Electrical bushing of a gas insulated electrical apparatus

An electrical bushing of a gas insulated electrical apparatus comprises an insulating tube, a central conductor extending through the insulating tube and a cylindrical shield disposed between the central conductor which is electrically connectable to ground at one end and has an electrical field moderating ring integrally formed therewith at the other end thereof. The central conductor has a reduced diameter portion in the vicinity of the electric field moderating ring of the cylindrical shield and large diameter portions and tapered portions connecting the reduced diameter portion to the large diameter portions. Thus, since the reduced diameter portion is formed in a region where the electric field strength is high, the concentration of the electric field in that region is moderated, thereby resulting in an improvement in the withstand voltage characteristic of the bushing. The reduced diameter portion can be formed as a separate member. Also, the reduced diameter portion may itself have portions of different diameters.

BACKGROUND OF THE INVENTION 
This invention relates to an electrical bushing of a gas insulated 
electrical apparatus and more particularly to an electrical bushing of a 
gas insulated electrical apparatus comprising a high voltage central 
conductor and a grounded shield within an insulating tube. 
FIG. 1 illustrates an example of a conventional electrical bushing which is 
disclosed in Japanese Patent Application Laid Open No. 56-30211. 
The conventional electrical bushing comprises an insulating tube 1 such as 
a porcelain tube, an upper mounting fixture 1a secured to one end of the 
tube 1, an upper terminal 10 hermetically secured to the tube 1 through 
the fixture 1a, a lower mounting fixture 1b secured at the other end of 
the tube 1, a flange 4 which is grounded and to which the fixture 1b is 
hermetically secured and a vessel 5 provided at the top end thereof with a 
flange member 5a which is hermetically secured to the flange 4. 
The electrical bushing further comprises a central conductor 2, which is 
used as a high voltage electrode and is axially disposed at the center of 
the hollow portion of the tube 1, a cylindrical shield 3, which is 
grounded and is secured at one end thereof to the inner circumferential 
edge of the flange 4 so as to surround the central conductor 2 and has an 
electric field moderating ring 3a formed at the other end thereof, and an 
insulating medium 6, such as SF.sub.6 gas, confined within the electrical 
apparatus. 
Since it has been recently attempted to minimize the size of the insulating 
tube so that the production cost thereof may be cut down, the inner space 
of the tube 1 is very limited and accordingly the insulating distance 
between the field moderating ring 3a of the cylindrical shield 3 and the 
central conductor 2 becomes very short. 
The thus constructed conventional electrical bushing has problems in that, 
when a high voltage is applied to the central conductor 2, the 
equipotential surfaces of the electric field are very dense between the 
field moderating ring 3a of the shield 3 and the central conductor 2 and 
the electric field strength around that portion is therefore considerably 
high, resulting in a limitation to the withstand voltage of the electrical 
bushing. Accordingly, the field moderating ring 3a must have a 
cross-sectional shape having as large a radius as possible and must be 
away from the central conductor 2 as far as possible. However, it is 
difficult to satisfy these conditions within the limited inner space of 
the tube 1. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide an electrical 
bushing of a gas insulated electrical apparatus free from the above 
discussed problems. 
Another object of this invention is to provide an electrical bushing of a 
gas insulated electrical apparatus having a superior withstand voltage 
characteristic. 
According to this invention, the electrical bushing comprises an insulating 
tube, a central conductor extending through the insulating tube, a 
cylindrical shield disposed between the insulating tube and the central 
conductor so as to surround the central conductor, the cylindrical shield 
being electrically connectable to ground at one end and having an electric 
field moderating ring at the other end thereof. The central conductor has 
a reduced diameter portion in the vicinity of the field moderating ring of 
the shield, large diameter portions and tapered portions conecting the 
reduced diameter portion to the large diameter portions. The reduced 
diameter portion preferably has an axial length equal to or greater than 
the inner diameter of the insulating tube. 
According to this invention, since the central conductor is provided with a 
reduced diameter portion in a portion where the electric field strength is 
high in the insulating bushing, the necessary insulating distance is 
provided in that portion, and the concentration of the electric field is 
moderated, so that the electric field strength is lowered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 illustrates an electrical bushing of a gas insulated electrical 
apparatus according to an embodiment of this invention. The electrical 
bushing is substantially similar to the bushing shown in FIG. 1 except for 
the configuration of a central conductor. In other respects, the structure 
is the same as that of the conventional bushing illustrated in FIG. 1. 
In the illustrated embodiment, the electrical bushing includes a central 
conductor 7 having a reduced diameter portion 7a in the vicinity of the 
field moderating ring 3a of the cylindrical shield 3. The reduced diameter 
portion starts from a point on the conductor 7 which is a little away from 
the field moderating ring 3a toward a ground side by a distance which 
raises no problems in terms of the internal electric field, and extends 
toward the source side by a distance at least equal to the inner diameter 
of the insulating tube 1. The reduced diameter portion 7a is smoothly 
connected at both ends thereof to large diameter portions through tapered 
portions 7b. 
Thus, the reduced diameter portion 7a, which is formed in the vicinity of 
the field moderating ring 3a of the cylindrical shield 3, and the 
insulating tube 1 are sufficiently away from each other, whereby the 
equipotential surfaces which were dense in the conventional electrical 
bushing shown in FIG. 1 are less dense, whereby the concentration of the 
electric field can be moderated and at the same time the electric field 
strength between the field moderating ring 3a and the central conductor 7 
can be lowered. 
The equipotential surfaces between the field moderating ring 3a of the 
shield 3 and the reduced diameter portion 7a of the central conductor 7 
are balanced in comparison with those of other portions within the 
insulating tube 1 because the diameters of the central conductor 7 and the 
cylindrical shield 3 are appropriately selected. When the diameter of the 
central conductor 7 is reduced in the vicinity of the field moderating 
ring 3a around which the electric field is strongest within the insulating 
tube 1, the electric field strength on the reduced diameter portion 7a of 
the central conductor 7 is high, on one hand, when only the round 
configuration of the field moderating ring 3a is taken into consideration. 
However, on the other hand, since the insulating distance between the 
reduced diameter portion 7a and the field moderating ring 3a is large, the 
electric field established therebetween has a strength substantially equal 
to the field strength of the conventional arrangement. 
FIG. 3 illustrates an electrical bushing of a gas insulated electrical 
apparatus according to another embodiment of this invention. 
According to this embodiment, the electrical bushing includes a central 
conductor 8 having large diameter portions 8a which are made of aluminium 
and a separate reduced diameter member 8b made of a material having a 
higher electrical conductivity, such as copper. The large diameter 
portions 8a and the separate reduced diameter member 8b are welded to each 
other. According to this embodiment, the problem of non-uniform 
temperature rise when a large current flows through the narrow reduced 
diameter portion 8b is eliminated. 
FIG. 4 illustrates an electrical bushing of a gas insulated electrical 
apparatus according to still another embodiment of this invention, in 
which the electrical bushing includes a central conductor 9 having a first 
reduced diameter portion 9a, and a second reduced diameter portion 9b 
which has a smaller reduced diameter than the first reduced diameter 
portion 9a. The second reduced diameter portion 9b is provided in a 
position which is away from the field moderating ring 3a of the shield 3 
so that the electric field strength is high between the central conductor 
9 and the inner wall of the electrical bushing 1, thereby making it 
possible to further improve the withstand voltage characteristic of the 
electrical bushing. 
As can be seen from the above description, according to this invention, the 
electrical bushing includes a central conductor having different 
diameters, i.e. a reduced diameter portion in the vicinity of the field 
moderating ring at one end of the cylindrical shield, large diameter 
portions and tapered portions connecting the reduced portion to the large 
diameter portions. The reduced diameter portion may start from a point on 
the conductor which is away from the field moderating ring, namely, a 
point where the electric field strength between the field moderating ring 
and the central conductor is less high, and may extend over an axial 
length at least equal to the inner diameter of the insulating tube. 
Therefore, the withstand voltage characteristic of the electrical bushing 
is significantly improved.