Resin impregnated electrical bushing

This application claims priority from Indian Patent Application no 797/MUM/2010 filed on 23 ^rd March 2010.

FIELD OF THE INVENTION

This invention relates to a resin impregnated electrical bushing, BACKGROUND OF THE INVENTION

Electrical bushings find applications in electrical devices or apparatuses like transformers, circuit breakers or gas insulated switchgears. Electrical bushings are very essential and critical components to facilitate or establish electrical connectivity between outside supply and internal parts of electrical apparatuses or devices and to avoid electrical break down with the enclosures of the apparatuses or devices. An electrical bushing comprises a wound structure formed by winding electrical grade crepe paper or craft paper on a central conductor rod with conductor foils inserted at regular intervals radially within the paper insulation. The wound structure is then impregnated with an electric grade oil or with an electric grade epoxy resin. The former is known as oil impregnated or OIP bushing and the latter is known as resin impregnated paper or RIP bushing. The conductor rod is hollow in the case of the RIP bushing. RIP bushings are dry and explosion proof and have high reliability besides being pollution-free. RIP bushings are especially suitable for outdoor applications and in areas prone to fire.

A main problem with RIP bushings is cross-sectional delamination between the current carrying central conductor (which is metallic) and resin impregnated solid mass of paper. Delamination occurs mainly because of metal and resin body interface offering poor coefficient of adhesion due to metal and resin body having different temperature coeficients of expansion. Delamination becomes more during working of the bushings as an effect of temperature cycles and different expansion and contraction rates of the metallic current carrying conductor and resin impregnated mass of paper. Due to delamination, separation of the current carrying conductor and resin impregnated paper body takes place. The current carrying conductor may get completely separated and slip down from the solid mass of impregnated paper causing failure of the electrical bushing and associated electrical apparatus or device. As a result, the reliability and life of the electrical bushing are reduced. In order to prevent delamination it is known to provide roughened surface or scratchy discrete surface patterns on the current carrying conductor by complex machining operations. Such matching operations are time consuming and costly. Further the roughened surfaces ' or scratchy discrete surface patterns on the current carrying conductor are also not very effective against delamination. Therefore, the reliability and life of electrical bushing comprising roughened surfaces or scratchy surface patterns on the current carrying conductor are also reduced.

OBJECTS OF THE INVENTION

An object of the invention is to provide a resin impregnated electrical bushing, which is simple in construction and has increased reliability and life. Another object of the invention is to provide a resin impregnated electrical bushing, which is cost effective.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention there is provided a resin impregnated electrical bushing comprising a wound structure formed by winding electrical grade crepe paper or craft paper on a hollow central conductor rod with conductor foils inserted at regular intervals radially within the paper insulation and impregnated with an electric grade epoxy resin, wherein the central conductor rod comprises a plurality of circumferential grooves along the outer surface thereof in radially spaced apart relationship having the impregnating resin penetrating into the grooves to provide a mechanical interlock between the resin impregnated solid mass of paper and the central conductor rod and to prevent delamination between the resin impregnated solid mass of paper and the central conductor. The following is a detailed description of the invention with reference to the accompanying drawings, in which:

Fig 1 is a crossectional view of the resin impregnated electrical bushing according to an embodiment of the invention; and

Fig 2 is a crossectional view of the central conductor rod of the electrical bushing of Fig 1 ;

The electrical bushing 1 as illustrated in Fig 1 of the accompanying drawings comprises a wound structure 2 formed by winding electrical grade crepe paper or craft paper 3 wound on a hollow central conductor rod 4 with conductor foils 5 inserted at intervals readilaly within the paper insulation. The wound structure is impregnated with an electrical grade epoxy resin 6. The conductor is preferably made of aluminium and the conductor foils are preferably aluminium foils. The conductor as shown in Fig 1 and also in Fig 2 of the accompanying drawing comprises a plurality of circumferential grooves 7 along the outer circumference thereof in spaced apart relationship. Preferably the depth of the grooves is 0.5 to 1.5mm. During impregnation of the wound structure with the resin, the resin penetrates into the grooves on the outer surface of the conductor and forms a mechanical interlock between the resin impregnated solid mass of paper and the conductor. As a result, delamination and disengagement between the conductor and the resin impregnated solid mass of paper during working of the bushing due to temperature cycles and different expansion and contraction rates of the conductor and resin impregnated solid mass of paper and vertical slip off of the conductor from the resin impregnated paper body are prevented. The engagement between the conductor and the resin impregnated paper body remains unaffected to a great extent as the depth of the grooves is higher than the dimensional changes experienced by the conductor and resin impregnated body due to temperature cycles and different expansion and contraction rates of the conductor and resin impregnated mass of paper during working of the bushing. As a result, the electrical bushing of the invention is reliable and has increased life. The grooves can be easily provided on the conductor with a simple parting tool. Therefore, the bushing of the invention is simple in construction and is very cost effective.