WO1999029026A1

WO1999029026A1 - A method in electric machines

Info

Publication number: WO1999029026A1
Application number: PCT/SE1998/002165
Authority: WO
Inventors: Mats Leijon
Original assignee: Abb Ab
Priority date: 1997-11-27
Filing date: 1998-11-27
Publication date: 1999-06-10
Also published as: SE9704382L; SE9704382D0; AU1516599A

Abstract

The present invention relates to a method of repairing a magnetizable core of an electric machine for high voltage, which core is provided with a winding and where the core has a damaged area (5). The method is characterized in that a repair portion (10) of the core, which includes the damaged area (5), is separated and removed, and in that the repair portion (10) is replaced with a replacement part (11, 14). The invention also relates to a replacement part (11, 14, 17, 25) which is manufactured and installed in the core in accordance with the method and an electric machine for high voltage, the core of which has been repaired in accordance with the method.

Description

A METHOD IN ELECTRIC MACHINES

The present invention relates to a method of repairing the magnetizable core of an electric machine for high voltage in accordance with the preamble to claim 1.

The invention also relates to a replacement part for use when repairing the magnetizable core of an electric machine for high voltage in accordance with the preamble to claim 44, and to a rotating electric machine in accordance with claim 45. The expression "magnetizable core" in this context refers both to the sta- tor core of a rotating electric machine and to the core of a power transformer or reactor.

The rotating electric machines referred to in this context comprise synchronous machines used primarily as generators for connection to distribution and transmission networks, generally known as power networks. The synchronous machines are also used as motors and for phase compensation and voltage control, and in that case as mechanically open-circuited machines. This technical area also includes normal asynchronous machines, double-fed machines, alternating current machines, asynchronous static current converter cascades, external pole machines and synchronous flux machines. These machines are intended for use at high voltages, by which is meant here electric voltages in excess of 10 kV. A typical operating range for such a rotating machine may be 36 to 800 kV, preferably 72.5 - 800 kV.

According to prior art, rotating electric machines have conventionally been designed for voltages in the range 6-30 kV, and 30 kV has normally been considered to be an upper limit. This usually means that a generator must be connected to the power network via a transformer which steps up the voltage to the level of the network, which is in the range of from approximately 70 kV and above.

Various attempts have been made over the years to develop especially synchronous machines, preferably generators, for higher voltages. Examples of this are described, for instance, in Electrical World, 15 October 1932, pages 524-525, the article entitled "Water-and-Oil-cooled Turbogenerator TVM-300" in J. Elektrotechnika, No. 1 , 1970, pages 6-8 and the patent publications US 4,429,244 and SU 955,369. However, none of these attempts has been successful, nor have they led to any commercially available product.

Transformers are available within all power ranges from W up to around 1000 MW. However, the term power transformers usually refers to transformers with a rated power from some 100 kW up to over 1000 MW, and with rated voltage from 3-4 kV up to extremely high transmission voltages.

Conventional power transformers in the lower part of the above- mentioned power range are sometimes provided with air cooling for the removal of the unavoidable losses in the form of heat. However, most conventional power transformers use oil cooling, generally forced oil cooling. This applies particularly to high-power transformers. Oil-cooled transformers have a number of drawbacks which are well known. They are, for instance, large, ungainly and heavy, thus entailing considerable transport problems. They also place extensive demands as regards safety and peripherals, an important demand being an outer tank in which the transformer must be enclosed when oil-cooling is used. Large reactors are also oil-cooled.

In conventional types of rotating electric machines the stator body often consists of a welded steel plate construction. In large machines the stator core, also known as the laminated core, is normally made of so called electric sheet, preferably 0.35-0.50 mm, divided into stacks having an axial length of approximately 50 mm, separated from each other by spacers forming ventilation ducts 5 to 15 mm wide. In large machines each laminated stack is formed by placing sheet segments, punched to a suitable size, together to form a first layer. The sheet segments in each subsequent layer are placed with overlap on the segments in the preceding layer. The laminated part of a stator core has been formed when all the layers, possibly with ventilation ducts between them, have been placed. The parts and spacers are held together with existing axial clamping means in the form of pressure brackets pressed by means of pressure devices against pressure rings, fingers or segments. The stator winding is arranged in slots in the stator core. A conventional power transformer comprises a transformer core, in the following referred to as a core, of laminated, oriented sheets, usually of ferrosili- con. The core consists of a number of core legs joined by a yoke. Around the core legs are a number of windings, generally designated primary, secondary and con- trol windings. In the case of power transformers these windings are practically always arranged concentrically and distributed along the core legs.

With regard to reactors, they are not intended for transferring power between the windings, but to create an inductance. Otherwise what has been stated above concerning transformers is substantially also relevant to reactors. A considerable number of all breakdowns occurring in electric machines of the above-mentioned type are localised to the magnetizable (or magnetic) core and are caused in most cases by electrical faults occurring in the winding. Generators will be primarily discussed in the following, but the equivalent is also applicable to other rotating electric machines as well as to transformers and reactors. By far the majority of all breakdowns occurring in generators are localised to the stator and thus primarily to the stator winding. An electrical fault, usually an earthing fault, that appears in the stator winding results not only in the winding being destroyed and having to be replaced, but also often to the stator core being damaged in the area concerned. With voltages of up to a few tens of kilovolt, the damage will be relatively slight, e.g. in the form of a small hole in the core, and in accordance with known technology it is customary to be satisfied with grinding and polishing in and around this hole and, if necessary, making sure that the sheets are not in contact with each other if they have been burned together. However, in the case of major damage, arising in the case of corona which is not checked, for instance, the damage to the core may be so important that the method described above does not give satisfactory results. Major damage may occur in the electrical sheet in the form of holes, sheet fire may occur when the sheets melt and carbonise, the cooling may be affected by a core damage so that the cooling water boils, etc. The only possibility in such cases so far has been to replace the entire core and the winding, or at least substantial parts thereof. This is both time-consuming and costly since it may cause interruption in operation of the machine for a considerable period, in the order of a year or more. An electric machine with a core provided with a winding of the type described above, however, opens the way to a new method of repairing damage to said core, which constitutes the object of the present invention. The object of the present invention is thus to solve the problems described above and the object is achieved by means of the method in accordance with the preamble to claim 1 with the addition of the features defined in the characterizing part of the claim, and by means of a replacement part as claimed in the characterizing part of claim 44.

The present invention thus relates to a method of repairing the magnetizable core of an electric machine for high voltage, which core is provided with a winding and which core has a damaged area, characterized in that a repair portion of the core, which includes the damaged area, is separated and removed, and in that said repair portion is replaced with a replacement part. By "repair portion" is in this specification meant a part of the core which part includes the damaged area that needs to be repaired, or, which is more likely, replaced. The new method has the advantage that in many cases it can be performed directly on the site of operation. The repair time is also considerably reduced when only a small part of the core must be replaced, thereby resulting in greatly reduced costs in the form of shorter shutdowns. The cost of the replacement part is also greatly reduced, primarily thanks to decreased material costs. It should be mentioned that the separation and removal of the damaged part can naturally be performed in the same operation.

Additional advantages and characteristics are revealed in the dependent claims.

In a particularly preferred feature the removed repair portion substantially corresponds to the damaged area. In most cases it is for obvious reasons advantageous to remove a repair portion comprising almost exclusively the damaged area so that, as far as possible, no more than is absolutely necessary of the core is removed. However, in some cases it may be advantageous to remove a repair portion that is considerably larger than the damaged area, as will be described below.

An advantageous feature is stated to be that the winding adjacent to the damaged area or the repair portion is removed. This can be done either by simply moving aside the winding or by taking away part of the winding in the area. This is normally performed before the repair area is removed.

According to the invention, the windings are preferably of a type corresponding to cables having solid, extruded insulation, of a type now used for power distribution, such as XLPE-cables or cables with EPR-insulation. Such a cable comprises an inner conductor composed of one or more strand parts, an inner semiconducting layer surrounding the conductor, a solid insulating layer surrounding this and an outer semiconducting layer surrounding the insulating layer. Such cables are flexible, which is an important property in this context since the tech- nology for the arrangement according to the invention is based primarily on winding systems in which the winding is formed from cable which is bent during assembly. The flexibility of an XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable with a diameter of 30 mm, and a radius of curvature of approximately 65 cm for a cable with a diameter of 80 mm. In the present application the term "flexible" is used to indicate that the winding is flexible, at least down to a radius of curvature in the order of four times the cable diameter, preferably eight to twelve times the cable diameter.

The winding should be constructed to retain its properties even when it is bent and when it is subjected to thermal or mechanical stress during operation. It is vital that the layers retain their adhesion to each other in this context. The material properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. In an XLPE-cable, for instance, the insulating layer consists of cross-linked, low-density polyethylene, and the semiconducting layers consist of polyethylene with soot and metal particles mixed in. Changes in volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers in relation to the elasticity of these materials, the radial expansion can take place without the adhesion between the layers being lost. The material combinations stated above should be considered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range of 10^_1-1θ6 ohm cm, e.g. 1-500 ohm cm, or 10-200 ohm cm, naturally also fall within the scope of the invention.

The insulating layer may consist, for example, of a solid thermoplastic material such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polymethyl pentene ("TPX"), cross-linked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.

The inner and outer semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in.

The mechanical properties of these materials, particularly their coefficients of thermal expansion, are affected relatively little by whether soot or metal powder is mixed in or not - at least in the proportions required to achieve the conductivity necessary according to the invention. The insulating layer and the semiconducting layers thus have substantially the same coefficients of thermal expansion.

Ethylene-vinyl-acetate copolymers/nitrile rubber (EVA/NBR), butyl graft polyethylene, ethylene-butyl-acrylate copolymers (EBA) and ethylene-ethyl- acrylate copolymers (EEA) may also constitute suitable polymers for the semiconducting layers. Even when different types of material are used as base in the various layers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with the combination of the materials listed above.

The materials listed above have relatively good elasticity, with an E- modulus of E<500 MPa, preferably <200 MPa. The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbed in the radial direction of the elasticity so that no cracks appear, or any other damage, and so that the layers are not released from each other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as in the weakest of the materials. The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconducting layer is sufficiently high to enclose the electrical field within the cable, but sufficiently low not to give rise to significant losses due to currents induced in the longitudinal direction of the layer.

Thus, each of the two semiconducting layers essentially constitutes one equipotential surface, and these layers will substantially enclose the electrical field between them.

There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulating layer.

By using the above described insulated electric conductors or cables as windings in rotating electric machines, the considerable advantage is obtained that it allows the voltage of the machine to be increased to such levels that it can be connected directly to the power network without intermediate transformers. The extremely important advantage is thus gained, inter alia, that the conventional transformer can be eliminated.

The new type of winding mentioned above has also proved advantageous in transformers and reactors. It is thus possible to a great extent to replace oil- cooled power transformers with dry transformers of a new type. The term "dry" transformer or reactor refers to a transformer/reactor that is oil-free, preferably air- cooled. An important consequence of this is that dry transformers and reactors can be used at considerably higher powers than has been previously possible. In accordance with the claims the winding is preferably designed to enclose a generated electric field inside the winding, at least for one turn of the winding. The winding is further characterized in that it is flexible or bendable, thus facilitating removal of the winding. This flexibility of the winding is to a great extent a condition for the present method since it allows the winding to be handled in a completely different way than was possible with previously known substantially rigid windings. Thanks to the flexibility of the winding it can easily be removed locally, giving access to the damaged area. It can also be spliced so that new winding turns can be spliced into remaining original winding turns when repair of the core has been completed. The winding is also characterized in that it is provided by means of an insulated electric conductor comprising at least one current-carrying conductor and also comprising a first layer having semiconducting properties arranged to sur- round the current-carrying conductor, a solid insulation layer arranged surrounding said first layer and a second layer having semiconducting properties arranged surrounding the insulation layer.

The high-voltage insulated electric conductor may be designed in many favourable ways. One advantageous feature is stated to be that the insulated conductor consists of a cable, preferably a high-voltage cable. The first semiconducting layer is also at substantially the same potential as the current-carrying conductor. The second semiconducting layer is preferably arranged so that it constitutes a substantially equipotential surface surrounding the current-carrying conductor(s) and the layer of insulation. It is also connected to a predetermined potential, preferably earth potential. According to other features at least two adjacent layers have substantially equal coefficients of thermal expansion, and the current-carrying conductor may comprise a number of strand parts, only a few of the strand parts being uninsulated from each other. Furthermore, each of the three layers, i.e. the two semiconducting layers and the insulating layer, may be firmly joined to the adjacent layer along substantially the entire connecting surface. According to a particularly important feature the layers are arranged to adhere to each other even when the insulated conductor or cable is bent. Finally, it may be mentioned that it preferably has a diameter in the range of 20-250 mm and a conducting area in the range of 80-3000 mm². The insulated conductor or cable used in the present invention is flexible and of a kind which is described in more detail in WO 97/45919 and WO 97/45847. Additional descriptions of the insulated conductor or cable concerned can be found in WO 97/45918, WO 97/45930 and WO 97/45931. According to a preferred embodiment of the invention the repair portion is separated by means of laser cutting or by means of water cutting. Other similar methods are of course feasible, e.g. more conventional methods such as gas cutting and various types of cutting machining.

According to an advantageous feature the method is characterized in that the parts of the core against which the replacement part is intended to abut are at least partially coated with an adhesive material, such as a binder with inverted thermal coefficient of linear expansion. This is of course to facilitate reliable at- tachment of the replacement part to the walls of the core. It is thus also stated that the replacement part is preferably also attached in the core with the aid of the adhesive material. The adhesive material is also preferably electrically insulating and especially arranged to provide insulation between the replacement part and unin- sulated core parts, preferably the ends of the core laminations, that have appeared as a result of the removal of the repair portion. This is particularly important when the replacement part is made of electrical sheets. A suitable adhesive may be a foil of the type known as Nomex adhesive film. If applicable it may be applied so that it also covers the inside of the mould which will be described be- low.

According to a preferred embodiment, the next step is characterized in that a replacement part for the repair portion is manufactured, positioned in the core and secured there. The replacement part may be manufactured from an element in a magnetic material which is formed to a shape substantially corre- sponding to the shape of the repair portion. Alternatively it may be shaped so that it is smaller than the repair portion. Any space arising between the replacement part and the core itself, when the replacement part has been positioned in the core, is filled with a suitable casting material, either non-magnetic or magnetic, to be described later. The replacement part may be manufactured from conventional core material, preferably laminated electrical sheet. Another possibility is to manufacture the replacement part from small particles of magnetic material, e.g. magnetic powder, which is formed, e.g. by baking, compressing, casting or the like, to a shape corresponding to the shape of the repair portion. Examples of suitable ma- terials will be mentioned later. It should be noted that the replacement part is not necessarily made of the same material as the core.

The replacement parts may be prefabricated, i. e. replacement parts of for example different sizes and embodiments are prefabricated and kept available from a stock, having the advantage of enabling the repair to be carried out even more quickly.

According to another preferred embodiment the method is characterized in that a replacement part for the repair portion is cast in a casting material directly in the core where the repair portion has been removed. This is performed advantageously by the replacement part being produced by a mould being arranged in connection with the area in the core where the repair portion has been removed and the replacement part being cast in a casting material directly on site. The re- placement part is preferably cast by the casting material being injected into the core where the repair portion has been removed, through a hole in the mould, under positive pressure. However it is perfectly possible, particularly in the case of minor damage, to cast the replacement part directly in the core without the use of any particular mould. The casting material can then be introduced by means of an extrusion gun.

It should be pointed out that the term "casting" also includes such procedures as die casting, injection moulding and even compacting, i.e. generally processes whereby a casting material is introduced into a moulding cavity in some way (with or without a completely delimiting mould), in order to be shaped there. The term "casting material" applies to any suitable material whatsoever that can be used for casting in accordance with the above definition of casting.

The casting material is preferably a magnetic material or a mixture of both magnetic and non-magnetic material- One example of suitable magnetic materials is a magnetic powder. As an alternative the particles in the magnetic powder may be electrically insulated by means of an outer, electrically insulating layer. This has the advantage that a minimum of basic compound is necessary and the degree of magnetic packing will be high. Another alternative is for the magnetic material to be a magnetic powder in a basic compound. Another example of a feasible material may be a polymer matrix, i.e. an intrinsically magnetic polymer or a non-magnetic polymer with a magnetic filler. According to an advantageous feature the basic compound comprises a thermoplastic and the casting material is introduced, while heat is supplied, into the area of the core where the repair portion has been removed. In such case, however, it is important to ensure that insulating material included in any adjacent winding, cable insulation or sheet insulation is not damaged. According to another feature, the magnetic material may be a soft- magnetic composite material. An example of such material is that described in a brochure from Hόganas, SMC UPDATE, Vol. No. April 1997. This is a soft- magnetic composite material composed of a soft-magnetic, electrically insulated powder.

According to another example the magnetic material may comprise a bundle of wires, in magnetic material, that is wholly or partially baked into a composite comprising soft-magnetic powder material, for example the material described in the above-mentioned Hόganas brochure, or a soft-magnetic slurry, i.e. a castable, viscous dispersion of magnetic powder, coated with an insulating layer, in a liquid. Such a slurry is also a compactable material. This has the advan- tage of giving good mechanical support. The said materials may of course also be combined in various ways.

In the case when the core is built up of several prefabricated separate parts that have been fitted together, the method of the present invention may be characterized in that that entire part where the damaged area is located consti- tutes the repair portion and is thus separated, removed and replaced by a new, identical part. This method is thus particularly suitable when the core is built up of segments, sections or in modular form and means that a complete segment, section or module can be exchanged if this is advantageous.

The repair procedure is suitably completed by the winding being re-instal- led. The winding previously removed may be re-used if it is undamaged or is repaired locally. Alternatively the winding is repaired by new winding turns being installed and connected to the remaining winding. It is naturally also feasible to replace more or less the entire winding.

A particularly advantageous feature is stated to be that the method is performed at the site of the electric machine. The machine need not then be moved or dismantled.

The method may be characterized in that the magnetizable core is a stator core in a rotating electric machine, or a core in a transformer or reactor.

The present invention finally relates to a replacement part for use when repairing a magnetizable core of an electric machine for high voltage, which core is provided with a winding and where said core has a damaged area, said re- placement part being characterized in that it is manufactured and installed in the core in accordance with the method as claimed in any one of claims 18-35.

In conclusion it should be mentioned that all the materials mentioned can be used both for the magnetizable core and for the replacement part. It should also be pointed out that the winding, in the form of the described flexible, insulated electric conductor or cable, constitutes an important condition for the new repair method in accordance with the invention. Finally, it should be mentioned that the term "magnetic", for instance in connection with magnetic powder, does not necessarily mean that the powder itself is magnetic. It is sufficient if it is affected by a magnet or is magnetizable.

To increase understanding of the invention it will now be described in detail with reference to the accompanying drawings illustrating non-limiting embodiments, in which:

Figure 1 shows schematically a partial view in perspective of a stator with a damaged tooth,

Figure 2 shows schematically a partial view in perspective of the stator in Figure

1 in the process of being repaired in accordance with the invention, Figure 3 shows schematically a partial view in perspective of the stator in Figure 1 which has been repaired in accordance with a first embodiment of the method according to the invention,

Figure 4 shows schematically a partial view in perspective of the stator in Figure 1 which has been repaired in accordance with a second embodiment of the method according to the invention, Figure 5 shows schematically a view in perspective of a stator of particular de- sign, in which the repair method in accordance with the invention can be used, Figure 6 shows schematically a view in perspective of a detail of the stator shown in Figure 5, Figure 7 shows schematically a view in perspective of another stator of particu- lar design, in which the repair method in accordance with the invention can be used, Figure 8 shows a cross section through an insulated electric conductor suitable for use as winding. The same designations have been used in ail the figures to denote components which are the same or equivalent to each other. Figure 1 illustrates a part of a stator, more specifically a part comprising three stator teeth 1 , 2, 3, of which one tooth 2 reveals a damaged area 5. The teeth are shown only schematically and between the teeth are schematically indicated stator slots 7 for the winding which is presumed to have been removed. We will not here go into how the damage may have arisen but, due to its location, ad- jacent to the stator slot and the winding therein during operation, it is probable that the damage is a result of an electrical fault in the winding.

Figure 2 illustrates how a repair portion 10, including the damaged area 5, has been separated and removed. There is now a corresponding empty space 9 in the tooth. The repair portion 10 can be removed by means of laser or water cutting, for instance. In accordance with a first embodiment of the invention a replacement part 11 is manufactured with a shape corresponding to the repair portion. The replacement part may be made of electrical sheet, the most usual core material, or of some other material such as some form of magnetic powder as mentioned earlier. The replacement part need not necessarily be made of the same material as used in the original core. This replacement part is then positioned in the existing empty space 9 in the stator tooth 2, as shown in Figure 3. It may be glued in place, for instance, or it may be attached by some other suitable means.

Figure 4 shows the same damaged stator tooth 2 as in Figure 1 , but here it has been repaired in accordance with a second embodiment. A considerably smaller repair portion has been removed, corresponding in principle to only the damaged area 5. A replacement part 14 has been manufactured directly on site in the damaged tooth 2 by means of casting. This can be effected by a mould being applied close to the area in the tooth where the repair portion has been removed. To ensure that the casting material adheres to the core it is advisable to treat the relevant area in the core with some type of adhesive or binder, preferably with inverted thermal coefficient of linear expansion. The casting material, e.g. of mag- netic powder metal in a polymer basic compound, is then suitably injected into the mould through a hole in the mould under positive pressure. If it is found that some of the core laminations have partially fallen down into the gap formed when the repair portion was removed, because they no longer had sufficient support, these laminations will be pushed back into place when the casting material is injected, which is an advantage of not inconsiderable value.

The magnetic particles may possibly be treated so that they become insulating, preferably by providing them with an outer insulating layer, e.g. a thermoplastic. This allows the use of a minimum of basic compound and the magnetic degree of packing will thus be advantageously high.

Alternatively the replacement part 14 may be cast without the use of any mould. This is particularly suitable when the damage is small. The casting material is then suitably a mixture of magnetic particles and particles of a thermoplastic, and it is introduced at the same time as heat is supplied, suitably being injected by means of an injection gun, in the area where the repair portion has been removed. This method, too, causes laminations that have fallen down to be pushed back into place.

The stator shown schematically in Figure 5 has a special design. It is built up of stator teeth 16 consisting of stator teeth slices or planks 17. Figure 6 shows such a stator tooth plank 17 in detail. Each tooth plank consists of a number of part-teeth 18 joined together axially to form a tooth plank 17. The tooth planks are then joined together side by side to form a stator core 20. Winding slots provided with winding 21 are arranged between the stator teeth 16. These are shown only schematically in the figure. In the event of damage to one or more stator teeth 16 it is thus both simple and advantageous to replace the whole stator tooth plank or planks affected by the damage.

Figure 7 shows another stator with special design. This comprises a stator core 24 built up of a number of substantially annular segments 25 joined together in axial direction to form the core. Each segment 25 comprises a set of stator teeth 27, between which winding 28 is arranged in slots therefor. In the event of damage to the stator, e.g. to one or more stator teeth or to the stator yoke, a complete segment 25 can be simply and advantageously removed and replaced with a new segment.

Figure 8, finally, shows a cross section of a cable that is particularly suitable for use as winding in the stator according to the invention. The cable 30 comprises at least one current-carrying conductor 31 surrounded by a first semiconducting layer 32. Around this first semiconducting layer is an insulating layer 33 and around this is a second semiconducting layer 34. The electric conductor 31 may consist of a number of strand parts 35. The three layers are constructed so that they adhere to each other even when the cable is bent. The cable shown is flexible and it retains this property throughout its service life. The cable illustrated also differs from conventional high-voltage cable in that the outer, mechanically protecting sheath and the metal screen normally surrounding such a cable may be eliminated.

The above description of embodiments refers only to the stator core in a rotating electric machine but the invention is not limited to this area of application and can, as stated, be applied in corresponding manner when repairing the magnetic core in a power transformer or reactor. Naturally the repair of other parts of a stator core than the illustrated repair of a stator tooth may also be performed. The invention can also be modified in other ways within the scope of the following claims.

Claims

C L A I M S

1. A method of repairing a magnetizable core of an electric machine for high voltage, which core is provided with a winding and where the core has a damaged area (5), characterized in that a repair portion (10) of the core, which portion includes the damaged area (5), is separated and removed, and in that the repair portion is replaced with a replacement part (11 , 14).

2. A method as claimed in claim 1 , characterized in that the removed repair portion (10) substantially corresponds to the damaged area (5).

3. A method as claimed in any of the preceding claims, characterized in that the winding adjacent to the damaged area or the repair portion is removed.

4. A method as claimed in any of the preceding claims, characterized in that the winding comprises an insulated electric conductor and that said winding encloses a generated electric field within the winding, for at least one turn of the winding.

5. A method as claimed in claim 4, characterized in that the winding is flexible.

6. A method as claimed in any of claims 4-5, characterized in that the winding is provided by means of an insulated electric conductor (30) comprising at least one current-carrying conductor (31) and a first layer (32) having semiconducting properties arranged to surround the current-carrying conductor, a solid insulation layer (33) arranged surrounding said first layer and a layer (34) having semiconducting properties arranged surrounding the insulation layer, said layers being in contact with each other.

7. A method as claimed in claim 6, characterized in that the insulated conductor consists of a cable, preferably a high-voltage cable.

8. A method as claimed in either of claims 6-7, characterized in that the second semiconducting layer (34) is arranged so that it constitutes a substantially equipotential surface surrounding the current-carrying conductor(s) (31).

9. A method as claimed in claim 8, characterized in that the second semiconducting layer (34) is connected to earth potential.

10. A method as claimed in any of claims 5-9, characterized in that at least two adjacent layers have substantially equal coefficients of thermal expansion.

11. A method as claimed in any of claims 5-10, characterized in that each of the three layers (32, 33, 34) is firmly joined to the adjacent layer along substantially the entire connecting surface.

12. A method as claimed in any of claims 5-11 , characterized in that said layers are arranged to adhere to each other even when the insulated conductor (30) or cable is bent.

13. A method as claimed in any of the preceding claims, characterized in that the repair portion (10) is separated by means of laser cutting.

14. A method as claimed in any of claims 1-12, characterized in that the repair portion (10) is separated by means of water cutting.

15. A method as claimed in any of the preceding claims, characterized in that the parts of the core against which the replacement part (11 , 14) is intended to abut are at least partially coated with an adhesive material.

16. A method as claimed in claim 15, characterized in that the replacement part is secured in the core with the aid of the adhesive material.

17. A method as claimed in either of claims 15-16, characterized in that the adhesive material is electrically insulating and especially arranged to provide insulation between the replacement part (11 , 14) and uninsulated core parts that have appeared as a result of the removal of the repair portion (10).

18. A method as claimed in any of the preceding claims, characterized in that a separate replacement part (11) for the repair portion (10) is manufactured, positioned in the core and secured there.

19. A method as claimed in claim 18, characterized in that the replacement part (11) is manufactured from an element in a magnetic material which is formed to a shape substantially corresponding to the shape of the repair portion (10).

20. A method as claimed in claim 18, characterized in that the replacement part (11) is manufactured from an element in a magnetic material which is shaped so that it is smaller than the repair portion (10).

21. A method as claimed in any of claims 18-20, characterized in that any space arising between the replacement part (11) and the core itself, when the re- placement part has been fitted in the core, is filled with a casting material.

22. A method as claimed in any of claims 18-21 , characterized in that the replacement part (11) is manufactured from laminated electrical sheet.

23. A method as claimed in any of the preceding claims, characterized in that the replacement part (11) is prefabricated.

24. A method as claimed in any of claims 1-17, characterized in that a replacement part (14) for the repair portion (10) is cast in a casting material directly in the core, where the repair portion (10) has been removed.

25. A method as claimed in claim 24, characterized in that the replacement part (14) is produced by a mould being arranged in connection with the area in the core where the repair portion (10) has been removed and that the replacement part is cast in a casting material directly on site.

26. A method as claimed in claim 25, characterized in that the replacement part (14) is cast by the casting material being injected into the core where the repair portion (10) has been removed, through a hole in the mould, under positive pressure.

27. A method as claimed in any of claims 21 , 24-26, characterized in that the casting material comprises a magnetic material.

28. A method as claimed in any of claims 19-20, 27, characterized in that the magnetic material is a magnetic powder.

29. A method as claimed in any of claims 19-20, 27, characterized in that the magnetic material is a magnetic powder in which the powder particles are provided with an externally electrically insulating layer.

30. A method as claimed in either of claims 28-29, characterized in that the magnetic material is a magnetic powder in a basic compound.

31. A method as claimed in claim 30, characterized in that the basic com- pound comprises a thermoplastic and that the casting material is introduced, while heat is supplied, into that area of the core where the repair portion has been removed.

32. A method as claimed in any of claims 19-20, 27, characterized in that the magnetic material is a soft-magnetic composite material.

33. A method as claimed in any of claims 19-20, 27, characterized in that the magnetic material comprises a bundle of wires that is wholly or partially baked into a composite comprising soft-magnetic powder material.

34. A method as claimed in any of claims 19-20, 27, characterized in that the magnetic material comprises a bundle of wires that is wholly or partially baked into a soft-magnetic slurry.

35. A method as claimed in any of claims 19-20, 27, characterized in that the magnetic material is a combination of several of the magnetic materials specified in claims 28-34.

36. A method as claimed in any of claims 1-17, characterized in that if the core is built up of several prefabricated separate parts (17, 25) that have been fit- ted together, that entire part where the damaged area is located, constitutes the repair portion and is thus separated, removed and replaced with a new part.

37. A method as claimed in any of the preceding claims, characterized in that the winding is again installed.

38. A method as claimed in any of the preceding claims, characterized in that it is performed on the site of the electric machine.

39. A method as claimed in any of the preceding claims, characterized in that the magnetizable core is a stator core in a rotating electric machine.

40. A method as claimed in any of claims 1-38, characterized in that the magnetizable core is a core in a transformer or a reactor.

41. A method as claimed in any of claims 6-40, characterized in that the said layers (32, 33, 34) in the insulated electric conductor are of materials having such elasticity and such a relation between the coefficients of thermal expansion of the materials that the changes in volume occurring in the layers during operation, as a result of temperature fluctuations, can be absorbed by the elasticity of the materials so that the layers retain their contact with each other at the temperature fluctuations occurring during operation.

42. A method as claimed in claim 41 , characterized in that the materials in said layers (32, 33, 34) have high elasticity.

43. A method as claimed in either of claims 41 or 42, characterized in that each semiconducting layer (32, 34) constitutes substantially one equipotential surface.

44. A replacement part (11 , 14, 17, 25) for use when repairing the magnetizable core of an electric machine for high voltage, which core is provided with a winding and where said core has a damaged area, characterized in that it is manufactured and fitted in the core in accordance with the method as claimed in any of claims 18-35.

45. An electric machine for high voltage comprising a magnetizable core pro- vided with a winding, characterized in that said core has been repaired in accordance with the method claimed in any of claims 1-43.