Resin-insulated dry transformer

Abstract

 A resin-insulated dry transformer is described, said transformer comprising: a magnetic core (20); an electrically conductive primary winding; and an electrically conductive secondary winding. The secondary winding is electrically insulated from the primary winding and is linked to the primary winding by means of the magnetic core. The secondary winding comprises a cooling channel (33) with a number of spacers (10,10'). According to the invention, the spacers are made, at least partially, of an electrically conductive material.

Description

[0001] The present invention relates to the sector of electrical transformers. In particular, the present invention relates to a medium voltage to low voltage (MV-LV) resin-insulated dry transformer. Moreover, the present invention relates to a method for reducing the temperature of the low voltage winding of such a resin-insulated dry transformer.

[0002] As is known, a transformer is an electrical machine able to transform an input voltage (and the input current associated with it) into an output voltage (and into an output current associated with it).

[0003] Typically, a transformer comprises a primary winding and a secondary winding. The primary and secondary windings are insulated electrically from each other and are linked together by means of a magnetic circuit; for example, the primary and secondary windings may be wound concentrically around a magnetic core.

[0004] An alternating input voltage applied to the terminals of the primary winding induces a magnetic flux. In turn, this magnetic flux induces an alternating output voltage at the terminals of the secondary winding. As is known, the ratio between the amplitude of the output voltage and the amplitude of the input voltage is equal to the ratio between the number of turns of the secondary winding and the number of turns of the primary winding.

[0005] For example, the electric mains supplies users with electric power in the form of an alternating voltage, generally called "low voltage". This low voltage generally has an amplitude Vb of about a few hundred volts. However, in the case of large-scale users, such as factories, shopping centres or the like, the electric power may be provided in the form of "medium voltage", the amplitude Vm of which is in the region of a few tens of thousands of volts. In this case, the user typically uses a transformer to convert the medium voltage which is supplied by the electric mains into low voltage. In these MV-LV transformers, the number of turns Nm of the primary winding (or medium voltage winding) and the number of turns Nb of the secondary winding (or low voltage winding) are such that:

[0006] Since electric power is generally in the form of three-phase voltage (for example in Italy and in other European countries, the three phases are called U, V, W, while in the USA they are called X, Y, Z), these MV-LV transformers typically comprise three parts, commonly called "columns"; each column of a three-phase transformer functions as described above. Each column of a three-phase transformer therefore transforms a phase of the medium voltage input into a respective phase of the low voltage output.

[0007] In order to make a transformer safer and more compatible with the environment in which it is installed, it is known to enclose the windings inside a casing of insulating material. In particular, resin-insulated dry transformers are known where this casing is made of epoxy resin.

[0008] As will be explained in greater detail below, in a MV-LV resin-insulated dry transformer, the medium voltage winding is situated outside the low voltage winding and in a concentric manner with respect thereto. The turns of the medium voltage winding are embedded in the resin casing. The low voltage winding typically comprises turns formed by a sheet of conductive material rolled up tightly together with a sheet of insulating material. In this way, a low voltage winding substantially in the form of a hollow cylindrical body is created. The turns are thus situated around a magnetic core in a concentric manner.

[0009] Since the windings are resistive, they dissipate energy; therefore, during operation of the transformer, their temperature tends to increase. This increase in the temperature could, for example, damage the resin casing, with consequent malfunctioning of the transformer and/or danger for the surrounding environment. Special regulations establish the maximum over-temperature at which a transformer may operate. For example, MV-LV resin-insulated dry transformers are insulated in the class F defined by the standard IEC 60076-11; according to this standard, this type of transformer may operate at a maximum over-temperature of 100 K.

[0010] In the art solutions which allow reduction of the temperature of a transformer in order to keep the over-temperature of a transformer below the limits established by the standards are known.

[0011] In particular, in MV-LV resin-insulated dry transformers it is known to provide the low voltage winding with a cooling channel, in the form of a space with an annular cross-section which separates a certain number of inner lying turns from a certain number of outer lying turns. The turn facing onto the cooling channel, i.e. that in contact with the air present in the said channel, evacuates a part of the energy dissipated from the low voltage winding, releasing it to the air; in this way, the temperature of the low voltage winding is reduced.

[0012] A cooling channel is typically achieved by means of spacers. These spacers are typically in the form of battens. They are positioned between a turn of conductive material and an insulating layer adjacent thereto, with an axis directed parallel to the axis of the winding. The number of spacers used and their positions along the perimeter of the turn are chosen so that the annular cross-section of the cooling channel has a radial dimension which is substantially constant along the perimeter of the said annular cross-section.

[0013] Typically, the spacers are made of insulating material, for example fibreglass, epoxy resin or the like.

[0014] This, however, results in some disadvantages. First of all, the presence of spacers made of insulating material in contact with the surface of the turn made of conductive material reduces the surface area by means of which the turn releases dissipated energy to the air of the cooling channel. At the insulating spacers, the turn is in fact thermally insulated from the cooling channel. Therefore, the surface of the turn facing the cooling channel is not used in an optimum manner.

[0015] Moreover, the efficiency with which the cooling channel reduces the temperature of the low voltage winding depends on the effective cross-sectional area of the cooling channel. Since the cooling channel contains the spacers made of insulating material, the effective cross-sectional area of the cooling channel is the area of the annular cross-section from which the area of the cross-sections of the spacers must be subtracted. Therefore, so that the cooling channel has a sufficiently high effective cross-sectional area, it is necessary to increase the radial dimension of the annular cross-section, with a consequent increase in the size of the low voltage winding and therefore the entire transformer.

[0016] Therefore, in general, the object of the present invention is to provide an MV-LV resin-insulated dry transformer which solves the abovementioned problems.

[0017] In particular, one object of the present invention is to provide an MV-LV resin-insulated dry transformer in which the temperature of the low voltage winding and, more generally, of the entire transformer, is reduced in a more efficient manner than in the known transformers.

[0018] A related object of the present invention is also to provide a MV/LV resin-insulated dry transformer which has dimensions smaller than those of known MV/LV resin-insulated dry transformers.

[0019] These and other objects are achieved by a transformer according to Claim 1 and by a method for reducing the temperature of a low voltage winding in a transformer according to Claim 5. Further advantageous characteristic features of the invention are described in the respective dependent claims. All the claims are to be regarded as forming an integral part of the present description.

[0020] According to a first aspect of the present invention a resin-insulated dry transformer is provided, said transformer comprising: a magnetic core; an electrically conductive primary winding; and an electrically conductive secondary winding. The secondary winding is electrically insulated from the primary winding. The secondary winding is linked to the primary winding by means of the magnetic core. The secondary winding comprises a cooling channel with a number of spacers. According to the invention, the spacers are made, at least partially, of an electrically conductive material.

[0021] Therefore, advantageously, in the transformer according to the present invention, the evacuation of the dissipated energy is more efficient than in the known transformers. On the one hand, in fact, the spacers made of electrically conductive material do not thermally insulate the turn facing the cooling channel; therefore, there is an increase in the surface area by means of which the turn releases dissipated energy to the air present in the cooling channel.

[0022] On the other hand, the spacers made of electrically conductive material are preferably profiled and do not consist of bodies with a full cross-section. As a result of this, it is possible to optimize the use of the cooling channel and if necessary reduce the radial dimension of the channel.

[0023] In this way, a low voltage winding with smaller radial dimensions is obtained. It is therefore possible to obtain a magnetic core with smaller dimensions and therefore lighter than that in known transformers. The reduction in the dimensions of the low voltage winding also results in a reduction in the quantity of conductive material necessary for making the turns, with a consequent reduction in the cost and time needed for manufacture of the transformer. Alternatively, it is possible to obtain, for the same transformer size, a low voltage winding with a larger number of turns; the increase in the number of turns allows advantageously a reduction in the cross-section of the magnetic core and consequently both windings, with a consequent reduction in the weight and the overall volume of the transformer.

[0024] Finally, advantageously, in the transformer according to the present invention there is a reduction in the dispersed power. The dispersed power of a transformer is generally defined as the difference between the power input into the transformer and the power output from the transformer. The dispersed power decreases with a reduction in the electric resistance of the windings, namely with the increase in the cross-section of the conductors. Since the spacers, according to the present invention, are made of electrically conductive material, they constitute, from an electrical point of view, a second conductor situated in parallel with the low voltage winding. In other words, the low voltage winding according to the present invention has a larger conductive cross-section. Therefore the electric resistance of the low-voltage winding decreases, resulting in a reduction in the dispersed power and therefore an increase in the efficiency of the transformer.

[0025] Further characteristic features and advantages of the present invention will become clear from the following description, provided by way of a non-limiting example, to be read with reference to the accompanying drawings in which:

• Figure 1 is a partially sectioned, axonometric view of a known MV/LV resin-insulated three-phase dry transformer;
• Figures 2 is a schematic cross-sectional view of a low voltage winding of a known resin-insulated dry transformer;
• Figure 3 is a schematic cross-sectional view of a low voltage winding of a resin-insulated dry transformer according to the present invention;
• Figure 4 is a schematic longitudinally sectioned view of the low voltage winding shown in Figure 3 along the line A-A;
• Figure 5a is a cross-sectional view of a spacer according to the present invention; and
• Figure 5b is an axonometric view of the spacer according to the present invention.

[0026] Figure 1 shows a MV/LV resin-insulated three-phase dry transformer T comprising three columns 1. The three columns 1 of the transformer T have a substantially identical structure. Therefore, in the remainder of the present description, only one column 1 of the transformer T, shown partially cross-sectioned in Figure 1, will be described in detail. The column 1 comprises a magnetic core 2. Preferably, the magnetic core 2 comprises a plurality of plates 2a of magnetic material (for example directed-grain silicon). Preferably, the plates 2a are first packed together and then fastened together by means of bands 2b.

[0027] As shown in Figure 1, the magnetic cores 2 of each column 1 of the transformer T are not independent of each other, but are branches of a single three-branch magnetic circuit 20; each branch is the magnetic core of a respective column 1. In this way, the phase relationship between the three phases of the output low voltage is similar to the phase relationship between the three phases of the input medium voltage.

[0028] The column 1 also comprises a low voltage winding 3; this winding has essentially the form of a hollow cylinder, as will be explained in greater detail with reference to Figure 2. The low voltage winding 3 surrounds the magnetic core 2. Moreover, the low voltage winding 3 has a pair of low voltage terminals 3a.

[0029] The column 1 also comprises a casing 4. This casing 4 has essentially the form of a hollow cylinder. The casing 4 is typically made of insulating material, such as an epoxy resin. The casing 4 surrounds the low voltage winding 3. Turns of a medium voltage winding 5 are embedded in the thickness of the casing 4, as shown in Figure 1. These turns are also connected to a pair of medium voltage terminals 5a, to which a phase of the input medium voltage is applied. The casing 4 has, protruding from it, the pair of low voltage terminals 3a from which a respective phase of the output low voltage is taken.

[0030] The low voltage winding 3 has, as already mentioned, essentially the form of a hollow cylinder. It comprises a sheet of electrically conductive material (for example an aluminium based alloy or copper) wound around an axis z (shown in Figure 4), so as to form a hollow cylindrical body having the axis z as the longitudinal axis. Each complete turn of the sheet about the axis z constitutes a turn of the low voltage winding. Therefore, the various turns are electrically connected in that they are part of the same sheet of electrically conductive material. In order to wind up the turns tightly but at the same time prevent short-circuits between the various turns, the sheet of conductive material is wound together with a sheet of insulating material with high heat and flame resistance properties. Materials considered suitable for this purpose are, for example, Nomex® and prepeg. Therefore, viewed in cross-section, the low voltage winding comprises two spirals, a spiral of electrically conductive material and a spiral of insulating material nested together and tightly wound. Each revolution of the spiral of electrically conductive material forms a turn; each revolution of the spiral of insulating material forms the electrical insulation between two turns.

[0031] Figure 2 shows a schematic cross-sectional view of a low voltage winding 3 of a known type. In Figure 2 the individual turns are not shown. The low voltage winding 3 comprises an inner half-winding 31, a cooling channel 33 and an outer half-winding 32. The cooling channel 33 has a substantially annular cross-section and separates the half windings 31 and 32.

[0032] A first terminal 31 a is fixed to the inner surface of the inner half-winding 31, while a second terminal 32a is fixed to the outer surface of the outer half-winding 32. The terminals 31 a, 32a extend longitudinally along the low voltage winding 3 and project at one end thereof, so as to form the pair of low voltage terminals 3a shown in Figure 1.

[0033] As mentioned above, the cooling channel 33 is formed by means of a number of insulating spacers 9, 9'. The insulating spacers 9, 9' are essentially battens with a solid cross-section. Figure 2 shows eleven spacers 9 and two spacers 9'. These spacers 9, 9' extend longitudinally parallel to the axis z (shown in Figure 4). The two spacers 9' are positioned opposite the terminals 31 a, 32a and typically have a radial dimension smaller than the radial dimension of the spacers 9. The spacers 9 are distributed in a substantially uniform manner.

[0034] With reference to Figures 3 and 4 an embodiment of a low voltage winding of a resin-insulated dry transformer according to the present invention is now described.

[0035] The low voltage winding 3' according to the present invention comprises, in a manner similar to the low voltage winding 3 shown in Figure 2, an inner half-winding 31, a cooling channel 33 and an outer half-winding 32. The inner half-winding 31 is connected to a first terminal 31 a, while the half-winding 32 is connected to a second terminal 32a. As shown in Figure 4, the two terminals 31 a and 32a extend longitudinally along the low voltage winding 3' and project at one end thereof so as to form the pair of low voltage terminals 3a shown in Figure 1. The portions of the two metal contacts 31 a and 32a which project from the winding 3' are shaped and are electrically insulated from each other by an insulating body 30a.

[0036] In the low voltage winding 3 according to the present invention, the cooling channel 33 is formed by a certain number (nine in the embodiment of Figure 3) of electrically conductive spacers 10, 10'. These electrically conductive spacers extend longitudinally along the axis z (shown in Figure 4). The electrically conductive spacers 10' are positioned opposite the terminals 31 a, 32a. The electrically conductive spacers 10 are distributed in a substantially uniform manner at angles b of about 45º along the perimeter of the cooling channel.

[0037] Conveniently, the electrically conductive spacers 10, 10' are sections made of aluminium or an alloy containing aluminium. This is particularly advantageous when the turns of the low voltage winding are also made of aluminium or an alloy thereof. In any case, the aluminium is advantageous also from the point of view of mechanical strength and heat transmission as well as from a cost-related point of view. As an alternative, other metals, typically copper or copper alloys, may be used.

[0038] The electrically conductive spacers of the present invention have the following advantages compared to the insulating spacers used in the known transformers.

[0039] Firstly, the spacers according to the present invention constitute, from an electrical point of view, a second conductor situated in parallel with the low voltage winding. The low voltage winding according to the present invention thus has a greater conductive cross-section. Therefore, the electrical resistance of the low voltage winding decreases, resulting in a reduction in the dispersed power and therefore an increase in the efficiency of the transformer. Secondly, the surface area for evacuation of the dissipated energy increases considerably compared to the known solution.

[0040] With reference now to Figures 5a and 5b, an embodiment of an electrically conductive spacer 10 according to the present invention will be described in detail.

[0041] The conductive spacer 10 extends longitudinally and its longitudinal extension is substantially equal to the longitudinal dimension of the low voltage winding 3'.

[0042] The spacer 10 is essentially a C-shaped section and comprises a curved surface 11, two radial legs 12 and two contact flanges 13. The curved surface 11 is essentially a portion of a cylindrical wall having a radius of curvature substantially equal to the internal radius of the outer half-winding 32. The radial legs 12 extend radially inwards from the ends of the curved surface 11. The length of the radial legs 12 substantially determines the width of the cooling channel 33. The flanges 13 act as support feet resting against the outer surface of the inner winding. Preferably, the spacer 10 has a longitudinal dimension substantially equal to the longitudinal dimension of the winding 3'.

[0043] According to a preferred embodiment, the spacer 10 shown in Figures 5a and 5b comprises cooling baffles 14 for increasing the heat dispersion surface area. Preferably, the cooling baffles 14 extend radially inwards and extend over the whole length of the spacer. Figures 5a and 5b show three baffles 14, but obviously they could consist of a number greater than or less than three.

[0044] The spacer 10 shown in Figures 5a and 5b is an example of embodiment of a spacer according to the present invention. However, a spacer according to the present invention may have a cross-section with a form different from the cross-section of the spacer 10 shown in Figures 5a and 5b. Preferably, this cross-section is open, so as to avoid the formation of currents in the plane of the cross-section, which may increase the temperature of the spacer and therefore of the whole low voltage winding.

[0045] Returning to Figure 3, it can be seen that the electrically conductive spacers 10 are inserted and form the cooling channel 33 so that the curved surface 11 rests against the inner wall of the outer half-winding 32, while the flanges 13 rest against the outer wall of the inner half-winding 31.

[0046] Since, as already mentioned, a turn which releases energy dissipated into the air present inside the cooling channel 33 faces onto the cooling channel 33, this turn is in contact, at least along a portion of its surface, with the conductive spacers 10. Turn portions in direct contact with the air release dissipated energy directly to the air; turn portions in contact with a spacer 10 release dissipated energy to the air via the spacer 10, which is electrically and thermally conductive. Advantageously, the cooling baffles 14 of the spacer 10 increase the efficiency with which the spacer 10 releases to the air of the cooling channel 33 the dissipated energy supplied from the turn.

[0047] Preferably, the electrically conductive spacers 10' are sections with an open profile, having a radial dimension smaller than the radial dimension of the spacers 10. For example, the spacers 10' may be C-shaped sections, as shown in Figure 3.

[0048] In other embodiments of the invention, the electrically conductive spacers 10' may be replaced by insulating spacers similar to the spacers 9' in Figure 2.

[0049] Tests were carried out on a three-phase transformer according to the present invention ("transformer "B" in short). In particular, measurements of the dispersed power and over-temperature were performed. The same tests were carried out on a three-phase transformer of the known type (with spacers made of insulating material), referred to in short as "transformer A".

[0050] Transformer A is a MV/LV resin-insulated three-phase dry transformer marketed by the same applicants of the present patent application under the identification code TFB 160015A 1600 kvA 15000/400 V with a Dyn 11 connection unit. Transformer B differs from transformer A owing to the absence of the insulating spacers 9 and owing to the presence of conductive spacers 10 in place of them. The operating parameters of the transformer TFB 160015A are summarised below:

• power: 1600 kvA
• cooling: natural air
• insulation class: F
• type: TTR (resin-insulated three-phase dry transformer)
• frequency: 50 Hz
• maximum over-temperature: 100 K
• medium voltage at input: 15,000 V
• input current: 61.58 A
• low voltage at output: 400 V
• output current: 2309.4 A
• connection of medium voltage winding: delta
• connection of low voltage winding: star

[0051] The low voltage windings according to the known art contained in transformer A and the low voltage windings according to the present invention present in transformer B have the following common features:

• conductive material for the turns: 1050 aluminium alloy
• thickness of the sheet of conductive material: approx. 1.6 mm
• insulating material for insulating the turns: prepeg
• thickness of the sheet of insulating material: approx. 0.18 mm
• total number of turns: 14
• number of turns of the first half-winding: 7
• number of turns of the second half-winding: 7
• radial dimension of the inner half-winding: approx. 14.3 mm
• radial dimension of the cooling channel. approx. 22.0 mm
• radial dimension of the outer half-winding: approx. 14.3 mm
• inner diameter of the winding: approx. 287.0 mm
• outer diameter of the winding: approx. 388.0 mm
• longitudinal dimension of winding: approx. 1150.0 mm

[0052] Each of the low voltage windings of transformer A according to the known art comprises thirteen insulating spacers 9, 9' arranged as shown in Figure 2. These insulating spacers are made of fibreglass.

[0053] Each of the low voltage windings of transformer B according to the present invention comprises seven conductive spacers, similar to the spacer 10 shown in Figures 5a and 5b, and two insulating spacers similar to the spacers 9' in Figure 2. The seven conductive spacers are arranged as shown in Figure 3. These conductive spacers are made of 6060 aluminium alloy.

[0054] Various measurements were performed on the two transformers A and B and some of these are listed hereinbelow: a) measurement of power dispersed under load; b) measurement of the over-temperature of the medium and low voltage windings under zero load (no load applied to the low voltage terminals); c) measurement of the over-temperatures of the medium and low voltage windings during shortcircuiting (low voltage terminals shortcircuited together); d) measurement of the over-temperatures of the medium and low voltage windings under rated conditions (rated current in the windings and normal excitation conditions of the magnetic core namely at the rated voltage). Table 1 hereinbelow shows the values obtained for the two transformers A and B.

Table 1

	Transformer A (prior art)	Transformer B (invention)
Power dispersed under load	14974 W	14817 W
Over-temperature of medium voltage windings under zero load	6.35 V	4.57 K
Over-temperature of low voltage windings under zero load	18.63 K	14.57 K
Over-temperature of medium voltage windings shortcircuited	95.43 K	95.55 K
Over-temperature of low voltage windings shortcircuited	92.41 K	84.10 K
Over-temperature of medium voltage windings under rated conditions	98.00 K	97.25 K
Over-temperature of low voltage windings under rated conditions	99.70 K	91.54 K

[0055] From a comparison of the results obtained for transformer A and for transformer B, it can be seen that in transformer B there is reduction in the over-temperature of the low voltage windings compared to transformer A, under zero load, during shortcircuiting as well under rated operating conditions. In particular, under rated conditions, the over-temperature of the low voltage windings of transformer B (91.54 K) is about 10% less than the over-temperature of the low voltage windings of transformer A (99.70 K).

[0056] This occurs because, for the same dimensions of the low voltage windings (inner and outer diameter, radial dimension of the half-windings, radial dimension of the cooling channel), in transformer B according to the present invention a greater quantity of dissipated energy is evacuated than in transformer A according to the prior art.

[0057] Therefore, it is, for example, possible to reduce the dimension of the channel for cooling the low voltage windings so that the over-temperature of the latter has a value close to the maximum over-temperature, namely 100 K. For example, using the conductive spacers of the invention and reducing the radial dimension of the cooling channel from 22 mm to 16 mm, an over-temperature of about 100 K is obtained. The low voltage windings are thus more compact; the magnetic cores may therefore have a smaller cross-section and may therefore be lighter.

[0058] It can be noted, moreover, that the over-temperature of the medium voltage windings of transformer B is, in the case of operation under zero load and in the case of operation under rated conditions, also less than the over-temperature of the medium voltage windings of transformer A. In particular, the reduction in over-temperature is about 1% in the case of operation under rated conditions.

[0059] Finally it can be noted that the power dispersed under load in transformer B is less than the power dispersed in transformer A; this reduction is equal to about 0.1%. The spacers made of conductive material according to the invention also have a high mechanical strength and low air resistance.

[0060] Obviously, the present invention may be subject to numerous variants, modifications, adaptation and replacement of parts with other functionally equivalent parts. In particular, the form of the spacers made of conductive material may be different from that described and illustrated. The cooling baffles may be entirely absent or may be present in a number greater than or less than three. Finally, an additional cooling channel may be provided, in addition to the existing cooling channel. In this case there would be, for the low voltage winding, a cooling channel between an inner half-winding and a middle half-winding and an additional cooling channel between the middle winding and an outer winding.

Claims

1. A resin-insulated dry transformer (T) comprising:

- a magnetic core (2);

- an electrically conductive primary winding (5); and

- an electrically conductive secondary winding (3),

- wherein said secondary winding (3) is electrically insulated from the primary winding (5),

- wherein said secondary winding (3) is linked to the primary winding (5) by means of the magnetic core (2), and

- wherein said secondary winding (3) comprises a cooling channel (33) with a number of spacers (10),

characterized in that said spacers (10) are made, at least partially, of an electrically conductive material.

2. The transformer according to Claim 1, in which said spacers (10) are sections with an open profile.

3. The transformer according to any one of the preceding claims,
characterized in that said spacers (10) comprise a curved surface (11) and at least one cooling baffle (12).

4. The transformer according to any one of the preceding claims,
characterized in that said electrically conductive material is chosen from the group comprising: aluminium, aluminium alloy, copper, copper alloy or a combination thereof.

5. A method for reducing the temperature of a secondary winding (3) in a resin-insulated dry transformer (T), said transformer (T) comprising:

- a magnetic core (2);

- an electrically conductive primary winding (5); and

- an electrically conductive secondary winding (3),

- wherein said secondary winding (3) is electrically insulated from the primary winding (5), and

- wherein said secondary winding (3) is linked to the primary winding (5) by means of the magnetic core (2),

said method comprising the steps of:

- providing a number of spacers (10); and

- arranging said spacers (10) in said secondary winding (3) so as to form a cooling channel (33);

characterized in that the step of providing said spacers (10) comprises the step of providing said spacers (10) at least partly made of electrically conductive material.

6. The method according to Claim 5, in which the step of providing said spacers (10) comprises the step of providing sections with an open profile.

7. The method according to any one of Claims 5 to 6, in which the step of providing said spacers (10) comprises the step of providing said spacers (10) made of an electrically conductive material chosen from the group comprising: aluminium, aluminium alloy, copper, copper alloy or a combination thereof.

Drawing