Note: Descriptions are shown in the official language in which they were submitted.
IMPROVF.D S~RII~ GAPPED MOV SIJR~E ARR~;TER
This application is a continuation-in-part of copending application Serial
No. 07/420,069, filed on October 11, 1989.
Backr~round of the Invention
The present invention relates generally to surge arresters. More particularly, the
invention relates to a new design for the internal components of arresters. Still more
particularly, the invention relates to a new combination of elements and method of
rnanufacture which increase the reliability and manufacturability of the arrester and which
have particular applicability to dead front arresters.
Under normal operating conditions, electrical transmission and distribution equipment
is subject to voltages within a fairly narrow predetermined range. Due'to lightning strikes,
switching surges or other system disturbances, portions of the electric system may experience
momentary or transient voltage ievels that greatly exceed the levels experienced by the
equipment during normal operating conditions. Left unprotected, critical and costly equipment
such as transformers, switching apparatus, cables and electrical machinery may be damaged
or destroyed by such overvoltages and the resultant current surges~ Accordingly, it is routine
practice within the electrical industry to protect such apparatus from dangerous overvoltages
through the use of surge arresters.
To provide such protection, a surge arrester is commonly connected ;n parallel with the
equipment requiring overvoltage protection so as to shunt or divert the overvoltage-induced
current surges safely to ground and thereby protect the equipment. When caused to operate,
an arrester forms a current path to ground having a very low impedance relative to the
impedance of the equipment that it is protecting. In this way, current surges which would
otherwise be conducted through the equiprnent are instead diverted ~hrough the arrester to
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ground. Once the transient condition has passed, the arrester must operate to open the
recentiy-formed current path to ground and again isolate or "reseal" the distribution or
transmission circuit in order to prevent the nontransient current of the system frequency from
"following" the surge current to ground, such system frequency current being known as
"power follow current." If the arrester did not have this ability to interrupt the flow of power
follow current, the arrester would continue to operate as a short circuit to ground, forcing
protective relays and circuit breaker devices to open or isolate the now-shorted circuit from
the electrical distribution system, thus causing inconvenient and costly outages on the sub-
network in which the arrester is located.
Conventional arresters typically include an elongated enclosure or housing made of an
electrically insulating material, a stack of voltage-dependent, nonlinear resistive elements
retained within the enclosure, and a pair of electrical terminals at opposite ends of the
enclosure for connecting the arrester between a line-potential conductor and ground. The
nonlinear resistive elements are chosen to have a much higher resistance than the protected
equipment at the normal steady-state voltage and a much lower resistance when the arrester
is subjected to high magnitude transient overvoltages. Depending on the type of arrester, it
may also or alternatively include one or more spark gap assemblies housed within the
insulative enclosure and electrically connected in series with the electrical terminals.
Present-day arresters are typically one of two basic types and are generally classified
according to the type ot nonlinear resistive elements they contain. The first type of
conventional arrester is commonly referred to as the series gapped silicon carbide (SiC)
arrester. The nonlinear resistive elements in this arrester are relatively short cylindrical blocks
of silicon carbide which are stacked one atop the other within the arrester housing in series
with spark gap assemblies which are generally resistance graded gap assemblies. A
resistance graded gap assembly comprises a resistor electrically in parallel with the spark gap
and may include one or more resistors in series with the gap. This network of resistors is
employed to control the voltaga level at which the spark gap will begin to conduct. The
second type of arrester commonly used today is know as the gapless metal-oxide varistor
(MOV) arrester. In this type of arrester, the nonlinear resistive elements comprise disks
formed of a metal oxide compound which are again stacked within the arrester housing in
series.
In both types of prior art arresters, the voltage-current relationship for the nonlinear
elements is expressed as I = kEn, where i is arrester current, k is a constant, E is the arrester
voltage, and n is the nonlinear exponent or coefficient. The older series gapped SiC arrester
uses low exponent silicon carbide blocks in series with low exponent nonlinear graded gaps,
the exponent n of both silicon carbide components being less than 10 and typically being
within the range of 4 to 5. The more modern MO\,' arrester typically uses only high exponent
nonlinear elements of the metal-oxide variety and, as described below, does not require series
gap assemblies to opera~e properly as is the case of SiC arresters. In the case of MOV
arresters, the exponent n is usually greater than 10 and typically 25 or greater.
Because of the different degrees of nonlinearity of the rasistive elements employed in
silicon carbide and MOV arresters, these arresters differ in structure and operation. The
silicon carbide blocks are designed to provide a very low resistance to surge currents, but a
higher resistance to the power-follow current which continues to flow through the arrester
after the transier.t condition has passed. Despite the higher resistance, the silicon carbide
blocks will still conduct large currents at the normal, steady-state line-to-ground voltage.
Accordingly, gap assemblies are employed in series with the silicon carbide blocks to
effectively isolate the blocks from the steady line voltage. The resistance of the gap
assemblies at the nominal operating voltage is in the order of megohms while the resistance
of the silicon carbide blocks is in the order of ohms. For this reason, essentially 100% of the
nominal operating voltage appears across the gap assemblies. When a transient overvoltage
occurs, the voltage across the gaps increases until it reaches the gaps' sparkover voltage.
Whan this occurs, the transient overvoltage appears across the silicon carbide blocks and the
blocks conduct the surge current to ground. As a transient overvoltage condition ceases, the
resistance of the silicon carbide blocks increases so as to limit the magnitude of the power
follow current. The reduced current flow and the corresponding decrease in the voltage
across the spark gaps provide the gap assemblies the opportunity to open the current path
to ground and thus "reseal" the power circuit after the surge has passed. This type arrester
has been in use for many years and is described in many earlier patents, such as U.S. Patent
Nos. 4,161,763 and 4,174,530.
With an MOV arrester, the MOV elements provide either a high or a low impedance
current path between the arrester terminals depending on the voltage appearing across the
varistor elements themselves. More specifically, at the power system's steady-state or
normal operating voltage, the varistors have a relatively high impedance. As the applied
voltage is increased, gradually or abruptly, the varistors' impedance progressively decreases.
When the voltage appearing across each varistor reaches the elements' breakdown voltage,
the varistor impedance dramatically decreases, and the varistors become highly conductive.
Accordingly, if the arrester is subjected to an abnormally high transient overvoltage, such as
may result from a lightning strike, for example, the varistor elements becorne highly
conductive and serve to conduct the resulting transient current to ground. As the transient
overvoltage and resultant current dissipate, the varistor elements' impedance once again
increases to a very high value, thereby reducing the current through the MOV arrester to a
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negligible flow and restoring the arrester and electrical system to their normal, steady-state
condition. The vety low magnitude of current which flows through the arrester to ground
during steady-state conditions is often referred to as the "leakage current." A variety of MOV
arresters have been described in many earlier patents, such as U.S. Patent Nos. 4,930,039
and 4,240,124.
The series gapped SiC artesters suffer from a variety of undesirable traits. First,
because the SiC elements are highly conductive at normal operating voltages, the gap
assemblies are required to support the full system line-to-ground voltage over the life of the
arrester, the SiC elements being used only to limit current which, in turn, assists the gaps in
returning to their non-conductive mode during a discharge operation as described above.
Because the gap assemblies must support the full line-to-ground voltage, SiC arresters are
typically comprised of many such assemblies, each of which must withstand its proportionate
share of the voltage. This type of construction results in more consistent impulse or spark-
over characteristics than can be achieved through the use of MOV arresters; however, the
undesirable result is that the design yields higher than desired impulse protective
characteristics.
Another deficiency characteristic of the series gapped SiC arrester is that its high
current discharge characteristic and the power follow current levels are both controlled by the
same nonlinear elements, I.e., the SiC elements. To achieve lower high current discharge
voltages during a transient overvoltage, also referred to as an "impulse condition," it is
desirable to have silicon carbide elements with a low resistance. Yet, to provide lower levels
of power-follow current, it is desirable to have silicon carbide elements with a high resistance.
Due to these diametrically opposed requirernents of the same components, design
compromises have resulted in less than desirable protective characteristics. Still another
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inherent problem with the series gapped SiC arrester is its comparatively large size and
weight.
The MOV arrester was developed to eliminate the higher-than-desired impulse
characteristic of the series gapped silicon carbide arrester and has replaced the silicon carbide
arrester in many applications. In the MOV arrester, the nonlineal MOV elements eliminate the
need for a series gap by remaining highly non-conductive at normal, steady state system
voltages. As the voltage applied to the arrester is increased, the MOV element, which is a
semiconductor, gradually begins to conduct, without a disruptive discharge as is characteristic
with th0 series gapped SiC arrester. This switch-like characteristic enables the MOV arrester
to shunt all fundamental transient overvoltage energies to ground. The inherent problem ~,vith
this type of arrester, however, is that both breakdown voltage and high current discharge
voltage are controlled or dictated by the same nonlinear elements. It is desirable to have
higher breakdown voltages so that power frequency overvoltages can be sustained for longer
periods of time. These power frequency overvoltages can be destructive to a normal MOV
arrester. Power frequency overvoltages can commonly occur at up to 1.5 per unit of MCOV,
where MCOV is the maximum continuous operating voltage oF the arrester. At the same time,
it is also desirable to have lower high current discharge voltages to provide better equipment
protection. Again, as with the series gapped SiC arrester, two diametrically opposed
requiremants of the same component result in a compromise of characteris~ics. In some cases
the discharge voltage capability of the MOV arrester is compromised. In other instances, the
arrester's ability to withstand a temporary, relatively low overvol~age condition, defined as the
arrester's temporary overvoltage capability, is reduced.
More recently, a hybrid arrester has been developed which combines the gap
assemblies previously use din the silicon carbide gapped arresters with the MOV elements of
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the metal oxide varistor arrester. Such hybrid arrester is described in co-pending U.S. patent
application, serial no. 07/420,069, and in the publication entitled New Surge Arrester
rechnology Offers Substantia/ /mprovement in Protection and Reliability as presented to the
SEE Overhead Committee, Annapolis, Maryland, May 10, 1990, such written disclosures
being disclosures being incorporated herein by reference. The hybrid arrester has been shown
to have superior performance characteristics as compared to both the SiC gapped arresters
and the MOV arresters.
Power distribution networks are normally comprised both of above-ground wires
or conductors strung on insulators along towers or poles, or underground systems in which
the conductors are buried in the ground. The advent of modern suburbs, office parks and light
industrial parks where streets and utilities are laid out over open fields and existing structures
do not interfere with the construction and planning of networks, has led to widespread use
of underground residential distribution networks, or URD's. In a URD, power is distributed to
homes, offices and other buildings through underground cables which are linked and supply
power to transformer banks. These transformers may be located within covered enclosures
located above ground adjacent a group of service users, or in underground vaults accessible
by manholes. Each transformer bank will service a group of homes, businesses or buildings
typically through buried service conductors.
Like overhead distribution systems, the components in an underground distribution
system must be physically and electrically protected. However, underground systems present
special problems which arise from their physical environment and close proximity to ground.
Because the transforrners and other components of URD's are located adjacent the earth,
special insulative protection components must be used to prevent faulted circuits and electric
shock to service personnel. Further, the underground cables must be specially manufactured
p
and insulated to prevent corrosion or physical deterioration caused by freeze and thaw cycles.
It should be appreciated that the buried cable environment may have temperature and
moisture attributes which create a severe corrosion hazard.
In addition to the environmental factors unique to URD's, these systems are also
subject to more severe electrical surge duty than overhead distribution networks. As known
to those skilled in the art, underground systems may experience transient voltage levels that
are twice the magnitude of the transient levels experienced by above-ground equipment when
a surge-induced voltage reaches the open point and is reflected back to its origin. Moreover,
the specialized insulation required to environmentally protect underground cables is subject
to breakdown as a result of surge currents in the system. It is known that surges will
accelerate the creation of "cable trees," which are paths of carbonized insulation which are
created by the field effects of power surges on the carbon-based insulative materials typically
employed in cable insulations. These carbon trees may eventually result in cable failure. If
a cable fails, a new trench must be dug along the length of the old cable and a new cable
must be installed. Digging and replacing cables in developed areas is a costly, disruptive and
time-consuming process.
A typical UI~D will include a group of transformers connected in parallel to an
underground distribution cable and physically spaced-apart along the length of the
underground cable. The location where the last transformer is connected to the underground
cable is known as the "open point" of the network. To protect the transforrners, underground
cable and other URD system components from transient overvoltages, it is common to employ
a "riser pole" arrestet in parallel with the underground cable, at the location where the
overhead distribution conductor and underground cable connect, and to employ a "dead front"
surge arrester at the "open point" of an underground network in paraliel with the last
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transformer in the network. A "dead front" arrester has a shielded outer surface that is
directly connected to ground to ensure that the surface of the arrester is always at ground
potential. This feature is incorporated to ensure the safety of utility personnel who may be
working in close proximity to the arrester-equipped transformer.
To reduce the extent of any service interruption which would occur when an
underground cable is severed, or a line component improperly operates, utilities utilize a
double loop distribution system in URD's. In a double loop system, a first transmission line
feeds one series of transformers and a second line feeds a second series of transformers. The
transformer at the open point of the first series of transformers also has an energized cable
disposed thereon, but the cable does not energize the transformer. This is known as
"parking" the cabie. If a service interruption occurs upstream in the first series of
transformers, the parked cable may be loaded onto the transformer, thereby re-energizing the
particular transformer, and those disposed between the transformer and the service
interruption, to re-establish service to the customers served thereby.
In addition to the problems and design considerations common to other surge arresters,
there are additional environrnental factors which impact the performance and reliability of
dead front arresters. The open point of the URD may be located below ground in a vault or -
other protective enclosure. These locations tend to maintain high humidity levels and may
enter the dead front arrester during installation of the arrester into the URD, or during on-site
repair or replacement o the arrester. If moisture enters the arrester and is absorbed by the
non-linear resistive elements of the gap assemblies, the variable resistance characteristics of
the component will change, leading to improper arrester performance. Likewise, the presence
of air may be detrimental to performance of the gapped arrester components, as the air may
ior,ize and create partial, unintended, discharges. If precautions are not taken, moisture may
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aiso enter the gap assemblies during their manufacture, in the period of time between the
manufacture and assembly of the gap assemblies or between assembly and their placement
into the arrester body. Presently, to maintain low moisture in the gap assemblies, after the
resistive elements are fired, they are stored and assembled in a dry environment. An improper
assembly sequence or unsealed arrester can cause moisture-related performance problems.
Such improper performance may include a failure of the arrester to reseal. These performance
inadequacies may lead to equipment damage which may require localized network shutdown
for repair or replacement of the arrester. These performance inadequacies may lead to
equipment damage or lower-than-desired protective levels for the equipment and will require
arrester replacement, which often requires localized network shutdown.
An additional reliability consideration of dead front arresters is related to the failure
mode of housing or cover. The housing or cover portion of the dead front arrester is typically
comprised of a polymer, which is molded to shape and placed over the internal components
for component protection and insulative purposes. During the infrequent occurrence where
an arrester fails and is unable to reseal after dissipating a surge, the high current passing
through the arrester will create a conductive plasma of ionized gas components, which must
be vented out of the arrester. If the arrester is not designed to or fails to safely vent the gas,
a catastrophic failure of the arrester may occur and arrester components may be violently
expelled in all directions, posing a hazard to personnel and equipment. Further, the conductive
plasrna, if not vented in the desired direction, can flare out of the arrester and conduct the
current onto the adjacent transformer, or other equipment which the arrester is intended to
protect, again causing possible damage and safety concerns.
Additionally, some prior art, such as U.S. Patent 4,161,012, ~_ninqham, has taught
th3t the housing must adhere to the interior components of the arrester, i.e., that there must
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be no air gap between the components and the arrester. The structure resulting from such
teachin~ has limited heat transfer capability, and the polymer in the housing softens and loses
its physical strength and elasticity. This may allow the sides of the housing to burst, causing
plasma to be uncontrollably expelled therefrom.
SUMMARY OF THE INVENTION
The protection system of the present invention includes a riser pole surge arrester and
a dead front arrester that is located at the open point of the underground distribution network
to protect the underground distribution network from surges, and a parking stand arrester
disposed at the transformer parking a second live cable. Each of the arresters include a series
combination of metal oxide disks and low exponent, nonlinear, resistively graded gap
assemblies, which together share the voltage across the arrester during steady state and
temporary overvoltage conditions. During transient overvoltages, the series gaps in the gap
assemblies will conduct, and the entire overvoltage will be supported by th0 metal oxide disks.
To improve the performance and reliability characteristics of the dead front arrester,
the gap assemblies are disposed within a subassembly which includes a housing which seals
the gap assemblies against moisture ingress while allowing electric and mechanical connection
between the gap assemblies and the remainder of the arrester components. The
subassembly, along with the metal oxide disks and remaining components, are enclosed in a
shrink wrap tube having mastic therein. The body includes an annulus formed by a
circumferential groove along the inner perimeter thereof, which is disposed adjacent the outer
perimeter of the metal oxide disks. The annulus in the body may comprise an air-filled void
or cover may be filled with moisture-reducing materials, materials which increase the heat
transfer from the gap assemblies and MOV disks, or materials to control the dielectric
properties of the structure. An insulative sleeve or par~ial sleeve may be placed about the
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periphery of the annulus to provide rigidity and added support and to prevent or directionally
control the pressure relief of the arrester during a failure.
In alternative embodiments, the housing which contains the gap assemblies may be
replaced with a polymeric body therein, or the gap assemblies may be held in position and
sealed within a material such as epoxy. Adjacent electrodes in the gap assemblies may be
welded, soldered or brazed together to eliminate nuisance high frequency noise which might
otherwise be generated.
Thus, the prPsent invention comprises a combination of features and advantages which
enable it to substantially advance arrester technology by providing an arrester having improved
performan~e characteristics, increased reliability in harsh environments and significant
manufacturing advantages. These and various other characteristics and advantages of the
present invention will be readily apparent to those skilled in the art upon rbading the following,
detailed description and referring to the accompanying drawings.
For an introduction to the detailed description of the preferred embodiments of the
invention, reference will now be made to the accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic view of a riser pole arrester, parking stand arrester and dead
front arrester of the present invention connected in an underground electrical distribution
network;
FIGURE 2 is a perspective view of the transformer connection of the dead front and
parking stand arrester of Figure 1;
FIGURE 3 is a cross-sectional view of the parking stand arrester of Figure 1 at 2-2:
FIGURE 4 is an elevation view partly in cross-section of a gap assembly retainer shown
in the arrester shown in Figure 2;
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FIGURE 5 is an exploded sectional view of the gap assemblies of Figure 3;
FIGURE 6 is a cross-sectional side view of a pair of gap assemblies shown in Figure 3;
FIGURE 7 is a perspective view of a metal oxide disk of the arrester shown in Figure 3;
FIGURES 8A and 8B are graphical representations of the performance characteristic
of the d0ad front arrester shown in Figure 3;
FIGURE 9 is a graphical representation of the system voltage and arrester voltage
characteristics of the dead front arrester shown in Figure 3 when experiencing an overvoltage;
FIGURE 10 is an alternative embodiment of the gap assembly retainer shown in
Figure 4;
FIGURE 11 is a partial elevational view, partly in cross-section, of an alternative
embodiment of the dead front arrester of Figure 3;
FIGURE 12 is an elevation view, partly in cross-section, of another alternative
embodiment of the surge arrester of Figure 3;
FIGURE 13 is an elevational view, partly in cross-section, of another alternative
embodiment of the surge arrester of Figure 3;
FIGURE 14~is an elevational view, partly in cross-section, of another alternative
embodiment of the dead front arrester of Figure 3; and
FIGURE 15 is a cross-sectional view of an alternative embodiment of the surge arrester
of Figure 11;
FIGURE 16 is a cross-sectional view of an alternative embodiment of the arrester of
Figure 1;
FIGURE 17 is a cross-sectional view of the gap assembly retainer of the surge arrester
of Figure 16;
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FIGURE 18 is an end view of the Bellville washer on the assembly retainer of Figure
17.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to Figure 1, a riser pole surge arrester 8 and dead front arrester 200,
structured in accordance with the present invention, are shown connected to first
underground distribution network 7. As shown, riser pole arrester 8 is connected at the entry
point which is the transition point between an overhead line 4 into an underground distribution
network 7. Dead front arrester 200 is disposed on transformer 12 at the open point 6 of the
underground distribution network 7. Both of arresters 8 and 200 are located in an electrical
parallel relationship with the transformers and components b~ing protected thereby. A second
overhead distribution line 4a supplies power to secondary line 7a. Riser pole arrester 8a is
connected at the entry point of the secondary line distribution system, and a plurality of
transformers 1 2b, c are connected thereto in series. Parking stand arrester 10 (which likewise
functions in a "dead front" configuration) is disposed on transformer 12, and line 7a
terminates thereon. In the following description, the arrester of the present invention is
described with reference to parking stand arrester 10 and dead front arrester 200. This is for
purposes of example only and not by way of limiting the scope of the invention in any way.
The various features of the invention are also applicable for use in riser pole arrester 3, as well
as with many other arresters.
Referring now ts Figures 2 and 3, parking stand arrester 10 is shown in greater detail
and generally comprises an L-shaped body 28 which encloses transformer interconnection
components 34, arresting components 46 and ground circuit interconnection components 33.
Body 28 is sized to receive components 33, 3a, and 46 and configured into an L-shaped body
to interconnect transformer interconnection components 34 onto a transformer 12 and
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connect ground circuit interconnection components 33 to a grounà. Interruption components46 are disposed within body 28 between transformer interconnection components 34 and
ground circuit interconnection components 33, to selectively divert secondary line 7a to
ground in the presence of surges. To envelope and support components 33, 34 and 46, body
28 includes an upper interface portion 17 and a lower extending portion extending therefrom
at a substantially right angle thereto and interconnected thereto at mounting plate 15.
Mounting plate 15 is configured to be mounted on a bracket on transformer 12. In the
preferred embodiment, arrester 10 is configured as a parking stand arrester, and interface
portion 17 therefore is configured as a bushing over which an elbow on line 7a is connected.
Referring again to Figures 1 and 2, the use and operation of arrester 10 and elbow
arrester 200 are shown. Transformer panel 300 is located adjacent transformer 12 and
includes feed bushing 302, arrester bushing 14 and parking stand bracket 304. Transformer
feed bushing 302 and arrester bushing are interconnected, or bussed, by buss bar 306, which
in turn is interconnected to the high voltage feed 308 of transformer 12. Arrester 200 is
mounted on arrester bushing 14. A load elbow 301, interconnected to high voltage feed line
7 is mounted on feed bushing 302. When surges occur, they travel through buss bar 306 to
arrester 200, to ground. Parking stand bracket 304 is a "dead," i.e., non-conducting
connection. Parking stand arrester 10 is mounted on bracket 304, and receives a load elbow
310 into bushing 17 thereof. Load elbow 310 is interconnected to a live secondary feed line
7a, which is parked on panel 300. If a surge occurs on line 7a, arrester 10 will divert the
surge to ground and then reseal.
If a failure occurs in feed line 7, such as when a cable is accidentally cut by a
construction crew, the parked line 7a may be used to energize the transformer and supply
power to the end user. To do so, arrester 200 is removed from arrester bushing 302, and
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elbow 310 on secondary feed line 7a is loaded thereon. Arrester 200 is then located at the
last transformer prior to the cable break, or "open point," of the system.
The internal arrangement of arrester 10 is best shown in Figure 3. Referring to Figure
3, neck portion 19 interconnects to head portion 17 at mounting plate 15 and houses
arresting cornponents 46 in columnar fashion therein. Neck portion 19 terminates at body
lower end 20 where ground circuit interconnection portion 33 is disposed. Ground circuit
interconnection portion 33 includes ground cap 31 to which a threaded ground stud 22 is
interconnected. An eye terminal 24 which is crimped to the end of a ground lead 14 is placed
over s~ud 22 and is connected thereto by threading a nut 26 over stud 22 and terminal 24.
Ground lead 14 is grounded, such that transient overvoltages on line 7a will be diverted
through interconnection components 34 and arresting components 46 to ground through
ground lead 14. Ground cap 31 is a cylindrical cap member which fits over end 20 and is
crimped onto body 20 at crimp 21. To support body 28 at end 20 an inner crimp support 23
made from a plastic material such as PVC, is disposed on the inner periphery of end 20.
When cap 31 is crimped onto body 28, crimp support 21 prevents body 28 from collapsing
and permits crimp to pinch body 28.
To support, envelope and protect the surge arresting components 46 of the arrester,
arrester 10 includes a molded polymeric body 28 having the aforementioned L-shaped profile.
80dy 28 is comprised of an insulative material, preferably a moldable material such as EPDM.
Body 28 should preferably be conformable, and thus capable of being stretched to receive and
thereafter seal interconnection components 34 and arresting components 46 from the ambient
atmosphere. Body ~8 must also be substan~ial enough to withstand internal pressure which
may accumulate therein during an arrester failure.
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Body 28 includes an upper bore 38 and a lower bore 32 formed therein. Bores 30, 32
are disposed substantially perpendicular to each other and intersect as shown generally at 31.
As shown, upper bore 30 is disposed substantially concentric about a portion of nose piece
18 and lower bore 32 is disposed substantially concentric with the axis of stud 22.
Interconnection components 34 are disposed within upper bore 30, and include right angle
brass connector 38 disposed at the rearward portion of bore 30 at the intersection 31 with
bore 32. Connector 38 includes a nose piece bore 40 for receiving nose piece 18, and a
conductive terminal base 36 into which the terminal rod on the transformer bushing 13
interconnects when it is positioned through nose piece 18. Connector 38 further includes
a lower receiving portion 44 adjacent to the upward terminus of lower bore 32. A cage insert
45, haviny a cup-shaped configuration 47 with a central bore 43 therethrough is disposed
about lower receiving portion 44 of connector 38. Insert 45 is formed of a semi-conductive
material such as semi-conducting EPDM. Insert 45 and connector 38 are configured to
minimize air ioni?ation by maintaining equal electrical potential therethrough.
Surge arresting components 46 are disposed in lower bore 32, and include a series of
metal oxide varister disks 49 electrically and physically in series, and a series-connected stack
of gap assemblies 80 retained in gap assembly retainer ~8. The uppermost metal oxide
varister (MOV) disk 49 is disposed in electrical and physical engagement with a braided lead
51 which passes through Faraday cage insert 45 and connected to brass connector 38. Lead
51 is coiled through the Faraday cage insert 45, and is adhesively attached to the uppermost
(MOV) disk ~9 and brass connector 38 with a conductive silver epoxy 51. Retainer 48 is
disposed in the lower portion of bore 32, and includes a Bellville washer 52 at the upper end
50 thereof, being in electrical and physical engagement with the lowermost metal oxide disk
49. At the lower end of body 28, bore 32 is closed with a conductive ground cap 31 made
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of copper or brass, for example, and disposed in electrical engagement with stud 22 as
previouslY discussed. A conductive spring 54 is disposed against ground cap 31, and bears
against the base 56 of retainer 48. To ensure electrical interconnection between the base 56
of retainer 48 with ground lead 14 and to prevent spring 54 from acting as an inductor, a
conductive strap ~7a is provided which electrically and mechanically interconnects stud 22
and gap assembly retainer 48 by means of a conductive silver epoxy which attaches each end
of strap 47a to stud 22 and retainer 48, respectively.
~ ody 28 further includes at least one annulus 58 disposed in the outer circumference
of the lower bore 32. Annulus 58 creates an air void 55 adjacene the surge arres~ing
components 56, and creates an extra pliable area in body 28 which aids in the placement of
the body 28 over the surge arrester components 46. It should be appreciated that the smaller
thickness of the walls of body 28 adjacent annulus 58 will make body 28 easier to bend as
the body is fitted over components 46. Additionally, the smaller thickness of the walls of
body 28 at the annulus 58 may be used to create a predetermined location for pressure relief
and the venting of ionized ga-s in instances of failure of the dead front arrester 10. The use
of an air void 55 in body 28 results in a body having greater pliability, better electrical
characteristics but requiring less EPDM or other such insulative material. The thin wall of the
body 28 adjacent annulus 58 allows body 28 to bend and deform as MOV disks 49 are placed
therein, and presents no physical resistance to the insertion of the disks in void 55 area. :
Additionally, the body 28 may be manufactured using less EPDM, as measured by the volume
of the void 55. Further, the void 55 will lower the electrical stress in the adjacent portions
of body 28, because the electrical stress will concentrate in the air within the void which has
a lower dielectric constant than ~hat of the EPDM. As a result, the degradation caused by
electrical stress in the ~PDM is reduced, which increases the useful life of the arrester 10.
- 18 -
Referring briefly to Figure 14, annulus 58 may alternatively be filled or partially filled
with performance enhancing material such as hygroscopic materials like calcium or sodium
hydroxide or a silicon gel to draw and retain moisture form within arrester 10 and prevent
such moisture form being absorbed by arresting components 46 which might alter the
performance characteristics of arrester 10. Alternatively, the materials disposed in annulus
58 may comprise thermally conductive materials, such as silicone, having a higher coefficient
of heat transfer than the body 28 to increase and improve the heat transfer from the arresting
components 46 to enable the components to cool and thermally recover faster after-surge
duty.
Referring again to Figure 3, to physically protect body 28, further improve the dielectric
strength of the arrester, and provide a measure of safety for utility personnel, a semi-
conducting shield 60 is disposed over the body 28 in areas where the body 28 will be
exposed after installation thereof on transformer 12. Preferably, shield 60 is made from a
semi-conducting EPDM approximately .1 inches thick. Shield 60 includes a tongue portion
62, having a ground strap connection hole 64 therein. A ground strap 66 is connected to
tongue portion 62 through hole 64 to ground the shield 60 and thereby ensure that the shroud
60 is at ground potential (a zero voltage condition) after the arrester 10 is connected to the
transf ormer 1 2 .
Under normal operating conditions, where the voltage potential in the underground
distributiort network 7 is witi-in pre-determined design operating parameters, parking stand
arrester 10 operates as an open circuit between parked cable 7a and ground lead 14 because
the resistance of the surge arresting components 46, when exposed to a normal systern
operating voltage, is several orders of magnitude greater than the overall resistance of the
underground distribution network. However, during surge conditions, the resistance of the
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; ' ' ' ~,
"
, 3 f ~ f
surge arresting components 46 quickly changes to a value which is several orders of
magnitude less than that of the underground distribution network cable 7a. As a result, the
overvoltage-induCed surse is discharged to ground through interconnection components 34,
arresting components 46 and through strap 47, stud 22 and ground lead 14. Once the
transient condition has passed, the arresting components 46 again change their resistance to
that the arrester 10 again functions as an open circuit.
Referring now to Figure 17 and 18, the structure of the gap assembly retainer 48 is
shown. Alternative embodiments are shown individually in Figures 4, 10, 12 an 13. For
purposes of illustration only, and not to limit the invention, the embodiment of Figure 4 is
shown in Figure 1 and Figure 17 includes the embodiment incorporated in Figure 1~. Retainer
48 is an integral unit in which a discrete number of gap assemblies 80 are disposed. Gap
assembiies 80, shown in diagrammatic form in Figure 17, are best shown in Figures 5, 6 and
18 and are described in more detail with reference to those figures. Retainer 48 includes a
moisture-proof housing 82 having a cylindrical outer surface 83. Housing 82 is preferably a
heat shrink wrap, or tube 40, having a mastic coating 81 on the inner surface thereof, which
is disposed circumferentially about gap assemblies 80 and heated to shrink the tubing and seal
the housing 82. Housing 32 may alternatively be formed from fiberglass or an insulative
plastic or other material capable of withstanding the energy and heat which will be generated
therein and which will resist carbonizing, such as glass-filled nylon or other thermally stable
plastics, or may be made by dipping a stack of gap assemblies in an epoxy to coat the outer
periphery thereof. Epoxy dip and heat-shrink tubing with mas~ic coatings will resist moisture
ingress and eliminate small air voids on the circumference of the gap structures, which can
cause partial discharges.
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A ,~
In the preferred embodiment, retainer 48 includes an upper end plate 84 and a lower
end plate 85, manufactured from a conductive material such as brass, and a shrink-wrap tube
outer body. End plates 84, 85 are disposed at opposite ends of a group of gap assemblies
80. Upp0r end plate 84 further includes a nesting shoulder 84A, which is a concentric
projection thereon. Bellville washer 52 (best shown in Figure 18), having a series of radial
slots 53 therein, is mounted over and connected to end plate 84 on nesting shoulder 84A.
End pieces 84 and 86 provide a sharp circumferential corner, or edge, over which the shrink
wrap body 49 or epoxy outer surface can grip. Further, as they are solid washers, when
combined with the wrap body 49 or coating, they provide a sealed container 82 or housing
for gap assemblies 80.
Referring to Figure 4, an alternative embodiment of housing 82 is shown. Housing 82
includes an upper aperture 84 formed at the terminus of an inward projec'ting lip 91 at the top
of housing 82 and a lower aperture 86. Upper aperture 84 is closed with a metallic plate 94,
preferably a copper or brass disk, which is sealed against lip 91 with an o-ring 85 disposed
around the periphery of disk 94 against the inner surface of lip 91. Threads 88 are disposed
on the outer surface 83 of housing 82 adjacent frustroconical face 92. A molded
frus~roconical face 92 is formed on the outer surface 83 of housing 82 adjacent aperture 86.
A cover g~, preferably molded from a thermoplastic material such as glass-filled nylon, is
disposed over lower aperture 86 and includes mating threads g8 to engage threads 88.
Referring still to Figure 4, to assemble gap assembly retainer 48 and secure gap
assemblies 80 in moisture-proof housing 82, o-ring 85 and plate 94 are first positioned within
housing B2 through lower aperture 86 with o-ring 85 disposed against the inner face of lip 91
of aperture 84. A series of gap assemblies 80 are then placed into housing 82 through lower
aperture 86, such that the first of the gap assemblies 80 physically bears aging plate g4. A
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,
coil spring 100 formed of conducting material such as copper or brass is then loaded into
lower aperture 86 with the innermost end thereof bearing aging the base of the last inserted
gap assembly 80. An o-ring 102 is then disposed about frustroconical surface 92, and a
second metallic plate 104, preferably a copper disk, is placed over the protruding portion of
spring 100. Molded cover 96 is then threaded over threads 88, and tightened into place.
Molded cover 96 includes an overhanging circumferential lip 106, which bears upon second
metallic plate 104. Therefore, as molded cover 96 is tightened into place over threads 88,
lip 106 pushes second plate 104 which presses against both spring 100 and o-ring 102.
Spring 100 impacts a compressive force against gap assemblies 80 which, in turn, tend to
squeeze o-ring 85 between lip 91 and first metallic place 94. In this manner, the threading
of cover 96 onto housing ~2 seals gap assemblies 80 inside housing 82 from the ambient
environment. Once assembled, an electrical conductive series path is crated through retainer
~8 comprising upper plate 94, gap assemblies 80, spring 100 and second metallic plate 104.
To ensure integrity of the series path, a conductive strap 101 may be positioned between the
top and lowermost coil of spring 100 so as to be in electrical contact with lowermost gap
assembly 80 and with lower plate 104.
Referring now to Figures 4, 5, 6 and 7, the structure of gap assemblies 80 is shown,
two such assemblies being depicted in Figures 4 and 5. Each gap assembly 80 includes a
voltage dependent, low exponent non-linear silicon carbide ring 1 10, a dimpled electrode 112
and a cupped electrode 114 or 116. One dimpled electrode 112 and one cupped elec~rode
1 14, 1 16 are disposed on opposite sides of ring 1 10.
Each silicon carbide ring 110 is preferably a right circular annular member, having
opposed annular faces 1 1~, inner circumferential tace 120 and outer circumferential face 122,
although other profiles, such as ellipses, may be used. Each annular face 118 preferably
,
.: ~ i .;
~ ~ .
.
includes conductive annular electrode 124 formed thereon and disposed adjacent the outer
circumferential face 122. As best shown in Figure 5, electrode 124 forrns an annular ring that
does not cover the entire surface of face 118. Annular electrode 124 is preferably a suitably
conductive material, such as aluminum or copper, which is deposited on face 118 by arc or
film spraying. An annular step, or inset 126, is disposed in the inner annular portion of one
of faces 118 between electrode 124 and inner circumferential face 120.
Each of electrodes 112,114 and 116 are metallic, electrically conductive members,
and are preferably manufactured as copper for brass stampings. Dimpled electrode 112 is a
generally square member, with flat sides 128 and rounded corners 130. The dimension
across a pair of diametrically opposed rounded corners 130 is slightly less than that across
the outer diameter of face 118. Three round dimples 132 are stamped into each side of the
electrode 112, forming a recess 136 at each dimple on the stamped side'of the electrode 112
and a crown 134 at each dimple on the opposite face thereof. The dimples 132 are
impressed in a triangular pattern, such that lines ~ormed to connect the centers of each
circular dimple 132 would form an equilateral triangle. Further, the diameters of dimples 132
is such that a circle drawn to proscribe all the dimples on a face should be slightly smaller in
diameter than the greatest diameter 125 of the step 126 on gap member 110. Each dimple
132 is also substantially equidistant from a through hole 138 disposed through electrode 112
substantially at the center thereof. Upon assembly of gap assembly 80, the face 118
containing step 126 of ring 110 is placed against dimpled electrode 112, and dimples 132 are
partially received into s~ep 126 such that the outeYmost extension of the dimple 132 does not
touch the base 127 of step 126, as further described herein.
Electrodes 114 and 116 are generally round stampings, in the form of a hat. Each
includes an annular rim portion 140 and a protruding central bowl portion 142 protruding
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' - ~ ,1 ~ i f . r
outward from the rim 140 and terminating in a generally flat top portion 144. Tvp portion
144 includes a through hole 146 therein, disposed at substantially the center of the electrode
114 or 116. Each rim 140 further includes an embossed set off 148, which is an annular
raised portion thereon protruding therefrorn opposite the direction of bowl portion 142. Each
bowl portion 142 is sized to be received within inner circumferential face 120 of ring 110,
such that set off 148 touches face 118 of ring 110 which does not include the step 126.
An assembly of two gap assemblies 80 is best shown in Figure 6. The space between
each top portion 144 and each dimpled electrode 112 forms a spark gap 150. Further, the
space between each dimple 132 and the adjacent step 126 forms a preionizing gap 152. The
size of the ionization gap is preferably approximately .007 inches. Preionization gap 152
generates ions which are dispersed throughout spark gap 150 to provide more consistent
sparkover when the voltage stress is increased across gap 150. Preioni~ing gap 152 may be
varied from .0035 to .028 inches, and gap 150 may be varied from .010 to 1.0 inches,
depending upon the desired gap breakdown voltage. However, spark gap 150 should not be
sized larger than the thickness of ring 1 iO. Preferably, spark gap 150 is .10 inches. It is
contemplated that ring 110 may be sized from 125 to 1.0 inches in height. Preferably, ring
110 is 11/32 of an inch in height.
Referring now to Figure 7, metal oxide disks 49 are shown. Each MOV disk 49 is a
generally right circular member, having a dielectric collar or sleeve 160 disposed
circumferentially about its outer surface. Collar 160 is preferably made of epoxy. MOV disks
49 further inciude a suitably conductive face 164 of disk 49 by arc or film spraying.
MOV disks 49 must be capable of withstanding high energy surge currents. The metai
oxide for MOV disks 49 may be of the same material used for any high energy, high voltage
MOV disi~, and is preferably made of a formulation of zinc oxide. See, for example, U.S.
- 24 -
,
f'~
Patent 3,778,743 of the Matsushita Electric Industrial Co. Ltd., Osaka, Japan, incorporated
herein by reference- In the preferred embodiment, MOV disk 49 will have a uniform
microstructure throughout the MOV disk and the exponent n for the zinc oxide formulation
of MOV 49 will be approximately equal to 25.
MOV 49 must be capable of discharging the high energy surge currents caused by
lightning strikes and then thermally recovering so as to be capable of enduring repetitive high
surge currents. It is desirable for MOV disks 49 to be able to thermally recover from a high
energy surge current while it remains energized at the power system's maximum continuous
operating voltage (MCOV). MQV disks 49 of the present invention is capable of conducting
lightning surge currents of up to 40,000 amps. MOV disks 49 will recover from a 40,000
amp surge current of a short duration such as 4/10 waive ~four microseconds to crest and
decaying to half crest in 10 microseconds). The cross-sectional area of MOV disk 49 will
dictate its durability and recoverability from high surge currents. It is preferred that the
circular cross section of MOV disks 49 have a diameter between 1 to 2 inches to insure that
there is sufficient surface area of between .8 and 3.2 square inches to maintain the desired
durability and recoverability. At the same time, it is also desirable that MOV disks 49 have
as small a cross-sectional area as possible in order to reduce the size, weight and cost of the
arrester. As size is reduced, however, the durability and recoverability of the disk is
decreased. Given these considerations, a diameter of 1.3 inches is the most preferred. The
thickness or height of MOV disk 49 as measured between faces 164 is preferably 1.3 inches.
As understood by those skilled in the art, given a particular metal oxide formulation and a
uniform or consistent microstructure throughout the MOV disk, the thickness of the MOV disk
determines the operating voltage level.
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~ , -
.
Referring again to Figure 3, three metal oxide disks 49 are disposed in lower bore 32
in a series with gap assembly retainer 48. The conductive coating 162 on one face 164 of
the MOV disk 49 adjacent gap assembly retainer 48 electrically engages gap assemblies 80
by a means of a conductive leaf spring 54 disposed on upper plate 84. Alternatively, other
interconnection techniques, such as conductive epoxies, may be used to electrically
interconnect the elements lowermost MOV disk 49 to gap assembly retainer 48. Likewise,
the connection between second plate 86 at the base retainer 48 and stud 22 and lower cap
31 may be accomplished with a conductive spring 54, a conductive strap 47, a conductive
epoxy or other methods of achieving a conductive circuit between the elements. It is
currently preferred that a spring 54 and a conductive strap 47 be employed. Each MOV disk
49 is connected to the next adjacent MOV disks by a silver conductive epoxy disposed on end
faces 164 thereof. As shown in Figure 3, three metal oxide disks 49 are matched with 12
gap members in the gap assembly 48. The number of metal oxide disks 49 and gap
assemblies 80 in retainer 48 must be selected to ensure proper surge arresting characteristics
and balanced impedance between the total impedance of MOV disks 49 and the impedance
of gap assemblies 80, depending upon the properties of the specific disk, 49 and gap
assemblies 80. It is preferred that these elements be chosen such that approximately 50/O
of the nominal steady-state operating voltage be supported by MOV disks 49 and 50% be
supported by gap assemblies 80.
Referring now to Figures 8A and 8B and 9, the electrical characteristics of the arresters
10 and 200 are shown. During steady state conditions, a first portion of the steady-state
operating voltage is impressed across disks 49 and a second portion is impressed across gap
assembly retainer 48. Because the total steady-state operating voltage is shared ~y these
elements, the total number of spark gaps 150 required to sustain normal operating voltage
- : :
~ ' :~ . , , . ~
'
without flash over is less than would be required with older prior art arresters where the total
system voltage was supported entirely by the spark gaps. This balanced condition persists
only so long as the total impedance of assemblies 80 is proportionately balanced with the
total impedance of the metal oxide disks 49. Because gap assemblies 80 have a lower
expollential impedance factor than the metal oxide disks 49, as the voltage across the arrester
increases, the voltage across the gap assemblies 80 rises faster than the voltage across the
MOV disks 49. This causes the percentages of voltage carried across the respective
components to shift so that the assemblies 80 carry a greater percentage of the voltage seen
by the arrester than they would at the nominal operating voltage. By switching a percentage
of the overvoltage from MOV's 49 to assemblies 80, the turn on of the metal oxide disks 49
is delayed, giving the arrester 10 a higher temporary overvoltage capacity.
Referring now to Figures 17 and 33, an alternative embodiment of the invention is
shown, wherein elbow connector 200 is configured to engage over a bushing (not shown) on
transformer 12. The outer profile of the bushing is configured substantially similar to upper
portion 19 of arrester 10, having a conical outer profile and a central receiving bore
therethrough. Elbow connector 200 includes an upper shroud portion 208 and lower arresting
component portion 210 interconnected therewith and projecting therefrom at a substantially
right angle thereto.
Shroud portion 208 and lower arresting component portion 210 are preferably formed
of a single, continuous molded piece of EPDM or other insulative material, and includes an
upper exiting bore 212 and lower exiting bore 214 intersecting at the elbow 215, or righ~
angle intersection, of shroud portion 208 and arresting component portion 210. Shroud
portion 208 includes a generally cylindrical outer profile 218 and lower exiting bore 212
comprises an inner, frusttoconical inner profile 220 which terminates inward shroud portion
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,
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'7 i
208 in a terminal retainer cavity 222. Arresting component portion ~10 likewise has a
generally cylindrical outer profile 224 leading to a step down portion 226 at the lower
terminus 228 thereof. Step down portion 226 is a reduced diameter portion of arresting
portion. Lower exiting bore 214 is a generally cylindrical bore having a first major diameter
portion 230 which intersects terminal retainer cavity 222, a lower enlarged diameter crimp
support bore 234, and an intermediate diarneter spacer bore 232 disposed therebetween.
Prior to assembly of arresting components 46 into arrester 200, spacer bore 232 and first
major diameter bore 230 are diametrically equal, but the insertion of arresting cornponents 46
stretches the inner diameter of first major diameter portion of 230. This results in a snug fit
of arresting components 46 in first major diameter portion 230. First major diameter portion
230 may include the annulus, either void or filled with moisture-reducing or heat-transfer
enhancing materials, as described with respect to the parking stand arrester 10.
To interconnect arrester 200 to transformer 12, shroud 208 includes a pin connector
236 retained in brass terminal 238 and projecting outward shroud 208 ~hrough the open end
of upper exiting bore 212. Pin connector 236 is a longitudinal cylindrical member, disposed
substantially at the centerline of upper exiting bore 212, and is threaded, at its inner terminus
248. Brass terminal 238 is secured within terminal cavity 222, and includes through bore
240 into which pin connector 226 is received, and a connection bore 242, at a right angle
thereto, into which transfer connector 244 is received. Connector 244 includes a threaded
bore 246 therethrough, which aligns with bore 240 in brass connector 238. to support and
retain pin connector 236 in shroud 208, threaded end 248 thereof is inserted through bore
240 and threaded into threaded bore 246 in transfer connector 244.
To interconnect arrester 200 to ground, lower arresting component portion 210
terminates in cover 250, which closes off lower exiting bore 212 at the lower terminus of
. ` ` ! ~ 'i . ' :
, ' : ~' ' ' ' ` - : , -
' '' ' ~ ' '' ~ '
arresting component portion 210. To secure cover 250 on lower arrester component portion
210, a cyiindrical crimp support 252, preferably manufactured from an insulative material such
as PVC, is received in lower enlarged diameter crimp support bore 234. Crimp support 252
forms an inner diarnetrical suppor~ which bears against the web 2~4 of material of lower
arresting portion 210 disposed about crimp support bore 234. Cover 250 is crimped to form
crimp 258, which pinches web 254 against crimp support 252 to support cover 250 on lower
arresting portion 210.
Arresting components 46, including MOVs 49 and gap assemblies 80 in retainer 82
are received in first major diameter bore 230 to secure arresting components 46 in arrestor
200 and ensure a connection therethrough to ground, cover 250 includes stud 256 projecting
therefrom, and spring 260 projecting inward arrester 200 therefrom. Spring 260 bears on a
series of copper or brass spacers 262, which in turn bear upon lower contact 86 of gap
retainer 82. As the number of gap assemblies 80 in retainer 84 may be varied to tailor the
arrestor characteristics to a particular transformer or network, spacers 262 are used to bridge
the gap between spring 260 and lower contact 86 which varies based on the number of
arresting components 46 used in the particular application. To ensure that spring 260 does
not act as an inductor during surge arresting cycles, a strap 263 is disposed across the gap
from the lower most spac0r 262 to cover 250. to interconnect the arresting components 46
to the transformer, the uppermost MOV 49 engages a brass spacer 282, and a wire rope 284
is welded thereto and to transfer connector 244. Wire rope is compressible, which ensures
sufficient electrical contact between the spacer 282 and transfer connector 284.
Arrester 200, when attached over a transformer bushing, is iocated in a high voltage
area. Therefore, to protect service personnel, the outer surface of the arrester is enclosed in
a thin polymeric cover 264 which is located over the o~.ter portion of upper shroud portion
- 29 -
: : . , .. , -.- ,
.: : , : .
.~ ; , . . :, :
, ~:
:; : ~ ~ : ': ' :
.
'.
208 and lower arresting componant portion 210. Cover 264 is preferably a semi-conducting
EYDM material, and includes a ground tab 266 to which a ground strap 268 is attached.
Ground strap 268 is connected to ground, so that cover is also at ground, or zero voltage,
potential. Cover also includes a pulling eye 270, which may receive a hot stick to pull the
arrestor 200 from the bushing 202 or insert it thereover.
To direct surges to ground, a ground lead 272, having an eye terminal 274 attached
to the end thereof, is connected to stud 258 by placing the eye, or hole, 276 of eye terminal
274 thereover and tightening eye terminal 274 against the outer surface of cover 250 by
tightening a nut 280 over stud 250.
When arrester 200 is disposed over bushing 202, pin connector 236 is exposed to line
potential. When a surge occurs, surge arresting components 46 react as herebefore
described, directing the surge to ground through ground lead 272 and then resealing to
reinitiate normal steady-state operation.
In response to a transient overvoltage, the arrester functions to first shunt the surge
to ground and subsequently to reseal the system. During a surge, the voltage shift described
above occurs. When the voltage across gaps 150 of gap assemblies 80 reach the gap spark
over voltage, gaps 150 spark over, shunting the load off of ring members 110 and causing
all the voltage to now appear across MOV disks 49. The metal oxide disks 49 absorb a
portion of the impulse energy as the s~rge is dissipated, and as a result they heat up. As the
metal oxide disks 49 heat, their structure causes them to tend to draw more leakage current
once the system has returned to steady-state conditions. Because rings 110 did not carry the
surge, they remain relatively cool and take up a higher percentage of the voltage load after
the surge passes, relieving that on the metal oxide disks 49. This voltage shift allows the
metal oxide disks 49 to cooi more quickly. Additionally, the relatively cool rings 110 also
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..
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.. '. ' ~ ~ '` '
.'S
serve as a heat sink to draw heat out of the disks ~9. as a result, fewer metal oxide disks
49 are required as compared to prior are gapless MOV arresters.
Referring now to Figure 10, an alternative embodiment of the arrester is shown
wherein an odd number of rings 110, in this instance five rings 110, are disposed in gap
assembly retainer 48. In this embodiment lower cupped electrode 1 14 and dimpled electrode
112 are disposed on the extra ring 1 10 above the numbet required to make even sets of two
as shown in Figure 5. In this configuration, spark gap 150 between electrodes 1 12 and 1 14
and preionizing gap 152 between electrode 112 and ring 110, are present. Alternatively,
although not shown, the extra ring 1 10 may be placed at the bottom on retainer 48, so that
dimpled electrode 112 bears against lower metallic plate 104 and cupped electrode bears
against tha lower most coil of spring 100. Any number rings 1 10 between 1 and 18 are
contemplated for use within gap assembly retainer 48. It is believed that'this number of rings
will be sufficient, when properly sized and matched to metal oxide disks ~9, to address most
surge arresting applications. More rings 1 10 could easily be employed by simply increasing
the length of the gap assembly retainer 48a.
Referring now to Figure 11, an alternative embodiment of the dead front arrester 10
i5 shown. In these embodiment, arrester body 28 includes annulus 58 having a rigid sleeve
200 disposed therein. It is preferred that sleeve 200 be made of fiberglass. Annulus 58 is
bounded by an outer right cylindrical recess wall 202, a pair of frustroconical tapered faces
204, 206 which project inward in the direction of the center of neck portion 19 body 28 and
are offset from wall 202 by setback 201, and by the outer surface of adjacent MOV disks 49
to form an air void 203. Fiberglass sleeve 200 is sized to be received against and cover
cylindrical wall 202 between set backs 201. The wall thickness of sleeve 200, and depth of
setbacks 201, is preferably .075 inches. The span between setbacks, and corresponding
~ , :
.
..
,~", ~ :.3 ~ ~ f
sleeve height, vaties with different rated arresters, but may cover 30% to 75/O of the
interfacial length of the arresting elements 1 i. This combination of sizes will expose the
complete outer surface of arresting elements 46 over 30% to 75% of their length. With MOV
disks 49 having an outer diameter of 1.30 inches, sleeve 200 preferably has an inner diameter
of 1.76 inches, leaving an air void 203 that is .230 inches wide and projecting cylindrically
continuously around MOV disks 49.
Fiberglass sleeve 200 supplies rigidity and support to body 28 to help prevent body 28
from bunching up at the inner wall membrane created by annulus 58, and also provides a non-
conductive barrier having higher strength than the adjacent EPDM structure so as to aid in
preventing rupture of body 28 at annulus 58. Tube 200 is inserted into body 28 prior to the
insertion of disks 49 therein, thus further stabilizing body 28 to improve the assembly thereof
into a complete dead front arrester 10.
Where sleeve 200 is employed, the energy of hot plasma induced by arcing can be
absorbed, in part, by sleeve 200, thus preventing a discharge of charged plasma through the
sidewall of body 28 adjacent annulus 58. In this manner, use of the sleeve 200 may be used
to direct the discharge downward toward the lower end of body 28 where the pressure can
be vented by means of conventional pressure relief mechanisms (not shown) typically located
at the end 20 of the arrester. Referring briefly to Figure 15, a sleeve segment 220 formed
of an arcuate cylindrical segment of the arrester adjacent the protected equipment, such as
transformer 12, or other conductive components, thereby directing the very high pressure
plasma to vent from body 28 on the opposite side thereof in a direction unlikely to cause an
arc to adjacent equipment.
Referring now to Figures 12 and 13, further alternative embodiments of the invention
are shown. In these embodiments, the gap assembly retainer 48 is eliminated, and the gap
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assemblies 80 and MOV disks 49 are maintained in series relationship and longitudinal
alignment by alternative reta;ning means which, like retainer 48, also prevents the entrance
of moisture. Referring first to Figure 12, the gap assemblies 80 and metal oxide disks 49 are
encased in a heat-reactive, shrink-fit tube 168 having a mastic coating 170 over the interior
surface 172 thereof. Tube 168 is made of a therrnally-actuatsd material such as polyolefin.
To ensure contact between the metal oxide disks 49 and the upper electrode 116 of gap
assemblies, a conductive rnetallic contact, in the form of a metal disk 174, is placed at the
interface thereof. Further, to ensure better contact between adjacent upper and lower cup
electrodes 1 14, 1 16, and eliminate high frequency contact noise, the annular lip portions 140
of each adjoining upper and lower electrode 114 and 116 are welded together, as shown
generally at 173, prior to assembly thereof into shrink fit tube 168. Alternatively, adjacent
electrodes 1 14, 1 16 may be brazed, soldered, metallized, epoxied or otherwise interconnected
where the interconnection mechanism creates both a mechanical and electrically conductive
bond. To secure gap assemblies 80 therein, mastic 170, such as a polyanide adhesive, is
applied within tube 168 and heat-shrink tube 168 is then heated causing it to shrink over
retainer gap assemblies 80. The rim 140 of lower most electrode 1 14 is covered by tube
168, and the electrode 1 14 is modified such that aperture 146 therethrough is blocked by
welding a disk 147 thereover. Alternatively, lowermost electrode 1 14 may be manufactured
without aperture 46 therein, or a plug may be inserted therein. Closing off aperture 146
ensures that moisture will not enter the gap assemblies 80 therethrough. Referring now to
Figure 13, another alternative embodiment of dead front arrester 10 is shown in which gap
assemblies 80 and disks 49 are retained in position in an epoxy film 176 applied to the outer
circumference of assemblies 80 and disks 49. Again, a conductive disk 174 is located
between the uppermost electrode 114 and lower most and MOV disk 49 to ensure good
- 33 -
electrical contact among the arresting components 46, and lowermost electrode 114 is
mordified to close or seal aperture 146.
To apply epoxy film 176, the MOV disks 49 and gap assemblies 80 are stacked
together in columnar fashion and the ends of the column are then covered with a removal
cover, such as adhesive tap or paper. Thereafter, the column is disposed into liquid epoxy so
as to completely coat the outer surface of the stacked components. The epoxy-coated
column is then cured and dried and the covering on the ends of the column are removed.
Alternatively, rather than applying the coating by means of a liquid bath, a power epoxy may
be employed. In this method, the ends of the column are again covered. Thereafter a static
charge is placed on the column of elements and the charged column is placed in a bath of
powdered epoxy, the column is then removed and cured, causing the outer surfaces of the
components to be coated and sealed by a thin glazing of epoxy that is of sufficient strength
to maintain the elements in columnar fashion to hermetically seal the column.
Of the three means described herein for hermetically sealing and retaining gap
assemblies 80 and MOV disks 49 in series relationship, the heat shrink tube 168 described
with reference to Figure 12 is presently considered the best way to practice the invention.
While the preferred embodiments of this invention have been shown and described,
modifications thereof can be made by one skilled in the art without departing form the spirit
or teachings of the invention. The embodiments described herein are exemplary only and are
not limiting.
Many variations and modifications of the system and apparatus are possible and are
within the scope of the invention. Accordingly, the scope of protection is not limited by the
above description, but is only limited by the claims which follow, that scope including all
equivalents of the subject matter of the claims.
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,
