Note: Descriptions are shown in the official language in which they were submitted.
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Axial air-cooling of transformers
TECHNICAL FIELD:
The present invention relates to an air-cooled, conductor-
wound power transformer and to a method of air-cooling
conductor-wound power transformers.
BACKGROUND ART:
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. 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
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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.
OBJECT OF THE INVENTION:
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. In a first embodiment, 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.
SUMMARY OF THE INVENTION:
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.
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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.
In spite of the fact that "air" is mentioned as coolant, also
other gas coolants are suitable, for example helium gas
coolant.
In a power transformer according to the invention 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. Such a cable comprises an inner
conductor composed of one or more strand parts, an inner
semiconducting layer surrounding the conductor, a solid
insulating layer surrounding this and an outer semiconducting
layer surrounding the insulating layer. Such cables are
flexible, which is an important property in this context since
the 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. 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.
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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. In a 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
_ 20 specified and also the condition of being semiconducting, i.e.
having resistivity within the range of 10-1 - 106 ohm-cm, e.g.
1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the
scope of the invention.
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.
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
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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
5 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.
Thus, each of the two semiconducting layers essentially
constitutes one equipotential surface, and these layers will
substantially enclose the electrical field between them. There
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is, of course, nothing to prevent one or more additional
semiconducting layers being arranged in the insulating layer.
BRIEF DESCRIPTION OF THE DRAWINGS:
The invention will now be described in more detail with
reference to the accompanying drawings.
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.
DESCRIPTION OF THE 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 in 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-lying turn of the winding. The
distribution between the spacers 4 in the embodiment shown is
such that six spacers are obtained in 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,
___._~_.~._.~ ___._....._~W_.~___..-.._.. .._.._...___.__~_.
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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. To achieve a correct distribution of coolant 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. In this
embodiment, it is also clear that a fan is mounted for each
coil. The air can be withdrawn from or forced through the coil
by each fan 10. The fan duct 7 at the side of the coil
opposite to the fan 10 is completely open for air to flow
either in or out depending on the suction or pressure function
of the fan. The fan duct 7 on the fan side is provided with
openings having a corresponding function.
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Figure 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
S 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.
Figure 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. Around the strands 112
is a first semi-conducting layer 113. Around the first semi-
conducting layer 113 is an insulating layer 114, e.g. XLPE
insulation. Around the insulating layer 114 is a second semi-
conducting layer 115. Thus the concept "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 mm2.
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.
Similarly, neither the number of spacers nor their shape is
fixed and several different spacer variants are possible to
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achieve the correct cooling. Neither need the spacers in the
first embodiment described run entirely axially but may be
placed in several ways.
S 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.
