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
  • H01F27/08—Cooling; Ventilating
  • H01F27/085—Cooling by ambient air
• • 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
  • H01F27/08—Cooling; Ventilating
• • 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
  • H01F27/32—Insulating of coils, windings, or parts thereof
  • H01F27/322—Insulating of coils, windings, or parts thereof the insulation forming channels for circulation of the fluid

Definitions

• the present invention relates to an air-cooled, conductor- wound power transformer and to a method of air-cooling conductor-wound power transformers.
• Modern power transformers are usually oil-cooled.
• the core consisting of a number of core legs joined by yokes, and the windings (primary, secondary, control), are immersed in a closed container filled with oil. Heat generated in coils and core is removed by the oil circulating internally through coils and core.
• the oil circulates out to an external unit where it is cooled.
• the oil circulation may either be forced, the oil being pumped around, or it may be natural, produced by temperature differences in the oil.
• the circulating oil is cooled externally by arrangements for air-cooling or water- cooling. External air-cooling may be either forced or through natural convection.
• the oil also has an insulating function in oil-cooled transformers for high voltage.
• Dry transformers are usually air-cooled. They are usually cooled through natural convection since today' s dry transformers are used at low power loads.
• the present technology relates to axial cooling ducts produced by means of a pleated winding as described in GB 1,147,049, axial ducts for cooling windings embedded in casting resin as described in EP 83107410.9, and the use of cross-current fans at peak loads as described in SE 7303919-0.
• the cooling requirement is greater for a conductor-wound power transformer. Forced convection is necessary to satisfy the cooling requirement in all the windings . Natural convection is not sufficient to cool the conductor windings . A short transport route for the heat to the coolant is important, and also that it is efficiently transferred to the coolant. It is therefore important that all windings are in direct contact with sufficient quantities of coolant.
• a conductor is known through US 5 036 165, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glassfiber. It is also known to provide conductors in a dynamo-electric machine with such an insulation, as described in US 5 066 881 for instance, where a semiconducting pyrolized glassfiber layer is in contact with the two parallel rods forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glassfiber.
• the pyrolized glassfiber material is described as suitable since it retains its resistivity even after the impregnation treatment.
• the object of the invention is to provide a device according to the present claims, i.e. of the type described in the introduction which will enable air-cooling of a cable-wound power transformer comprising a high-voltage conductor of the type presented in the description.
• the invention aims at producing axial cylindrical ducts between each turn of the winding in windings where the coolant is correctly distributed in order to satisfy different cooling requirements of the windings.
• the cylindrical ducts are created by inserting spacers during winding of the coil.
• the flow of coolant is achieved with fans and the spacers are dimensioned to provide a flow through the ducts which will satisfy the cooling requirements of the individual windings.
• the present invention relates to a power transformer comprising a transformer core wound with cable, arranged so that the winding is provided with spacers separating each cable turn in radial direction in the winding in order to create axial cylindrical ducts.
• a first embodiment of the invention thus comprises axial cylindrical cooling ducts between each winding turn placed one above the other, said ducts being created by spacers being inserted during winding of the coil.
• a cylindrical duct is also arranged between the legs of the core and the first layer of cable nearest the core.
• the embodiment also comprises fans for transporting air through the axial cylindrical ducts .
• the spacers in the ducts are dimensioned to give varying resistance, thus distributing the flow of coolant so that it covers the cooling requirement in the individual axial ducts since the cooling requirement is different for the windings.
• air is mentioned as coolant
• other gas coolants are suitable, for example helium gas coolant .
• the windings are composed of 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 context since the technology for the device 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 a XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable 30 mm in diameter, and a radius of curvature of approximately 65 cm for a cable 80 mm in diameter.
• 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.
• Windings in the present invention are constructed to retain their properties even when they are bent and when they are subjected to thermal 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.
• the insulating layer consists of cross-linked, low-density polyethylene
• 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 insulating lay 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 (PMP) , 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
• PMP polymethyl pentene
• XLPE cross-linked polyethylene
• 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 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, butyl graft polyethylene, ethylene-butyl-acrylate-copolymers and ethylene- ethyl-acrylate copolymers 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 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 or other damage appear 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 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 large to contain the electrical field in the cable, but sufficiently small 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. There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulating layer.
• Figure 1 shows one embodiment of a power transformer according to the invention, in perspective.
• Figure 2a shows a view from above of the windings with cooling ducts, spacers and outer cover in a first embodiment according to the present invention.
• Figure 2b shows a side view of the embodiment in Figure 2a provided with one fan per coil .
• Figure 3 shows a section through a coil according to the embodiment in Figure 1 with its axial ducts between the windings .
• Figure 4 shows a section through a high-voltage cable according to the present invention.
• Figure 1 shows an embodiment of the invention relating to a power transformer 1 provided with three winding coils 2, each having a number of windings arranged m winding turns radially separated by axial spacers 4 to produce axial concentric cooling ducts 3.
• the transformer is provided with an iron core in conventional manner.
• Figure 2a shows a view from above of a three-phase power transformer 1 provided with windings 2 constituting coils with cooling ducts 3 produced by axially extending spacers 4 placed between each radially-lymg turn of the winding.
• the distribution between the spacers 4 in the embodiment shown is such that six spacers are obtained m each concentric cooling duct 3. From the cooling aspect the shape and material of the spacers are of minor significance.
• the mechanical, magnetic and electrical aspects of the transformer determine the shape, number and material of the spacers.
• the figure also shows the yoke 5 of the transformer, which constitutes a part of its iron core. The yoke is shown in section with its longitudinal cooling tubes 6 indicated.
• Each winding coil is also surrounded by a fan duct 7 inside which cooling air is arranged to flow.
• the cooling requirement is different for the windings, which means that the cooling flows in the concentric ducts differ.
• the ducts have different dimensions in radial direction in order to give different resistance in the ducts and thus distribute the flow in accordance with the needs of the ducts.
• Ducts with little cooling requirement thus have a smaller radial distance than ducts with greater cooling requirement which therefore have a larger radial distance.
• the cable-wound transformer described in the embodiment has larger spacing between the low-voltage windings, the windings closest to the core, than between the high-voltage windings .
• Figure 2b shows a side view of the power transformer in Figure 2a, provided with corresponding windings and a corresponding yoke 5 together with its three legs 8 forming the iron core.
• the fan duct 7 is at one end of the coils and forms a fan cowl 9 in which at least one fan 10 is mounted.
• the embodiment in the figure shows three fans, closed in relation to their respective coils, in order to produce air flow in the axial cylindrical cooling ducts 3.
• the coils are encased in an outer cylindrical casing 11 to prevent radial leakage of air and to guide the air axially through the coils.
• the casing 11 around the outermost cable winding produces an outer duct for cooling of the outer part of the outermost cable winding.
• FIG. 3 shows a cross section of a coil with axial cylindrical cooling ducts 3 between each radial winding 2. Spacers are also arranged to form an axial cooling duct between the legs 8 of the core and the winding nearest the core. The cooling ducts are created by the spacers placed between the windings, see Figure 2a.
• the spacers are placed around the circular cross section and run in axial direction.
• the spacers are placed between the turns of the winding while the coil is being wound.
• the arrows in the figure indicate air flow through the windings of the coil. The air can flow in either direction, depending on the suction or pressure action.
• FIG. 4 shows a cross-sectional view of a high-voltage cable 111 for use as transformer winding in accordance with the present invention.
• the high-voltage cable 111 comprises a number of strands 112 of copper (Cu) , for instance, having circular cross section. These strands 112 are arranged in the middle of the high-voltage cable 111.
• a first semi-conducting layer 113 Around the first semiconducting layer 113 is an insulating layer 114, e.g. XLPE insulation.
• Around the insulating layer 114 is a second semiconducting layer 115.
• the high-voltage cable has a diameter within the range of 20-250 mm and a conducting area within the range of 40-3000 mm 2 .
• the invention is not limited to the examples shown. Several modifications are feasible within the scope of the invention.
• a fan need not be provided for each coil, for instance.
• An arrangement is feasible with one fan supplying all three coils with sufficient air. The air can be either sucked in or forced through the coils in order to achieve the desired cooling.
• neither the number of spacers nor their shape is fixed and several different spacer variants are possible to achieve the correct cooling. Neither need the spacers in the first embodiment described run entirely axially but may be placed in several ways.
• Another modification is to arrange speed control of the fan with the aid of temperature sensors in order to enable a varied cooling requirement, depending on the load in the transformer .
• the casing may also be arranged in a number of other ways than shown in the embodiments described above.
• the outermost cable winding can be used as outer casing and cool the outside by means of natural convection.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Coils Of Transformers For General Uses (AREA)
• Transformer Cooling (AREA)
• Motor Or Generator Cooling System (AREA)
• Coils Or Transformers For Communication (AREA)
• Insulated Conductors (AREA)

Priority Applications (9)

Applications Claiming Priority (4)

Publications (1)

Family

ID=26662864

Family Applications (1)

Country Status (14)

Cited By (4)

Families Citing this family (10)

Citations (2)

Family Cites Families (1)

• 1997
  • 1997-11-28 SE SE9704414A patent/SE9704414D0/sv unknown
• 1998
  • 1998-02-02 EA EA199900703A patent/EA001869B1/ru not_active IP Right Cessation
  • 1998-02-02 WO PCT/SE1998/000155 patent/WO1998034238A1/en not_active Application Discontinuation
  • 1998-02-02 EP EP98902352A patent/EP1016098A1/en not_active Withdrawn
  • 1998-02-02 KR KR1019997006652A patent/KR20000070417A/ko not_active Application Discontinuation
  • 1998-02-02 CN CN98801964A patent/CN1244283A/zh active Pending
  • 1998-02-02 TR TR1999/01700T patent/TR199901700T2/xx unknown
  • 1998-02-02 AU AU58906/98A patent/AU724972B2/en not_active Ceased
  • 1998-02-02 PL PL98334617A patent/PL334617A1/xx unknown
  • 1998-02-02 NZ NZ337098A patent/NZ337098A/en unknown
  • 1998-02-02 BR BR9807134-3A patent/BR9807134A/pt not_active IP Right Cessation
  • 1998-02-02 CA CA002276620A patent/CA2276620A1/en not_active Abandoned
  • 1998-02-02 JP JP53279798A patent/JP2001509959A/ja active Pending
• 1999
  • 1999-07-28 NO NO993673A patent/NO993673D0/no not_active Application Discontinuation

Patent Citations (2)

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