Pressure vessel for a switch

Abstract: A pressure vessel for a switch comprises an insulative tube (3) serving as a tank for containing a fixed contact (13) and a moving contact (8) of a switch and sealing an insulation gas therein; and a reinforcement member (22) having higher rigidity than that of said insulative tube (3) and being closely fixed on an inner surface of said insulative tube (3) for enclosing said fixed contact (13) and said moving contact (8). As a material of the reinforcement member (22), a conductive material having a larger thermal expansion coefficient than that of the material of the insulative tube (3) is suitable. Thereby, absolute values of the difference of the stresses acting to the insulative tube (3) and the reinforcement member (22) can be reduced. Accordingly, pull-out type fracture occurring at a boundary between the insulative tube (3) and the reinforcement member (22) can be prevented.

Description: The present invention relates to a pressure vessel for a switch, and especially relates to an improvement of a pressure vessel for a puffer-type gas switch for opening and closing an electric circuit, and is a divisional application from EP-A-0 311 017. A conventional puffer-type gas switch which is, for example, shown in published unexamined Japanese Utility model application Sho 59-88842 is described with reference to FIG. 5. Fig. 5 is a cross-sectional view showing a conventional puffer-type gas switch in an opening state of the contacts thereof. A lower tank 101 is fixed on a bottom flange 102. The lower tank 101 generally contains driving shafts (not shown) of three-phases which are connected to an operation mechanism and levers which connect the driving shafts and insulative rods 105 of the respective three-phases. As the above-mentioned constitution is generally known, the driving shafts, levers and operation mechanism are not shown for simplicity. An insulative tube 103 contains elements 104 for arc-extinction and is filled with insulation gas such as SF₆. The insulative tube 103 has a double casing consisting of an inner arc-proof material 103a and an outer normal material 103b. An end of an insulative rod 105, which is connected to the driving lever (not shown in the figure) in the lower tank 101, is connected to an end of a conductive piston rod 106 which is reciprocatively driven in directions shown by arrows A and B. On the other end of the piston rod 106, a disc-shaped piston 107 and a moving contact 108 are fixed. The piston 107 closely slides on an inner surface 103C of the insulative tube 103, and thereby the piston 107 compresses and expands the insulation gas in a lower space 109 and an upper space 110. An insulative nozzle 111 is fixed on the piston 107 co-axially with the moving contact 108 by a nozzle joiner 112. A fixed contact 113 to be connected to the moving contact 108 is fixed on an upper cover 115. When the moving contact 108 is in contact with the fixed contact 113, the electric circuit whereto the switch is provided is closed. A midway position of the moving contact 108 contacts a sliding contact 114, and thereby an electric current flows from the sliding contact 114 to the moving contact 108 and vice versa. Operation of the above-mentioned conventional switch is described in the following. When a closing command is issued from a control apparatus (not shown in the figure), the insulative rod 105 is linearly driven by the operation mechanism. In closing operation of the contacts 108 and 113, the insulative rod 105 is pushed up in a direction shown by arrow A. When this action continues, the moving contact 108 and the fixed contact 113 are closed at a position near to the final position of the closing operation. For opening the contacts 108 and 113, the reverse action to the above-mentioned takes place. In the above-mentioned conventional switch, electric current is capable of flowing when the moving contact 108 and the fixed contact 113 contact each other, and the actural path of electric current is from the sliding contact 114 to the piston rod 106. The pressure vessel of the above-mentioned conventional switch is filled by an insulation gas normally having pressure of 2-5 kg/cm². The pressure of the insulation gas builds up 10-20 kg/cm² when the electric current is cut off. Therefore, the thickness of the insulative tube 103 must be sufficiently thick for withstanding such a high pressure. And also, when the insulative tube 103 is made as a double casing and the inner part 103a is made of an arc-proof insulative material, it is difficult to make the thickness of the insulative tube 103 thin because the mechanical strength of the insulative material against pressure becomes relatively weak. FR-A-2 266 285 discloses a circuit breaker having a pressure vessel which is internally partially coated with a metallic coating. This metallic coating is provided in the region of the contacts and has the function of radiators, temperature distributors, electric field rings, protecting screens for protection against corrosion and diffusion, and as deflectors for the gas flows. An object of the present invention is to provide an improved pressure vessel for a switch for overcoming the above problem. A pressure vessel for a switch in accordance with the present invention comprises the features of claim 1. Further details of the implementation are mentioned in the subclaim. The following is a more detailed description of the present invention taken in conjunction with the accompanying drawings, in which: FIG. 1 is a cross-sectional view showing a construction of a pressure vessel for a switch with the switch mechanism contained therein, wherein the contacts are opened; FIG. 2 is a cross-sectional view showing the pressure vessel of FIG. 1 wherein the contacts are closed; FIG. 3 is a cross-sectional view showing details of the pressure vessel of FIG. 2; FIG. 4 is a cross-sectional view showing a preferred embodiment of the pressure vessel in accordance with the present invention seen alone; and FIG. 5 is a cross-sectional view showing a conventional pressure vessel of a switch. First, a construction of a pressure for a switch in conjunction with the switch mechanism is described making reference to FIGs. 1 to 3. For more details of the switch mechanism, reference is made to the above-mentioned EP-A-0 311 017. FIG. 1 is a cross-sectional view showing the switch mechanism and the pressure vessel under a condition that the contacts are opened, FIG. 2 is a cross-sectional view showing the switch mechanism and the pressure vessel shown in FIG. 1 under a condition that the contacts are closed and FIG. 3 is an enlarged cross-sectional view showing details of FIG.2. In the figures, a lower tank 1 is fixed on a bottom flange 2 and contains driving shafts of each three phases driven by an operation meehanism and insulative rods which are connected to the driving shafts. As the driving shafts and the operation mechanism are known in the art, they are not shown in the figure for simplifying the drawings. Furthermore only one insulative rod 5 is shown in the figure. An insulative tube 3 contains arc-extinction elements 4 and is filled with insulation gas such as SF₆. The arc-extinction elements 4 consist of, for example, an insulation rod 5, a conductive piston rod 6, a cylindrical piston 7 and a moving arc-contact 8. The insulative rod 5 is not connected to the driving lever. The conductive piston rod 6 is reciprocatively driven in directions shown by arrows A and B and is connected to an end of the insulative rod 5. The cylindrical piston 7 and a moving arc-contact 8 are fixed to the other end of the piston rod 6. The insulative tube 3 is molded with a tubular conductor 15. The piston 7 and a sliding contact 14 which is co-axially provided on the outer surface of the piston 7 slide on an inner surface 15a of the tubular conductor 15. The insulation gas in a lower space 9 and an upper space 10 is expanded and compressed by the motion of the piston 7. An insulative nozzle 11 is fixed on the piston 7 co-axially with the moving arc-contact 8 by a nozzle joiner 12. A fixed contact 13 to be connected to the moving arc-contact 8 and having a tubular shape is fixed on an upper terminal 18. When an outer surface 8a of the moving arc-contact 8 is in contact with inner surface 13a of the fixed contact 13, an electric circuit, which is to be connected to the switch, is closed. Plural current collectors 16 are circularly provided in the cylindrical piston 7 around the moving contact 8. When the moving contact 8 is in contact with the fixed contact 13, the current collectors 16 are also in contact with an external surface 13b of the fixed contact 13. The current collectors 16 serve as a main moving contact. A lower terminal 17 is electrically in contact with the tubular conductor 15 and is provided at a midway position of the insulative tube 3. An upper tank 19 is fixed on the upper terminal 18 and thereby the insulation gas is sealed in the insulative tube 3. As shown in FIG. 3, two compression springs 30 and 31 are provided between an inner surface 7a of the piston 7 and an outer surface 16a of each current collector 16 so as to apply contact pressures at positions C and D. In a switch mechanism which is constituted as mentioned above, when the contacts 8 and 13 contact each other, the electric current flows in the order of from the upper terminal 18, through the fixed contact 13, the current collector 16 which serves as a main moving contact, the piston 7, the sliding contact 14, the tubular conductor 15 to the bottom terminal 17. When a trip signal is issued (for example, by flow of an accident over-current), movable elements of the arc-distinction elements 4 such as the piston 7, the moving arc-contact 8, the current collectors 16 and so on are driven in a direction shown by arrow B by action of the operation mechanism (not shown in the figure because of being known in the art). When the piston 7 moves in the direction shown by arrow B, the insulation gas in the lower space 9 is compressed and the insulation gas in the upper space 10 is expanded. Then, the current collector 16 departs from the fixed contact 13 according as movement of the movable element of the arc-extinction elements 4 in the direction shown by arrow B. When the moving arc-contact 8 departs from the fixed contact 13, an arc is discharged. By such actions, the pressure of the insulation gas in the lower space 9 becomes higher than those of the gases in other spaces. When pressure buildup due to the arc discharge is above about zero point of the current, the insulation gas in the bottom space 9, where the pressure of the insulation gases is high, flows to other space where the pressures are lower than in the bottom space 9. For example, gas passing through a hole 7b of the piston 7 flows through a hole 11a of the nozzle 11 and a hole 13c of the fixed contact 13 to the upper space 10 and the upper tank 19, and another gas passing through a gap 6a between the insulative tube 3 and the piston rod 6 flows to an inner space la of the bottom tank 1. At that time, the insulation gas flowing from the bottom space 9 to the upper space 10 collides with the arc made by discharge between the fixed contact 13 and the moving arc-contact 8. Accordingly, the arc is cooled and diffused by the flow of the insulation gas, and finally the arc is extinguished. When the arc is extinguished, the switching off of the circuit is completed. In an operation for closing the switch, the movable elements of the arc-distinction elements 4 moves in a reverse direction shown by arrow A, and the switch is closed by contact of the current collectors 16 (which serve as a main moving contact) and the fixed contact 13. In the above-mentioned construction, the insulation gas is sealed in the insulative tube 3 in a pressure of about 2-5 kg/cm². Therefore, a stress δ γ in radial direction and a stress δ  in circumferential direction corresponding to the pressure of the insulation gas always act onto the insulative tube 3. Generally, the insulative tube 3 having the inner tubular conductor 15 is manufactured by a cast molding process in a temperature range of 150-200°C. When the insulative tube 3 is cooled to the normal temperature from the above-mentioned high temperature range, the insulative tube 3 is hardened and contracts, and the tubular conductor 15 also contracts in proportion to the temperature difference. Hereupon, when the thermal expansion coefficient of the tubular conductor 15 is larger than that of the insulative tube 3, the stress δ  in circumferential direction of the insulative tube 3 is always in compressing state (since the stress δ  in circumferential direction is generally larger than the stress δ γ in radial direction). When pressure of the insulation gas acts on the inner surface of the insulative tube 3 (the highest pressure part is in the lower space 9, where the insulation gas is compressed), the stress in circumferential direction of the insulative tube 3 effects as a tension stress. However, the compression stress due to the thermal contraction has already acted on the insulative tube 3. Therefore, by selecting an insulative material such as epoxy resin and a conductive material such as aluminum as materials of the insulative tube 3 and the tubular conductor 15, which have a larger thermal expansion coefficient than that of the insulative material, in the pressure vessel the above-mentioned compression stress and the tension stress may be canceled. Therefore, creep fracture of the insulative tube 3 or destruction of the insulative tube 3 due to the sudden pressure buildup at break of the circuit can be prevented. Furthermore, the tubular conductor 15 receives abnormal high pressure of the insulation gas which may occur at accidental over-current breaking. Namely, the tubular conductor 15 serves as a reinforcement of the insulative tube 3 for partially charging the internal stress of the insulative tube 3. As a result, the thickness of the side wall of the insulative tube 3 can be made thin. A preferred embodiment of the pressure vessel in accordance with the present invention is described with reference to FIG. 4. In FIG. 4, a second tubular conductor 22 is provided co-axially with the outer surface of the tubular conductor 15. The second tubular conductor 22 is fixed to the tubular conductor 15 with electric conductivity thereto. Hereupon, as a material of the second tubular conductor 22, a conductive material having a larger thermal expansion coefficient than that of the material of the insulative tube 3, and smaller than that of the conductive material of the tubular conductor 15 is suitable. Thereby, absolute values of the difference of the stresses acting to the insulative tube 3 and the second tubular conductor 22 or acting to the second tubular conductor 22 and the tubular conductor 15 can be reduced. Accordingly, pull-out type fracture occurring at a boundary between the insulative tube 3 and the second tubular conductor 22 can be prevented. In the above-mentioned embodiment, the insulative tube 3 for containing the switch mechanism and the arc-extinction elements of a switch, can be utilized for any types of pressure vessels made of resin. Also, in the above-mentioned embodiment, the tubular conductor 15 is provided on a peripheral part of the lower space 9 where the pressure of the insulation gas will be the highest. However, there is a case that the pressure of the insulation gas surrounding the arc-discharging part between the moving arc-contact 8 and the fixed contact 13 becomes the highest. Therefore, a constitution similar to the above-mentioned can be adopted thereto. Furthermore, in the above-mentioned embodiment the insulative tube 3 is molded with the inserted tubular conductor 15. The tubular conductor 15, however, is not necessarily conductive when a method for collecting electric current similar to the prior art for correcting the electric current from the midway portion of the piston rod 6 is adopted.

Claims: A pressure vessel of a switch comprising: an insulative tube (3) serving as a tank for containing a fixed contact (13) and a moving contact (8) of a switch and sealing an insulation gas therein; and a reinforcement member (15, 22) being closely fixed on an inner surface of said insulative tube (3) for enclosing said fixed contact (13) and said moving contact (8), characterized in that said reinforcement member (15, 22) having higher rigidity than that of said insulative tube (3) comprises: an inner and an outer tubular member (15, 22), the outer tubular member (22) being arranged between the inner tubular member (15) and the tube (3), wherein the thermal expansion coefficient of the outer tubular member (22) lies between same of the inner tubular member (15) and the tube (3). The pressure vessel in accordance with claim 1, wherein the thermal expansion coefficient of said reinforcing outer tubular member (22) is higher than that of said insulative tube (3). The pressure vessel in accordance with claim 1 or 2, wherein said reinforcing outer tubular member (22) is electrically conductive for serving as an element of said switch.

Inventors: HASEGAWA HIROSHI C O MARUGAME, MARUYAMA TOSHIMASA C O MARUGAM, NISHITANI JUNICHIRO C O MARUGA, HASEGAWA, HIROSHI, C/O MARUGAME SEISAKUSHI, MARUYAMA, TOSHIMASA , C/O MARUGAME SEISAKUSHI, NISHITANI, JUNICHIRO, C/O MARUGAME SEISAKUSHI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: H01H33
IPC codes: H01H33