Note: Descriptions are shown in the official language in which they were submitted.
11523
BACKGROUND OF THE INVENTION
The present invention relates to electrical bushings.
More particularly, the present invention relates to bushings such
as gas filled bushings for introducing high voltage conductors
into a housing, such as a gas filled circuit breaker.
In bushings of the foregoing type, the central conductor
serves both a mechanical and electrical function. In addition to
providing an electrical connection between the conductive flanges
on either side of the hollow dielectric
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housing, the conductor provides the mechanical connection for
holding the bushing together. In order to be certain that the
central conductor serves both functions properly, it is
necessary to design the bushings to accommodate relative
expansion or dimensional changes between the metallic central
conductor and the porcclain insulative housing due to thermal
expansion and contraction. This is a major problem since
bushings of the present type are ordinarily subjected to wide
ranges of temperatures and since the coefficients of thermal
expansion of the metallic conductor and porcelain housin~ are r
quite divergent.
The standard solution of this problem has been to
provide a spring assembly connecting the central conductor
to one of the two conductive flanges. Bushings of this type
are illustrated in U.S. Patent No. 3,566,001 and will be
described in some detail with reference to Figure 1,
below.
The apparatus dlsclosed eliminates the need for s~ring
type systems of the prior art by providing a unique through-
rod assembly, the effective coefficient of thermal expansion
of which is approximately equal to the coefficient of thermal
expansion of the hollow insulator column within which the
through-rod assembly is s-ituated.
The through-rod assembly
comprises first, second and third coaxial cylinders which
cooperate to bias first and second conductive flanges, located
adjacent first and second ends of a hollow insulator column,
against their respective ends of the insulator column. The
innermost of the three cylinders is connected at one end to the
first conductive flange and extends internally into the insulator column.
~n external shoulder extends from the distal end oE the first cyLinder and
supports one end of the second cylinder. The remaining end oE the second
cylinder terminates at an internal flange extending from the distal end of
a third cyclinder which is connected to the second conductive flange.
The coefEicient of thermal expansion of the through-rod assembly
is chosen such that the effective coefficient of thermal expansion thereof
is approximately equal to the coefficient of thermal expansion of the
ho]low insulator column. By way of example, the coefficient of thermal
expansion of the second cyclinder will be slightly less than twice as great
as the coefficient of thermal expansion of the first and third cylinders.
Since the coefficient oE thermal expansion of the hollow insulator column
is typically low, the effective coefficient of thermal expansion of both
elements will be approximately equal and both elements will expand or
contract an equal distance during normal temperature excursions.
A significant feature of the apparatus disclosed is that any
suitable flexible conductor may be utilized to electrically connect the
first and second conductive flanges on either end of the insulator column.
When such an arrangement is utilized, the conductor is not subjected to any
mechanical load and can be made from the most suitable material in terms
of current carrying capacity.
More particularly in accordance with the invention there is
provided a bushing for leading an electrical connector through a wall or
the like, comprising an intermediate flange for attachment in the wall,
two metallic end flanges arranged on either side of the intermediate
flange and two hollow insulating bodies clamped between the intermediate
flange and the end flanges, and abushing conductor running ~he length of
the whole bushing, saidbushing conductor being movable in relation to at
least one of the end flanges, and a drawing member connected between the
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end flanges comprising a first and a second elongated metallic body, each
of which is attached at one end to a corresponding end flange, said bodies
overlapping each other for a certain distance, and a pressure-transmitting
member forming a power-transmitting connection between the ends of the
metallic bodies which are not connected to the end flanges. The insulating
bodies may be of ceramic material with the pressure transmitting member
made of a material of greater average thermal expansion per unit of
length than the first and second metallic bodies. There may be a third
elongated metallic body constituting the pressure transmitting member,
and the first, second and third metallic bodies may be coaxial cylinders
with the third body radially interposed between the other two. The
bushing may be a tube surrounding the drawing member and the tube together r
with a metal cylinder included in the drawing member may define a hollow
cylindrical space communicating with a space radially outside the tube
for circulation of coolant.
Specific embodiments of the invention will now be described
having reference to the accompanying drawings in which:
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Figure 1 is a plan cross-sectional view of a prior art
bushing.
Figure 2 is a plan cross-sectional view of a novel bush-
ing constructed in accordance with the teaching herein.
Figure 3 is a cross-sectional view of the bushing of
Figure 2 taken along line 3-3 of Figure 2.
Referring now to the drawings wherein like numerals indi-
cate like elements there is shown in Figure 1 a typical prior art
gas-filled bushing 10. Bushing 10 consists of an insulator housing
12, a pair of conductive flanges 14 and 16 and a central conductor
18.
Insulator housing 12 comprises two conical insulator
columns 20 and 22 which are separated by an annular mounting flange
24. The sections of insulator housing 12 are biased together by
centrla conductor 18 which applies a tensile force to conductive
flanges 14 and 16.
Central conductor 18 is threaded at 26 and connected to
conductive flange 16 in a manner described below. The distal end
of conductor 18 is provided with an external flange 28 which is
electrically connected to conductive flange 14 by flexible conduct-
ors 30. External flange 28 is mechanically connected to conductive
flang 14 by a spring assembly comprising studs 32, rings 34 and
springs 36. Studs 32 depend from conductive flange 14 and termin-
ate at expanded heads 38. Head 38 of each stud 32 supports a ring
34 of sufficient size to seat one end of spring 36. Springs 36
are compression springs and force conductive flanges 14 and 16
inwardly towards annular mounting flange 24. The particular force
exerted as well as the distance "A" between external step 28 and
conductive flange 14 is adjusted by tightening conductive flange
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16 about the threaded end 26 of central conductor 18.
In the foregoing bushing, the force applied to flanges
14 and 16 by springs 36 varies for different operating temperatures
particularly when the length of the insulator column increases for
increased voltage rating. As the operating temperature increases,
the length of central conductor 18 increases at a faster rate than
the length of insulator housing 12 causing the length of springs
36 to increase. The converse is, of course, also true. Since the
spring rate is a function of the length of the spring and the
length of the spring is a function of temperature, the spring rate
will vary for varying operational temperatures. In practical
applications, this requires that the force exerted by the springs
be excessively high when the conductor 18: is at its shortest length
to insure adequate force when the conductor 18 is at its maximum
length.
In the prior art design the central conductor 18 also
serves both a mechanical and electrical function. Thus its
material must be chosen such that the central conductor can with-
stand both the tensile forces applied thereto during the normal
operation and at the same time, have the highest possible conduct-
ivity.
Referring now to Figures 2 and 3, there is illustrated a
new bushing design designated generally as 40. Bushing 40 comprises
five major components; insulator housing 42, conductive flanges 44
and 46, through-rod assembly 48 and conductor 50. Insulator hous-
ing 42 consists of two insulator columns 52 and 54 which may be of
any standard configuration and which are joined in end-to-end re-
lation through an annular
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mountin~ flange 56. Columns 52 and 54 are normally made of
porcelain but may be constructed of any other suitable
insulative material. The annular mounting flange 56 is of
the standard type and contains numerous bolt hole openin~s,
such as bolt hole 58, which permit bushing 40 to be mounted
to any suitable enclosure such as the fragmentarily shown
enclosure 6~ which could, for example, represent the sealed
housing of a gas circuit breaker. When so mounted, the
entire insulator column 54 is immersed within the enclosure,
and the insulator housing 42 may also be gas filled.
Bushings of the type disclosed herein may be rated
at extremely high voltages, for example 550 k~' and above.
For this reason, it is desirable to fill bushing 40 with an
insulation gas such as sulfur hexafluoride to pro~erly insulate
annular mounting flange 56 (which will normally be grounded)
From the high voltage conductor 50. To this end, conductive
flange 46 may be provided with an aperture 64 which permits
gas to communicate between the enclosure 60 into bushing 40.
Since insulator column 5Z is positioned above the
exterior of enclosure 60, seals 68 are provided between
flanges 44 and 56 and insulator column 52. Suitable seals
are described in U.S. Patent No 3,566,001, assi~ned to ~ould
Inc , the assignee of the present invention.
Conductive flanges 44, 46 are situated adjacent
opposite ends of insulator housing 42 and are biased towards
each other by througll-rod assembly 48 ~hrough-rod assembly
48 comprises three cylindrical rods 70, 72 and 74 which
clamp the two flanges 44, 46 to insulator housing 42 .~ith a
sufficiently high force to insure a sound mechanical design.
Specifically, the force which must be exerted by the through-
rod assembly 48 must be sufficiently great to overcome the
following loads~ load due to gas pressure ~ithin bushing
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40, (2) load due to wind forces, (3) load due to line pulls,
(4) load due to short circuit forces, and ~5) load imposed
during a seismic event.
The innermost cylindrical rod 70 is fitted into an
appropriately threaded opening 76 in conductive flange 44 and
extends internally into insulator housing 42. The distal end
78 of cylindrical rod 70 is provided with an external flange
78 which supports cylindrical rod 72. Cylindrical rod 72 is
coaxial with cylindrical rod 70 and, as will be shohn below,
cooperates with cylindrical rods 70 and 74 to act as a spring
member which biases conductive flanges 44, 46 together. The
upper end 80 of cylindrical rod 72 abuts an internal flange
82 on the distal end of cylindrical rod 74. The proximal end
84 of cylindrical rod 74 is externally threaded and mates with
~n internally threaded aperture 85 in conductive flan~e 46.
The desired force between conductive flanges 44 and 46 is
adjusted by rotating conductive flange 46 on the tllreaded end
84 of cylindrical rod 74. ~s conductive flange 46 is rotated,
cylindrical rod 74 is drawn away from conductive flange 44
and cylindrical rod 72 is compressed between flanges 78 and
82. This increases the tensile force applied to conductive
flanges 44, 46 by through-rod assembly 48 and makes it
possible to adjust the force with ~hich flanges 44, 46 press
against housing 42.
Significantly, the effective length of through-rod
assembly 48 is approximately three times the length of the
bushing. This length provides an effective spring rate which,
although relatively high, keeps the force to be used on
assembly to an acceptable level. Particularly, the force required
is such that under the worst temperature conditions the force
generated by through-rod assembly 48 is the minimum required
to overcome the externally applied loads described above.
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Although through-rod assembly 48 is normally
metallic, and therefore provides an electrical connection
between conductive flanges 44 and 46, it is pTeferable to
provide a separate conductor, such as cylindrical conductor
50, to electrically connect flanges 44, 46. In the strueture
illustrated in the drawings, conductor 50 is a cylinder of
extremely high conductivity which is coaxial to rod assembly
48. One end of cylindrical conductor 50 includes an
external flange 86 which is bolted to conductive flange 46 by
appropriate fasteners 88. As best seen in Figure 3, flange 86
is provided with a notch 90 which is coextensive with aperture
64. Although conductor 50 serves no mechanical function, it
still must accommodate dimensional changes in the bushing
structure. Accordingly, a plurality of flexible connectors 92
connect conductor 50 to conductive flange 44. While conductor
50 has been shown as a cylindrical conductor, any other suit-
able arrangement could be utilized without departing from the
spirit of scope of tlle present invention.
It should be obvious from the foregoing, that the
full mechanical load between flanges 44 and 46 is applied to
through rod assembly 48 and that conductor 50 may be designed
with only electrical characteristics in mind. Accordingly,
conductor 50 may be made of any material exhibiting high
conductivity regardless of the relative strength of such
material. Similarly, through-rod assembly 48 can be designed
with only mechanical cha~acteristics in mind. Accordingly,
the cylindrical rods 70, 72, 74 can be made from any material
exhibiting high tensile strength.
As noted above, bushing 40 will normally be subjected
to large temperature excursions due to both ambient conditions
and I2R losses within the bushing itself. Since bushings of
the type described herein are often used in high voltage
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applications in the 550 kV range and above in hostile
environments, tl-e temperature excursions can be quite extreme.
The insulator housing 42 is normally ma~le of porcelain for its
good insulative characteristics. The through-rod assembly
will normally be made of metallic elements for their strength
and good sprin~ characteristics. The coefficient of thermal
expansion of procelain is relatively low while that of metals
is relatively high. If this variance is not compensated for,
the structural integrity of the bushing will be jeopardized.
To avoid this possibility, the thermal coefficients
of expansion of cylindrical rods 70, 72, 74 are chosen such
that the overall coefficient of thermal expansion of through-
rod assembly 48 is approximately equal to the coefficient of
thermal expansion of insulator housing 42. In this manner,
the pressure applied by conductive flanges 44 and 46 against
the ends of insulator housing 42 will remain approximately
constant over the entire range of operating temperatures
of bushing 40. In the embodiment illustrated in Figure 2,
cylindrical rod 72 is chosen to have a coefficient of thermal
expansion which is slightly less than twice the coefficient
of thermal expansion of cylindrical rods 70 and 74, the co-
efficient of thermal expansion of the latter two rods being
essentially identical. By this arrangement, the distance
between conductive flanges 44 and 46 will be permitted to
increase an amount approximately equal to the distance between
the two ends of insulator housing 42 during any temperature
excursion and the force applied by flanges 44 and 46 against
insulator housing 42 will remain approximately constant.
~lthough this invention has been described with
respect to a preferred embodiment, it should be understood
that many variations in modifications will now be obvious
to those skilled in the art, and, therefore, the scope of
this invention is limited not by the specific disclosure
- herein, but only by the appended claims.
