Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F27/00—Details of transformers or inductances, in general
  • H01F27/28—Coils; Windings; Conductive connections
  • H01F27/288—Shielding

Definitions

• 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.
• transformers 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.
• 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.
• 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.
• 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.
• the power transformer/reactor 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.
• 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.
• 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.
• 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.
• 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,
• 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
• 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 ;
• Figure 7 shows a flow chart of the method of adapting a high voltage cable according to the present invention.
• FIG. 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.
• Cu copper
• first semiconducting layer 14 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 layerl ⁇ .
• 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.
• 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.
• 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.
• 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.
• LDPE low-density polyethylene
• HDPE high-density polyethylene
• PP polypropylene
• PB polybutylene
• TPX polymethyl pentene
• XLPE cross-linked materials
• EPR ethylene propylene 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 EBA
• EBA ethylene-butyl-acrylate copolymers
• EAA ethylene-ethyl-acrylate copolymers
• 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.
• each of the two semiconducting layers essentially constitutes one equipotential surface, and these layers will substantially enclose the electrical field between them.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 Jardindon.
• 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.
• 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 Jardindon, 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.
• 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.
• FIG 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 10 ⁇ , 10 2 , 10 3 .
• the second semiconducting layer of each high voltage cable 10 ⁇ ,10 2 ,10 3 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.
• 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.
• the electric contact in the second semiconducting layer is inter- rupted two times 20n, 20 2 ⁇ , 20 31 , 20- ⁇ 2 , 20 22 , 2O 32 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.
• 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 2O1 1 , 20 2 ⁇ , 2O 3 1, 20 ⁇ 2 , 20 22 , 20 32 is earthed in a cross-connected manner.
• 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.
• 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, 20 2 ⁇ , 20 3 ⁇ , 20 ⁇ 2 , 20 22 , 20 3 2 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, 20 ⁇ 2 ; 20 2 ⁇ , 20 22 ; 2O31, 20 32 per phase 1 , 2, 3 and thus three continuous sec- tions 18-n, I812, 18 ⁇ 3 ; I821, 18 22 , 18 23 ; I8 31 , 18 32 , 18 33 of the second semiconducting layer 18 per phase 1 , 2, 3.
• the first section 18n of the second semiconducting layer 18 of the first phase 1 is connected to the second section 18 22 of the second phase 2.
• the first section 18n of the first phase 1 is connected to the first section 18 2 ⁇ , 18 3 of the remaining phases 2, 3 and connected to indirect earthing by means of a spark gap 40.
• the first section 18 2 ⁇ of the second phase 2 is connected to the second section 18 32 of the third phase 3.
• the second section 18 ⁇ 2 of the first phase 1 is connected to indirect earthing by means of the spark gap 40.
• cross-connected earthing is applied to the second interruption 20 ⁇ 2 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 18 22 of the second phase 2, which section 18 22 is connected to the third section 18 33 of the third phase 3, which is connected to direct earth via the point 34.
• sections 18 2 ⁇ -18 32 -I8 13 are connected between both of the direct earthing points 32, 34.
• sections 18 3 ⁇ -18 ⁇ 2 -18 23 are connected between both of the direct earthing points 32, 34.
• 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.
• 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.
• 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.
• section r of the first phase is connected to section r of the remaining phases.
• 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.
• 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 IH 3 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.
• 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.
• 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.
• 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.
• two interruptions 20 are achieved between each pair of directly earthed points in the electric contact in the second semiconducting layer 18.
• 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.
• the second semiconducting layers of the different phases 1 , 2, 3 are earthed in cross-connected manner at each said inter- ruption 20.
• 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.
• 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.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Coils Of Transformers For General Uses (AREA)
• Regulation Of General Use Transformers (AREA)
• Insulated Conductors (AREA)

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Citations (2)

• 1997
  • 1997-09-30 SE SE9703563A patent/SE511361C2/sv not_active IP Right Cessation
• 1998
  • 1998-09-29 AU AU93714/98A patent/AU9371498A/en not_active Abandoned
  • 1998-09-29 DE DE19882712T patent/DE19882712T1/de not_active Withdrawn
  • 1998-09-29 JP JP2000514287A patent/JP2001518698A/ja active Pending
  • 1998-09-29 WO PCT/SE1998/001749 patent/WO1999017312A2/en active Application Filing

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