until the filament surrounding such sections
has an outer peripheral surface at least equal to the
outermost extension of the flange. The outer surface of the
assembly is then abraded to the extent necessary to provi~-e
a uniform cylindrical s~rface along its entire length.
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The insulating material 130 fills the openings 128 to
maintain the desired uniform cylindrical surface of assembly
52. However, insulating material 130 can readily break or
separate upon the development of adequate internal pressure
within the winding, which pressure exceeds the threshold
level permitted by epoxy or other insulating material
~ against rupture, to permit gas to vent.
- Upon electrical component failure, gas is released
developing tremendous gas pressure within the fiber filament
winding. This pressure causes the epoxy or other insulating
- - material to fracture and the gas to escape to the inside of
weathershed housing 58. Due to the flexible and resilient
nature of elastomeric weathershed housing 58, the housing
will expand, permitting the gas to flow along the length of
the housing inner surface and out its axial ends. The gas
can also vent between adjacent housings in a stacked
arrangement, or through a split in the elastomeric housing.
Once the gas is released, the housing will contract and
again tightly bear against assembly S2. Without this
venting of the gas, the gas would be entrapped within the
winding until the increasing gas pressure causes an
explosion of the assembly. After venting, ionized gas
causes an external arc bridging the damaged arrester to
relieve the internal fault.
2S To mechanically and electrically connect a plurality of
the electrical assemblies together in an aligned, straight
end-to-end serial array, externally threaded, metallic, and
conductive studs can be used. These studs are
advantageously substantially identical and interchangeable,
as well as substantially rigid and formed of stainless
steel. The studs couple the adjacent ends of adjacent
assemblies by being threadedly received in the threaded
sockets in each assembly's adjacent end member. The
adjacent ends of adjacent assemblies are screwed tightly
together on the studs to provide a substantially gap-free
engagement between the facing planar, axially-directed outer
ends of the end members thereon. This provides an
advantageous electrical and mechanical interface by reducing
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possible separation during bending of the device. Plural
weathe shed housing sections, or a larger, one-piece housing
can be used.
To provide sealing against water invasion, preferably a
gasket 140 is interposed between each end member and the
adjacent end plate, and silicone grease is interposed
between each adjacent end plate and end member, between
adjacent end members, and between the outer surfaces of the
electrical assemblies and the inwardly facing surfaces of
the throughbore in each weathershed housing section. Use of
grease between the weathershed housing section and the
electrical assembly aids in construction and assembly by
reducing friction and also reduces any radial gaps
therebetween.
Advantageously, the longitudinal axes of the studs, the
electrical components in each assembly, and the weathershed
housing 58 are coaxially aligned. Preferably, the planar
ends of the end members are perpendicular to these aligned
longitudinal axes.
Preferably, with regard to the electrical device 50,
the axial load on the electrical components before winding
is about 750 pounds per square inch, and the filament or
stranded element of fibers is wet, epoxy coated fiberglass
which is wound through about 100 turns and is cured for
about two hours at 150~C.
While a particular embodiment has been chosen to
illustrate the invention, it will be understood by those
skilled in the art that various changes and modifications
can be made therein without departing from the scope of the
invention as defined in the appended claims.
