WO2011029965A1 - Method for assembling a prefabricated concrete tower of a wind turbine and structure for the pre-assembly of the tower segments - Google Patents

Abstract

The invention relates to a method for horizontally pre-assembling the annular segments or portions (2) that form a prefabricated concrete tower of a wind turbine, in which the portions are formed by connecting arched elements side-by-side, and in which the prefabricated tower is assembled using the portions obtained in accordance with the above-mentioned horizontal pre-assembly method. The structure used allows: a lower arched element (2.1) to be positioned with the longitudinal axis thereof arranged essentially horizontally and secured in this position; the side-by-side positioning of lateral arched elements (2.2) placed adjacently on the lower arched element (2.1); and the positioning and securing of a mould (23) having a suitable shape and orientation such that upper arched elements (2.3) can be subsequently installed, which upper elements, together with the lower arched element (2.1) and the lateral arched elements (2.2), form the annular segment (2).

Description

 ASSEMBLY PROCEDURE OF PREFABRICATED TOWER OF CONCRETE OF A WIND TURBINE, AND STRUCTURE OF

 PREMONTAGE OF THE SEGMENTS OF THE TOWER D E S C R I P C I O N

OBJECT OF THE INVENTION The present invention has for object a pre-assembly procedure in horizontal position of the sections or segments that constitute a prefabricated concrete tower of a wind turbine, in which the sections or segments are formed by the collateral union of voussoirs. Also the object of the invention is the assembly procedure of the prefabricated tower from the sections or segments obtained according to the horizontal pre-assembly procedure described above.

Another object of the invention is the structure used to carry out the horizontal pre-assembly of each section of the tower.

BACKGROUND OF THE INVENTION In general, several types of towers are used to support a wind turbine, among which include: steel cylindrical towers, steel lattice towers, concrete towers with in situ spillage, prefabricated concrete towers. The steel towers have several drawbacks, since they show a lower resistance to weather influences than the towers of concrete, which makes it necessary to use a paint-type protection, with the associated problems, especially in relation to the associated maintenance costs. In the case of heavy wind turbines, steel towers require very thick walls and large diameters, which means high production costs.

The transport of steel towers involves many problems due to the large diameter of the sections of the tower, which involves a great work of installation, the use of large number of pieces of union, as well as the need to use large cranes.

The procedure to build towers capable of supporting heavy machines is the on-site pouring consisting of making the formwork and the armor on site, pouring the concrete next. This procedure has the following disadvantages: low quality of concrete and therefore less resistance, construction depends on atmospheric conditions, a large face crane and scaffolding is necessary, and it requires a lot of time. On the other hand the prefabricated concrete towers used to support wind turbines are raised from cylindrical elements that are connected together by tension and mortar cables, using large cranes that stack these elements. The transport of these elements is also of great complication due to its high diameters.

The invention patent ES 2272954 describes a wind turbine that is supported by a tower that can be lifted from prefabricated voussoirs, which are transported and manipulated more simply than the sections or complete elements described above, so they do not require a Special road transport and / or heavy equipment for lifting. The tower described in said invention patent is composed of segments or annular segments composed of adjacent prefabricated wall parts or segments, placed side by side forming said annular segment, in which the voussoirs are more than twice as tall as the largest dimension of the voussoirs in the direction of the width, and the horizontal edges of the annular segments are placed one on top of the other. The voussoirs are pre-assembled on site to form the segments that are hoisted with large cranes, the pre-assembly being carried out in the final location or in a provisional location.

The different patents or patent applications on the subject explain that said pre-assembly and the joining of the voussoirs is carried out with them in vertical position. This is the case of patent application EP 1889988 which proposes a method of vertical assembly of concrete towers for wind turbines, in which the tower is divided into annular segments constituted by voussoirs. The procedure includes the pre-assembly of each segment, consisting of placing the voussoirs that form one next to the other, on a base located on the ground and with the help of props and jacks held in a vertical position until forming the entire perimeter of the segment , proceeding then to the poured of mortar in the vertical joints between voussoirs. Each of the segments is obtained in the same way and is moved for vertical assembly with the help of a crane on the lower segment already installed. This vertical assembly procedure involves major drawbacks, especially the following:

- Difficulty in securing the geometry of the segment when positioning a voussoir next to another in a vertical position.

 - Difficulty to seal and cover the wet joints between voussoirs.

- Difficulty filling these joints, because when pouring concrete or mortar in vertical, the pressure is very high.

 - Difficulty to execute works in vertical. These segments can measure between 20 and 30 m in height.

 - Difficulty in introducing and evacuating the necessary material means inside the segment.

 - Difficulty in applying the vertical prestressing in your case.

On the other hand, the invention patent ES 224734 describes a modular prefabricated tower that also uses prefabricated elements that are mounted in vertical position and that are reinforced by an internal structure of horizontal and vertical ribs, preferably made of reinforced concrete, being tensioned by metallic cables. flexible that are tensed once placed in a vertical position each segment and the voussoirs that form it.

DESCRIPTION OF THE INVENTION

The present invention proposes to carry out the pre-assembly of each segment or annular segment of the tower in a horizontal position, thus eliminating the problems described above.

A first object of the invention is the pre-assembly procedure of each ring segment of the tower from the segments. A second object of the invention relates to the method of assembling the tower by superposition of segments, in which each segment forming the tower is obtained according to said pre-assembly procedure.

The preassembly method proposed according to the first object of the invention makes it possible to obtain the desired annular configuration of the segment from the positioning and fixing of the voussoirs, in which the voussoirs may have a partially frustoconical shape, or partially cylindrical or flat or partially troncopiramidal or others.

According to the first object of the invention the pre-assembly procedure would take place according to the following phases:

- location of a lower segment with its longitudinal axis arranged essentially horizontally and fixing this position,

 - collateral and horizontal positioning of a first lateral voussoir in an adjacent arrangement on one of the edges of the lower voussoir and fixing its position establishing a continuity with the lower voussoir with adequate orientation to progressively form the segment,

 - collateral and horizontal positioning of a second lateral voussoir in an adjacent arrangement on the other edge of the lower voussoir and fixation of its position establishing a continuity with the lower voussoir and with the first lateral voussoir with adequate orientation to continue with the progressive formation of the segment ,

 - positioning and fixing a mold with the appropriate geometry and orientation for the subsequent arrangement of keystone or key / s that together with the lower keystone, and lateral segments form the segment with the desired annular geometrical configuration,

- horizontal arrangement of upper segments on the mold and adjacent arrangement on the free edge of the side segments and fixing its position to close the segment,

 - execution of the longitudinal joints between segments,

 - optionally, post-tensioning the segment once the joints have hardened.

According to the second object of the invention, the assembly procedure of the tower consists in the turning operation from the horizontal position and the elevation of each of the segments obtained in accordance with the horizontal pre-assembly procedure described above, and its vertical mounting, in which the first segment is mounted in vertical position and the other segments are also mounted in vertical position on the segment previously installed.

A third object of the invention constitutes a structure designed to carry out the described procedure of pre-assembly of the segments of the tower from the segments. The structure according to this third object of the invention comprises at least two frames adapted to support and constitute a horizontal support of the segments from which the segment is obtained, in which the frames are located in a distant and level position. In the case of having two racks it is contemplated that these are located at a distance of approximately 3/5 of the length of the segment.

Each of the frames has essentially a base cross and vertical beams that depart from said base cross, incorporating on the base cross at least two supports adapted to support and fix a voussoir in a horizontal position, in this case the lower voussoir , so that its longitudinal axis is located in an essentially horizontal position. In the case of voussoirs of curved configuration, such as those of a partially frustoconical or cylindrical shape, the longitudinal axis is understood as its axial axis, and its positioning on the supports is made with its concavity oriented superiorly.

Between the lower segment and the supports, it is foreseen the incorporation of first mooring means that facilitate the support and fixation of the lower segment on the supports. These first mooring means also facilitate the reception of the lateral voussoir that is located in a horizontal and collateral arrangement with the edge of the lower voussoir. The structure also incorporates fastening means designed to fix the lateral segment by one of its ends on the vertical beam in a position such that a continuity of the lateral segment is established with the lower segment with adequate orientation to progressively form the segment with the intended ring configuration. Complementarily, the incorporation of second mooring means is envisaged which together with the first mooring means collaborate in the fixation of the lateral voussoir in collateral arrangement with the lower voussoir. On the other hand, the structure comprises the above-described mold, which is supported on the second mooring means and shows a suitable geometry and orientation for the subsequent arrangement of key or superior voussoirs which together with the lower voussoir and lateral voussoirs will form the segment with the desired ring geometrical configuration.

The mold is preferably formed by a supporting arch that can be a curved steel beam or a lattice and by two adjustable struts that start from the ends of the supporting arch and whose opposite ends are adapted to be fixed to the second means of Mooring The mold also has fixing means that allow its connection to the lateral segments.

The use of this mold allows to obtain an exact geometry of the segment.

Once the supporting arch is positioned and fixed, the key or upper segments are mounted horizontally, for which an auxiliary or retaining crane or some auxiliary steel structure can be used to facilitate the turning of the upper segment smoothly. and progressive. Once the upper segments are assembled, the segment would be ready to proceed to execute the longitudinal joints between segments. It is planned that the mold is configured in such a way as to allow the free circulation of people, wheelbarrows and the necessary material to execute said longitudinal joints between segments. The realization of the joints in horizontal position of the segment is executed much easier than in the case of vertical mounting, since the workers work at much lower heights, the joints can be filled at one time and without excessive hydrostatic pressures, improving therefore the speed of work and the quality of the finish.

It is also noteworthy that the procedure of horizontal pre-assembly of the segments in combination with the use of frames and molds allows the obtaining of a more precise geometry of the segment than that obtained in vertical assembly procedures.

Another advantage associated with the horizontal pre-assembly of the tower segment is that low-tonnage cranes are required. Subsequently, for the assembly of the already formed segments, to configure the tower, large tonnage cranes are used that place one segment on another in a turning and lifting maneuver.

It is envisaged that in general a structure is used for the conformation of each section or segment of the tower, although it is not excluded that the frames have means of regulation in distance of the vertical beams and can be used a same pair of frames for the configuration of different segments.

DESCRIPTION OF THE DRAWINGS To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, of according to a preferred example of practical realization thereof, an assembly of drawings is included as an integral part of said description, in which the following is represented by way of illustration and not of limitation:

Figure 1 shows a view of the segment formed horizontally according to the pre-assembly process object of this invention, which also observes the frames of the structure used to execute this procedure.

Figure 2.- Shows a side view of the frame.

Figure 3.- Shows an elevation view of the frame. Figure 4 shows a plan view of the frame.

Figure 5.- Shows the assembly sequence of the lower segment from its unloading from the truck to its horizontal positioning and mooring on the frame.

Figure 6 shows an elevation view in which the first mooring means fixing the lower segment by one of its ends is shown in detail. Figure 7 shows a side view of one of the pieces that make up the first mooring means.

Figure 8 shows a plan view of a joint washer that forms part of the first mooring means.

Figure 9.- Shows the assembly sequence of one of the voussoirs from the unloading of the truck to its horizontal positioning on the lower segment already assembled, and its subsequent mooring on the frame.

Figure 10.- Shows a side view of a joint part that forms part of the second mooring means.

Figure 11.- Shows an elevation view of the joining part of figure 10. Figure 12.- Shows a plan view of the joining part of the figure

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Figure 13.- Shows an elevation view of a mold formed by truss lattice and adjustable struts.

Figure 14.- Shows an elevation view in which the frame with the lower and lateral segments already mounted on the frame is observed, and a mold formed by a curved beam and adjustable struts that is positioned to receive the upper segments.

Figure 15.- Shows the assembly sequence of the upper segments from the truck unloading to its horizontal positioning on the support arch. Figure 16.- Shows a view of a segment formed by three segments on the horizontal pre-assembly structure.

Figure 17.- Shows a view of a segment formed by four segments on the horizontal pre-assembly structure.

Figure 18.- Shows a view of a segment formed by five voussoirs on the horizontal pre-assembly structure, which would constitute the assembly solution subsequent to that shown in figure 15.

Figure 19.- Shows a view of a segment formed by six segments on the horizontal pre-assembly structure.

Figure 20.- Shows the assembly sequence of the tower from the segments from its horizontal position.

PREFERRED EMBODIMENT OF THE INVENTION

In view of the figures, a preferred embodiment of the structure object of the present invention is described below, which is valid for segments formed by any number of segments, but especially between 3 and 6 segments, as shown below. .

It is contemplated that for each of the segments (2) of the tower to be configured, the structure will present geometries and curvatures adapted to the measurements, shapes and number of segments that will form said segment (2), so that it will be possible to have a structure for each segment (2) to conform to the tower.

As shown in Figure 1, the structure comprises at least two frames (1) adapted to support and constitute horizontal support of the segment (2) to be formed on it, in this case from five voussoirs: lower voussoir (2.1), two lateral voussoirs (2.2) and two upper voussoirs (2.3). Said frames (1) are located in a distant and level position, each of which, as shown in Figures 2 to 4, essentially has a crossbeam base (3) and vertical beams (4) that start distant from said base cross member (3), also observing bracing beams (5) that support the vertical beams (4) on the base cross member (3) and on a few stringers (6) perpendicular to the base cross member (3).

Figures 3 and 4 show that on the base cross member (3) there are at least two supports (7) adapted to support and fix the lower segment (2.1), as shown in Figure 5, so that its axial axis is located in an essentially horizontal position.

In said figure 5 the lower segment (2.1) is observed on a truck (8) from which it is lifted with slings (9) and is then positioned on the frame (1) horizontally, being tied on the supports (7) of the frame (1) with the aid of first mooring means (11).

Figure 6 shows in detail the first mooring means (11) linked to one of the ends of the lower segment (2.1), in which said first mooring means (11) comprise a joint part (12) formed by: a flat bar (13) of curvature coinciding with that of the lower segment (2.1) and adapted to receive said lower segment (2.1) and the lateral segment (2.2), each of the lugs (14), which can also be seen in the figure 7, intended for the introduction of a pin, not shown, which crosses and establishes the connection with the anchor (2), and a joint washer (15) adapted for its location on the face of the lower segment (2.1) opposite to the contact with the connecting piece (12) that facilitates the fixation of the lower segment (2.1) with the first joint piece (12), with the additional collaboration of a bolt (16) and nuts (17).

Figure 9 shows the assembly of one of the side segments (2.2) which is moved from the truck (8) by means of a sling (9) and then positioned collaterally and horizontally with the lower segment (2.1). The structure has fixing means (30) that establish the connection of the lateral segment (2.2) thus positioned with the vertical beam (4). On the other hand, the incorporation of second lashing means (20) intended to fix the relative position of the side segment (2.2) with respect to the lower segment (2.1), as shown in Figure 14. and as seen in Figure 6, the above-described joint washer (15) incorporates a recess (18) on one side intended for its tongue and groove joint with a second joint part (19), shown in figure 11, which forms part of the second lashing means (20). This second connecting piece (19), shown in figures 10 to

12, comprises a curved plate (21) showing on one side a recess (22) adapted to be coupled to the recess (18) of the joint washer (15) in its tongue and groove joint, as well as has lugs (23). On the other hand, these second clamping means (20) additionally incorporate to the second joining part (19), nuts (17) and bolt (16) that facilitate the relative positioning between lower (2.1) and lateral (2.2) segments, as seen in figure 14. In this same figure 14 the two lateral voussoirs (2.2) and the lower voussoir (2.1) arranged in the structure are observed and additionally the incorporation of a mold (23) that is fixed is observed to the lateral segments (2.2) and to the second securing means (20), specifically to the second joining pieces (19), forming on the supporting and reference surface for the subsequent positioning of the upper segments (2.3).

The mold (23) represented in figure 14 comprises a support arch (24, 25), fixing means (27) that establish the union of the support arch (24, 25) with the lateral segments (2.2) and both struts (26) adjustable in height starting from the ends of the supporting arch (24, 25) and fixed to the lugs (23) of the second connecting pieces (19). In in this case the supporting arch (24, 25) consists of a curved beam (24), while in another alternative embodiment of the mold (23), shown in figure 13, the supporting arch (24, 25) consists of a latticework (25).

Figure 15 shows the final assembly operation of the upper segments (2.3) that rise from the truck (8) by means of a sling (9) and are positioned horizontally on the curved beam (24) and in collateral arrangement with the lateral voussoirs (2.2). Figure 18 shows the upper segments (2.3) already positioned forming the segment (2) with either the lateral voussoirs (2.2) and the lower voussoir (2.1).

At a later stage the formwork would be made in the same horizontal position of the segment to obtain the joints (28) between segments (2.1, 2.2, 2.3), represented in figure 1, thus configuring the segment (2) in its entirety in position horizontal.

Figure 16 shows the application of the structure (1) to the case of a segment (2) made up of three voussoirs: upper voussoir (2.3) and two lateral voussoirs (2.2), in which the voussoirs have been foreseen or lateral (2.2) are linked to the vertical beams (4) by fixing means (30) and have a support stop (31) between edges of the lateral segments (2.2) fixed to the crossbar (3) of the structure . Likewise, the incorporation of junction plates (32) and anchoring means that facilitate the continuity and connection between each of the lateral segments 5 (2.2) and the upper segment (2.3) has been envisaged.

Figure 17 shows that the structure (1) also allows the shaping of the segment (2) from four voussoirs: a lower voussoir (2.1), two lateral voussoirs (2.2) and an upper voussoir (2.3), in the that the lower voussoir (2.1) is connected to the lateral voussoirs (2.2) by means of the previously described first and second mooring means (11, 20), and the lateral segments (2.2) are connected to the upper segment (2.3) by the connecting plates (32) and corresponding anchoring means.

Figure 19 shows the structure for its application in the conformation of a segment (2) of six voussoirs: two lower voussoirs (2.1) that are mounted on the support stop (31), two lateral voussoirs (2.2) and two upper segments (2.3), where the incorporation of the first mooring means (11) joining the lower segments (2.1) with the lateral segments (2.2), in which said first mooring means (11) has been envisaged. they are mounted on supports (7) that in this case start from the vertical beams (4) instead of the crosspiece (3) of the structure (1). Also this structure (1) incorporates the second mooring means (20) that link the lower segments (2.1) with the lateral segments (2.2.), The aforementioned mold (23) and joining and support means (31) between edges. of the lateral segments (2.2) on the crossbar (3) of the structure (1).

Figure 20 shows the assembly procedure of the tower from the segments (2), which are initially in a horizontal position, are turned, raised and are superimposed on each other with the help of cranes (29) .

Claims

1.- Pre-assembly procedure for the segments (2) of a prefabricated concrete tower of a wind turbine, characterized in that it comprises the following phases:  - location of at least one lower segment (2.1) with its longitudinal axis arranged essentially horizontally and fixing this position,  - collateral and essentially horizontal positioning of at least two lateral segments (2.2) in an adjacent arrangement on at least one of the edges of the lower segment (s) (2.1) and fixing its position, establishing continuity with the / s bottom segment (s) (2.1) with adequate orientation to form the segment (2) of annular configuration,  - essentially horizontal arrangement of at least one upper segment (2.3) in an adjacent arrangement on the free edge of the side segment (s) (2.2) and fixing its position to close the segment (2), - execution of the longitudinal joints (28) between segments (2.1, 2.2, 2.3). 2. Procedure for preassembly of the segments (2) of a prefabricated concrete tower of a wind turbine according to claim 1, characterized in that the upper segment (s) (2.3) are placed on the lateral segments (2.2). ) with the help of a mold (23) with the appropriate geometry and orientation so that the arrangement of the upper segments (2.3) in combination with the lower (2.1) and lateral segments (two. 2) form the segment (2) of annular geometric configuration. 3.- Pre-assembly procedure of the segments (2) of a prefabricated concrete tower of a wind turbine, characterized in that it comprises the following phases: - collateral and essentially horizontal positioning of two voussoirs laterals (2.2) and fixation of their position,  - essentially horizontal arrangement of an upper segment (2.3) in an adjacent arrangement on the free edge of the side segment (2.2) and fixing its position to close the segment (2),  - execution of the longitudinal joints (28) between segments (2.2, 2.3). 4. - Procedure for preassembly of the segments (2) of a prefabricated concrete tower of a wind turbine according to claim 1 or 3 characterized in that once the joints (28) are hardened, the post-tensioning of the segment (2) in position is proceeded horizontal. 5. - Assembly procedure of a prefabricated concrete tower of a wind turbine characterized in that it comprises the turning operation from the horizontal position and the elevation of each one of the segments obtained according to the horizontal pre-assembly procedure described in any one of claims 1 to 4 and their vertical mounting, in which the first segment is mounted in a vertical position and the other segments are also mounted in vertical position on the segment previously installed. 6. Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine characterized in that it comprises at least two frames (1) adapted to support and constitute a horizontal support of lower (2.1), lateral (2.2) and upper ( 2.3) during the forming of the segment (2) in a horizontal position from said segments (2.1, 2.2, 2.3), in which said frames (1) are located in a distant position and level. 7. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 6, characterized in that each of the frames (1) comprises: a base cross member (3) and vertical beams (4) that depart from said base cross member (3), and  fixing means (30) that establish the linking of each lateral voussoir (2.2) in essentially horizontal position with each of the vertical beams (4). 8. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 7, characterized in that the frame additionally comprises:  at least two supports (7) adapted to support and fix the lower segment (s) (2.1) so that the axial axis of the lower segment (s) (2.1) is located in an essentially horizontal position,  first mooring means (11) that establish the union of the lower segment (2.1) with each of the supports (7),  second lashing means (20) intended to fix the relative position of the side segment (2.2) with respect to the lower segment (2.1), 9. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 8, characterized in that the supports (7) are mounted on the base crosspiece (3). 10. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 8, characterized in that the supports (7) are mounted on the vertical beams (4). 11. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 8, characterized in that the first mooring means (11) comprise a connecting piece (12) formed by a plate (13) of curvature coinciding with the of the lower segment (2.1) and which is adapted to receive said lower segment (2.1) and the lateral segment (2.2), two lugs (14), intended for the introduction of a pin that crosses and establishes the connection with the anchor ( 2 and  a joint washer (15) adapted for its location on the face of the lower segment (2.1) opposite that of contact with the connecting piece (12), and nuts (17) and bolt (16) which in collaboration with the joint washer (15) facilitate the union of the lower segment (2.1) with the first joint part (12). 12. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 1 characterized in that the joint washer (15) has a recess (18) in one of its sides. 13. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 8, characterized in that it additionally comprises a mold (23) adapted to be fixed to the lateral segments (2.2) and to the second mooring means ( 20) and to become a reference surface and support for the location of the upper segments (2.3). 14. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 13, characterized in that the mold (23) comprises:  a supporting arch (24, 25),  fixing means (27) that establish the union of the supporting arch (24, 25) with the lateral segments (2.2), and struts (26) adjustable in height starting from the ends of the supporting arch (24, 25) adapted to be fixed to the second joining parts (19). 15. Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 15, characterized in that the support arch (24, 25) consists of a curved beam (24). 16. - Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 15, characterized in that the support arch (24, 25) consists of a lattice (25). 17. Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claims 8, 12 and 13, characterized in that the second clamping means comprise: a second joint part (19) formed by  a curved plate (21) showing on one side a recess (22) adapted to be coupled to the recess (18) of the joint washer (15) at its tongue and groove joint,  some lugs (23) intended for fixing the mold (23), and  Nuts (17) and bolt (16) that facilitate the connection between the first joint part (12), the second joint part (19) and the side segment (2.2). 18. Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 7, characterized in that it comprises stop means and support (31) between edges of the lateral segments (2.2) on the crosspiece (3) of the structure.  19. Pre-assembly structure of the segments of a prefabricated concrete tower of a wind turbine according to claim 7, characterized in that it comprises connecting plates (32) and fixing elements that link the lateral segments (2.2) to the upper segment ( 2.3)