GB2311418A

GB2311418A - Conductors mounted in electrical insulation

Info

Publication number: GB2311418A
Application number: GB9605723A
Authority: GB
Inventors: David Arnold Peck
Original assignee: HAWKER SIDDELEY POWER TRANSFOR
Current assignee: HAWKER SIDDELEY POWER TRANSFOR
Priority date: 1996-03-19
Filing date: 1996-03-19
Publication date: 1997-09-24
Anticipated expiration: 2016-03-19
Also published as: WO1997035327A1; EP0891625A1; JP2000507030A; GB2311418B; GB9605723D0

Abstract

A high voltage apparatus 13 comprises an insulating support 14 made of a moulded first electrically insulating material and carrying rigid electrical conductors 11, 12 and a homogenous second electrically insulating material 16, 17 at least partially surrounding the conductors. The second insulating material may be epoxy resin or polyester filled with silica, and may cast around the conductors or provided as a preformed sleeve. The first insulating material may be dough moulding compound such as a filled thermosetting polyester or epoxy composition, and is moulded around the sleeved conductors. The apparatus may be a transformer, a tapchanger or switchgear.

Description

Electrical Apparatus Having Electrical Conductors Mounted in Electrical Insulation This invention relates to electrical apparatus having rigid electric conductors mounted in electrical insulation and to a method of manufacturing such electrical apparatus.

In particular, but not exclusively, the invention relates to high voltage apparatus, such as tapchangers, transformers or switchgear, in which the electrical conductors comprise electrodes, tappings or the like.

In the manufacture of high voltage tapchangers it is known to fix tappings in a preformed electrically insulating support, e.g. of epoxy resin or the like. Such known epoxy resin insulating supports are required to provide a good standard of electrical insulation at the high operating voltages experienced in use and, as such, are relatively expensive to manufacture.

It is known that so-called dough moulding compounds (DMC), often referred to as "bulk moulding compounds", provide good, relatively cheap electrical insulation for low voltage applications. With such DMCs, rigid electrical conductors can be held in desired positions while the DMC is heated and moulded around the conductors in compression heated dies to produce electrical apparatus in which the electrical conductors are mounted in a moulded, electrically insulating support. However, up until now such electrical apparatus incorporating DMC as the electrically insulating material has proved to be unsatisfactory for high voltage applications since the insulation tends to break down when a high voltage potential is applied between the conductors mounted in the insulation. It is believed that this break down in the electrical insulation is due to the nonhomogeneous nature of the electrically insulating DMC where "flow lines" are created during formation of the insulation by semi-cured skins of material from the heated moulds meeting and creating non-homogeneous joins. This type of join or "flow line" greatly weakens the electrical "withstand" strength between the electrical conductors or electrodes causing internal tracking or puncture between them.

An aim of the present invention is to enable the use of mouldable electrically insulating material in high voltage electrical grade apparatus.

According to one aspect of the present invention high voltage electrical apparatus comprising an insulating support made of a moulded first electrically insulating material and carrying rigid electrical conductors electrically insulated from each other, is characterised in that each rigid electrical conductor is at least partly surrounded with a homogeneous second electrically insulating material.

Conveniently the first electrically insulating material is moulded from a dough moulding compound, e.g. a thermosetting composite moulding material such as a polyester moulding compound or the like.

Suitably the second electrically insulating material comprises an epoxy or polyester material with filler material, e.g silica. Preferably the second insulating material will be the same insulating material for each rigid electrical conductor but this is not essential. However it is important that the second insulating material should be homogeneous and have no "flow line" faults. Typically it will be formed around the electrical conductors by vacuum pouring. Alternatively, however, it may, for example, be cast over a relatively long period of time to ensure that it sets as a homogeneous mass with no internal voids or fault lines.

Preferably the second electrically insulating material for each rigid electrical conductor comprises a sleeve surrounding the conductor and fixed thereto.

Preferably each sleeve is cast, e.g. vacuum or gravity cast, onto its associated electrical conductor.

According to another aspect of the present invention a method of manufacturing a high voltage electrical apparatus comprising arranging rigid electrical conductors in an insulating support made of a moulded first electrically insulating material, is characterised in that each rigid electrical conductor is initially at least partly surrounded by a homogeneous screen of second electrically insulating material and thereafter the first electrically insulating material is moulded around the previously insulated electrical conductors.

Conveniently the second electrically insulating material is cast, e.g. vacuum or gravity cast, about the rigid electrical conductors. However, alternatively, the second electrically insulating material may comprise, for each rigid electrical conductor, a pre-formed homogeneous sleeve which is connected, e.g. via screw-threads, in an interference fit or even with adhesive, to its associated electrical conductor.

The first electrically insulating material preferably comprises a dough moulding compound such as a thermosetting polyester moulding compound, but other mouldable electrically insulating materials may be used. Dough moulding compound is moulded in moulds to provide the desired shape of insulating support and these are heated to enable curing of the dough moulding compound. Since the dough moulding compound attains temperatures typically of about 1300C during this curing process, the second electrically insulating material must be able to withstand such temperatures without melting.

Embodiments of the invention will now be described, by way of example only, with particular reference to the accompanying drawing, in which: Figure 1 is a schematic sectional view of part of a known high voltage electrical tapchanger; and Figure 2 is a schematic sectional view of part of a high voltage electrical tapchanger made in accordance with the present invention.

Figure 1 shows two tappings or electrodes 1 and 2 of a known high voltage electrical tapchanger, generally designated by the reference numeral 3, which are securely mounted in an electrically insulating support 4. Heretofore the support 4 has been made of a high grade insulating material, e.g. epoxy resin or the like, for high voltage electrical grade apparatus to ensure that breakdown in the insulating properties of the support does not occur when high voltage potential are applied between the electrodes 1 and 2.

It has been proposed to use a cheaper dough moulding compound for the insulating material of the support 4.

However attempts to use dough moulding compounds in electrical grade apparatus have not proved successful up until now. In particular, if the support 4 is made from such a dough moulding compound flow lines 5 are likely to be created in the support. This is because of the nonhomogeneous nature of the insulating material in which the flow lines 5 are created by semi-cured skins of material from heated moulds containing the dough moulding compound meeting and creating non-homogeneous joins. The joins or flow lines 5 greatly weaken the electrical "withstand' strength between the electrodes 1 and 2 causing internal "tracking" or "puncture" in the insulation between the electrodes 1 and 2.

Figure 2 illustrates how the apparatus shown in Figure 1 can be modified in accordance with the principles of the present invention. In particular, Figure 2 shows a modified tapchanger 13 having two electrodes 11 and 12 securely mounted in an electrically insulating support 14 moulded from a heat cured, dough moulding compound, such as a thermo-setting composite moulding material. Prior to moulding the electrodes 11 and 12 in the support 14, sleeves 16 and 17 of electrical grade, electrically insulating material, e.g. epoxy material or polyester material with filler material such as silica, are cast, e.g. vacuum cast or gravity cast, around the electrodes 11 and 12, respectively, to form homogeneous cast sleeves around the electrodes. The electrically insulating sleeves 16 and 17 are thus securely retained on the electrodes 11 and 12, respectively. The electrode/sleeve assemblies 11/16 and 12/17 are then moulded in their correct locations into the support 4. This is achieved by heating the dough moulding compound, which is retained in a compression mould (not shown), to an elevated temperature of about 1300C and at an elevated pressure, e.g. from 17 to 140 bar, so that the compound flows around the electrode/sleeve assemblies 11/16 and 12/17. On curing,the dough moulding compound provides a relatively cheap, rigid electrically insulating support 4 for the electrodes 11 and 12. Although flow lines 15 may be created in the support 4 during curing of the dough moulding compound, the presence of the homogeneous insulating sleeves 16 and 17 containing no fault line or internal voids around the electrodes 11 and 12 guarantees a relatively high electrical withstand level.

The dough moulding compound conveniently comprises a mixture of unsaturated polyester resin or epoxy resin, chopped strands of glass fibre reinforcement, mineral, e.g.

silica, fillers, and preferably a shrinkage control additive and a hot press cure additive. A suitable polyester moulding compound is available from BIP Chemical Limited under the trade name "Beetle". Other mouldable compounds can be employed instead.

Although the sleeves 16 and 17 have been described as being vacuum or gravity cast around the electrodes 11 and 12, respectively, other means of providing a sleeve/electrode connection may be employed. For example mechanical screw or interference connections may be used or, alternatively, the sleeves may be adhesively bonded to the electrodes. Adequate electrical insulation may also be provided if only part of the electrodes (i.e. the part or parts facing an adjacent electrode or electrodes) are surrounded or covered with electrical grade insulating material.

In practice the sleeves 16 and 17 will generally be made of the same electrical grade insulating material although this is not essential. The material of the sleeve must, however, be able to resist without melting the curing temperatures experienced during moulding of the support 14 and must result in a homogeneous sleeve.

Claims

1. A high voltage electrical apparatus comprising an insulating support made of a moulded first electrically insulating material and carrying rigid electrical conductors electrically insulated from each other, characterised in that each rigid electrical conductor is at least partly surrounded with a homogeneous second electrically insulating material.

2. An electrical apparatus according to claim 1, characterised in that the first electrically insulating material is moulded from a dough moulding compound.

3. An electrical apparatus according to claim 2, characterised in that the dough moulding compound comprises a thermosetting composite moulding material.

4. An electrical apparatus according to any one of the preceding claims, characterised in that the second electrically insulating material comprises an epoxy or polyester material with filler material.

5. An electrical apparatus according to any of the preceding claims, characterised in that the second insulating material is the same insulating material for each rigid electrical conductor.

6. An electrical apparatus according to any one of the preceding claims, characterised in that the second electically insulating material for each rigid electrical conductor comprises a sleeve surrounding the conductor and fixed thereto.

7. An electrical apparatus according to claim 6, characterised in that each sleeve is cast onto its associated electrical conductor.

8. A method of manufacturing a high voltage electrical apparatus comprising arranging rigid electrical conductors in an insulating support made of a moulded first electrically insulating material, characterised in that each rigid electrical conductor is initially at least partly surrounded by a homogeneous screen of second electrically insulating material and thereafter the first electrically insulating material is moulded around the previously insulated electrical conductors.

9. A method according to claim 8, characterised in that the second electrically insulating material is cast about the rigid electrical conductors.

10. A method according to claim 8, characterised in that the second electrically insulating material comprises, for each rigid electrical conductor, a pre-formed homogeneous sleeve connected to its associated electrical conductor.

11. A method according to any one of claims 8 to 11, characterised in that the first electrically insulating material comprises a dough moulding compound such as a thermosetting polyester moulding compound.

12. A high voltage electrical apparatus constructed and arranged substantially as herein described with reference to, and as illustrated in, Figure 2 of the accompanying drawing.

13. A method of manufacturing a high voltage electrical apparatus substantially as herein described with reference to, and as illustrated in, Figure 2 of the accompanying drawing.