The present invention relates to a method of producing an insulating core body of a high voltage resistant device, e.g. a bushing, a measurement transformer, a switching device, a breaker or the like, the core body being produced by winding a web of insulating material, such as paper, onto a mandrel and impregnating the wound web material with a curable resin in a mould.
There are many types of high voltage resistant devices for outdoor and/or indoor use. As an example, in a bushing, a high voltage current is passed through a partition, such as a wall or a tank, via a central conductor extending inside a hollow insulating core body. The latter is normally produced by winding a strip of paper onto a mandrel and impregnating the wound paper with a curable resin in a mould.
Consequently, for each kind of bushing, and for each particular dimension, a specific set of moulds have to be prepared and stored. Since there are many kinds and many different dimensions, a great number of moulds have to be procured, which is costly and time consuming.
The object of the present invention is to drastically reduce the number of different moulds and to provide a flexible system enabling the production of a great variety of insulating core bodies for each type of high voltage resistant device.
This object is achieved in that at least a part of the mould is formed by an adhesive, flexible tape which is wound externally onto the wound web material so as to form a mould shell and which is resistant and impermeable to the resin. The tape will form at least one tight layer which encloses the insulating core body so as to function as a mould shell when the core body is impregnated with a curable resin. Of course, the dimensions and the geometrical configuration of such a mould formed by a flexible tape can be adjusted at will in accordance with the specific conditions and requirements. The mould shell can be combined with other mould parts, e.g. in combination with an external tubular member which is to form an integral part of the finished device.
According to a further aspect of the invention, two kinds of material can be used to form the mould shell or a part thereof. For example, a first kind of tape may be wound directly onto the insulating core body to form an inner tight layer and a second kind, to be used several times in the production, is then wound externally to form a strong and rigid outer layer of the mould shell. As a modification, the inner layer may be formed by a flexible hose which is slipped onto the core body, the outer layer being formed by a tape which is wound onto the hose.
Upon curing of the resin, the mould shell is preferably removed mechanically, e.g. by a machining operation, such as by turning.
These and other advantageous features are stated in the dependant claims.
The invention will now be described more fully with reference to the appended drawings illustrating three embodiments.
The bushing shown in figure 1 comprises an insulating, hollow core body 1 and a central conductor 2 extending along the axis of the bushing. At the axial end portions, there are top and bottom end walls 3 and 4, respectively, serving as parts of a mould during production of the bushing.
The insulating core body 1 is produced by winding a strip of paper onto a mandrel, formed by the central conductor 2 in the illustrated embodiment, and impregnating the wound paper with a curable resin in a mould. In accordance with the invention, the mould is partly formed by an adhesive, flexible tape 5 which is wound externally onto the core body 1 and which is resistant and impermeable to the resin. Preferably, the tape has a large width, e.g. 20 cm or even wider, and is wound with overlap in a helical configuration so as to form at least one tight layer. The tape may alternatively be as wide as the length of the core body. Then, it is sufficient to wind it around the core body in one lap.
Generally, the tape comprises a flexible foil material and an adhesive coating on at least one side thereof. The foil material may be paper or a thermoplastic material, such as polypropylene or polyester, whereas the adhesive coating may be a silicone-based or acrylic-based glue or natural rubber.
In order to achieve sufficient strength and rigidity of the mould shell, it is possible to apply an outer flexible strip externally onto the adhesive tape 5. Such a strip 5a may be constituted by a stronger and more expensive adhesive tape or a foil material without adhesive properties, such as a thermoplastic foil material, possibly reinforced by fibers to make it stronger and less elastic but yet flexible. The outer flexible strip 5a is wound externally with overlap and can be removed and used over again several times in the production.
Upon forming the mould shell constituted by the adhesive tape 5 and possibly the external layer 5a, a curable resin, such as an epoxy resin with a curing agent, is fed through a flexible hose 6 from a funnel 7 to a nozzle 8 in the lower mould part 4, whereby the paper core 1 is slowly impregnated in its entirety. Excess resin will flow out through an outlet channel 9 in the upper mould part 3 to an outlet collecting funnel 10. When the epoxy resin has cured, typically in about ten hours, the mould parts 3, 4 and 5a are removed upon detaching upper and lower hose clamps (not shown) having being used to securely fix the upper and lower ends of the mould shell 5, 5a.
The adhesive tape 5, which is firmly bound to the insulating core 1, is preferably removed by a machining operation, such as by turning.
The embodiment shown in figures 3 and 4 is quite similar to the one shown in figures 1 and 2, the only difference being that the upper part of the insulating core 1 is enclosed by a prefabricated tube member 11 of elastomeric material, which forms a permanent integral part of the finished bushing. The tube member 11 is used as a part of the external mould during the impregnating and curing operation. Thus, the mould shell formed by the adhesive tape 5 and a possible outer layer 5a is only applied onto the lower part of the insulating core 1.
In fig. 5 there is shown another embodiment, viz a measurement transformer for measuring a current flowing therethrough. The transformer includes a central conductive rod 2', having terminal end portions 2'a, 2'b for external connection, and a tubular insulating core body 1' which has been produced substantially in the same way as the bushing shown in figs. 1 and 2. The outside surface 5' has been machined so as to remove the mould shell. A transformer ring core 20, with inductive coils (not shown) and an annular insulating casing 21, is arranged so as to enclose the core body 1'.
The method and the bushing described above may be modified by those skilled in the art within the scope of the appended claims. For example, the insulating core body may be formed by a different material than paper, such as a plastic foil, preferably treated so as to form voids or channels upon being wound onto a mandrel. The resin may be fed into the core body directly from a moulding apparatus, if desired under pressure so as to accelerate the impregnating process. The mould shell may include two or more flexible layers applied to the core body, at least one such layer being formed by the flexible tape. Also, one of the layers, in particular an inner layer, may be formed by a flexible tube or hose of a thin thermoplastic material, the tube or hose being slipped onto the core body.