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Oct 8, 1946.
J. E. M. MITCHELL‘
' 2,408,990
BUSHING INSULATOR
Filed July 16, 1941
2 Sheets-Shee't 1
INVENTOR.
(1.15M M/fmg //;l
Get. 8, 1946.
J. E.
2,408,990
MITCHELL
BUSHING INSULATOR
Filed July 16, 1941
2 Sheets-Sheet 2
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Patented Oct. 8, 1946
2,408,990
UNITED STATES PATENT OFFICE
2,408,990
BUSHING INSULATOR
John E. M. Mitchell, Parkersburg, W. Va., as
signor to Porcelain Products, Inc., Findlay,
Ohio, a, corporation of Delaware
Application July 16, 1941, Serial No. 402,671
3 Claims.
(Cl. 174-31)
1
2
The invention described in detail below relates
primarily to insulators of the bushing type and
more particularly to bushings made of porcelain
tablishing intimate connection with the porcelain
body and at the same time producing improved
voltage distribution without local differences of
potential existing between the glaze and the adja
cent porcelain. This glaze is moreover high in
silica reaching its optimum at approximately
or other suitable material and employing oil or
other insulating media. Such bushings are de
signed for insulating the conductor which car
ries electric current through a grounded zone
such as the cover of a transformer tank. Under
such circumstances there is a concentrated Ilux
or potential difference which may give rise to
sparkover or even corona formation in quantities
sufficient to cause disturbances for example, in“
terference with the reception of radio frequency
currents. Numerous means have been suggested
70 %’ so that the resultant compound has a coeffi
cient of expansion conforming closely to that of
the porcelain body and thus there is no tendency
to “crazing” or “shivering.” Glazes of this type
are said to “?t” the porcelain. It is to be under
stood that when the glaze is thus attached to
the porcelain body there results an actual increase
in the mechanical strength of the porcelain mem
for the purpose of improving the voltage gradient 15 ber. In fact the glaze becomes an integral part
as Well as the distribution of the ?ux density.
of the porcelain. .
p
Prominent among these is the idea of applying a
In respect to the conducting character of the
conductor of relatively low resistance for an ex~
silicate glaze it is to be noted that the resistance
betweenpoints one-half inch apart range from
one-tenth up to ?fty megohms which is an ex
ceptionally low resistance compared with com
tended distance along the high tension conductor.
When this takes the form of a metallic sleeve
around the conductor and within the bushing, it
parable electricalporcelain glazes.
adds materially to the complication of the device
The application of this conducting glaze may
and its cost. Furthermore, it in turn gives rise
be made to any portion of the insulator where
to stresses existing between this sleeve and the
nearest insulating ba?ie or other part of the 25 there is likelihood of over-stressing or abnormally
high voltage gradient.‘ In the example illustrated
bushing. Where it has been the practice to‘bond
on the attached drawing I have shown it applied
such a metallic sleeve to the porcelain of the
to several areas any one of which may in itself
bushing or its baffle, this can be accomplished only
be the locality of sparkoverv or discharge.
at the expense of. the strength of the porcelain
30
The protection of any one of these areas is
which in the treatment is materially weakened.
bene?cial by itself. At the same time the pro
My invention has been developed for the pri
mary purpose of obtaining a high electrical effi
tection of all of these areas is an added advantage
ciency as well as attaining higher mechanical em
ciency and greater strength.
and improvement in operating characteristics.
An important area to be protected by the con
quately eliminate the generation of voltage of the
type causing radio interference.
A still further object of my invention is to ob
ducting glaze from overstressing, as has already
been suggested, is the surface of the bushing and
speci?cally its internal baille, where it surrounds
the high tension conductor. I have obtained
tain a more improved voltage gradient than’ is
improved voltage gradient as well as distribution
It is a further object of my invention to ade
present in similar devices.
. '
The method. used in this invention, in addition
to greatly improving the voltage gradient and
of stresses by giving the inner surface of the
baffle surrounding the conductor a uniform and
complete coverage of the conducting silicate glaze.
Another area likely to be affected by abnormal
eliminating to a greater degree over stressed
voltage characteristics, is the outer surface of
areas, actually increases the mechanical strength
45 the insulating bushing extending downwardly
of the dielectric porcelain members.
from the mounting ?ange to below the surface
Speci?cally my invention applies a special glaze,
of the oil surrounding the bushing. By applying
upon insulating parts of the bushing. Thisglaze
a glaze of good conductivity over‘ the outer sur
is selected for the special purpose of materially
face of the bushing and extending this from below
reducing the'electrical resistance and this tak
ing the place of any grounding sleeve or other 50 the surface of the liquid upwardly to the support—
metallic conducting member. This glaze is prinj
cipally formed of silicates containing. conducting“
of a discharge between any part of the bushing
materials and which ?res to an integral part of
and the adjacent areas.
in. g ?ange or other structure I avoid the possibility
This eliminates the necessity of a special outer
the porcelain body without the reduction of the
conducting metallic oxides present, thereby cs 55 grounding sleeve and also grading down the con
2,408,990
4
3
ductivity of the latter at its lower extremity as
in Rorden 2,209,003. It also eliminates the neces
is given a glaze 6.
This glaze is a silicate spe
sity of providing special flux control structure
known to have de?nitely low resistance.
at the lower end of the grounding sleeve as in
glaze is fired on the bushing d and becomes an
integral part of this. It is characteristic of such
?red glazes that the porcelain may thus be given
ci?cally one of the silicate compositions which are
This
Figure 3 of Rorden 2,188,417. In these prior
structures, the lower ends of the grounding sleeves
an increased strength of as much as 40% over
are maintained at ground potential, and the entire
similar unglazed articles. This glaze is of a sili
voltage drop between the high tension conductor
cate type similar to those applied to the other
passing through the insulator bushing and ground
is concentrated within the oil space between the 10 portions of the bushing, with the exception that
it possesses conductivity to the degree already
lower end of the ground sleeve and the lower
terminal of the high tension conductor. The con
mentioned.
Glazes of this type may be prepared by intro
ducing titanium dioxide
one or more other
for a different distribution of the voltage drop
between the lower terminal of the high tension 15 metallic oxides in a silicate glaze which would be
a non-conducting glaze without the addition of
conductor and the supporting ?ange of the insu
these particular oxides. As examples, titanium
lator, and the present invention is distinguished
dioxide with iron oxide or chromium oxide, or
from these prior structures by the fact that instead
combinations of these oxides, will produce the
of concentrating the entire voltage drop below the
surface of the oil, the glazed outer surface of 20 required conductivity of the glaze. The resistivity
of the glaze may be varied between wide limits hy
the insulator bushing provides for a substantial
the variation of the content of the titanium dioxide
voltage drop along the outer surface of the bush~
and the other metallic oxides introduced for the
ing within the air space between the top of the
purpose of increasing conductivity. Dependent
oil and the supporting ?ange. The remainder
upon the proportions of the oxides, the resistance
of the voltage drop is taken up below the surface
of the glaze over a one-half inch surface will
of the oil. This distribution of the potential drop
vary from one-tenth megohm to ?fty megoh'ms.
reduces the amount of concentration of voltage
The bottom of the bushing shell 4 is closed by
at any one point and thereby reduces the tendency
means of a ring ‘I. This is separated from the
for the development of corona discharge.
These bushings contain an insulating oil up to 30 bottom of the tubular bushing shell 4 by means
of the oil-proof gasket 8 made of synthetic resin
within a short distance from the top.
such as Corprene (from neoprene) or the like
An electrostatic charge may develop within
and the ring is cemented or otherwise attached
the bushing and thus subject the space above
to the outer wall of the shell as shown in Fig. 1.
the oil to an overstress. To reduce this condi
Within the bushing the ring ‘I is extended to
tion I propose to provide an oil reservoir at the 35
form a ?ange support upon which a gasket 9 rests.
top of the bushing. This reservoir will be of con
A vertical tubular baffle ill rests upon the gasket
ducting material, for example, a metal casting.
9 and extends to the top of the bushing. As the
It will have the same potential as the lead-in
ducting glaze of the present invention provides
conductor and as high as any potential Within
the bushing itself. Hence there will be no differ
ence in potential to cause a discharge, Where
the size of the bushing does not require a reser
voir and the oil level does not come above the
outer porcelain wall of the bushing then the latter
may have a conducting glaze applied from a
point below that oil level upwardly to the metal
cap of the bushing and electrically bonded to this
metal cap.
In like manner the surface inside of the bush
ing may be subject to overstress in the area oppo
site the fiange. To reduce this I may provide a
conducting glaze on this surface extending above
and below the ?ange and also on the opposite
surface of the inner bai’?e.
To illustrate my invention I have shown it in
shell and baiiie are concentric, there is thus pro
40 duced an annular oil chamber H.
The inner surface of the tubular baffle I!) has
a conducting glaze it, This is coextensive with
the inner surface of the ba?le. It is also fired
upon the baffle in the manner already indicated.
45 The ba?ie provides a passageway for the high U311“
sion conductor l3.
The metallic cap M is provided to rest upon
the shell 4, to center the inner baiiie it, to sup
port the high tension conductor I3 and ?nally
50 to permit ready inspection of the oil with which
the bushing is ?lled.
This cap i4 is supported
upon an oil-proof gasket i5 which in turn rests
upon the upper edge of the shell 4. The exten
sion l6 of cap [4 is cemented
indicated at
55 I‘! around the upper end of the shell 6.
The cap I4 is substantially cylindrical and pro
its preferred form in the accompanying drawings
vides a chamber l3 which is in communication
in which
with the space I i in the shell 4.
Fig. 1 is a vertical half section of the improved
One portion of the side wall of the cap 14 is
bushing partly reduced in length;
Fig. 2 is an enlarged plan view of the reservoir 60 recessed as shown at H! to receive a vertical gauge
glass 20. This gauge glass rests upon the gasket
with the cover removed and
2| in the seat 22. Its upper end projects through
Fig. 3 is an enlarged vertical section of the
the hole 23 at the top of the cap I4. A passage
cap on the line 3-3 of Fig. 2.
way 24 connects the lower end of the gauge glass
The bushing is mounted upon a ?ange I. This
?ange is of suitable form and shape to be attached
to the chamber H3.
to the top of a transformer tank A or the like.
The ?ange has an upstanding collar 2 upon
which a gasket 3 rests. The outer tubular bush
The cap [4 has a plurality of inwardly extend
ing lugs 25 upon which an open spider 20 rests
and is bolted by means of screws 21. This open
spider 26 rests upon an oil-proof gasket 28 which
to the collar 2 by means of a body of cement 5 70 is thus clamped ?rmly against the end of the
baffle l0 and compresses the latter against the
or the like.
Commencing with the enlargement of the bush
gasket 9 at the bottom of the bushing. In this
ing shell 4 which rests upon the gasket 3, and
way the chamber If is rendered substantially
extending to a point below the oil level B of the
leak proof at the bottom and top of the inner
transformer tank, the outer surface of the bushing 75 baffle.
‘
ing shell 4 rests upon the gasket 3 and is attached
5
2,408,990
6
Another oil-proof gasket 29 overlies the spider
‘ 26 and the top of the cap l4. A metallic cover
30 ?ts over the gasket 29 and has a rim 3| ?t
ting within a stepped portion 32 of the side wall
of the cap M. Bolts 33 pass through the cover
39 and seal the top of the cap M. A passage
way 34 is provided to connect the upper part of
the gauge glass 20 with the chamber l8 through
gasket 29. A suitable vent may be provided in
complished by providing a zone of intermediate
conductivity.
~ While I have illustrated by way of example my
preferred form of an oil ?lled bushing insulator
having integral stress-reducing facilities, the
principle involved in the invention may be em
bodied in numerous other speci?c arrangements
of details and materials and the invention is to
be considered as limited only by the scope of the
the cover 3%}.
10 appended claims.
A spring contact 35 is screwed to the cover 30
In the annexed claims the term “high resist
by means of screw 36. Y The free end of the con
ance path” is intended to apply to a path of sum
tact rests against the inner surface of the ba?le
ciently high resistance to produce an appreciable
H) where it is in contact with the conducting
voltage drop along its length, as distinguished
glaze E2. The high tension conductor [3 has a
a low resistance path provided by the
screw-threaded extension 31 extending through 15 from
grounding
sleeve or metalized sleeve used in the
a center opening in the cover 39. A nut 38 on
prior art where there is substantially no voltage
the extension 31 supports the conductor I3 on
drop along the length of the sleeve.
the cover.
What I claim is:
A metallic cap nut 39 is also threaded on the
1. In electrical apparatus having a conducting
20
extension 31 and by means of the oil-proof gasket
housing containing insulating liquid and provided
49 seals the passageway through the cover 30.
with an opening above the level of said liquid
The usual terminal 4! is formed integrally with
through which a high voltage conductor extends
the cap nut 39.
to a point below the level of said liquid, said con
This arrangement of a bushing insulator is a
great simpli?cation over existing types. In the 25 ductor being supportedby an insulator bushing
mounted in said opening and extending below
?rst place by the use of a conducting glaze on the
the level of said liquid, the combination of means
inner surface of the baf?e, it is possible to elimi
for establishing a substantial voltage drop along
nate entirely the conducting metal sleeve which
the outer surface of said bushing between said
has ordinarily been used for ?ux distribution at
this point. Additionally the use of the conduct 30 opening and the level of said liquid, said means
comprising a high~resistance path formed solely
ing glaze gives added mechanical strength to the
porcelain.
The top of the oil adjacent to the bushing is
of a conducting glaze forming the exterior sur
face of said bushing and having electrical contact
said opening and extending
- with said housing at
maintained at the same ground potential as the
mounting ?ange by means of the conducting glaze
6. The glaze coatings 6 and I2 extend parallel
to a point below the level of said liquid.
reducing materially local electrical stresses.
housing containing insulating liquid and provided
2. Electrical apparatus according to claim 1
wherein said conducting glaze has a resistance
for a considerable distance and thus distribute
value not substantially less than one-tenth meg
the ?ux in large degree.
The arrangement described in detail above, per 40 ohm and not substantially greater than ?fty
megohms as measured between points separated
mits the support of a high tension conductor
by a distance of one-half inch.
through a partition such as the cover of a trans
3. In electrical apparatus having a, conducting
former casing, a metallic deck or the like while
Thus the outer shell of insulating porcelain which
would otherwise establish a zone of high electrical
4 .3
to a point below the level of said liquid, said con
stress, is sheathed with a conducting glaze 6 in
contact with the oil or other media surrounding
it. This prevents objectionable electrostatic dis
ductor being supported by an insulator bushing
turbances.
The same property is possessed by the con
ducting glaze 12 extending throughout the length
of the inner ba?le l3. Not only does each con;
ducting glaze surface perform a useful purpose
by itself but also the extension of these surfaces
in parallel serves to distribute the stresses be
tween them and vavoid electrostatic disturbances.
In some situations, for example where the con
ducting glaze is carried beneath the level of the
oil, a corona glow may still occur if the glaze ends
abruptly. Hence the glazed areas may be grad
ed off in conductivity at their edges. This is ac
with an opening above the level of said liquid
through which a high voltage conductor extends
50
mounted in said opening and extending below
the level of said liquid, the combination of means
for distributing a substantial portion of the volt
age of said conductor along the outer surface of
said bushing between said opening and the level
of said liquid, said means comprising a conduct
ing glaze forming the exterior surface of said
bushing and having electrical contact with said
housing at said opening and extending to a point
below the level of said liquid, said glaze constitut
ing a resistance path having a resistance value
not less than one-tenth megohm as measured be
tween points separated by a distance of one-half
inch.
JOHN E. M. MITCHELL.
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