EP0014967B1

EP0014967B1 - Bushing

Info

Publication number: EP0014967B1
Application number: EP19800100757
Authority: EP
Inventors: Mitsuhiro Kishida
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1979-02-16
Filing date: 1980-02-14
Publication date: 1983-12-14
Also published as: DE3065859D1; EP0014967A1

Description

The present invention relates to a bushing according to the first part of claim 1 and more particularly but not exclusively to a bushing to be used at the outlet of e.g. a super high voltage transformer in the more than 500 KV class, or a tank type electric machinery and apparatus wherein switchgears are immersed in insulating fluid such as insulating gas, insulating oil, etc.
FR-A-10 40 087 discloses a bushing having a porcelain portion with one end thereof being spaced from a cylindrical mounting flange, a connecting portion with a predetermined gap being left between it and said mounting flange, a sealing means and a spring mechanism means to elastically connect said connecting portion and said mounting flange. The porcelain portion is axially spaced from said cylindrical mounting flange; and the connecting portion is fixedly secured to the porcelain portion.
In a case where the electric voltage of the electric machinery and apparatus is high and the environment in which it is used is subject to salt or dust contamination, a long porcelain bushing having a long creepage is used at its connecting portion with the overhead cable so as to resist this contamination environment. When such an electric machinery or apparatus is used in regions where the frequency of occurrence of earthquakes is high (such as in Japan), the machinery or apparatus is designed with a view to such risk of earthquakes. In general, when a bushing mounted on electric machinery or apparatus is affected by earthquake, there is an amplification of the earthquake amplitude at the ground on which the base and the tank portion of the machinery, and the portions of the mounting seat of the bushing, etc., are mounted. Further, when the frequency of the machinery which is determined by the weight distribution and the rigidity, is near to, or corresponds to, the frequency of the earthquake, a resonance phenomenon occurs, subjecting the bushing portion to a very large oscillation by the amplification of the oscillations of the base, tank and bushing mounting seat, which may result in the breakage of the porcelain (portion) by exceeding the breakage strength of the bushing.
Generally, the frequency of an earthquake lies principally in the range of 1 to 10 Hz, and bushing of the plus 220 KV class may have a natural frequency, in its mounted state in electric machinery or apparatus, of 10 Hz, this frequency coinciding with that of an earthquake. According to experience, it has been proved that in a bushing of less than 5 m in length the breakage strength of its porcelain portion will not be exceeded even if subjected to the strongest earthquake ever experienced, so that it has sufficient strength; but in the case of the 500 KV plus class using a long porcelain bushing of the contamination proof type, the natural frequency of the bushing is low, so that it may coincide with the earthquake frequency, and so that there is a risk of breakage of the porcelain during a strong earthquake.
Therefore, it proved necessary to improve the strength of the porcelain portion under the effect of earthquakes. When a bushing of the 1000 KV class is utilized as a countermeasure for increasing its strength against earthquakes, it is conceivable to have the bushing strengthened at its forward end by stay insulators in three or four directions. In this case, the oscillation of the bushing portion becomes a string oscillation so that a phenomenon superimposed by an oscillation other than that of the bushing occurs, so the analysis of the strength against earthquakes is made difficult, leaving a question mark on its reliability. Further, in this case, it is necessary to consider the contamination withstand voltage value with the stay insulator portions connected in parallel with a bushing device. The adhesion of the contamination substance differs between the bushing portions and the stay insulator portions, and generally the stay insulator portions each having a smaller drum diameter are easily contaminated. At any rate, it is necessary to assure an electric voltage value resisting against contamination when connected in parallel. Therefore, from this point of view such a system likewise decreases in reliability.
When a bushing of the anticontamination type of the 500 KV plus class is designed in conventional manner, as shown in Fig. 1 of the attached drawings, a core electrode 1 is surrounded circularly by a condenser part 2 which comprises a cylindrically wound insulating paper with an electrical field regulation electrode introduced therein so as to equalize the inner and the outer electrical fields, core electrode 1 supporting a lower porcelain portion 3 at the lowest portion by means of a screwable support member 4 which acts also as a terminal. Lower porcelain portion 3 is placed onto support member 4 with a gasket being interposed therebetween, while a flange 5 for fixedly mounting the bushing to an electric machinery or apparatus is set upon the porcelain portion 3. An upper porcelain portion 7 with porcelain fixing elements 6a, 6b cemented thereto at its upper and lower portions is placed on flange 5 with a gasket interposed therebetween, to be fixedly secured by a bolt and nut assembly. Set upon the upper end of the upper porcelain portion 7 is a stop member 8 which receives a coil spring 9 to compress porcelain portions 3, 7 and flange 5, a spring compressor plate 10, and a ring nut 11 to adjust the compressive force of coil spring 9. Further, flexible lead wires 13 are provided for the connection of a terminal member 12 and central conductor 1, insulating oil 14 being filled in the space formed between central conductor 1 and upper as well as lower porcelain portions 3, 7. In this case, a gas space having an appropriate volume is left within top member 8 so as to avoid an abnormal change in the volume of insulating oil 14, and inert gas, such as nitrogen, is filled within the space under a suitable pressure. When the bushing having such a construction is subjected to an earthquake when mounted on an electric machinery or apparatus, the vibration is amplified across a distance from the earth to the porcelain portions of the bushing. Therefore, if the rigidity of the various portions is increased, the degree of the amplification is made small so that the strength against earthquake of the electric machinery or apparatus is improved. Consequently, it is generally necessary that the rigidity of the mounting flange of a bushing is designed to be as high as possible.
The earthquake vibration response of a bushing having such a configuration when mounted on an electric machinery or apparatus may be amplified by more than two times at the portion of the mounting flange of the bushing, even if the electric machinery or apparatus as a whole has been designed with a view to earthquake resistance. Further, it is conceivable that an amplification with a factor of three or more may occur at the top of the bushing. Therefore, the bushing must be able to resist such amplified vibration, but when the bushing vibrates, mechanical stresses are added to the various parts of the bushing. Although the magnitudes of the stresses differ from part to part, the bending stress at the upper surface part of the lower fixing member 6a of upper porcelain portion 7 shown in Fig. 1, i.e. at the inner side of the part indicated by an arrow, is maximum. The bending breakage strength of porcelain is in the order of 200 to 250 kg/cm², so that when the bending stress exceeds this value, the porcelain portion may be broken.
A bushing in which the dimensions of the porcelain portions are not too large has a high natural frequency, seldom developing the resonance phenomenon, and has sufficient earthquake strength due to the large drum diameter of the porcelain portion relative to the bushing weight. However, in the case of a bushing of the anticontamination type for more than 500 KV, an elongated porcelain portion is used because of the necessity to lengthen the surface leakage distance, and while the weight becomes higher, the drum diameter is not so large. Therefore, its natural frequency is low, easily coinciding with the earthquake frequency to cause resonance and, hence, large oscillation. It may be assumed that in such an event the internal stress in the porcelain portion would easily exceed the breakage stress. Therefore, in a conventional bushing of the more than 500 KV class of the contamination-proof type, a countermeasure such as reinforcing by arranging stay porcelains in three directions from the top is necessary. However, as stated above, the stay porcelains present difficulties with regard to reliability, and are not practical.
The invention as claimed is intended to provide a remedy by proposing a bushing of superior vibration proof characteristics.
In accordance with the present invention, the bushing according to the first part of claim 1 is characterized in that said connecting element is rigidly secured to said porcelain portion at said one end, that said sealing means is provided to seal the gap formed between said mounting flange and said connecting element, that damping means is interposed between said connecting element and said mounting flange and that said spring mechanism means comprises a plurality of springs, the damping means and springs serving to absorb energy tending to cause the porcelain portion to vibrate.
In a preferred aspect of the present invention, the damper mechanism incorporates a so- called dash-pot construction to provide a more effective absorption of the vibration energy, so that breakage of the bushing due to vibration is more effectively prevented.
The advantages offered by the invention reside mainly in the fact that, since in a bushing the adapter and the mounting flange are connected by the spring mechanism, the oscillation energy applied to the mounting portion of the bushing under earthquake conditions can be absorbed by the spring mechanism so that the earthquake proof characteristic of the bushing is remarkably improved.
One way of carrying out the invention is described in detail below with reference to drawings which illustrate a few specific embodiments of the invention in addition to one example of a conventional bushing, in which:

Fig. 1 is a cross-sectional view of a bushing of conventional design;
Fig. 2 is a cross-sectional view of the essential portion of an embodiment of the present invention; and
Fig. 3 to 7 are respectively cross-sectional views of the essential portions of other embodiments of the present invention.

In summary, according to the present invention, a spring mechanism is disposed between a porcelain portion and a mounting flange in a bushing, thereby to absorb the vibration energy by friction when the spring mechanism is compressed, so that when a large vibration is applied to the mounting portion of the bushing as the result of a large earthquake, the vibration energy is absorbed by the spring mechanism thereby protecting the bushing against breakage.
Now the present invention will be explained in more detail with reference to the attached drawings.
In Fig. 2, reference numerals 1,2,3, 6a and 7 show parts similar to those shown in Fig. 1, with 15 being a cylindrical adapter connected to one end of porcelain portion 7 through a rigid member 6a, and having a connecting portion 15a extending orthogonally to the axis. 16 is a cylindrical mounting flange receiving therein adapter 15 and having a mounting portion 16a extending orthogonally to the axis and adapted to be connected to flange 5, as well as a connecting portion 16b opposing the connecting portion 15a. A plurality of bolts 17 pass through said confronting connecting portions 15a and 1 6b, and a spring mechanism 18 is held by each of bolts 17 and nuts 19 screwed thereon so as to maintain a predetermined fastening force, with spring mechanism 18 being constituted by a number of belleville springs piled up so as to absorb vibration energies by friction when compressed. A sealing element 20 comprising an 0- ring is disposed between adapter 15 and mounting flange 16 so that adapter 15 and mounting flange 16 sealingly separate the inside of porcelain portion 7 from the outside. A damping element 21 is interposed between connecting portions of 15a and 16b of adapter 15 and mounting flange 16.
In a bushing having such a construction, if the fastening or clamping force of spring mechanism 18 is previously set so that the internal stress of the base of porcelain portion 7, i.e. the portion represented by the arrow in Fig. 2, at the time of application of a bending force to the bushing is sufficiently large relative to the breakage stress of porcelain portion 7, when the bushing resonates and oscillates strongly, and at the moment when the amplitude due to the bending load comes to exceed the previously set pressure of spring mechanism 18, spring mechanism 18 begins to compress, and, upon oscillation with a larger amplitude, the vibration energy is absorbed by the function of the spring mechanism 18, thus damping the vibration. Thus, the response of the bushing to the acceleration does not become excessive from the time of the beginning of the compression of spring mechanism 18, and the internal stress of porcelain 7 is also prevented from becoming too large, thereby to keep it below its breakage stress. Therefore, in comparison with a case where porcelain portion 7 is rigidly secured to mounting flange 16, the bushing is prevented from breakage even in the case of a heavy earthquake.
Although it is conceivable that a gap may arise at the portion of damping element 21 interposed between both connecting portions 15a and 16b when spring mechanism 18 begins to move, sealing element 20 interposed between mounting flange 16 and adapter 15 can prevent the insulation fluid from flowing out at that portion. Further, when the phase of the oscillation is inverted, the gap between both connecting portions 15a and 16b closes so that a high impact force is applied thereto, but it impinges upon damping element 21 so that the impact force is moderated by the cushioning effect of element 21.
In a bushing having such a construction and being mounted on electric machinery or apparatus having a tank, such as a transformer, when a large amplitude is applied to the bushing as the result of the amplification of the earthquake amplitude at the foundations, the body of the electric machinery, and mounting flange 16 of the bushing, etc., the oscillation period varies from the moment of the operation of spring mechanism 18. Therefore, the natural frequency of the bushing changes, and when the amplitude tends to become larger, the vibration energy is absorbed by the friction of spring mechanism 18 to increase the damping effect so that the response to the acceleration at the lower portion of porcelain portion 7 does not increase, such that the internal stress within the porcelain portion is suppressed below its breakage stress even when subjected to severe earthquakes.
In a modification of the embodiment shown in Fig. 2, if there is mounted by welding, as shown in Fig. 3, a bellows 23 between mounting flange 16 and adapter 1 5 in the place of 0- ring 20 shown in Fig. 2, a perfectly sealed configuration is always provided even should a clearance occur therebetween.
Although in the embodiments shown in Fig. 2 and 3 spring mechanism 18 is arranged outside mounting flange 16, a similar effect can be expected if spring mechanism 18 is provided inside mounting flange 16 as shown in Fig. 4, wherein 25 is a connecting plate.
Further, although in the embodiment described above and shown in the drawings it is assumed that there is an insulating oil contained within the porcelain portion, similar effects can be expected also when an insulating gas is contained therein.
Fig. 5 shows another embodiment of the present invention wherein reference numerals 1, 2, 3, 6a and 7 correspond to those shown in the previous figures, 16 being a mounting flange ridigly secured to lower porcelain portion 3, 25 being a connecting plate secured to the upper surface of the mounting flange 16 by bolts and nuts 26, 15 being an adapter secured to a fastening element 6a by bolts, and 27 being connecting rods arranged in adapter 15 at regular intervals to transfer a dimensional change caused between adapter 15 and connecting plate 25 when porcelain portion 7, fastening element 6a and adapter 15 oscillate integrally. 28 are dampers to give a dash pot effect when connecting rods 27 move up and down, each damper comprising a cylindrical pot 29 encircling connecting rods 27 and a piston 30 spaced by a predetermined gap from pot 29. 31 is a coil spring disposed around connecting plate 25, and a nut 32 is screwed to connecting rod 27, said nut 32 being adapted to secure piston 30 as well as to provide a predetermined fastening or clamping force between connecting plate 25 and adapter 19. 33 is a damping member interposed between connecting plate 25 and adapter 15 to relieve the collision phenomena caused by the occurrence of an opening between them as a result of the oscillation. 34 is a sealing element comprising a bellow to prevent insulating fluid filled within the porcelain portion from leaking out if an opening is formed between connecting plate 25 and adapter 15.
Even if a bending force is applied to a bushing having the above-mentioned configuration at a portion represented by the arrow A in Fig. 5, the internal stress of the portion shown by arrow A is previously set so as to be sufficiently below the breakage stress, by suitable selection of the fastening force of spring 31. In this state, when the bushing oscillates with a large amplitude, at the moment when the amplitude due to the bending force exceeds the previously set pressure of spring 31, an opening is formed between connecting plate 25 and adapter 15, resulting in a change in position of piston 30, as the result of which a damping effect is generated to absorb vibration energy by damper 28 and the insulating fluid filled within it.
Although in the above embodiment a configuration wherein a damping effect is exhibited by the combination of a coil spring and a dash pot has been explained and shown, when ring springs or belleville springs are used, a superior vibration damping effect is obtainable on the expectation of a damping effect due to the internal friction of the coils themselves.
Moreover, although in the above embodiment the sealing is maintained by sealing member 34 against the opening phenomenon at the time of vibration, the sealing effect can be maintained also by an 0-ring 35, as shown in Fig. 6.
Alternatively, in the place of the peripheral arrangement of a number of dash pot constructions as shown in Fig. 6, a number of springs 31 can be arranged peripherally at equispaced intervals as shown in Fig. 7, and a cylindrical ring 36 can be disposed inside a circle encircling the radially inner surfaces of springs 31 so as to be coaxial with the center of the bushing, so that double cylinders are constituted together with the drum portion 16a of mounting flange 16, whereby an annular piston 30 is disposed within double cylinders 16a, 36 with suitable gaps being left between their confronting surfaces, and springs 31 being suitably compressed by nuts screwed on connecting rods 27. Thus, a dash pot effect is realized by the double cylinders and the piston, damping the vibration energy even more efficiently.
When a bushing having such a configuration as explained above and shown is mounted in an electric machinery or apparatus having a tank, such as a transformer, upon the occurrence of an earthquake a large amplitude is applied to the bushing as the result of the amplification of the amplitude at the foundations; the body, and the mounting flange of the bushing, etc., but since the oscillation system changes at the moment an opening between connecting plate 25 and adapter 15 commences, the natural frequency of the bushing becomes low. Further, when the amplitude increases, the vibration energy is absorbed by the damper mechanism such that the damping is increased, and the magnification of the acceleration response is decreased so that the internal stress within the porcelain portion can be suppressed below its breakage stress even if the bushing is subjected to severe earthquake.
From the foregoing it will be appreciated that, in accordance with the present invention, since the bushing is constituted such that the adapter and the fastening flange are fastened by a plurality of springs with a dash pot construction being provided around the springs, when the bushing is subjected to severe earthquake and vibrates with a large amplitude, the vibration energy is absorbed by the dash pot construction simultaneously with the occurrence of an opening between the adapter and the mounting flange. Therefore, it is made possible to suppress the internal stress of the porcelain portion below its breakage stress.

Claims

1. A bushing having a porcelain portion (7) with one end thereof being axially spaced from a cylindrical mounting flange (16), a central conductor (1) passing through said porcelain portion (7) and said mounting flange (16), an insulating fluid sealed within said mounting flange (16) and said porcelain portion (7), a connecting element (15) rigidly secured to said porcelain portion at said one end and adapted to have said central conductor (1) passing therethrough with a predetermined gap being left between said connecting element (15) and said mounting flange (16), a sealing means (20; 23; 34; 35), and a spring mechanism means (18; 31) to elastically connect said connecting element (15) and said mounting flange (16), characterized in that said sealing means (20) is provided to seal the gap formed between said mounting flange (16) and said connecting element (15), that damping means (21; 28; 16a, 30; 36) is interposed between said connecting element (1 5) and said mounting flange (16) and that said spring mechanism means (18; 31) comprises a plurality of springs, the damping means (21; 28; 16a, 30, 36) and springs serving to absorb energy tending to cause the porcelain portion to vibrate.

2. A bushing as claimed in claim 1 characterized in that said sealing means (20) comprises an O-ring.

3. A bushing as claimed in claim 1, characterized in that said sealing means comprises a bellows (23; 34).

4. A bushing as claimed in claim 1, characterized in that said spring mechanism means (18; 31) is provided outside said mounting flange (16).

5. A bushing as claimed in claim 1, characterized in that said spring mechanism means (18; 31) is provided inside said mounting flanged 6).

6. A bushing as claimed in claim 1, characterized in that said spring mechanism means (31) is surrounded by a cylindrical pot (29), and that a piston (30) is fixedly secured to said spring mechanism means (31) at its one end with a predetermined gap being left between said pot (29) and the periphery of said piston (30), whereby a dash pot construction is formed by said pot (29) and said piston (30).

7. A bushing as claimed in any one of claims 1 to 6, characterized in that said springs are belleville-type springs (18).