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
  • H01F27/28—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 circulating oil is conveyed 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, created 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. Besides its role as conveyor of heat, 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 cable of the type presented in the description.
• the invention aims at producing horizontal cooling ducts between each layer of cables where the coolant is correctly distributed in order to satisfy the cooling requirements of the cable layers.
• the core and winding are cooled by cooling air flowing radially through horizontal ducts in the winding.
• the air is distributed in the ducts according to their cooling requirement by varying the height of the horizontal ducts.
• 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 axial direction in the winding in order to create cooling ducts.
• the invention thus comprises radial disc-shaped cooling ducts between each layer of cable, i.e. each winding turn in axial direction, created by means of spacers arranged in various ways during winding of the coil, as can be seen in the embodiments illustrated.
• the embodiments also comprise horizontally acting fans for the transport of air through the ducts.
• the coolant "air” also can be other gas coolants, such as helium gas.
• 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.
• 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 m or not - at least m 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.
• Etnylene-v nyl-acetate copolymers/nit ⁇ le 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.
• Tne 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 oetween the coefficients of thermal expansion for the materials in the layers to be absorbed m 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 la shows a view in perspective of a winding coil in a first embodiment according to the invention showing horizontal ducts between the layers of cable.
• Figure lb shows a corresponding view to that in Figure 1 but including the iron core.
• Figure 2a shows a view from above of the first embodiment according to Figure 1, with spacers constituting horizontal ducts for air flowing through at right angles to the spacers.
• Figure 2b shows a second embodiment corresponding to Figure 2a but where the spacers are oriented parallel to the direction of flow.
• Figure 2c shows a view from above of a third embodiment of the present invention, with four radial spacers forming ducts, with arrows indicating the direction of air flow.
• Figure 2d shows a view from above of a fourth embodiment of the present invention having eight radial spacers forming ducts, with arrows indicating the direction of air flow.
• Figure 3 shows schematically a radial section through the embodiment according to Figure 2c, with a fan arrangement according to the invention.
• Figure 4 shows a section through a high-voltage cable used in the winding coil according to the present invention.
• Figure la shows a winding coil 1 arranged with cable.
• the cable is wound m a disk winding, each new turn being wound radially outside the previously wound layer.
• the winding coil 1 is wound with spacers 3 inserted between each turn.
• the spacers are in the form of blocks 30 shaped to abut the outer surface of the cable.
• Figure lb shows part of a transformer with a cable-wound coil 1, but arranged around a laminated iron core in the form of a leg 2 of the transformer core.
• the winding coil 1 is wound with spacers 3 as above inserted between each winding turn.
• Figure 2 shows windings provided with a different type of spacer, beam-shaped spacers, placed in four different ways.
• the spacers can be arranged n several different ways in order to obtain spaces between the disc-snaoed winding turns in order to enable horizontal cooling through the winding coil with the aid of an air flow.
• Figure 2a shows a first embodiment corresponding ro that shown n Figure 1, in which the spacers 3 are placed between each winding turn with an orientation perpendicular to the flow direction.
• the spacers are arranged with one spacer diametrically, like the one m the middle of the Figure, and/or as at least one chord through the winding between each winding turn, like the two spacers on each side of the iron core.
• Tne spacers are oriented perpendicularly to the direction of flow. The air flow is indicated by arrows in the Figure and is perpendicular to the spacers and horizontal in the case of upright winding coils.
• Figure 2b shows a second embodiment substantially identical to that in Figure 2a but with the spacers instead oriented parallel to the direction of flow.
• the air flow is indicated by arrows in the Figure and is substantially parallel with the spacers m this embodiment.
• Figure 2c shows a third embodiment with spacers 3 placed radially outwardly from the iron core between each winding turn.
• the embodiment is arranged with at least four spacers 3 distributed uniformly around the leg 2 of the transformer core.
• the air flow is indicated by arrows m the Figure and is directed perpendicularly to the iron core, parallel to two of the spacers and perpendicular to the other two spacers.
• Figure 2d shows a fourth embodiment provided with spacers 3 placed radially out from the iron core between each winding turn.
• eight spacers 3 are distributed uniformly around the leg 2 of the transformer core.
• the air flow is indicated by arrows in the Figure and is directed perpendicularly in towards the iron core, parallel with two of the spacers, perpendicular to another two of the spacers and partly parallel to the remaining four spacers.
• the air flow is the same as in Figure 2c, but around additionally four spacers.
• Figure 3 is a view from above of part of a three-phase power transformer provided with windings 1 constituting coils which, by means of the spacers shown above, have been provided with cooling ducts . 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 ron core.
• the yoke 5 is shown in section and is provided with a laminar structure corresponding to the leg 2 of the transformer core.
• Each winding coil is also surrounded by a fan cowl 6 inside which cooling air is arranged to flow.
• the winding 1 is provided with a fan 7 arranged to either force or suck cooling air through the winding 1.
• the fan cowl is provided with an opening 8 which may be shaped m many ways but preferably extends along the entire length of the winding coil.
• the width of the opening 8 is 0.1 0 - 0.6 0, where 0 is the outer diameter of the coil.
• the cooling requirement is different for the various turns of the winding, which means that the flows of coolant in the horizontal ducts differ.
• the ducts nave different dimensions in axial 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 axial extension than ducts with greater cooling requirement.
• Arrows indicate a cooling a r flow m the Figure, effected by the fan 7 forcing air through the winding coil.
• the fan cowl 6 is arranged to seal at both ends of the coil against either an end plate 9, indicated in broken lines in Figure 3, or curving in to seal against the outermost winding turn so that cooling air can flow out axially along the leg of the iron core.
• the emoodiment in Figure 3 has one fan per winding coil .
• FIG. 4 shows a cross-sectional view of a high-voltage cable 111 for use as transformer winding m 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 cab b ie 111.
• a first semi-conducting layer 113 Around the strands 112 is a first semi-conducting layer 113.
• an insulating layer 114 e.g. XLPE insulation.
• Around the insulating layer 114 is a second semiconducting layer 115.
• high-voltage cable in the present application does not include the outer sheath that normally surrounds such cables for power distribution.
• 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 is not necessary for each coil, for instance. An arrangement is feasible with one fan supplying all three coils with sufficient air. As also indicated above, the air can be either sucked or forced through the coils in order to achieve the desired cooling. Similarly, neither the number of spacers nor their shape is fixed and several different spacer variants are possible to achieve the correct cooling.
• 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 thus cool the outside by means of natural convection. It is also possible to use the uppermost or lowermost cable layer as end plates. The uppermost and lowermost sides of the cable layers would then be cooled through auto-convection.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Coils Of Transformers For General Uses (AREA)
• Transformer Cooling (AREA)

Applications Claiming Priority (5)

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• 1997
  • 1997-11-28 SE SE9704415A patent/SE9704415D0/sv unknown
• 1998
  • 1998-02-02 JP JP53279898A patent/JP2001509960A/ja active Pending
  • 1998-02-02 AU AU58907/98A patent/AU5890798A/en not_active Abandoned
  • 1998-02-02 WO PCT/SE1998/000156 patent/WO1998034239A1/en not_active Application Discontinuation
  • 1998-02-02 EP EP98902353A patent/EP1016099A1/en not_active Withdrawn

Non-Patent Citations (1)

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