GB1602970A - Three phase transformers - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THREE PHASE TRANSFORMERS (71) We, THE ENGLISH ELECTRIC COMPANY LIMITED, of 1, Stanhope Gate, London W1A 1EH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to three phase transformers of the so called dry type having one or more windings encapsulated in solid insulation, usually a thermosetting resin, and relates also to the manufacture of encapsulated windings for such apparatus.

It is known to encapsulate a high voltage winding of a dry type power distribution transformer by locating the coils which form the winding in an annular cylindrical mould, for example of stainless steel, introducing a thermosetting resin into the mould to impregnate and enclose the coils and, following the curing of the resin, removing the completed winding assembly from the mould.

This is satisfactory for producing large numbers of standard sized winding assemblies, but in view of the cost of the moulds it is uneconomic where only a small number of assemblies of a particular size are required.

According to the invention, a high voltage winding of a three phase power distribution transformer is encapsulated by locating the coil or coils forming said winding in the annular space between two concentrically supported tubular mould members, introducing a solidifiable insulating material in liquid form into said space to cause it to impregnate and enclose the coil or coils, and allowing or causing the material to solidify, the material being bondable to the mould members, is such that the members form part of the completed winding assembly.

The insulating material is preferably a thermosetting resin, for example an epoxy resin, and is introduced into the annular space between the mould members after reducing the pressure in said space to a low value, for example less than 0.1 mm Hg, the external pressure subsequently being increased to facilitate the impregnation of the coil or coils, and the assembly finally being baked to harden the resin.

The mould members are conveniently formed from glass fibres or woven glass fabric wound into the required tubular form and bonded by a thermosetting resin, the glass and the bonding resin having a coefficient of thermal expansion closely matching that of the encapsulating material. The thickness of the mould members can readily be chosen to provide the required strength for any particular application of the invention.

However the mould members which are preferably of cylindrical shape can be formed of any other suitable material.

The winding may comprise a plurality of generally flat coils, stacked one upon the other in the space between the two mould members with layers of insulating material conveniently in the form of glass fibre mats, interposed between adjacent coils, the coils conveniently being formed from wound strips of aluminium or other metal foil, with individual turns separated by insulating strip material, for example polyethylene terephthalate, or other suitable insulating material.

The invention includes within its scope winding assemblies for inductive apparatus encapsulated as above described, and to inductive apparatus such as transformers and reactors incorporating such winding assemblies.

The invention will be further explained by describing by way of example, with reference to Figures 1 to 3 of the accompanying schematic drawings, a dry type distribution transformer having high voltage winding assemblies constructed in accordance with the invention.

In the drawings Figure 1 illustrates the complete transformer in part section, and Figure 2 and 3 show two stages in the manufacture of a high voltage winding for the transformer.

The transformer illustrated in Figure 1 is a three phase distribution transformer comprising, in known manner, three vertical core limbs 1 of laminated steel sheets connected at the top and bottom by horizontal yokes 2,3, and each surrounded by an inner low voltage winding 4, and an outer high voltage winding 5, the two windings being supported coaxially with respect to each other on the respective core limb in any convenient way, and the whole assembly being secured in a frame 10.

Each high voltage winding 5 comprises a stack of pancake type coils 6 of annular form, separated by pads 7 of glass fibre, and located in the space between two concentric hollow cylinders 8, 9. This space is filled with epoxy resin enclosing and impregnating the stacked assembly and bonded to the cylinders 8, 9. Each cylinder is conveniently formed by winding glass fibres, coated with liquid epoxy resin, around a cylindrical mandrel of appropriate diameter to build up a cylinder of the required thickness, for example 2 to 3 mm and hardening the resin.

Each coil 6 consists of aluminium foil strip 11 wound to provide an annulus of the required dimensions, with adjacent turns interleaved with a thin strip 12 of polyethylene terephthalate or other suitable insulating material of slightly greater width. The coiled aluminium strips 11 in each stack are connected in series, and outer ends of the upper and lower coils are connected to terminals 13 attached to the side of the outer cylinders 9.

Tapping points can also be connected to respective terminals 16 carried by housings 14 contained within hollow projections 15.

The encapsulation of the windings is carried out as described with reference to Figures 2 and 3.

Thus in carrying out this process the inner cylinder 8 is initially mounted on a metal base plate 17 with its lower end in a respective locating groove 18, and is bonded to the plate with a temporary sealing compound 19 such as silicone RTV. An annular glass fibre mat 20 is then placed on the base plate 17 around the cylinder, and on this is built up the stack of coils 6 with intervening pads 7 of glass fibre, connections being made between corresponding ends of adjacent coils as the stack is formed, and a further glass fibre pad 21 being placed on the upper surface of the top coil, as shown in Figure 2.

The outer cylinder 9, with the terminal projection 15 bonded to the side thereof around an opening in the cylinder wall, is then slid -over the assembly, and positioned in a further locating groove 23 in the base plate so as to be coaxial with the inner cylinder 8, the outer cylinder 9 being similarly bonded to the plate with a temporary sealing compound as at 24 (see Figure 3). As the cylinder 9 is placed over the assembly, conductors 25 which are connected to the upper and lower coils 6 are threaded through the terminal projection 15 and ccnnected to the respective terminals 13. Further conductors 26 connected to various tapping points are similarly threaded through the terminal projection 15 and connected to respective terminals 16, the various conductors being separated by further pads of glass fibre (not shown).The terminal housing 14 is then fitted into the projection 15 and sealed in position.

The complete assembly is then heated to approximately 90"C for some hours and placed in an autoclave in which the pressure is reduced to about 0.065 mm Hg.

An epoxy resin for encapsulating the winding is then degassed and allowed to be drawn by the reduced pressure in the autoclave into the annulus above the uppermost glass fibre pad 21 of the winding assembly, the pressure within the autoclave being then gradually increased to produce a thorough impregnation of the assembly. The impregnated winding assembly is then heated to cure the resin 27 and cause it to bond to the cylinders 8,9 and to the terminal housings 14, the completed assembly being finally removed from the base plate 17.

The three high voltage winding assemblies, all constructed in the same way, can then be fitted around the low voltage windings 4 of the transformer and held in position in any convenient manner, the assembly of the transformer being completed by clamping the various elements, with the surrounding frame.

Each low voltage winding can also be formed from aluminium foil in the form of a single strip of appropriate width wound into a coil with adjacent turns of the coil separated by a thin strip of insulating material such as polyethylene teraphthalate, or glass fibre fabric or other suitable insulating material which may or may not be preimpregnated with thermosetting resin, and in some cases the coil may also be encapsulated by utilising inner and outer cylinders arranged to be bonded to the encapsulating medium in a similar manner to those of the high voltage winding.

The cylinders 8, 9 used for forming the high voltage winding assemblies are conveniently formed by winding glass fibres coated with liquid epoxy resin on to the mandrels with a winding angle of about 75o, and may be formed by winding into longer tubes and then cutting to the required lengths.

Instead of being formed by the winding of glass fibres the woven glass cloth could alternatively be used, as well as other materi als capable of being bonded by thermosetting resin.

WHAT WE CLAIM IS: 1. A high voltage winding of a power distribution three phase transformer encapsulated by locating the coil or coils forming said winding in the annular space between two concentrically supported tubular mould members, introducing a solidifiable insulating material in liquid form into said space to cause it to impregnate and enclose the coil or coils, and allowing or causing the material to solidify, the material being bondable to the mould members, is such that the members form part of the completed winding assembly.

2. A winding according to Claim 1 wherein the insulating material is a thermosetting resign.

3. A winding according to Claim 2 wherein the insulating material is an epoxy resign.

4. A winding according to Claim 2 or 3 wherein during encapsulation the insulating material is introduced into the annular space between the mould members after reducing the pressure in said space to a low value, the pressure is subsequently increased to facilitate the impregnation of the coil or coils, and the assembly is finally baked to harden the resign.

5. A winding according to any preceding claim wherein the mould members are formed from glass fibres or woven glass fabric wound into tubular form and bonded by a thermosetting resin.

6. A winding according to any preceding claim comprising a plurality of generally flat coils, stacked one upon the other in the space between the two mould members with layers of insulating material interposed between adjacent coils.

7. A winding according to Claim 6 wherein the coils are formed from wound strips of aluminium or other metal foil, with individual turns separated by insulating strip material.

8. A three phase power distribution transformer having an encapsulated high voltage winding substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. als capable of being bonded by thermosetting resin. WHAT WE CLAIM IS:

1. A high voltage winding of a power distribution three phase transformer encapsulated by locating the coil or coils forming said winding in the annular space between two concentrically supported tubular mould members, introducing a solidifiable insulating material in liquid form into said space to cause it to impregnate and enclose the coil or coils, and allowing or causing the material to solidify, the material being bondable to the mould members, is such that the members form part of the completed winding assembly.

2. A winding according to Claim 1 wherein the insulating material is a thermosetting resign.

3. A winding according to Claim 2 wherein the insulating material is an epoxy resign.

4. A winding according to Claim 2 or 3 wherein during encapsulation the insulating material is introduced into the annular space between the mould members after reducing the pressure in said space to a low value, the pressure is subsequently increased to facilitate the impregnation of the coil or coils, and the assembly is finally baked to harden the resign.

5. A winding according to any preceding claim wherein the mould members are formed from glass fibres or woven glass fabric wound into tubular form and bonded by a thermosetting resin.

6. A winding according to any preceding claim comprising a plurality of generally flat coils, stacked one upon the other in the space between the two mould members with layers of insulating material interposed between adjacent coils.

7. A winding according to Claim 6 wherein the coils are formed from wound strips of aluminium or other metal foil, with individual turns separated by insulating strip material.

8. A three phase power distribution transformer having an encapsulated high voltage winding substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawing.