Note: Descriptions are shown in the official language in which they were submitted.
5D-5416
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The present invention relates generally to electrical
overvoltage surge arresters of the type which include
power varistors and relates more particularly, but not
exclusively, to such arresters which have no power handling
arcing gaps connected in series with the varistors and which
have varistors of the zinc oxide type.
Overvoltage surge arresters can be considered to be
high speed voltage sensitive switches which are normally
in the open position and connected between an electrical
system and ground or some other reference potential.
Typically, they include an electrical series of one or
more varistors and one or more arc gaps in an insulating
housing. At higher voltages, there may be voltage grading
; resistors shunting the gaps and also certain other circuitry
to afford better control of the arrester response to a surge.
When the arrester is in the steady state, essentially
no current passes through it except for the steady state
current through the grading resistors. A voltage surge in
the system above a predetermined voltage, however, will
cause the arc gaps to arc over and pass a large current to
ground through the series power varistors, which are chosen
to have a low resistance at such a voltage. As the system
voltage returns to normal, the resistance of the power
varistors rapidly increases until there is insufficient
follow current through the arrester for the arcs to be
maintained in the gaps and the arrester then clears to
once again become an open switch. The gaps perform the
functions of providing a sharp control of the switching
function and of isolating the system voltage from the
varistors in the steady state. This isolating is needed
because the varistors may not have sufficient nonlinearity
in their current-voltage characteristic for keeping the
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steady state current at the normal system voltage to a low
enough value to prevent thermal damage to the arrester.
Recently developed varistors of the zinc oxide
compound type have made it feasible to eliminate series
arc gaps entirely from arresters. These varistors are
often referred to as "high exponent" varistors. The
"exponent" is the numerical exponent in the current-voltage
relationship I = KV for a varistor, where I is the current
; through the varistor, K is a constant, and V is the voltage
across the varistor. Such high exponent varistors can have
sufficient resistance at system voltage to pass a follow
; current which is not ordinarily significant, while neverthe-
less having a sufficiently rapid decreasing of resistance
at predetermined surge voltage to afford close control of
the arrester switching functions without any interposed
gaps.
Varistors used in arresters are generally subject to
a thermal runaway condition, and this is particularly true
for high exponent varistors used without series arc gaps.
The runaway condition is due to the tendency of the varistor
at a set voltage to pass more and more current with in-
creasing temperature.
An arrester without series gaps and with high exponent
power varistors will pass a certain steady state current
at the normal system voltage. The magnitude of this current
wlll be affected by the manner in which heat generated by
the current is dissipated from the arrester. If the steady
state current is too high, then the temperature of the
arrester will continue to rise and the current will increase
until the arrester fails, since the temperature dependence
of the varistor current is a higher order function than is
the heat dissipation from the arrester. On the other hand,
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even if the steady-state current is well below the instability
threshold, a series of surge currents might add so much
energy to the varistors that they are unable to recover to
the steady-state current and are thus pushed into a runaway
condition.
The problem of thermal runway in arresters has been
recognized previously. Prior approaches to preventing
runway have concerned primarily improving the heat transfer
between the varistors and the housing, so that the housing
would dissipate enough heat to keep the varistors well
below a temperature from which they might be pushed into
runaway by any normally anticipated surge currents. Such
a prior approach is described, for example, in U.S. Patent
No. 2,050,334 dated August 11, 1936 to D.R. Kellogg. In
Kellogg there is disclosed an arrester in which the space
between the varistors and the procelain housing is filled
with a nonflammable insulator to improve the heat transfer
to the housing. The insulator is cylindrical and is
provided after the varistors have been fitted into the
housing. Depending on the particular insulator form, it
may be packed around varistors, embed them, or be inserted
as a preformed cylinder.
A serious problem with the above prior approach is
that any arcing across the varistors in a failure mode
will be closely confined and will therefore result in a
rapid generation of large volumes of gas. Such gas genera-
tion presents an increased liklihood of a violent explosion
of the housing.
The novel arrester of the present invention comprises
between the varistors and the housing a heat transfer and
`~ sinking collar. The collar configuration is such as to
leave space for free arcing in the event of a failure of
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the varistors. This reduces the rapid generation of gases
which would result from a confined arc and thereby sub-
stantially reduces the likilhood of a violent failure of
the arrester. In addition to its function of transferring
heat from the varistors to the housing, the collar itself
acts as a heat sink to supplement the heat capacity of the
varistors and to thereby decrease the liklihood of the
varistors being pushed into a thermal runaway condition by
impluse energy.
FIGURE 1 is a side sectional view of a first example
of an arrester in accordance with a preferred embodiment
of the present invention.
E'IGURE 2 is a cross-sectional view of the arrester of
FIG. 1 taken through the central portion.
FIGURE 3 is a side sectional view of a longitudinal
fragment of an arrester of a second example in accordance
with a preferred embodiment of the present invention.
FIGURE 4 is an elevantional view of one of the
varistor units of the arrester of FIG. 3.
FIGURE 5 is an elevational view of a first alternate
configuration for a varistor unit of the genera:L type as
the unit of FIG. 4.
FIGURE 6 is a front sectional view of the varistor
unit of FIG. 5.
FIGURE 7 is a cross-sectional view of an arrester
with varistor units such as the unit shown in FIG. 5.
FIGURE 8 is an elevational view of a second alterna-
tive configuration for a varistor unit of the general type
as the unit of FIG. 4.
FIGURE 9 is a side sectional view of the varistor unit
of FIG. 8.
FIGURE 10 is a cross-sectional view of an arrester in
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which varistor units such as the unit of FIGS. 8 and 9 are
installed and held in place by a resilient biasing ball.
FIGURE 11 is a side sectional view of a longitudinal
fragment of the arrester.
FIGURE 12 is an elevational view of a third alterna-
tive configuration for a varistor unit of the general type
as the unit of FIG. 4.
FIGURE 13 is a front sectional view of the varistor
unit of FIG. 12.
FIGURE 14 is an elevational view of a metal thermal
shunt place which is included in the varistor unit of
FIGS. 12 and 13.
FIGURE 15 is a cross-sectional view of an arrester
showing a pair of varistor units such as the unit of
FIGS. 12 and 13 installed in the porcelain held in place
by a resilient biasing ball.
A first preferred embodiment of the present invention
is the electrical overvoltage surge arrester 10 shown in
the FIG. 1 of the drawings. The arrester 10 has a housing
which includes a skirted housing porcelain 12. The
porcelain 12 has fixed to its ends two metal terminal end
cap assemblies 14 which include means for venting of gas
from inside the arrester 10 when a predetermined gas
pressure is exceeded in the arrester. Inside the porcelain
12 and electrically connected in series between the end
cap assemblies 14 is a stack of discoid-shaped varistors
16 which are of high exponent zinc oxide compound ceramic
material. The varistors 16 are disposed to one side of
the central axis of the porcelain 12.
Filling a ma]or portion of the longitudinal space
between the varistors 16 and the interior wall of the
porcelain 12 is a heat transfer and sinking material 18
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which is a room-temperature-vulcanizing silicone rubber
compound loaded with a particulate aluminum oxide filler.
The unfilled portion of the longitudinal space in the
interior of the porcelain 12 defines an arcing and gas
venting channel 19~
In FIG. 2 is shown a cross-section through the
arrester 10 illustrating one of the varistors 16 embedded
in the heat transfer and sinking material 18. Each of the
varistors 16 is provided on its faces with a conductive
electrode coating, so that when the varistors 16 are stacked
together, they are connected electrically in series by the
contact between the adjacent faces.
The heat transfer material 18 may be poured into the
arrester 10 after the varistors 16 are installed, and the
arrester 10 then turned on its side during the curing of
the material 18 so that by self-leveling of the material
18 the venting channel 19 is left in the interior of
the porcelain 12.
The heat transfer and sinking material 18 provides
an improved thermal coupling between the varistors 16
and the porcelain 12 to permit more effective dissipation
of heat generated in the varistors 16 from the porcelain
during the steady state operation of the arrester 10 on
a system. It also augments the heat sinking capability
for the varistors 16 by adding to the total heat capacity
of the arrester 10 so that the varistors 16 are less
subject to being pushed into a thermal runaway condition
by the energy absorbed during the course of a single long
over-voltage impulse or by a series of impulses closely
spaced in time. A further function of the heat transfer
and sinking material 18 is the protection of the varistors
16 against mechanical shock damage during shipment or other
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handling of the arrester 10.
For all the embodiments described herein, a suitable
heat transfer and sinking material may be made by mixing
1.8 parts by weight aluminum oxide sand particulate filler
with 1 part low-viscosity two-component room~temperature-
vulcanizing liquid silicone rubber binder, such as for
example a product marketed in 1976 as RTV 627 by the
Silicone Products Department of the General Electric Company,
Waterford, New York, U.S.A. The sand is preferably a
mixture of equal parts 180 grit fine and 80 grit coarse as
defined by the U.S. National Burear of Standards for example
in U.S. Dept. of Commerce pub].ication 118-50, "Simplified
Practice Recommendations". The primary function of the
coarse component of the sand is to improve the thermal
conductivity, while the primary functions of the fine
component of the sand are to improve the structural
properties of the material, to inhibit settling out of
the coarse component during pouring and curing, and to
displace the more costly silicone rubber binder.
The venting channel 19 provides a space for unconfined
arcing across any of all the varistors 16 in case of a
failure of the arrester 10, so that a minimum of gas is
generated by the failure. The gas that is unavoidably
generated in such a failure can be vented through the
venting mechanism in the end cap assemblies 14 by passing
through the unrestricted venting channel 19 left by the
heat transfer material 18.
Example 2 - A second preferred embodiment of the
present invention is the arrester 20 shown in the FIG. 3
of the drawings. The housing of the arrester 20 includes
cap assemblies and a porcelain 22 and is similar to that
of the arrester 10 of Example 1. Stacked inside the housing
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~5~83 5D-5416
porcelain 22 of the arrester 20 are a plurality of varistor
units 24, one of which is shown in more detail in the EIG.
4. There is left by the varistor units 24 a venting space
25 which extends longitudinally along the interior arrester
20.
The varistor unit 24 of FIG. 4 is a zinc oxide com-
pound varistor 26 which is provided with an individual
heat transfer and sinking collar 27 of heat transfer
material of the same type as the material 18 of the ar-
rester 10 in Example 1. The collar 27 completely surrounds
the varistor 26 and has a flattened venting space section
28 which provides the incremental portion of the venting
space 25 of the arrester 20 for the individual varistor
unit 24.
There are several advantages to the combining of avaristor and an individual heat transfer and sinking
collar 27, rather than an arrangement such as in the
arrester 10 of Example 1, where the material 18 encapsulates
all the varistors 16 as a group. One advantage is that
the individually collared varistor units 24 are easier to
handle and to install in the arrester than are the varistors
26 themselves without the collar 27, since the collar 27
provides a supporting means for the varistors 26. Another
advantage is that the varistor units 24 can be readily
disassembled again if upon testing of the finished ar-
rester 20 it is found that one or more of the varistors 26
is faulty. A faulty one of the varistor units 24 can then
be replaced and the arrester reassembled without being
scrapped. A third advantage to combining the varistors 26
individually with a collar 27 to make a unit 24 is that
the configu,ation of the collar 27 can be easily modified
to save material and to be better adapted for other problem
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conditions.
One characteristic that can be a problem is the
difference in the coefficient of thermal expansion between
the material of the collar 27 and the varistors 26 and
porcelain 22. The coefficient of thermal expansion of the
collar 27 is considerably greater than that of the porcelain
22 or the varistor 26. This could mean that upon heating
- of the arrester 20 the collar 27 of adjacent varistor units
24 would push against each other so that the contact between
the faces of their respective varistors 26 is broken. In
order to prevent such an occurrence, the thickness of the
collar 27 is made less than the thickness of the varistor
26.
It is desirable that each of the varistor units 24
be firmly held in place within the porcelain 22, both for
simple mechanical stability and also for establishing a
good heat transfer relationship to the porcelain 22. Since
the material of the collar 27 can be made resilient, the
collar 27 can itself provide the mechanical and thermal
; 20 contact needed to establish the desired holding in place.
However, it is found that as the thermal conductivity of
the collar 27 is increased by increased loading with insu-
lating ceramic particulates such as aluminum oxide, the
resilience decreases to the point where excessive stresses
may result in the course of installation of the units 24
- and also upon heating of the arrester 20 after it is
completed.
There are described below several alternative con-
figurations of varistor units with collars modified to
avoid one or more of the above problem conditions. The
alternative units are of the same general type as the
varistor units 24 of the arrester 20 in that the units
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include a separate and individual heat sinking and transfer
collar and may be incorporated into an arrester porcelain
in one or more stacks. Therefore, the features of the
arrester other than the porcelain are not further dis-
cussed for each alternative unit. Also, the collar of each
alternative unit may be of the same material as described
for the arrester 10 of Example 1 above.
In the FIGS 5 and 6 there is shown a first alternative
varistor unit 30. The varistor unit 30 includes a varistor
32 and a collar 33 around the varistor 32. The collar
33 has a flattened venting space section 34 and is provided
with two expansion space indents 35. The indents 35 com-
pensate for a loss of resiliency in the collar material
when it is heavily loaded with filler. The varistor unit
30 is shown installed in a porcelain 36 in the FIG. 7 with
a venting space 37 left open. The indents 35 save collar
material and provide a space for the collar 33 to expand.
Also, the indents 35 make those portions of the collar 33
which are to either side of the venting space section 34
flexible, to permit a snug fit in porcelains of various
diameters. The faces of the varistor 32 are raised above
the collar 33 to allow for thermal expansion of the
collar 33 in that direction.
In the FIGS. 8 and 9 there is shown a second alter-
native varistor unit 38 which is adapted to be installed
inside the porcelain 36 as is shown in the FIGS. 10 and 11.
The unit 38 includes a varistor 40 and a collar 42. The
faces of the varistor 40 are raised above the collar 42 to
permit expansion of the collar 42. The collar 42 includes
includes a nose 44 which has a raised portion 46 and a
longitudinal bias channel 48. Four faceted portions 49 of
42 function as vent space sections 49. As shown in the FIG~.
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10 and 11, the varistor unit 38 is installed in the porcelain
36 together with a highly resilient bias ball 50, which is
cast of unfilled silicone rubber. The ball 50 is pushed
into place between the bias channel 48 of the varistor unit
38 and the inside wall of the porcelain 36, and is just
large enough in diameter to be slightly deformed when in
place, so that it exerts a constant bias on the varistor
unit 38 against the opposite inside wall of the porcelain
. 36. This provides mechanical stability and a good thermal
contact of the varistor unit 38 to the porcelain 36 by
forcing the collar 42 to conform to the wall of the porcelain
36. The vent space sections 49 provide for venting on both
sides of the nose 44, so that there are two venting spaces
52 formed in an arrester with units such as the varistor
units 38. The raised portion 46 of the nose 44 retains
: the proper spacing of the nose 44 when the varistor unit
38 is in a stack and biased by the ball 50. The part of
the nose 44 near the end and including the channel 48 may
have additional loading of particulate filler material to
further stiffen it, so that the force of the bias ball 50
is more evenly distributed in the collar 42.
The bias balls 50 hol.d the varistor units 38 in-
dividually in a stack inside a porcelain. The balls S0
can be read.ily pushed along the alogned bias channels 48
of the varistor units 38, one at a time, or even in groups,
and also be readily pulled out to release the varistor
units 38. The longitudinal dimension of the installed balls
50 is the same as the thickness of the varistors 40 of the
units 38, so that the balls 50 of a stack of the units 38
are necessarily in registry with the stacked varistor units 38.
The use of a bias ball for holding in place a varistor
unit is not a part of the present invention, and is separately
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5D-5416
l~95S~33
disclosed and claimed in U.S. Patent No. 4,092,694 dated
May 30, 1978 in the name of E.W. Stetson and entitled
OVE~VOLTAGE SURGE ARRESTER HAVING LATERALLY BIASED RESILIENTLY
CARRIED VARISTORS.
In the FIGS. 12 and 13 there is shown a third alternative
varistor unit 54 which includes a pair of varistors 56 stacked
together and surrounded by a single collar 58. The collar
58 has a nose 60 with a raised portion 62 and a bias channel
64 much as does the varistor unit 38 described above.
Two flat vent space sections 66 of the collar 58
are located to each side of the nose 60. In addition, there
is embedded in the mid-section of the collar 58 an aluminum
thermal shunt plate 67, shown separately in the FIG. 14
for increasing the thermal conductivity laterally in the
collar 58.
In the FIG. 15 there is shown how a plurality of the
varistor units 54 are installed and held in place by bias
balls 68 in an arrester porcelain 70. There are two parallel
stacks of the varistor units 54 oriented in diametrically
opposed relationship in the porceIain 70. The b.ias ball
68 between them and in both channels 64 provides a mutually
opposing force to the noses 60 to flrmly hold the units 54
in place and in intimate contact with the inside wall of
the porcelain 70. Such arrangement of parallel stacks of
the varistor units 54 is particularly suited for arresters
designed to withstand unusually high surge currents requires
in some cases that more than one stack of varistors be in
parallel to present a current path of sufficiently low
resistance. In addition, at high currents the surge voltage
across the individual varistor units 54 can be so high that
additional insulating surface is needed between the faces
to prevent flashover. For this reason, the varistors 56 of
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:1~95583
the units 54 are not closely spaced from that portion of
the collar 58 which is in contact with the porcelain 70,
although a close spacing would provide the better thermal
coupling of the varistors 56 to the porcelain 70. Instead,
the varistors 56 are moved away a sufficient distance to
provide the needed insulation surface. Because the thermal
coupling to the porcelain 70 is thereby decreased, the
thermal shunt plate 67 is embedded in each of the varistor
units 54 to correspondingly increase the thermal conductivity
of the vollar 58 in the general direction of the inside wall
of the porcelain 70.
Varistor units such as described in the preferred
embodiments may be used inside a metal enclosure of a gas-
insulated system directly in the insulating gas, with
sufficient spacing from the enclosure wall, to prevent
flashover. With such an arrangement, the insulating
housing can be considered to be the gas itself. The collars
of the varistor units will be in intimate contact with the
gas to provide cooling of the collar by the gas. Independently
of the cooling, the collar will provide a heat sinking
function for absorbing impulse energy to prevent thermal run-
away of the varistors. Thus, the term "insulating housing"
as used herein is intended to include an insulating fluid
environment in thermal contact with the collars of the
varistors units.
The collar of the varistor units may be of any material
which is electrically insulating and sufficiently thermally
conductive to give improved heat conduction over that norm-
ally du to the radiation and convection of the gas inside
an arrester. rrhese properties alone will provide heat sink-
ing. Further, it preferably has some resiliency, so that
intimate thermal contact can be made to the inside wall of the
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porcelain by having the material conform to the contours
there, and so that dif~erences in the coefficients of
-thermal expansion of the varistor and the collar are safely
absorbed by the elasticity of the material. The filled RTV
of the preferred embodiments is particularly suitable as a
collar material. However, other elastomers could be used
if they have a high enough long-term high-voltage electrical
resistance. Also, other particulate fillers, such as silicon
or magnesium oxides, etc., can be used, but aluminum oxide
has the desired electrical and thermal properties and is
readily available.
A single varistor unit may have any number o~ varistor
elements, depending upon the convenience of manufacturing
and assembly, and taking into consideration the desired
electrical and thermal factors for the particular application.
The collar of a varistor unit need not extend about
the entire perimeter of the varistor, but should extend
about the portion which is to make contact with the porcelain
or housing wall to cushion against mechanical shocks and to
20 provide the thermal contact to the wall by conforming to
the contours.
The ven-ting portion of the collar may be of any con-
figuration which is a sufficient departure from the cross-
sectional geometry of the interior of the housing to allow
for ready passage of gas longitudinally in the housing and
to provide an arcing space. The venting portions may,
for example, be simply holes punched through the collar in
various places to provide passages from the side of the
other. However, the venting portions should be made so that
they are in registry when the varistor units are stacked.
While for the arresters of the pre~erred embodiments
the varistor units were arranged in mechanical series stacks
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~955~3 5D-54l6
in which adjacent units were also connected electrically
in series with one another, the electrical circuit relation-
suip of units in mechanical series may be varied in
numerous way by interposing between adjacent varistor units
an insulating spacer and providing conductive connectors
between selected locations of mechanically parallel stacks
of units or between locations of the same stack to achieve
various other circuit connections as desired. Thus, the
present invention is not limited by an particular circuitry
of the internal components of the arrester, but relates
primarily to the relationship of the varistors to the collar
as a heat conducting and sinking means; to the relationship
the varistors to the collar as an electrical insulator, and
to the relationship of the varistors to the rigid housing
of an arrester for thermal contact and mechanical stability.
While the collars as described herein are primarily
designed for varistors, it is recognized that their electxical,
thermal, and mechanical features may also make them use-
ful for other electrical circuit components which might be
included in an arrester circuit. The collars are also
clearly applicable to other varistors than those of zinc
oxide varistor compound.
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