WO1999017312A2

WO1999017312A2 - Power transformer/reactor and a method of adapting a high voltage cable

Info

Publication number: WO1999017312A2
Application number: PCT/SE1998/001749
Authority: WO
Inventors: Li Ming; Mats Leijon; Rongsheng Liu; Albert Jaksts
Original assignee: Abb Ab
Priority date: 1997-09-30
Filing date: 1998-09-29
Publication date: 1999-04-08
Also published as: SE511361C2; SE9703563D0; AU9371498A; SE9703563L; DE19882712T1; WO1999017312A3; JP2001518698A

Abstract

The present invention refers to a power transformer/reactor comprising at least one winding (1, 2, 3). The winding/windings (1, 2, 3) are manufactured with a high voltage cable (10), comprising an electric conductor, around the conductor there being arranged a first semiconducting layer (14), around the first semiconducting layer (14) there being arranged a first insulating layer (16) and around the first insulating layer (16) there being arranged a second semiconducting layer (18). The second semiconducting layer (18) is directly earthed (32, 34) at n points of each winding (1, 2, 3) wherein n is an integral number and n≥2, and whereby two (32, 34) of said n directly earthed points are arranged at or in the vicinity of both ends of each winding (1, 2, 3) and the electric contact is interrupted (20) 2(n-1) number of times between both ends in the semiconducting layer (18). At each said interruption (20) there is arranged a means (24, 26) comprising a second insulating layer (24) and third semiconducting layer (26) in order to reduce the amplification of electric field strength at said interruption (20). The second semiconducting layer (18) of different phases (1, 2, 3) at each said interruption (20) is earthed in a cross-connected manner (42, 44). Besides, at least one point (36, 38) between both ends is indirectly earthed.

Description

POWER TRANSFORMER/REACTOR AND A METHOD OF ADAPTING A HIGH VOLTAGE CABLE

Technical field The present invention relates in a first aspect to a power transformer/reactor. A second aspect of the present invention relates to a method of adapting a high voltage cable for windings of a power transformer/reactor.

For all transmission and distribution of electric energy, transformers are used and their task is to allow exchange of electric energy between two or more electric systems having generally different voltage levels. Transformers are available in all power ranges from the VA up to the 1000 MVA range. With respect to the voltage range, there is a spectrum up to the highest transmission voltages which are being used today. Electromagnetic induction is used for the transmission of energy between electric systems. For the transmission of electric energy, reactors are also included as an essential component, for example for phase compensation and filtering.

The transformer/reactor relating to the present invention belongs to the so- called power transformers/reactors with a rated output ranging from a few hundred kVA up to more than 1000 MVA with a rated voltage ranging from 3-4 kV and up to very high transmission voltages.

Background of the invention

From a purely general point of view, the primary task of a power transformer is to allow exchange of electric energy between two or more electrical systems usu- ally having different voltages with the same frequency.

A conventional power transformer/reactor comprises a transformer core referred to below as core, made of laminated preferably oriented sheets, usually of silicon steel. The core consists of a number of core legs connected by yokes. Around the core legs there are a number of windings which are normally referred to as primary, secondary and regulation winding. As far as power transformers are concerned these windings are practically always concentrically arranged and distributed along the length of the core legs. Other types of core constructions occasionally occur such as those of the so-called shell-type transformer or the toroidal-type transformer. Examples relating to core constructions are described in for example DE 40414. The core may consist of conventional magnetizable material such as said oriented steel sheet, and of other magnetizable material such as ferrites, amorphous material, wire strands or metal tape. With respect to reactors, the magnetizable core is as known not necessary.

The above-mentioned windings constitute one or several coils connected in series, which coils are constructed of a number of turns connected in series. The turns of a single coil normally make up a geometrically continuous unit which is physically separated from the remaining coils.

The insulation system, partly on the inside of a coil/winding and partly between coils/windings and other metal parts is normally in the form of a solid cellulose or varnish based insulation closest to the separate conducting element and the insulation on the outside is in the form of a solid cellulose insulation, a fluid insulation and possibly an insulation in the form of a gas. Windings having insulation and possible bulky parts represent in this way large volumes that will be subjected to high electric field strengths occurring in and around the active electro-magnetic parts belonging to transformers. A detailed knowledge of the properties of insulation material is required in order to predetermine the dielectric field strengths which arise and in order to attain a dimensioning such that there is a minimal risk of electric breakdown. Furthermore it is essential to achieve a surrounding environment which does not change or lead to the deterioration of the insulation properties.

Today's predominant outer insulation system for conventional high voltage power transformers/reactors consists of cellulose material for the solid insulation and transformer oil for the fluid insulation. Transformer oil is based on so-called mineral oil.

Additionally, a conventional insulation is relatively complicated to construct and special measures need to be taken during manufacture in order to utilize the good insulation properties of the insulation system. The system should have a low moisture content, the solid phase in the insulation system needs to be well impregnated with the surrounding liquid, the risk for remaining gas pockets in the solid phase must be minimal. During manufacture a special drying process is carried out on the complete core with windings before it is lowered into the tank. After lowering the core and sealing the tank, the latter is emptied of all air by means of a special vacuum treatment before being filled with oil. This process is relatively time consuming seen from the entire manufacturing process in addition to requiring the extensive utilization of resources in the workshop.

The tank surrounding the transformer must be constructed in such a way that is able to withstand full vacuum since the process requires that all the gas be pumped out to almost absolute vacuum which involves extra material consumption and manufacturing time. Furthermore, the installation on site requires renewed vacuum treatment, a process to be repeated each time the transformer is opened for attention or for inspection.

Summary of the invention The power transformer/reactor, according to the present invention, comprises at least one winding arranged in most cases around a magnetizable core which is of varying geometry. The term "windings" will preferably be referred to below in order to simplify the following specification. The windings are formed of a high voltage cable having solid insulation. The cables consist of at least one cen- trally located electric conductor around which there is arranged a first semiconducting layer, around the first semiconducting layer there is arranged a solid first insulating layer and around the insulating layer there is arranged a second outer semiconducting layer.

An additional advantage is that said layers are arranged to adhere to one another even when the cable is bent. Hereby, good contact is achieved between the layers during the cable's entire life.

The second semiconducting layer is directly earthed at n points of each winding, where n is an integral number and n>2, and whereby two of said directly earthed points are arranged at or in the vicinity of both ends of each winding. The electric contact is interrupted 2(n-1) times in the second semiconducting layer. The second semiconducting layer of different phases at each said interruption is earthed in a cross-connected manner. A method for adapting a high voltage cable for windings of a power transformer/reactor, according to the present invention, comprises the following steps:

• directly earthing the second semiconducting layer at n points of each winding where n is an integral number and n>2, and whereby two of said points are arranged at or in the vicinity of both ends of each winding;

• achieving two interruptions in the electric contact in the second semiconducting layer between each pair of directly earthed points; and

• earthing in a cross-connected manner the second semiconducting layer at different phases of each said interruption.

The use of such a cable implies that those areas of the transformer/reactor which are subjected to high electric field stress are limited to the solid insulation of the cable. Remaining parts of the transformer/reactor, with respect to high voltage application, are only subjected to very moderate electric field strengths. Further- more, the use of such a cable eliminates several problematic areas described in the background of the invention. Consequently a tank is not needed for the insulating and cooling medium. Besides, the insulation also becomes substantially simple. Construction time is considerably shorter compared to that of a conventional power transformer/reactor. The windings may be manufactured separately and the power transformer/reactor may be assembled on site.

However, the use of such a cable presents new problems which must be solved. The outer semiconducting layer must be directly earthed at or in the vicinity of both ends of the cable so that the electric stress, which arises both at normal operating voltage and during transience, will primarily only load the solid insulation of the cable. The semiconducting layer in addition to these direct earthings form a closed circuit in which a current is induced during operation. The resistivity of the layer must be great enough so that the resistive losses arising in the layer are negligible.

Besides this magnetically induced current, a capacitive current will flow into the layer through the direct earthing in both ends of the cable. If the resistivity of the layer is too great, the capacitive current will become so limited that the potential in parts of the layer, during a period of alternating stress, may differ to such an extent from the earth potential that areas of the power transformer/reactor other than the solid insulation of the windings will be subjected to electric stress. By breaking the electric contact n amount of times, where n is an integral number and n>1 , in the second semiconducting layer between both ends of the cable and by earthing the second semiconducting layer at different phases in cross-connected manner at each said interruption, the current in the second semiconducting layer is eliminated and the power losses are minimized.

All interruptions in the second, outer semiconducting layer of a high voltage cable will result in an increase of the electric field strength at the edge of the second semiconducting layer at the interruption. This increase of the electric field strength clearly increases the risk for electric breakdown. By arranging a means comprising a second insulation layer and a third semiconducting layer at each interruption in the second semiconducting layer, the risk for electric breakdown is minimized.

In extreme cases the windings may be subjected to such rapid transient overvoltage that parts of the outer semiconducting layer assume such a potential that areas of the power transformer other than the insulation of the cable are subjected to undesirable electric stress. In order to prevent such a situation from arising a number of non-linear elements, e.g. spark gaps, gas diodes, zener-diodees or varistors are connected between the layer and earth for each phase. By connecting a capacitor between the outer semiconducting layer and earth, undesirable electric stress may also be prevented from arising. A capacitor reduces the voltage stress even at 50Hz. This principle of earthing will be referred to below as "indirect earth- ing .

The indirectly earthed points are connected to earth either via the following;

• a non-linear element e.g. a spark gap or a gas diode, • a non-linear element parallel to a capacitor,

• a capacitor or a combination of all three alternatives.

The invention will now be described in more detail in the description here- inafter of the preferred embodiments of the invention with reference to the accompanying drawings.

Brief description of the drawings Figure 1 shows a cross-sectional view of a high voltage cable;

Figure 2A shows a partly sectional view of a high voltage cable having interruptions in the second semiconducting layer in order to illustrate the amplification of the electric field at the edges of the interruption; and Figure 2B shows a perspective view of a part of the cable shown in Figure 2A; Figure 3 shows a cross-sectional view along the longitudinal axis of the cable on a high voltage cable having a means to reduce the amplification of the electric field strength at the interruption; Figure 4 shows a schematic principle of earthing a three phase power transformer according to the present invention;

Figure 5 is a diagram showing the potential of the second semiconducting layer in relation to the length of the cable; Figure 6a and 6b, respectively, show different elements in order to achieve indirect earthing ; and Figure 7 shows a flow chart of the method of adapting a high voltage cable according to the present invention.

Detailed description of the embodiments of the present invention

Figure 1 shows a cross-sectional view of a high voltage cable 10 tradition- ally used for the transmission of electric energy. The shown high voltage cable 10 may for example be a standard XLPE cable 145 kV but without a mantle and a screen. The cable 10 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 cable concerned can be found in WO 97/45918, WO 97/45930 and WO 97/45931. The high voltage cable 10 comprises an electric conductor which may comprise one or several strands 12 having a circular cross section of for example copper (Cu). These strands 12 are arranged centrally in the high voltage cable 10. Around the strands 12 there is arranged a first semiconducting layer 14. Around the first semiconducting layer 14 there is arranged a first insulating layer 16, of for example XLPE insulation. Around the first insulating layer 16 there is arranged a second semiconducting layerlδ.

In fig. 1 showing the detail of the invention relating to the cable 10, the three layers 14, 16, 18 are arranged to adhere to each other even when the cable 10 is bent. The cable 10 shown is flexible, and this property is maintained during the entire life of the cable.

Accordingly, the windings, in the arrangement according to the invention, 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 con- text since the technology 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 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. Ih~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 lay- ers, 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-modu- lus 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 substan- tially 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 semiconduct- ing layers being arranged in the insulating layer.

Figure 2A shows a view, partially cross-sectional, of a high voltage cable having interruptions in the second semiconducting layer in order to illustrate the amplification of the electric field strength at the edges of the interruption. The section shown in 2A extends along the longitudinal axis of the high voltage cable. Figure 2B shows a perspective view of a part of the cable shown in Figure 2A. Like parts in Figures 2A and B have been designated by the equivalent reference numbers. The strands 12 are only shown schematically in Figure 2A. As shown in Figures 2A and B the second semiconducting layer 18 has been removed in the shape of a ring around the periphery of the high voltage cable 10 so that a groove 20 is formed. In this way the first insulation layer 16 is exposed in the groove 20. By achieving this interruption in the electric contact located between two earthing points, in the second semiconducting layer 18, no current will flow and thus no loss of heat will occur due to induced voltage. However all interruptions in the second semiconducting layer 18 give rise to an amplification of the electric field strength at the sides of the interruption. As shown in Figure 2A, the electric field lines are illustrated (indicated by the reference number 22). At the edges of the groove 20 there is a concentration of field lines 22 which meansjhat the electric field strength shows a sharp increase. This unfortunately results in an increased risk for electric discharge, the aim being to strive towards avoiding this occurrence. Figure 3 shows a cross-sectional view along the longitudinal axis of the cable of a high voltage cable having a means to reduce the amplification of the electric field strength at the interruption. The high voltage cable 10 comprises, in the same way as the high voltage cable according to Figure 1 , the following: strands 12; a first semiconducting layer 14; a first insulating layer 16 and a second semiconducting layer 18. As shown in Figure 3 the second semiconducting layer 18 has been removed in the shape of a ring around the periphery so that a groove 20 is formed, exposing the first insulating layer 16. As shown in Figure 3 the groove 20 has downward sloping edges i.e. the groove 20 has a larger breadth at the upper edge of the second semiconducting layer 18 than that of the first insulating layer 16. The groove 20 may for example have straight edges even though downward sloping edges are advantageous. The distance between the edges of the second semiconducting layer 18 of the first insulating layer is indicated by b in Figure 3. The width b of the groove 20 is preferably 10 mm. Besides, the high voltage cable 10 comprises a second insulating layer 24 which is applied among other things onto the groove 20 so that the groove 20 is filled in this way. The reason for having sloping edges at the groove 20 is in order to avoid obtaining a hollow space at the edges when the second insulating layer 24 is formed by filling among other things the groove 20 with a suitable insulating material, for example insulating "self amalgamating" EPR-tape such as the insulating tape IV-tape®, IA 2332 from ABB Kabeldon. The second insulating layer 24 covers even the sloping edges of the second semiconducting layer 18 and a part of the second semiconducting layer 18 to the side of the sloping edges. Besides, the high voltage cable 10 comprises a third semiconducting layer 26, for example in the form of tape such as the semiconducting tape, HL-tape®, IA 2352 from ABB Kabeldon, which is applied over the second insulating layer 24 in such a way that the one end of the third semiconducting layer 26 covers one edge of the second insulating layer 24 and has electric contact to the second semiconducting layer 18. The other end of the third semiconducting layer 26 does not cover the other side of the second insulating layer 24 but stops at a distance c from the other edge of the second insulating layer 24. The second insulating layer 24 should at least be 1 mm thick at the edge where the third semiconducting layer 26 does not cover the second semiconducting layer 24. However, the third semiconducting layer 26 must be stretched at its other end over (overlapping) the second semicon- ducting layer 18 located under the second insulating layer 24. The distance between the edge of the third semiconducting layer 26 and the edge of the second semiconducting layer 18 in the longitudinal direction of the cable 10 is d as shown in Figure 3. The third semiconducting layer 26 should be at least 1 mm thick.

Figure 4 shows schematically the earthing principle for a three phase power transformer/reactor in accordance with the present invention. Windings are shown as drawn out cables in order to clarify the Figure. Besides, a possible core of the three phase power transformer has been omitted. Three phase power transformers comprise three windings 1 , 2, 3 representing the different phases 1 , 2, 3. Each winding 1 , 2, 3 is constructed with the high voltage cable 10 shown in Figure 1. The cables for the different phases are designated as 10ι, 10₂, 10₃. The second semiconducting layer of each high voltage cable 10ι,10₂,10₃ is directly earthed at the points 32, 34 which are located at or in the vicinity of both ends of each winding 1 , 2, 3. Generally, the second semiconducting layer 18 is directly earthed at n points of each winding 1 , 2, 3, where n is an integral number and n>2, and whereby two of said directly earthed points are arranged at or in the vicinity of both ends of each winding 1 , 2, 3. This direct earthing is performed by means of a galvanic connection to earth. Besides, the electric contact in the second semiconducting layer is inter- rupted two times 20n, 20₂ι, 20₃₁, 20-ι₂, 20₂₂, 2O₃₂ per winding 1 , 2, 3. The electric contact in the second semiconducting layer 18 is generally interrupted 2(n -1) times per winding 1 , 2, 3. Even if not shown in Figure 4 there may be found arranged at each such interruption 20 a means 24, 26 comprising a second insulating layer 24 and a third semiconducting layer 26 in order to reduce the amplification of the elec- trie field strength at said interruption 20. This means 24, 26 is shown in Figure 3. The second semiconducting layer 18 of the three phases 1, 2, 3 at each said interruption 2O1₁, 20₂ι, 2O₃1, 20ι₂, 20₂₂, 20₃₂ is earthed in a cross-connected manner. Besides, the second semiconducting layers 18 of the three phases 1 , 2, 3 are indirectly earthed at two points 36, 38. Generally speaking, the number of indirectly earthed points may vary. As in the shown case the indirect earthing is performed by means of spark gaps 40. The indirect earthing may be per ormed in a number of different ways as for example in the aforementioned under the heading "Summary of the invention" and as shown in the Figures 6a, 6b. Cross-connected earthing 42, 44 is achieved through the second semiconducting layers 18 of the different phases 1 , 2, 3 being connected at each said interruption 20n, 20₂ι, 20₃ι, 20ι₂, 20₂₂, 20₃2 and being indirectly earthed via a spark gap 40. A more detailed description of cross-connected earthing will be discussed hereinafter.

The power transformer 30 in Figure 4 is provided with two interruptions 2O11, 20ι₂; 20₂ι, 20₂₂; 2O31, 20₃₂ per phase 1 , 2, 3 and thus three continuous sec- tions 18-n, I812, 18ι₃; I821, 18₂₂, 18₂₃; I8₃₁, 18₃₂, 18₃₃ of the second semiconducting layer 18 per phase 1 , 2, 3. At the first interruption 20n, the first section 18n of the second semiconducting layer 18 of the first phase 1 is connected to the second section 18₂₂ of the second phase 2. Besides, the first section 18n of the first phase 1 is connected to the first section 18₂ι, 18₃ of the remaining phases 2, 3 and connected to indirect earthing by means of a spark gap 40. The first section 18₂ι of the second phase 2 is connected to the second section 18₃₂ of the third phase 3. Besides, the second section 18ι₂ of the first phase 1 is connected to indirect earthing by means of the spark gap 40. Correspondingly, cross-connected earthing is applied to the second interruption 20ι₂ and is not repeated herein. Another way of describing this cross-connected earthing is to follow the connections from a direct earthing point to the next earthing point. To start with the direct earthing point 32, is followed by the first section 18n of the first phase 1 , which section 18n is connected to the second section 18₂₂ of the second phase 2, which section 18₂₂ is connected to the third section 18₃₃ of the third phase 3, which is connected to direct earth via the point 34. Correspondingly, sections 18₂ι-18₃₂ -I8₁₃ are connected between both of the direct earthing points 32, 34. Correspondingly, sections 18₃ι-18ι₂-18₂₃ are connected between both of the direct earthing points 32, 34. However, a general description of cross-connected earthing in a power transformer/reactor will be described hereinafter where there are n number of direct earthing points per phase.

Generally speaking from the point of view of one case, the second semiconducting layer 18 is directly earthed at n number of points of each winding 1 , 2, 3 where n is an integral number and n>2, and whereby two of said n directly earthed points are arranged at or in the vicinity of both ends of each winding 1 , 2, 3. This means that the electric contact is interrupted 20 2(n-1) amount of times in the second semiconducting layer 18 between both ends, seeing that there are two interruptions 20 between each pair having directly earthed points. This means that there exists 3(n-1) sections of the second semiconducting layer 18 per phase 1 , 2, 3, whereby one section begins at a directly earthed point or interruption 20 and ends at an interruption 20 or a directly earthed point.

At interruption 20 number q, where 1 <q<2(n-1), of the different phases there is section r, where 1 <r<3(n-1),of the second semiconducting layer 18 of one phase which is connected to section (r+1)of the second semiconducting layer 18 of the consecutive phase. Besides, section r of the first phase is connected to section r of the remaining phases. Section r of the last phase and section (r+1) of the first phase are connected to the indirect earthing by means of a spark gap 40. The aforementioned does not apply to r evenly divisible by 3, except for the last section, i. e. r=3(n-1) for a given n.

Figure 5 shows a diagram illustrating the potential of the second semiconducting layer 18 extending along the length of the cable. A power transformer hav- ing a Y connected winding is referred to in this case. This results then in that the voltage on the second semiconducting layer of the cable winding reduces linearly from the HV-connection to the neutral point under AC-voltage. Let the direct earthing points be designated A and D, and the two points for cross-connected earthing be designated B and C. Designate the distance between the direct earthing points A and D as L, the distance between A and B as , the distance between B and C as l₂ and the distance between C and D as l₃. If the ratio between the distance I, l₂ and l₃ is lι< l₂< and the surface potentials of the second semiconducting layer at the points B and C have the same value, as indicated in Figure 5, the current will be 0 in the second semiconducting layer, which means that the power losses in the sec- ond semiconducting layer will be negligible. The distances IH₃ and L are dependent on the dimension of the winding cable in addition to the thickness and the resistivity of the second semiconducting layer.

Figures 6a and 6b respectively, illustrate different elements in order to achieve indirect earthing. In Figure 6a, indirect earthing takes place by means of a circuit 50 comprising one element 52 having a non-linear voltage-current characteristic which is connected in parallel with a capacitor 54. In the shown case, the element 52 having a non-linear voltage-current characteristic is designed having one spark gap 52. The element 52 may also be designed having a gas-filled gas diode, a zener-diode or a varistor. In Figure 6b, indirect earthing takes place by means of a zener-diode 56.

Figure 7 shows a flow chart illustrating a method for adjusting a high voltage cable 10 (compare to Figure 1) comprising an electric conductor around which there is arranged a first semiconducting layer 14, around the first semiconducting layer 14 there is arranged a first insulating layer 16, and around the first insulating layer 16 there is arranged a second semiconducting layer 18. The method in accordance with the invention comprises a number of steps which will be described hereinafter. The flow chart starts at block 60. The next step, at block 62, is to indirectly earth 32, 34 the second semiconducting layer 18 at n points of each winding 1 , 2, 3 where n is an integral number and n >2, and whereby two of said n points are arranged at or in the vicinity of both ends of each winding 1 , 2, 3. Thereafter, at block 64, two interruptions 20 are achieved between each pair of directly earthed points in the electric contact in the second semiconducting layer 18. Thereafter, at block 66, a means 24, 26 is applied at each said interruption 20 in the second semiconducting layer 18, which means comprises a second insulating layer 24 and a third semiconducting layer 26 in order to reduce the amplification of the electric field at said interruption 20. Thereafter, at block 68, the second semiconducting layers of the different phases 1 , 2, 3 are earthed in cross-connected manner at each said inter- ruption 20. Thereafter, at block 70 at least one point 36, 38 of the second semiconducting layer 18 of each phase 1 , 2, 3 is indirectly earthed between both ends. The method is concluded at block 72. Reference is made to Figures 2 - 6 regarding further details relating to the method.

It should be indicated that power transformers/reactors may be manufac- tured with a magnetizable core and also manufactured without a magnetizable core. The invention is not limited to the embodiments described in the foregoing, several modifications are possible within the scope of the appended claims.

Claims

1. A power transformer/reactor comprising at least one winding (1 , 2, 3) characterized in that the winding/windings (1 , 2, 3) are manufactured with a high volt- age cable (10), comprising an electric conductor, around the conductor there being arranged a first semiconducting layer (14), around the first semiconducting layer (14) there being arranged a first insulating layer (16) and around the first insulating layer (16) there being arranged a second semiconducting layer (18), whereby the second semiconducting layer (18) is directly earthed (32, 34) at n points of each winding (1 , 2, 3), where n is an integral number and n>2, and whereby two (32, 34) of said n directly earthed points are arranged at or in the vicinity of both ends of each winding (1 , 2, 3), whereby the electric contact is interrupted (20) 2(n-1) number of times between both ends in the second semiconducting layer (18) , whereby the second semiconducting layers (18) of different phases (1 , 2, 3) at each said in- terruption (20) is earthed in a cross-connected manner (42, 44).

2. A power transformer/reactor according to claim 1 , characterized in that at least one point (36, 38) between both ends is indirectly earthed (40).

3. A power transformer/reactor according to claim 1 , characterized in that at each said interruption (20) in the second semiconducting layer (18) there is arranged a third semiconducting layer (26) in order to reduce the amplification of the electric field strength at said interruption (20),

4. A power transformer/reactor according to any one of claims 1-3, characterized in that the electric contact in the second semiconducting layers (18) are interrupted in that the second semiconducting layer (18) being removed around the periphery of the high voltage cable (10) down to the first insulating layer (16) so that grooves (20) surrounded by the second semiconducting layer (18) are formed.

5. A power transformer/reactor according to claim 4, characterized in that the second insulating layer (24) is arranged onto each groove (20), besides which the layer (24) covers a part of the second semiconducting layer (18) on both sides of each groove (20), and that the third semiconducting layer (26) is arranged at the second insulating layer (24), whereby the one end of the third semiconducting layer (26) covers the one edge of the second insulating layer (24) and has electric contact to the second semiconducting layer (18), and that the other end of the third semi- conducting layer (26) does not cover the other edge of the second insulating layer (24) but extends along part of the second semiconducting layer (18) located under the second insulating layer (24).

6. A power transformer/reactor according to claim 5, characterized in that the edges of the second semiconducting layer (18) at said grooves (20) slope in such a way that the grooves (20) have the least width at the first insulating layer (16).

7. A power transformer/reactor according to claim 6, characterized in that the third semiconducting layer (26) at the end covering the edge of the second insulat- ing layer (24) has mechanical contact with the second semiconducting layer (18), and that the other end of the third semiconducting layer (26) does not have mechanical or electric contact with the second semiconducting layer (18).

8. A power transformer/reactor according to any one of claims 1 - 7, charac- terized in that the high voltage cable (10) is manufactured having a conductor area which is between 80 and 3000 mm² and an outer cable diameter which is between 20 and 250 mm.

9. A power transformer/reactor according to any one of claims 1 - 8, charac- terized in that there are two interruptions (20) between two consecutive direct earthing points (32, 34)

10. A power transformer/reactor according to any one of claims 1 - 8, characterized in that each cross-connected earthing is formed by the second semicon- ducting layers (18) of the different phases (1 , 2, 3) at each said interruption (20) being connected and indirectly earthed (40).

11. A power transformer/reactor according to claim 10, characterized in that there are 2(n-1) number of interruptions (20n, 20_i2; 20₂╬╣, 20₂₂; 20₃╬╣, 20₃₂) per phase and thus 3(n-1) connected sections of the second semiconducting layer (I8₁₁, 18₁₂, 18╬╣₃; 18₂╬╣, 18₂₂, 18₂₃; 18₃╬╣, 18₃₂, I8₃₃) per phase, and that also at inter- ruption (20) number q, where 1 <q<2(n-1), of the different phases (1 , 2, 3) section r, where 1 <r<3(n-1), of the second semiconducting layer (18) of one phase (1 ; 2; 3) is connected to section (r+1) of the second semiconducting layer (18) of the con- . secutive phase, and that section r of the second semiconducting layer (18) of the first phase (1) is connected to section r of the second semiconducting layer (18) of the remaining phases (2, 3), and that section r of the second semiconducting layer (18) of the last phase (3) and section (r+1) of the second semiconducting layer (18) of the first phase (1) are connected to the indirect earthing (40), whereby the aforementioned does not apply to r evenly divisible by 3, except for the last section, i. e. r=3(n-1).

12. A power transformer/reactor according to any one of claims 1 - 11 , characterized in that the direct earthing (32, 34) is performed by means of galvanic connection to earth.

13. A power transformer/reactor according to any one of claims 1 - 12, characterized in that the indirect earthing is performed by means of a capacitor which is connected between the second semiconducting layer (18) and earth.

14. A power transformer/reactor according to any one of claims 1 - 12, charac- terized in that the indirect earthing is performed by means of an element, connected between the second semiconducting layer (18) and earth, having a non-linear voltage-current characteristic.

15. A power transformer/reactor according to any one of claims 1 - 12, charac- terized in that the indirect earthing is performed by means a circuit (50) comprising an element, connected between the second semiconducting layer (18) and earth, having a non-linear voltage-current characteristic (52) connected in parallel with a capacitor (54).

16. A power transformer/reactor according to claim 15, characterized in that the indirect earthings are performed by means of a combination of alternatives according to claims 13 - 15.

17. A power transformer/reactor according to any one of claims 14 - 16, characterized in that the element (52) having a non-linear voltage-current characteristic may be designed with a spark gap (52), a gas-filled gas diode, a zener-diode (56) or a varistor.

18. A power transformer/reactor according to any one of claims 1 - 17, characterized in that the power transformer/reactor comprises a magnetizable core.

19. A power transformer/reactor according to any one of claims 1 - 17, charac- terized in that the power transformer/reactor is manufactured without a magnetizable core.

20. A power transformer/reactor according to any one of claims 1 - 19, characterized in that said layers (14, 16, 18) are arranged to adhere to one another even when the cable is bent.

21. A method for adjusting a high voltage cable (10) for windings in a power transformer/reactor which high voltage cable (10) comprises an electric conductor, around which there is arranged a first semiconducting layer (14), around the first semiconducting layer (14) there is arranged a first insulating layer (16), and around the first insulating layer (16) there is arranged a second semiconducting layer (18), which method comprises the following steps:

ΓÇó to directly earth (32, 34) the second semiconducting layer (18) at n points of each winding (1 , 2, 3), where n is an integral number and n>2, and whereby two (32, 34) of said n points are arranged at or in the vicinity of both ends of each winding (1 , 2, 3);

ΓÇó to achieve two interruptions (20) between each pair of directly earthed points in the electric contact in the second semiconducting layer (18); and ╬▓ to earth in cross-connected manner the second semiconducting layer (18) of the different phases (1 , 2, 3) at each said interruption (20);

22. A method according to claim 21 , characterized in that said method com- 5 prises furthermore the step:

ΓÇó to indirectly earth at least one point (36, 38) in each phase between both ends of the second semiconducting layer (18).

23. A method according to any one of claims 21 - 22, characterized in that said 10 method comprises furthermore the step:

ΓÇó to apply a third semiconducting layer (26) at each said interruption (20) in the second semiconducting layer (18) in order to reduce the amplification of the electric field strength at said interruption (20).

15 24. A method according to any one of claims 21 - 23, characterized in that each said interruption (20) is achieved by removing the second semiconducting layer (18) around the periphery of the high voltage cable (10) down to the first insulating layer (16) so that grooves (20) surrounded by the second semiconducting layer (18) are formed.

20

25. A method according to claim 24, characterized in that the step of applying said means (24, 26) comprises the following steps:

ΓÇó to apply a second insulating layer (24) over each groove (20) in such a way that part of the second semiconducting layer is (18) on both sides of each groove (20)

25 is additionally covered; and

ΓÇó to apply a third semiconducting layer (26) on the second insulating layer (24) in such a way that the one end of the third semiconducting layer (26) covers the one edge of the second insulating layer (24) and has electric contact to the second semiconducting layer (18), and the other end of the third semiconducting

30 layer (18) does not cover the other edge of the second insulating layer (24) but extends along a part of the second semiconducting layer (18) located under the second insulating layer (24).

26. A method according to any one of claims 21 - 25, characterized in that the step to earth in cross-connected manner comprises the following step:

ΓÇó to connect the second semiconducting layer of the different phases (1 , 2, 3) at each said interruption (20), and indirectly earthing the aforementioned.

5

27. A method according to claim 26, characterized in that the step to earth in cross-connected manner comprises furthermore the following steps; whereby the number of interruptions (20n, 20╬╣₂; 20₂╬╣, 20₂₂, 20₃₁, 2╬╕₃₂) per phase are 2(n-1) and the number of connected sections of the second semiconducting layer (18n, 18╬╣₂,

10 18₁₃; 18₂╬╣, 18₂2, 18₂₃; I831, 18₃₂, I833) per phase are 3(n-1):

ΓÇó to connect at interruption (20) number q, where 1 <q<2(n-1), of the different phases (1 , 2, 3), section r, where1<r<3(n-1), of the second semiconducting layer (18) of one phase (1 , 2, 3) to section (r+1) of the second semiconducting layer of the consecutive phase;

15 ΓÇó to connect section r of the second semiconducting layer (18) of the first phase (1) to section r of the second semiconducting layer (18) of the remaining phases (2, 3); and

ΓÇó to connect section r of the second semiconducting layer (18) of the last phase (3) and section (r+1) of the second semiconducting layer (18) of the first phase (1) to

20 the indirect earthing (40), whereby the aforementioned does not apply to section r, where r is evenly divisible by 3, except for the last section, i. e. r=3(n-1).