Note: Descriptions are shown in the official language in which they were submitted.
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HEAT SINK T~IERMAL TRANSFER SYSTEM
FOR ZINC OXIDE ~ARISTOR5__
BACKGROUND OF THE INVENTION
Zinc oxide varistors are employed in voltage
surgearres~er devices for shunting surge currents while
maintaining the ability to operate under line voltage
conditions. These varistors have a high exponent "n" in
the voltaye-current relationship I=KVn for a varistor,
where I is the current through the varistors, K is a
constant and V is the voltage across the varistor. High
exponent zinc oxide compound varistors can have sufficient
resistance at normal line voltage to limit the current
through the varistor to a low value, but resistance at
high currents is low so that the varistor voltage with
surgè~current flowing is held to a level low enough to
prevent damage to the insulation of the equipment being
protected by the varistor.
Because the varistors are continuously connected
from line-to-ground a continuous current flows through the
varistor, and the current causes a small amount of power to
be dissipated by the varistors at normal system voltage
and at normal operating temperature. The magnitude of
both the current and the resulting power ~issipation
increases as the varistor temperature increases. Some
means must therefore be provided to remove heat from the
varistor to prevent thermal runaway. The means must not
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only be capable of preventing thermal runaway under
normal conditions, but it must also be capable of
dissipating the heat resulting from high current surges.
One effective means Eor removing the heat from the
varistor bodies employs an aluminum oxide filled silicone
resin. Each individual varistor disk is cast within a
thick quantity of the resin material prior to insertion
within the surge arrester housing. The thick silicone
material carries heat away from the varistor to the walls
of the surge arrester body. The use of a silicone
encapsulant as a heat transfer means in zinc oxide
varistors is described within U.S. Patents 4,092,69~,
Earl W. Stetson, issued May 30, 1978 and 4,100,588,
Hiroshi Koike, issued July 11, 1978.
The process of silicone encapsulation is
extremely difficult to implement in a high production
operation. Varistor disks are encapsulated within the
silicone by means of a molding operation and individual
varistor disks or a pair of disks must be inserted within
a separate mold before the silicone encapsulant is added.
After a sufficient quantity of time has lapsed -for the
silicone material to cure, the en~apsulated disks must
then be manually removed from the molds. The high
; material costs ~or the quantity of silicone material
employed as well as the custom mold forming operation
have made the use of zinc oxide vaxistors in surge
arrester devices very expensive. One of the purposes
of this invention is to provide zinc oxide varistors
with an improved heat sink thermal transfer system at a
greatly reduced manufacturing cost.
U.S. Patent 2,870,307, Alan R. Milliken et al,
issued ~anuary 20, 1959, describes an early method of
providing a weatherproof resistor which is cooled by
means of a plurality of thin plates made from aluminum
or copper. Cooling air is freely circulated about the
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periphery of the plates during operation of the resistor.
The instant invention dif~ers substantially
from the device described within the aforementioned
patent for the reason that the amount of heat required
to be rapidly transferred away from a zinc oxide varistor
disk is too large to be carried by the aluminum or
copper plates. The mechanism of heat transfer required
by the resistors described within the aforementioned
patent is by the mechanism of radiation and convection
into the surrounding air and is in effect, a steady
state heat transfer system. The heat transfer mechanism
proposed within the instant invention is a "heat sink"
which rapidly absorbs heat and is, in effect, a transient
responding system. The heat sink elements proposed
within the instant invention can therefore have a
diameter corresponding to the zinc oxide varistors
employed whereas the radiating plates described within
the patent to Milliken et al should be laryer than the
resistors to be cooled to provide a large radiatiny
surface.
SUMM~RY OF THE INVENTION
Zinc oxide varistor disks are fitted with a
metal disk heat sink held in place by means of a
flexible elastic sleeve. The heat sink varistor
combination is held in thermal contact within the surge
arrester body by means of a resilient positioning member
and axially applied spring force~ The metal disk rapidly
removes and absorbs heat from the varistor body during
surge conditions thereby limiting the varistor temperature
rise and then transmits the heat to the arrester housing
through the flexible eIastic sleeve surrounding both
the varistor body and the metal disk.
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BRIEF DESCRIPT'ION' OF' THE' DR:AWINGS
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; FIGURE 1 is a sectional view of a prior art
surye arrester containing a plurality of varistor disks;
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FIGURE 2 is an enlarged top perspective view of
the zinc oxide varistor disk of FIGURE 1 in partial section;
FIGURE 3 is a front view in partial section of a
surge arrester containing -the heat sink transfer arrangement
of the invention;
FIGURE 4 is a graphic representation of the power
: dissipation capability of the surge arre~ter and the power
dissipation of the zinc oxide varistor as a function of
varistor termperature;
FIGURE 5 is a sectional view of the surge arrester
of FIGURE 3 through plane 5-5;
FIGURE 6 is a side sectional view of the heat sink
transfer arrangement according to the invention;
FIGURE 7 is a side view of the heat sink thermal
transfer arrangement of FIGURE 6;
FIGURE 8 is an enlarged cross sectional view of
the heat sink thermal transfer arrangement of the
varistors within the surge arrester of FIGURE 3;
: EIGURE 9 is a top view;of a further embodiment
of the varistor heat sink thermal transfer ~arrangement of
the invention; and
FIGURE 10 is a top perspective view of another
embodiment of the varistor heat sink thermal transfer
arrangement of PIGURE 9.
GENERAL DESCRIPTION OF THE PRIOR ART
FIGURE 1 shows a typical:surge arrester 10 of the
~: type consisting of a porcelain housing 11 having a top: end
: ~ cap 12 and a top terminal 13 electrically connected to a
plurality of zinc oxide varistors 16 by means of:a spring
30: 15. The arrester further contains a gas space 17 in order
~: : to provide for the reIease of gas in the e~ent of vari$tor
failure. The surge arrester is closed at the bottom by
means of a bottom cap 18 and electrical connections to
the bottom of the arrester are made by means of bottom
35 terminal 14. The varistors used within the arrester are
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of the type consisting of a sintered disk of zinc oxide
material 19 as shown in FIGURE 2 and having an electrode
layer 20 on the top and bottom faces.
The varistor is enclosed within a ceramic
collar 21 in order to prevent a discharge from occuring
between the electrode layer along the periphery of the
varistor and bypassing the zinc oxide material.
The size and composition of the metal plates
described within the aforementioned patent to Milliken
et al would render such plates inadequate for heat sinking
the heat energy requirement of zinc oxide varistors. A
typical zinc oxide varistor weighs approximately 385 grams
and has a specific heat of 0.14 cal/gm/C and a mass
density of 5.63 grams per cubic centimeter. The thermal
capacity is defined as the product of the specific heat
times the mass density, and, for such a zinc oxide
varistor, is, therefore 0.79 calper C per cubic
centimeter.
The absorption properties of a heat sink device
should therefore be of the same order as the thermal
capacity of the zinc oxide disk in order to be effective
for rapidly removing heat away from the disk and
controllin~ the varistor temperature rise and power
dissipation after surge current conduction. The specific
heats of the materials disclosed within the device of
Milliken et al, such as aluminum and copper, have values
of 0.21 and 0.09 cal~gm/C respectiveIy with mass density
values of 2.70 and 8.92 gm/cc. The thermal capacity for
aluminum is therefore 0.56 cal/C/cc and 0.80 cal/C/cc
for copper.
DESCRIPTION OF THE EMBODIMENT
In order to transfer the heat generated within
the varistor body during operation within a surge arrester
device similar to thè voltage surge arrester of FIGURE 1,
the heat transfer arrangement of FIGURE 3 is employed.
The arrester 10 of FIGURE 3 is similar to that of FIGURE
1 and like reference numerals will be used to describe
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similar elements. The heat sink disks 23 having a high
thermal capacity and the vari.stors 16 are surrounded hy
flexible elastic sleeves 22, such that varistors are
held in intimate thermal and eIectrical contact with
each other and with the end caps by means of spring 15.
The flexible elastic sleeve 22 is held in thermal
contact with the porcelain housing 11 by means of a
positioning member 24 which is inserted between the
housing 11 and the heat transfer assembly consisting of
elastic sleeve 22 and heat sink disk 23 on varistor 16.
For the embodiment of ~IGURE 3, the heat sink disk 23
can consist of a high thermal capacity material such.as
steel having a specific heat of 0.12 cal/gm/C and a
mass density of 7.86 gm/cc resulting in a thermal capacity
of 0.94 cal/C/cc. As described earlier, this thermal
capacity should be of the same order as that of zinc
oxide varistor 16 which was described earlier as
comprising approximately 0.79 cal/C/cc in order to
provide effective heat sinking under surge current
conditions.
Since the size of the heat sink is a critical
factor to be considered with the heat sink transfer
system of the invention, the heat sink vol~ume must be
at least 25% of thè volume of the zinc oxide disk to
be effective. The temperatur~ of the zinc oxide disk
~: rises directly with the amount of energy absorbed and
inversely with comblned masses of the zinc oxide disk
and the heat sink. For example, a steel heat sink
having a volume equivalent to that of the zinc oxide
~: 30 disk, the temperature rise would be less than 50~ that
of the zi.nc oxide disk alone. One other important
`: factor to be considered is the cost of the material
employed. Copper, for example, having a thermal
capacity close to that of zinc oxide would be too
experlsive to be used within surge voltage arresters.
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Aluminum for example, would have to be exceedingly large
in order to be effective ln view of its low thermal
capacity.
As described earlier, surge current flow
through a typical zinc oxide varistor causes the
temperature of the varistor to rise directly with the
amount of energy absorbed by the varistor and inversely
with both the mass and the specific heat of the varistor.
As the temperature of the varistor rises, the power
dissipation of the varistor at normal voltage rises as
shown in FIGURE 4 wherein the thermal dissipation
capability of the surge arrester A is compared to -the
varistor power dissipation per unit rating at operating
voltage B. After the flow of a high surge current, the
power dissipation requirement of the varistors can
increase to such an extent that the power dissipation
of the varistors B exceeds the heat dissipation
capability of the surge arrester A as shown at C so that
; thermal runaway results. When the heat sink 23, held to
the varistor by means of flexible elastic sleeve 22 as
; shown in the embodiment of FIGURE 3r is employed, the
temperature of the varistor rises directly with the
amount of energy absorbed ~y the ~aristor and inversely
with the combined masses and specific heats of both the
varistor 16 and heat sink 23 because of the rapid flow
of heat from the varistor into the heat sink and as
shown earlier, thè temperature rise o the varistor is
now held to`a sufficiently low leveI so that the
varistor effectiveIy cools to a normal operating
temperature. The functional relationship between positioner
24 and varistor 16 is shown in FIGURE 5 to include a gas
space ~ to provide for gas expansion in the event of
varistor failure. The heat sink thermal transfer
arrangement of FIGURE 3 can be seen in greater detail
~ 35 in FIGURES 6 and 8 wherein the varistor 16 having upper
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and lower electrode layers 20 on top and bottom sur~aces
of -the zinc oxide material and a ceramic collar 21
around the periphery of the zinc oxide material further
includes a metal disk 23 held in thermal cont~ct with
the varistor by means of elastic sleeve 22. The
material selected for the elastic sleeve is a silicone
resin having a high degree of flexibility and good heat
conducting p,roperties. As described earlier, metal
disk 23 comprises a steel composition but other alloys
of iron having a similar heat capacity can be employed.
The heat sink thermal transfer arrangement of
FIGURE 6 is shown in FIGURE 7 wherein the elastic sleeve
22 surrounds varistor 16 and heat sink 23. Electrical
continuity is achieved between each varistor in a
series arrangement of a plurality of varistors by means
of the electrode layer 20 of one varistor and the metal
disk 23 of the next succeeding varistor in the series.
In some applications the ceramic collar 21 can be
dispensed with and the elastic sleeve 22 provides
electrical insulation between the electrode layers of
the disk as well as holding the, metal disk and varistor
in good physical contact.
The positioning element 24 ~or holding the
varistor and metal heat sink in thermal contact within
the arrester of FIGUR~ 3 is shown in greater detail in
FIGURE ~. Once the eIastic sleeve 22 is ~itted around
the varistor and metal disk, the metal disk and varistor ;
combination is inserted within the arrester housing.
Positioning member 24 is then inserted to force the
varistor assembly directly against the housing. The
; positioning eIement 24 can be conveniently manufactured
from flexibIe polymer such`as silicone resin or other
good thermal conducting eIectrical insulating material.
, ~ FIGURE 9 shbws elastic sleeve 22 and positioning
', 35 member 24 wherein the positioning member is held between
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the varistor 16 and the sleeve. FIGURE 10 shows a
combination elastic sleeve 22' and extension 24' formed
in a sinyle unitary assembly. The extension 24' similar
to that o~ positioning member 24 also holds the assembly
against the housing.
With the novel heat sink thermal transfer
arrangement of the invention, the thermal transfer
characteristics can be tailored for each specific
varistor requirement.
The important requirements are that the hea-t
be transferred away from the varistor at a rapid rate,
and that the heat storage capacity of the sink disks
be sufficient to prevent excessive termperature rise
of the varistor.
The purpose of the elastic sleeve is to provide
good thermal contact between both the varistor and the
heat sink with the porcelain housing wall.
Although the heat sink thermal transfer system
of the invention is described for use within arrester
housings, this is by way of example only. The heat
sink thermal transfer system of the invention finds
application wherever zinc oxide varistors may be employed.
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