ELECTRIC INSULATOR

Abstract: A gas-filled electric insulator for high voltage comprising a tubular insulant (1) with a flanged annular pole (3) attached at each end. Each pole has an annular part (9) and a flange (4). The annular part (9) has an outside diameter which is smaller than the inside diameter of the insulant (1) and is inserted into the insulant such that only the flange of the pole is exposed on the outside. The larger safety distance with respect to flashover which is obtained on the inside of the insulator, which is filled with insulating gas, is then utilized for reducing the overall height.

Description: Electric insulator TECHNICAL FIELD The present invention relates to an electric insulator of the kind described in the preamble to claim 1. The insulator com¬ prises an elongated, hollow insulant provided at each end with an attachment or connection flange. The insulator may comprise a contact device with cooperating fixed and movable main and arcing contacts. The insulator may also be in the form of a support insulator for a circuit breaker and then comprises a continuous operating rod for the circuit breaker. The invention is primarily intended for rated voltages of the order of magnitude of 100 - 300 kv but may advantageously also be used at voltages both below and above this range. Prefe¬ rably, an insulator according to the invention is filled with an electrically insulating gas, preferably SFβ, but may also be filled with other types of insulating and arc-extinguishing media. In the following, the designations insulation, insu¬ lant, and the like, relate to insulation against electrical flashover. BACKGROUND ART An insulator usually comprises two attachment means retained by an insulant. Its primary task is to electrically separate two poles with different voltage potential. For applications within high-voltage engineering, insulants of porcelain or of polymeric material are preferably used. To attach them to each other or to another object, such as a base, a mechanism housing, a circuit-breaker unit, or a conductor, the poles of the insulator are equipped with attachment flanges. As dimen¬ sioning criterion for an insulator, there is primarily used that distance between the poles which provides sufficient safety against flashover, but also other aspects, such as resistance to mechanical forces, may occur. In the case of hollow insulants, also the thickness of the material in relation to the length and the diameter is of importance. In gas-filled insulators, also the tightness of the material and in pole connections must be taken into consideration. The attachment flanges must be able to be connected tightly to the insulants and also withstand a certain overpressure of the enclosed insulating gas. A gas-filled insulator of the above-mentioned kind with one attachment flange at each end is previously known from US 4 827 373. A support insulator here supports an extinguishing chamber insulator comprising an interrupting element, whereby a hermetically sealed space, filled with SFβ gas, extends through the units. The insulant of both the support insulator and the extinguishing chamber insulator is in the form of a hollow, circular-cylindrical porcelain body, the outer surface of which is enamelled. The porcelain body exhibits a number of water-repellent grooves along the main part of its length. At each end the porcelain body has a smooth cylindrical portion, each of which being surrounded by an annular pole of electri- cally conducting material provided with an attachment flange. The support insulator and the extinguishing chamber insulator are attached to each other by a screw joint, a connection flange for connection to the network being placed between the insulators. The connection flange is first attached to the pole ring of one of the insulators, in which case good tightness against the leakage of gas must be ensured. After this, the insulator, with the connection flange mounted thereon, is attached to the pole ring of the second insulator. Also with this attachment, the tightness against leakage of gas must be ensured. In this connection, a problem with the known insulator is that, during mounting, two cut surfaces must be sealed and that the time of mounting may be considerable. To achieve sufficient safety against electric flashover, the distance between the poles must be made large. This distance is counted between two adjacent electrically conducting parts of the poles. In this case, these consist of the pole rings externally fixed around the ends of the porcelain body. The total overall height of the insulator is thus the safety dis¬ tance plus the height of each pole ring. In this way, the insulator will have a height which is larger than that which . is defined by the safety distance. A problem arises in that the insulant, to be able to resist mechanical transverse forces in relation to the overall height, must be dimensioned stronger than what would be necessitated by the transverse forces in relation to the safety distance. An additional gas-filled insulator is known from SU 1557590. This insulator comprises an insulant wound or cast onto two end poles. The poles are designed with radially projecting elevations which engage into radial slots formed in the insulant during the casting. By the overpressure in the insulator, the elevations are brought to be pressed against the sides of the slots such that a self-sealing joint is formed by way of a coating. A disadvantage with the known insulator is that, during casting of the insulant, a mould must be arranged on the inside of the insulator. This mould must either be left or be able to be removed in some other way afterwards. The possibilities of achieving an insulant with good insulating properties, which may be controlled after- wards, are thus not good. In case of outdoor mounting, an insulator must be provided with grooves to increase the leakage path and reduce the risk of flashover. A further disadvantage of the known insulator is that such an outer insulant, provided with grooves, must be manufactured in an additional process, which increases the cost and increases risk of the occurrence of air pockets which may jeopardize the safety. SUMMARY OF THE INVENTION The object of the invention is to achieve an insulator of the above-mentioned kind, which has a lower overall height for a given rated voltage and which entails a saving of material and time during mounting. The lower overall height permits the thickness of the material and the length of the insulant to be made smaller, thus reducing the production cost. These proper¬ ties are achieved according to the invention by means of an insulator which exhibits the features described in the characterizing part of the claims. According to the invention, a flanged annular pole is inserted and attached with its annular part on the inside of a hollow insulant. In this way the better insulating capacity, in relation to that of the external atmosphere, of the insulating gas enclosed in the insulator is utilized. The safety distance in relation to flashovers is smaller with an insulating gas between two poles than with air between the poles. By placing the annular parts of the poles on the inside of the gas-filled insulant, the distance between the poles can be reduced without jeopardizing the safety with respect to flashover. On the outside of the insulant, only the flanges of the poles are exposed, which flanges thus constitute the conducting parts between which the safety distance in air is to be defined. The saving in height may amount to 10 % of the overall height of traditionally designed insulators. By designing the attachment flanges so that they can be directly connected to each other, the flanged connection plate may be completely omitted. This results in only one cut sur¬ face which has to be sealed as well as in one joint with fewer parts. The attachment flanges are formed with plane attachment surfaces, in which an O-ring placed in a slot is arranged for sealing. The plane flanges may be provided with projecting portions or be arranged with an oval plane section to make possible connection into the network. BRIEF DESCRIPTION OF THE DRAWING The invention will be explained in greater detail by descrip¬ tion of an embodiment with reference to the accompanying drawing, wherein Figure 1 shows a section through two insulators according to the invention which are attached to each other, whereof the lower insulator is a support insulator and the upper is an extinguishing chamber insula- tor. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows the upper part of a lower support insulator comprising a tubular inner insulant 1 of, for example, glass fibre-reinforced epoxy resin, onto which is cast an outer insulant provided with grooves and being, for example, of silicone rubber. Into the upper part of the tubular inner insulant 1, an upper pole 3 of electrically conducting material is inserted. The pole 3 comprises an annular part 9, the outer diameter of which is smaller than the inner diameter of the inner insulant 1. The annular part 9 is fixed to the insulant 1 with an adhesive 6, such as glue or the like. The pole further comprises a flange 4, the outer portion 5 of which is rounded to avoid concentrations of the electric field. The mounting surface 7 of the flange is plane and has a slot 8, in which an O-ring (for sealing against an end sur¬ face) may be placed. The annular part 9 of the pole, facing inwardly towards the insulator, has rounded edges to avoid electric field concentrations. An extinguishing chamber insulator, comprising a circuit breaker and the lower part of which is shown in Figure 1, is mounted against the support insulator. The extinguishing chamber insulator comprises an inner insulant 1' of, for example, glass fibre-reinforced epoxy resin, onto which there is cast an outer insulant 2" of, for example, silicone rubber provided with grooves. Into the tubular inner insulant 1' , a lower pole 3' of electrically conducting material is inserted. The pole 3' comprises an annular part 9', the outer diameter of which is smaller than the inner diameter of the inner insulant 1' and which is fixed to the insulant 1' with an adhesive 6' such as glue or the like. Further, the pole comprises a flange 4' , the diameter of which is larger than that of the corresponding flange 4 of the support insulator and the outer portion 5' of which is rounded to avoid concen¬ trations of the electric field. The mounting surface 7' of the flange 4' is plane and provided with a circular slot 8' with a square cross section, in which is placed an 0 ring 10 for sealing between the flanges 4, 4' which are mounted against each other. The extinguishing chamber insulator is fixed to the support insulator by fixing their attachment flanges 4, 4' to each other with a screw joint 11. The screws are threaded in the upper flange 4' and, upon tightening, press the two attachment surfaces 7, 7' against each other. The O ring 10 located in the slot 8" is thereby subjected to an elastic deformation which causes it to seal against the two flanges. A lower current collector 12 is connected to the upper pole 3 ' with a screw joint 13 to be further connected to the network by means of a conductor (not shown) connected to the flange 4' . The insulators which are assembled together in the manner described above have a common internal space which is filled with an insulating gas, for example SF6- The safety distance with respect to flashover is therefore smaller on the inside of the insulator than on the outside thereof. The poles inser¬ ted into each end of the insulants make use of this effec . The distance between the inserted parts 9, 9' of the poles and the parts inserted into the other end of the respective insulant is shorter than the corresponding distance between the flanges 5, 5' exposed on the outside of the insulant. Since a shorter safety distance may be allowed on the inside of the insulator, the dimensioning safety distance is thereby defined between those outer edge portions of the flanges which are exposed on the outside at the two ends of the insulator. The effect thus utilized permits the insulant to be made shorter. A saving in length of up to 10 % may thus be achieved, which entails a corresponding saving in costs. The shorter length gives the insulator greater mechanical resistance to transverse forces. If, therefore, the same resistance to transverse forces is desired as with a longer insulator, the thickness of the material may be reduced. In this way a saving in costs of about 5 % may be utilized. The connection flange which is dispensed with by the invention constitutes a further saving of material and a reduction of the mounting time, which entails a saving in costs of about 5 %. In total terms, thus, a saving of about 20 % may be obtained when utilizing the invention. The invention is not only intended to be utilized for support insulators or extinguishing chamber insulators, but may arbi¬ trarily be used with all types of hollow insulators, both for high voltage and at a lower voltage. The poles inserted into the insulant may also be attached by means of a shrinkage fit.

Claims: CLAIMS 1. A gas-filled electric insulator for high voltage com¬ prising a tubular insulant (1) with a pole (3) which is gas- tightly attached at each end and which comprises a flange (4) and an annular part (9), characterized in that the annular part (9) has an outside diameter which is smaller than the inside diameter of the insulant (1) and that the annular part (9) is inserted into the insulant (1) such that only the flange (4) of the pole is exposed on the outside of the insu¬ lator. 2. An insulator according to claim 1, characterized in that the insulant (1) comprises a body of porcelain. 3. An insulator according to claim 1 or 2, characterized in that the insulant comprises a glass fibre-reinforced epoxy tube (1) with an external casing (2), provided with grooves, of silicone rubber. 4. An insulator according to any of the preceding claims, characterized in that the flanges (4) of the poles are plane in a plane normal to the longitudinal axis of the insulator and that they comprise means for fixing (11) and sealing (8) . 5. An insulator according to any of the preceding claims, characterized in that it is filled with SFβ gas. 6. An insulator according to any of the preceding claims, characterized in that the flanges (4) of the poles are adapted to be electrically connected to a high-voltage net¬ work. 7. A method for manufacturing a gas-filled electric insulator for high voltage comprising a tubular insulant (1) with a pole (3) attached at each end, characterized in that the pole is designed with an annular part (9) which is adapted to be inserted into the insulant such that only the flange (4) of the pole is exposed on the outside, thus utilizing the larger safety distance with respect to flashover which is permitted inside the gas-filled insulator to reduce the overall height of the insulator.

Inventors: BENDIKSEN ODD, WUTHRICH PETER, BENDIKSEN, ODD, WUTHRICH, PETER
Applicants: ASEA BROWN BOVERI, BENDIKSEN ODD, WUTHRICH PETER, ASEA BROWN BOVERI AB, BENDIKSEN, ODD, WUTHRICH, PETER
Application year: 1996
Main classification: H01B17
IPC codes: H01B17