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Title:
VALVE UNIT FOR HVDC POWER CONVERTER
Document Type and Number:
WIPO Patent Application WO/2017/114545
Kind Code:
A1
Abstract:
The present disclosure relates to a valve unit comprising a container, a plurality of converter cells, a plurality of holding elements and a plurality of connecting elements for mechanically connecting the holding elements. The container may be at least partially filled with an electrically insulating gas and may extend along an axial direction. The plurality of converter cells may be arranged as at least one column within the container. A holding element is arranged to hold at least one converter cell. The connecting elements extend from a first holding element to another within a space delimited by an outer perimeter of a converter cell arranged between the holding elements.

Inventors:
LI MING (SE)
CHEN NAN (SE)
HJORTSTAM OLOF (SE)
PERSSON ERIK (SE)
ERIKSSON THOMAS (SE)
Application Number:
EP2015/081258
Publication Date:
July 06, 2017
Filing Date:
December 28, 2015
Export Citation:
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Assignee:
ABB SCHWEIZ AG (CH)
International Classes:
H05K7/14; H01L23/60; H01L25/11; H02B5/06; H02H9/04; H02M7/155; H03K17/72
Domestic Patent References:
WO2008034271A22008-03-27
Foreign References:
EP0172554A21986-02-26
JPH04208070A1992-07-29
EP0891648B12002-11-13
Attorney, Agent or Firm:
AHRENGART, Kenneth (Intellectual PropertyIngenjör Bååths Gata 11, Västerås, SE)
Download PDF:
Claims:
CLAIMS

A valve unit comprising:

a container at least partially filled with an electrically insulating gas, said container extending along an axial direction;

a plurality of converter cells arranged as at least one column within said container;

a plurality of holding elements, wherein a holding element is arranged to hold at least one converter cell; and

a plurality of connecting elements mechanically connecting said holding elements, said connecting elements extending from a first holding element to another within a space delimited by an outer perimeter of a converter cell arranged between said holding elements.

The valve unit of claim 1, wherein the column of converter cells is arranged coaxially to the axial direction along which the container extends.

The valve unit of claim 1 or 2, wherein the container has a cylindrical shape.

The valve unit of any one of the preceding claims, wherein the holding elements and said connecting elements together form a supporting structure for the converter cells.

The valve unit of any one of the preceding claims, wherein the holding elements define a number of positions for arrangement of the converter cells along the axial direction with a first position located at, or near, a first extremity of the container and a second position located at, or near, an extremity of the container opposite to the first extremity along the axial direction.

The valve unit of any one of the preceding claims, further comprising at least one attaching element for attaching at least one of the holding elements and/or said connecting elements to a bottom surface, a top surface and/or a wall surface of the container.

7. The valve unit of any one of the preceding claims, wherein at least part of a holding element is integrated in a body of the converter cell supported by that holding element.

8. The valve unit of any one of the preceding claims, wherein a converter cell includes a body extending in a radial direction between an outer perimeter and an inner perimeter, thereby defining an inner space.

9. The valve unit of claim 6, wherein said connecting elements extend within said inner space. 10. The valve unit of claim 6 or 7, wherein the body is divided in a plurality of pieces, wherein a piece forms a section of the body.

11. The valve unit of any one of claims 6-8, wherein a holding element includes a central through-hole with a perimeter corresponding to, or being larger than, the inner perimeter of the body of a converter cell arranged at that holding element.

12. The valve unit of any one of the preceding claims, wherein an outside surface of a converter cell is elliptic, circular and/or comprises at least one rounded corner.

13. The valve unit of any one of the preceding claims, wherein the converter cell may comprise at least one capacitor element arranged in a body extending from the outer perimeter to an inner perimeter of the converter cell and at least one switching device.

14. The valve unit of claim 8, wherein the intersection of a plurality of connecting elements with a holding element defines a number of compartments at the holding element corresponding to the number of pieces of the body arranged at the holding element, wherein a piece of the body is arranged at one compartment. 15. The valve unit of any one of the preceding claims, further comprising an

electrical shield for insertion of said at least one column of converter cells at one end of the container and/or an electrical shield for insertion of the stack of converter cells at an opposite end of the container. 16. The valve unit of any one of the preceding claims, further comprising at least one first connector including a first busbar element electrically connecting a cell of the valve unit at one end of said container to a cell of a second valve unit. 17. The valve unit of any one of the preceding claims, further comprising a

spacing element including insulating material and being arranged at one end of the container for separating the container from a container of another valve unit to which said valve unit is connected. 18. The valve unit of any one of the preceding claims, wherein the insulating gas is at least one of sulfur hexafluoride (SF6), Nitrogen (N2), air and dry air.

19. The valve unit of any one of the preceding claims, wherein a connecting

element at least partly includes solid insulating material.

20. The valve unit of any one of the preceding claims, wherein the axial direction is vertical such that the container is vertically oriented from ground.

21. A power converter station, comprising at least two valve units as defined in any one of the preceding claims.

Description:
VALVE UNIT FOR HVDC POWER CONVERTER

TECHNICAL FIELD

The present disclosure relates generally to the field of high voltage power converters and is concerned with a valve unit in which insulation is obtained by means of at least an insulating gas. The valve unit of the present disclosure may be applicable in for instance offshore platforms.

BACKGROUND

A high voltage direct current (HVDC) converter station is a type of station adapted to convert high voltage direct current (DC) to alternating current (AC) or the reverse. An HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), an alternating current switch gear, transformers, capacitors, filters, a direct current switch gear and other auxiliary elements. Electronic converters may be categorized as line- commutated converters using e.g. thyristors as switches or voltage source converters using transistors, such as insulated gate bipolar transistors (IGBTs), as switches (or switching devices). A plurality of solid-state semiconductor devices, such as thyristors or IGBTs, may be surrounded by capacitors and connected together, for instance in series, to form a building block or a valve unit of an HVDC converter. A challenge in the construction and design of an HVDC converter station is the electrical insulation of the different parts of the HVDC converter station since an increased in distance between the various parts of an HVDC converter station improves insulation but at the same time results in an HVDC converter station with larger dimensions. This may raise a number of difficulties relative to e.g. installation, transport and cost, in particular for offshore applications. For at least such

applications, there is a general need of more compact solutions. SUMMARY

An object of at least some embodiments of the present disclosure is to wholly or at least partly overcome the above disadvantage of prior art systems and to provide a more compact alternative to the prior art.

This and other objects are achieved by means of a valve unit as defined in the appended independent claim. Other embodiments are defined by the dependent claims. According to a general aspect, there is provided a valve unit. The valve unit comprises a container, a plurality of converter cells, a plurality of holding elements and a plurality of connecting elements for mechanically connecting the holding elements. The container may be at least partially filled with an electrically insulating gas and may extend along an axial direction. The plurality of converter cells may be arranged as at least one column within the container. A holding element is arranged to hold at least one converter cell. The connecting elements extend from a first holding element to another within a space delimited by an outer perimeter of a converter cell arranged between the holding elements. It will be appreciated that the holding elements and the connecting elements may together form a supporting structure. By the term supporting structure is meant a structure that serves to support the stack of converter cells. The supporting structure may also be referred to as for example a support, supporting means or holding structure.

With the term "container" is also meant an enclosure or tank in which a plurality of converter cells may be arranged to form the column of converter cells. The container may extend along the axial direction between a first end (or base or extremity) and a second end (or base or extremity) located at two distant positions along the axial direction. The distance between the two ends of the container along the axial direction corresponds to the height of the container. With the term "column" of convert cells is also meant a stack of converter cells, i.e. a plurality of converter cells stapled on top of each other. The plurality of converter cells is arranged as a stack along the axial direction (or stacking direction), e.g. a vertical direction thereby forming a column of converter cells. As mentioned above, such a column may be arranged within the container which is at least partially filled by the insulating gas and it will be appreciated that there may be insulating gas between two successive converter cells of the stack.

The present disclosure provides a valve unit including a container at least partially filled with an electrically insulating gas and a cell stack with an internal supporting structure. Arranging the connecting elements of the supporting structure within the outer perimeter delimited by a converter cell arranged between the holding elements reduces the requirement on insulation between adjacent converter cells in that the connecting elements (or the supporting structure in general) do not (or at least do less) disturb the electric field around the converter cells. This also reduces the requirement on distance between the converter cell and an inner wall surface of the container. As mentioned above, electrical insulation of the converter cells is obtained by means of an insulating gas enclosed in the container. As a result, a more compact valve unit is achieved.

In the present embodiments, the connecting elements are arranged within the stack of converter cells in that, between two (successive) holding elements, the connecting elements are arranged within the outer perimeter (e.g. a diameter) of a converter cell arranged between (or at one of) these two holding elements. It will be appreciated that the connecting elements may be located within the outer perimeter of the stack (as defined by the outer perimeter(s) of the converter cells forming the stack) such that an internal supporting structure is obtained. As a result, a more compact valve unit and consequently a more compact installation of a plurality of valve units to form e.g. an HVDC converter station can be achieved.

It will be appreciated that the holding elements (and the supporting structure in general) provides the capability to carry the weight of the converter cells, i.e. the installation. For this purpose, the holding elements are mechanically connected from one to another via the connecting elements.

Generally, at least some embodiments of the present disclosure provide for a more compact valve unit in that the converter cells surround the connecting elements or, at least, in that the connecting elements are located within the space delimited by the outer perimeter of the converter cells (i.e. the outer perimeter of the column of converter cells). The holding elements may define a number of positions for arrangement of the converter cells along the axial direction. It will be appreciated that more than one converter cell may be arranged at each position along the axial direction (in the column) and that, in some cases, a position may be empty. A position is determined by the intersection between the connecting elements and a holding element, wherein a holding element is arranged to receive (support) one or more converter cells.

It will be appreciated that in some embodiments the axial direction may be a vertical direction such that the container (and thereby the valve unit) is in a standing position (or vertically oriented) from ground.

According to an embodiment, the column of converter cells may be arranged coaxially to the axial direction along which the container extends, thereby providing an even more compact valve unit. In some embodiments, the container may have a cylindrical shape. Further, in some other embodiments, the column may be formed with disc-shaped converter cells, thereby forming a column having also a cylindrical shape, which provides an even more homogeneous electric field. As a result, the gas clearance (between the column and the sidewall of the container) may be further reduced, and an even more compact valve unit is provided. Generally, it will be appreciated that the container may include an electrically conductive material such as a metal. Alternatively, the container may be made of another material coated with an electrically conducting layer on its outer surface. The electrically conductive material or coating of the container may then be used for grounding.

In some embodiments, the holding elements define a number of positions for arrangement of the converter cells along the axial direction with a first position located at, or near, a first extremity of the container and a second position located at, or near, an extremity of the container opposite to the first extremity along the axial direction. In other words, the column of converter cells may extend from one extremity of the container to its opposite extremity.

In some embodiments, the valve unit may further comprise at least one attaching element for attaching at least one of the holding elements and/or the connecting elements to a bottom surface, a top surface and/or a wall surface of the container. In one embodiment, considering the valve unit (and thereby the container extending the axial direction) in a standing position, i.e. with the axial direction being substantially vertical, the supporting structure may at one end be attached at (or mounted to) the bottom of the container and, optionally, the supporting structure may at its other end be attached at (or mounted to) the top part of the container.

In some embodiments, the attaching element may include a shielding surface structure for smoothing the electrical field within the container, which improves insulation. In a more specific embodiment, the attaching element (or the shield) may be designed (or structured) to extend along a direction intersecting a radial direction (i.e. designed in a direction which is not substantially perpendicular to the axial direction along which the container extends). In other words, the attaching element may be designed in a way to avoid horizontal surfaces at which particles may drop (and accumulate), thereby impairing surface insulation. In some embodiments, at least part of a holding element is integrated in a body of the converter cell supported by that holding element. In other words, the holding elements may be integrated parts of the converter cells such that a holding element is part of a body of a converter cell supported by that holding element. In particular, the bottom part of (a body of) a converter cell may be made thicker or equipped with some means for being attached to the connecting elements. Still, whether the holding elements are integrated parts of the converter cells or not, it may in the following be referred to a supporting structure for referring to the connecting elements and the holding elements. In some embodiments, the holding element may be an integrated part of the converter cell and it may thus be considered that the arrangement of the converter cells itself defines positions along the axial direction.

According to some embodiments, the holding elements may be holding plates.

However, it will be appreciated that a holding element does not necessarily have to be a continuous plate and that a holding element providing contact points at the connecting elements may be sufficient for one or more converter cells to rest at a position along the stacking direction.

According to an embodiment, a converter cell may include a body extending in a radial direction between an outer perimeter and an inner perimeter, thereby defining an inner space. In other words, a converter cell may include a body with an inner perimeter (such as e.g. an inner diameter in the case of a circular or annular body) and an outer perimeter (such as e.g. an outer diameter), thereby defining the inner space. The body of the converter cell may include a hollow or cavity, e.g. a through hole. It will be appreciated that the outer perimeters of the converter cells determine the outer perimeter of the column of converter cells. Although some variation of the outer perimeter of the column of converter cells may be envisaged along the axial direction, in some embodiments, the converter cells may have substantially the same, or approximately the same, outer perimeter thereby defining, along the axial direction, a constant outer perimeter of the column of converter cells within which the connecting elements are located. According to an embodiment, the connecting elements may extend within the inner space (as defined by the superposition of the converter cells), which provides for an even more compact valve unit as the distance between two successive converter cells in the column may be reduced.

According to some embodiments, the holding element may have a cross section (or shape) across the axial direction corresponding to a cross section (or shape) of the body of the converter cell arranged at the holding element across said axial direction. According to an embodiment, a holding element includes a central through-hole with a perimeter corresponding to, or being larger than, the inner perimeter of the body of a converter cell arranged at that holding element. In particular, the central-trough hole of the holding element may have a shape matching a shape of the surface of the body of the converter cell defining the inner space. It will be appreciated that the through- hole of the holding element may have various shapes such as for example circular, elliptic, rectangular or square. Further, the size of the through hole may also vary.

In some embodiments, the perimeter of the central through-hole of the holding element may be larger than the inner perimeter of the body of the converter cell, in which case only part of the body of the converter cell rests on the holding element. In other embodiments, the perimeter of the central through-hole of the holding element may be substantially equal to the inner perimeter of the body of the converter cell. These embodiments provide the effect of preventing surface flashover assisted by particle contamination as there is a reduced area of horizontal surfaces of the holding elements exposed to such particles.

Still, in some other embodiments, the perimeter of the central through-hole of the holding element may in some embodiments be smaller than the inner perimeter of the body of the converter cell, in which case any device (or some of the devices) arranged within the inner space delimited by the inner perimeter may rest on the holding element. The body (or capacitor unit) of a converter cell may be formed of a single piece with a hole within which other electric components such as e.g. switching devices may be arranged. In other embodiments, the body may be divided in a plurality of pieces. A piece may then form a section of the body. The present embodiments provide a converter cell with a body including a plurality of pieces or sections (in particular capacitive pieces). These embodiments are advantageous in that it reduces Eddy currents (or Foucault currents) generated at the outside surface of the capacitor unit (i.e. on the capacitor box or capacitor enclosure/container) when it includes electrically conductive material. Eddy currents flow in closed loops within electrically conductive materials (conductors), in planes perpendicular to the magnetic field. The magnitude of the current in a loop is, among others, proportional to the area of the loop. As compared to a capacitor unit based on a single piece, the use of a plurality of smaller pieces breaks the induction loop into smaller parts, which thereby reduces the amplitude of the Eddy currents.

According to some embodiments, the intersection of the plurality of connecting elements with a holding element may define a number of compartments at the holding element corresponding to the number of pieces of the body arranged at the holding element. A piece of the body may then be arranged at one compartment.

According to some embodiments, the connecting elements extending between a first holding element and a second holding element may be fixed at the first holding element between two adjacent pieces of the body arranged at the first holding element.

According to an embodiment, an outside surface of a converter cell (i.e. the body or capacitor unit of a converter cell) may be elliptic, circular and/or comprises at least one rounded corner. In some embodiments, the converter cells may be disc-shaped. A circular shaped body provides a smoother converter cell profile, which reduces requirement on insulation design and provides other benefits such as a lower stray inductance in current commutation loop. The body (or capacitor unit) of the converter cell may for example be annular (or ring-shaped).

According to an embodiment, the valve unit may further comprise a first electrical shield at one end of the container and/or a second electrical shield at an opposite end of the container. The electrical shields may be removable parts of the container for facilitating insertion of the at least one column of converter cells in the container.

According to an embodiment, the valve unit may further comprise at least one first connector or a plug-in cable termination for electrically connecting a cell of the valve unit at one end of the container. According to an embodiment, the valve unit may further comprise a spacing element including insulating material and being arranged at one end of the container for separating the container from a container of another valve unit to which said valve unit is connected. According to a embodiment, the insulating gas may be at least one of sulfur hexafluoride (SF 6 ), Nitrogen (N 2 ), air and dry air. It will be appreciated that the gas may be a mixture of different gases such as a mixture of SF 6 and N 2 . It will be appreciated that the present disclosure is not limited to such gases and that other insulating gases may be envisaged. Further, the gas may be a compressed gas, i.e. a gas under a certain pressure higher than the atmospheric pressure.

According to some embodiments, a connecting element may at least partly include solid insulating material. In particular, each of the connecting elements of the plurality of connecting elements may include insulating material, which improves the potential electrical separation between successive converter cells. According to an embodiment, a power converter station is provided. The power converter station may comprise at least two valve units as defined in any one of the preceding embodiments. The power converter station may also be referred to as a high voltage direct current (HVDC) converter station.

It will be appreciated that other embodiments using all possible combinations of features recited in the above described embodiments may be envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments will now be described in more detail, with reference to the following appended drawings:

Figure 1 shows a schematic perspective view of a holding structure (or supporting structure) in accordance with an embodiment;

Figure 2 shows a schematic perspective view of a valve unit including a container at least partially filled with insulated gas, a cell stack and a supporting structure in accordance with some embodiments;

Figure 3 shows a schematic view of a cell in accordance with some embodiments;

Figure 4 shows a schematic general view of a valve stack in accordance with some embodiments;

Figure 5 shows a schematic perspective view of a body (or capacitor unit) of a converter cell in accordance with an embodiment;

Figures 6A and 6B show schematic top views of bodies of converter cells in accordance with some embodiments;

Figure 7 shows a schematic view of a piece of a capacitor unit according to an embodiment;

Figures 8A and 8B show schematic views of alternative embodiments for the assembly of piecewise converter cells on a holding structure;

Figure 9 shows a schematic view of a valve unit of an HVDC converter in accordance with an embodiment;

Figure 10 shows a schematic view of a valve unit in accordance with another embodiment; and Figure 11 shows a schematic perspective view of a valve unit in accordance with yet another embodiment.

As illustrated in the figures, the sizes of the elements, layers and regions may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of the embodiments. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION

Exemplifying embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

With reference to Figure 1, a supporting structure 100 comprising a plurality of holding elements 121-124 and a plurality of connecting elements 141-144 is described.

Without loss of generality, Figure 1 shows a supporting structure 100 comprising a plurality of holding plates 121-124 acting as holding elements (or holding members) and a plurality of rods 141-144 acting as connecting elements. The rods 141-144 extend along an axial direction denoted 110 in Figure 1. More specifically, the rods 141-144 extend parallel to the axial direction 110 and the holding plates 121-124 are arranged at different positions along the axial direction 110 in planes perpendicularly intersecting the axial direction 110. It will be appreciated however that the connecting elements do not necessarily have to be parallel to the axial direction 110 and that the holding plates do not need to be arranged in planes which are perpendicular to the axial direction 110. The holding plates 121-124 are separated from each other along the axial direction 110 by a gap permitting the arrangement of converter cells, as will be described in more detail in the following. In the embodiment shown in Figure 1, the holding plates 121-124 of the supporting structure 100 are circular or disc- shaped. However, other shapes may be envisaged. Further, each of the holding plates 121-124 includes a through hole so that they are shaped as rings. For example, a first holding plate 121 is disc-shaped with a through- hole denoted 151 arranged in a center portion of the holding plate 121. The central through-hole 151 of the holding plate 121 has a perimeter corresponding to the inner perimeter of a body of the converter cell to be arranged at the holding element 121. The rods 141-144 extend along the axial direction 110 and intersect the holding plates 121-124 close to the through-holes of the holding plates. As mentioned above, according to some embodiments, the holding element may have a cross section (or shape) across the axial direction corresponding to a cross section (or shape) of the body of the converter cell arranged at the holding element across said axial direction. A holding element may include a central through-hole with a perimeter corresponding to, or being larger than, the inner perimeter of the body of a converter cell arranged at that holding element. In particular, the central-trough hole of the holding element may have a shape matching a shape of the surface of the body of the converter cell defining the inner space. It will be appreciated that the through- hole of the holding element may have various shapes such as for example circular, elliptic, rectangular or square. Further, the size of the through hole may also vary.

In some embodiments, the perimeter of the central through-hole of the holding element may be larger than the inner perimeter of the body of the converter cell, in which case only part of the body of the converter cell rests on the holding element. In other embodiments, the perimeter of the central through-hole of the holding element may be substantially equal to the inner perimeter of the body of the converter cell. These embodiments provide the effect of preventing surface flashover assisted by particle contamination as there is a reduced area of horizontal surfaces of the holding elements exposed to such particles.

Still, in some other embodiments, the perimeter of the central through-hole of the holding element may in some embodiments be smaller than the inner perimeter of the body of the converter cell, in which case any device (or some of the devices) arranged within the inner space delimited by the inner perimeter may rest on the holding element. The body (or capacitor unit) of a converter cell may be formed of a single piece with a hole within which other electric components such as e.g. switching devices may be arranged.

Although Figure 1 shows holding plates as an example of holding elements, it will be appreciated that other types of holding elements may be used. Generally, the holding elements serve to hold one or more converter cell at a position along the axial direction along which the stack of converter cells extends (i.e. the stacking direction). The holding elements may therefore be fixed at a specific position along the stacking direction. The holding element of a specific position may not be a continuous body or plate but could be a number of holding members defining a number of contact points at each of the connecting elements such that a converter cell may be installed on them at a specific position along the stacking direction. However, it will be appreciated that a holding element with a single body, such a holding plate, physically connecting the connecting elements extending along the stacking direction is beneficial for carrying the weight of the converter cells, thereby improving mechanical stability.

Although Figure 1 shows a supporting structure with a plurality of connecting elements and a plurality of holding elements, it will be appreciated that the holding elements may be integrated parts of the bodies of the converter cells to be arranged at the positions defined by the holding elements along the axial direction. In these embodiments, the converter cells may be equipped with some attaching means for attachment to the connecting elements. In some embodiments, the connecting elements may be one or more rods, ropes or tubes. The connecting elements may be designed to mechanically connect one holding element to another. For example, the supporting structure may include a plurality of holding plates comprising a number of holes for inserting a plurality of rods acting as connecting elements. The holding plates may then be fastened by means of some fasteners (or fastening means) such as screws or clips on the rods. In another example, the holding elements may be soldered to the connecting elements. According to some embodiments, the connecting elements may at least partly include (or be made of) solid insulating material. In particular, each of the connecting elements of the plurality of connecting elements may include insulating material, which improves the potential electrical separation between successive converter cells. With reference to Figure 2, a valve unit 200 according to an embodiment is described.

Figure 2 shows a schematic perspective view of a valve unit 200 comprising a plurality of converter cells 130-139, a supporting structure similar to the supporting structure described with reference to Figure 1 with holding elements 121-124 (not all elements are denoted in the figure) and connecting elements 141-143, and a container 280 at least partially filled with insulated gas.

The valve unit 200 comprises a container (or enclosure) 280 and a plurality of cells 130-139 arranged as a stack (or column) within the enclosure 280. The cells 130-139 are arranged on top of each other and connected in series to form an electrical equipment or system (e.g. a converter) within the container 280.

The container 280 may extend mainly along an axial direction 110 and may for instance have a cylinder- like shape extending from one base surface or region 288 to another base surface or region 289 (i.e. between two distant positions along he axial direction 110). In a specific embodiment, the enclosure 280 may be a cylinder extending along the axial direction 110 and the cells 130-139 are arranged along the axial direction 110 using a supporting structure such as described with reference to Figure 1, thereby defining a number of cell positions along the axial direction 110.

As illustrated in Figure 2, in some embodiments, the valve unit may be equipped with at least one attaching element for attaching at least one of the holding elements and/or the connecting elements 141-143 to a bottom surface, a top surface and/or a wall surface of the container. In one embodiment, considering the valve unit (and thereby the container extending the axial direction) in a standing position, i.e. with the axial direction being substantially vertical, the supporting structure may at one end be attached at (or mounted to) the bottom of the container and, optionally, the supporting structure may at its other end be attached at (or mounted to) the top part of the container.

As will be further described in other embodiments, the attaching element may include a shielding surface structure for smoothing the electrical field within the container, which improves insulation. In a more specific embodiment, the attaching element (or the shield) may be designed (or structured) to extend along a direction intersecting a radial direction (i.e. designed in a direction which is not substantially perpendicular to the axial direction along which the container extends). In other words, the attaching element may be designed in a way to avoid horizontal surfaces at which particles may drop (and accumulate), thereby impairing surface insulation.

As will be described in more detail with reference to Figure 3, a cell may include at least one capacitor element and a switching device. The stack of converter cells 130- 139 may include high voltage capacitor shields in which the capacitor elements are arranged. The HV capacitor shield of a cell may surround the switching device. In other words, a capacitor shield of a cell acts as a casing for the capacitor element. The capacitor shield may be formed as a toroid with a square section. In the stack, two successive converter cells, each comprising a capacitor shield with a capacitor element and a switching device, are separated by the insulating gas located within the container. In this configuration, an HV capacitor shield is considered to be part of a cell such that a cell includes a switching device, a capacitor element and its capacitor shield, in which case the stack includes a succession of cells disposed on top of each other with some gas gap between two successive cells.

The outer surface of the enclosure 280 may be made of an electrically conducting material, such as a metal, or may be covered by an electrically conducting material such that the outside surface of the enclosure 280 may be grounded.

Further, for electrical insulation between the converter cells 130-139, the container 280 may be at least partially filled with an insulating gas 215, which may for example be SF 6 , N 2 , air, dry air or a mixture of such gases. It will be appreciated however that the present disclosure is not limited to these examples and that other gases, in particular SF6-free gases, with similar insulation properties may be used. Further, a compressed gas with pressure of approximately a few bars may be used. For example, the enclosure 280 may be filled with SF 6 at a pressure in the range of 2 to 6 bars.

In some embodiments, at least one of the cells, a group of cells or the stack may be detachably arranged such that it is removable from the container. In particular, the stack of cells may be detachable (or removable) from the enclosure, which is advantageous for example for repair or replacement of a cell of the stack, a group of cells or even the whole stack. Referring to Figure 2, the stack of cells 130-139 may be inserted or removed from the enclosure 280 by removing an upper part of the container 280. Figure 2 shows that the container 280 may be made of at least two pieces wherein a main part is a cylinder in which the cell stack is arranged and a top part is shaped as a bottle-neck resting on the main part by means of a flange 287 so as to form a bottle-like container 280. The cylinder-like stack with its internal supporting structure may then glide along the axial direction 110 within the cylinderlike enclosure 280.

In the valve unit 200 shown in Figure 2, the stack of converter cells may be arranged coaxially to the axial direction 110 along which the container 280 extends.

It will be appreciated that, although a cylinder-like shape provides some advantages for homogeneity of the electrical fields induced in such kind of electrical equipments, the present disclosure is not limited to such a shape of the enclosure and other geometries may be envisaged.

Further, the voltages between the capacitors of the cells 130-139 (which might be of approximately a few kV) will be exposed to the compressed gas enclosed within the enclosure 280.

In the valve unit 200, the plurality of converter cells 130-139 is arranged as a stack along the axial direction 110. A converter cell, for example the converter cell denoted 139, includes a body extending in a radial direction (in a direction substantially perpendicular to the axial direction 110 in the present example) between an outer perimeter and an inner perimeter (see also Figure 3 for more detail). As a result of the arrangement of the converter cells 130-139 on the holding plats 120-129 of the supporting structure, an inner space 170 is formed within the stack of converter cells.

In the example shown in Figure 2, each converter cell is arranged at its own holding plate. As such, the holding plates 120-129 define a number of positions for arrangement of the converter cells 130-139 along the axial direction 110. In particular, the converter cells denoted 130-139 are arranged at the holding plates denoted 120-129, respectively, of the supporting structure. The supporting structure includes also a number of connecting elements, in the present example four rods 141- 144 extending along the stacking direction 110. The four rods are arranged to mechanically connect the holding plates 120-129 from one to another. The rods 141- 144 extend within the inner space 170 defined by the stacking of the converter cells 130-139. Although the specific embodiment of Figure 2 shows connecting elements extending within the inner space of the stack, the connecting elements may more generally be described as extending within the outer perimeter (and in particular the outer diameter) of the stack (as defined by the outer diameter of the converter cells forming the stack).

Figure 2 illustrates that the converter cells surround the connecting elements 141-144 of the supporting structure. In particular, Figure 2 illustrates that the converter cells may be ring-shaped and thus has a circular outside surface, thereby resulting in a cylinder-like stack of cells. Other geometries may be envisaged; however, it is beneficial if the converter cells (and the resulting stack of converter cells) have a smooth outside surface with rounded corners.

With reference to Figure 3, a cell 130 according to an embodiment is described in more detail.

Figure 3 shows a cell 130 including a body (or capacitor shield) 332 and a switching device such as a semiconductor-based component 337. At least one capacitor element (not shown) is arranged or enclosed within the body (or capacitor shield 332), i.e. within the volume defined by the interior of the body 332. The body 332 of the converter cell 130 extends in a radial direction (in a direction substantially

perpendicular to an axial direction as defined by the ring-shaped body in the present example) between an outer perimeter 160 and an inner perimeter 162.

The capacitor shield 332 and its capacitor element arranged within it may be referred to as the capacitor unit (or body) of the converter cell 130. In the embodiment shown in Figure 3, the body 332 surrounds the semiconductor component 337 and is disc- shaped. The body 332 is annular and defines a center hole in which the

semiconductor component 337 may be placed. The cell 130 shown in Figure 3 is therefore particularly suitable for forming a cylinder-like stack of converter cells.

In the example shown in Figure 3, the body 332 forms an inner space 370 defined by the body 332 of the converter cell 130. The semiconductor component 337 is arranged within the inner space 370. The inner space 370 is shown to have a cross- section circular shape across the axial direction along which the converter cell extends. However, the inner space may be of a different geometry. The inner space defined by the body of a converter cell may for example have an elliptic cross-sectional shape, a circular cross-sectional shape, a polygonal cross- sectional shape, or a square cross-sectional shape across the axial direction. While it is advantageous but not always necessary that the outside surface of the body includes rounded corners and is circular, the inner space (or internal space) delimited by the bodies of the converter cells may have various shapes, depending on the desired arrangement of the electric components within the inner space. In a specific embodiment, the inner space delimited by the hollow body may be a square, which may provide an improved filling factor of the devices installed in it. Further, the shape of the inner space may be adapted to the configuration (number, positioning) of the connecting elements extending within the inner space. In some embodiments, the inner perimeter of the converter cells (i.e. the inner space) may be adapted to receive the connecting elements. For example, indents may be formed at the surface of the body facing the inner space to insert the connecting elements, thereby leaving more space to any electronic components and/or other devices to be arranged within the inner space.

It will be appreciated that in some embodiments the connecting elements may extend in (within) a space delimited between the outer perimeter of a converter cell and the inner perimeter of the converter cell. In other words, in some embodiments, the connecting elements may not be located within the inner space of the column (as defined by the superposition of the converter cells) but outside the inner space, yet within the outer perimeter of the column (or outer diameters of the converter cells forming the column).

As mentioned above, an outside surface of the converter cell (i.e. the body or capacitor unit of the converter cell) may be elliptic, circular and/or comprise at least one rounded corner. In some embodiments, the converter cells may be disc-shaped. In particular, the converter cell (or its body) may have a cylindrical shape or the shape of a parallelepiped. It will be appreciated however that, for the purpose of an HVDC converter cell, a circular shape or at least a shape with rounded corners is

advantageous since this provides a smoother surface, which in turn facilitates the HV insulation as there are less sharp turns and edges pointing out. As a result, insulation distances can be shortened and for example corona rings may be partly or completely avoided. The use of a capacitive body with an outside surface comprising rounded corners, and e.g. being circular, provides therefore the advantage that space can be more efficiently used, thereby reducing the size of the power station.

Further, according to a more general embodiment, the converter cell may comprise at least one capacitor element arranged in a body extending from the outer perimeter to an inner perimeter of the converter cell and at least one switching device.

In such embodiments, the body may include at least one capacitor element, i.e. the body may correspond to the capacitor part (or capacitor unit) of the converter cell. In this respect, the dimensions of the body may determine the properties, and in particular the possible capacitance and voltage, of the capacitor of the converter cell for a particular selection of materials. Further, the height of the body along the axial direction may be determined by the desired capacitance or desired voltage. The body may therefore also be referred to as the capacitive body or capacitor of the converter cell.

By the term capacitor element is meant a component functioning as a capacitor, i.e. acting as an electric component used to store energy electrostatically in an electric field. A capacitor element (or capacitor) is normally built by metal layers (or plates) between which an insulating media is arranged.

The switching device may for example be arranged within the inner space delimited by the body of the converter cell. The switching devices (e.g. semiconductor switches) may be arranged in a way to more evenly distribute the switched current around the area of the capacitive body, e.g. to reduce hot spot temperatures and to increase the long-term reliability of the capacitor.

The switching device may be a semiconductor-based switching device. By way of examples, the switching device may be an insulated-gate bipolar transistor (IGBT), a metal- oxide-semiconductor field-effect transistor (MOSFET), an integrated gate- commutated thyristor (IGCT), a gate turn-off thyristor (GTO), a high electron mobility transistor (HEMT) and a hetero junction bipolar transistor (HBT). Other types of transistors (or semiconductor-based switching devices) may be envisaged.

Further, it will be appreciated that the present disclosure is not limited to a specific semiconductor technology. It will be appreciated that switching devices based on silicon or silicon carbide may be employed, in particular for MOSFETs, IGBTs, IGCTs and GTOs as examples. Switching devices based on Gallium Nitride or Gallium Arsenide may also be employed, in particular for HEMTs or HBTs as examples. Other types of semiconductors providing switching devices for high power applications may be envisaged.

It will be appreciated that the converter cell may further comprise other electric components or devices. For example, the converter cell may also include a cooling device and/or a by-pass switch which allows a current to bypass the switching devices of a converter cell upon failure of a switching device, thereby reducing the risk of damages of the components of a converter cell, e.g. caused by short circuit currents. The bypass switch may be a mechanical switch or an electric switch such as for example a thyristor. Further, the converter cell may also include means for reducing the failure currents. Other components and devices than those listed herein may be arranged within the inner space defined by the cell stack.

A plurality of cells may be arranged on top of each other along an axial direction (in particular along a vertical direction but not necessarily) to form a stack of cells. The plurality of cells may be electrically connected via a busbar element together to form the desired electrical equipment.

Figure 4 shows an example of a valve stack 450 wherein a plurality of converter cells is arranged on top of each other along an axial direction. The valve stack 150 includes a plurality of cells 130-139, each of which may be equivalent to the converter cell 130 described with reference to Figure 3. The arrangement shown in Figure 4 may correspond to the arrangement depicted in Figure 2. Figure 4 shows a cross-sectional view of the stack 450 of converter cells with a switching device 337 surrounded by a body or capacitor shield 332. Still, it will be appreciated that two successive cells in the stack may be identical or different from one to another. Figure 4 shows also a busbar 440 electrically connecting the plurality of converter cells 130-139 in series to form a larger converter.

Figure 4 illustrates also that holding elements 120-129 may be arranged between the converter cells 130-139. In the present example, one holding element is arranged between two successive converter cells. In particular, the holding element may have a shape corresponding to a shape of the body of the converter cell arranged at the holding element.

In the present configuration, the stack of converter cells may be constructed by successively assembling a holding element (such as a holding plate) and a converter cell on the connecting elements.

In the following, other embodiments will be described using another type of converter cells.

With reference to Figure 5, a capacitor unit 500 of a converter cell according to an embodiment is described.

Figure 5 shows a schematic perspective view of the capacitor unit 500. The capacitor unit 500 comprises four pieces 501-504. When assembled together, the four pieces 501-504 form a body extending along an axial direction 110.

In these embodiments, the body (or capacitor unit) of the converter cell may therefore not consist of one single piece (or one single mechanical block), but several (at least two) pieces. The pieces, or "slices" in the case of a circular capacitor, form the capacitor unit when assembled together. It will be appreciated that each of the pieces or sections may be a sub-unit (or sub-element) of the capacitor unit and acts itself as a capacitor. The capacitor unit may be formed by assembling N pieces, which facilitates the installation of the capacitor unit in a valve unit of a power converter hall since one of the N pieces of the capacitor unit is more easy to handle than the full capacitor unit (i.e. if the capacitor unit was made of a single piece). A piece or sub-element of a capacitor unit has also a lower weight than the whole capacitor unit (as compared to a single piece making the full capacitor unit).

In these embodiments, the body is formed of a plurality of pieces which, when assembled, formed a hole or hollow at which other electric components may be arranged. The pieces may be arranged adjacent to each other, i.e. in a tight arrangement with a mechanical contact between two adjacent or successive pieces. However, the pieces forming the body may in some other embodiments be arranged close to each other, yet with a gap between two successive pieces. Thus, the body may also be formed by a loose arrangement of the pieces, i.e. with a gap between the pieces, which is advantageous as it releases some pressure. The arrangement of the pieces determines an outer perimeter of the resulting converter cell.

It will be appreciated that a piece of the capacitor unit (or the body) may itself include a plurality of capacitor elements or capacitive sub-elements connected together to form a "capacitive" piece (i.e. functioning as a capacitor).

Referring to Figure 5, the pieces 501-504 delimit a space or area 570, also referred to as inner space, which corresponds to the center portion or hollow of the capacitor unit 500. Each one of the pieces 501-504 forms a section of the body and at least one of the pieces is a detachable section of the body. Figure 5 illustrates that the piece denoted 504 is detached from the other pieces.

The capacitor unit 500 may have different shapes. In some embodiments, an outside surface 506 of the body of the capacitor unit 500 may be circular, such as represented in Figure 5, but it may be envisaged that the outside surface of the body of the capacitor unit may be elliptic and/or rectangular or square or any other form. It may however be appreciated that the outside surface of the body of the capacitor unit 500 may advantageously comprise rounded corners. According to an embodiment, each of the pieces 501-504 may define a section of a ring such that the body 500 is ring-shaped, thereby forming an annular capacitor, such as shown in Figure 5.

Still referring to Figure 5, the pieces 501-504 may be distributed around the axial direction 110. The pieces 501-504 extend in a plane intersecting the axial direction 110. In particular, Figure 5 shows that the pieces 501-504 are arranged in a plane which is perpendicular to the axial direction 110.

Although four pieces 501-504 form the capacitor unit 500 in the example shown in Figure 5, it will be appreciated that the capacitor unit 500 may be divided in another number of pieces. The capacitor unit 500 may be divided in at least two pieces such that at least one piece is detachable from the capacitor unit. By detachable is meant that the piece may be detached from the capacitor unit without having to disassemble the whole capacitor unit, i.e. without having to detach all the other pieces. The detachable piece 504 is removable from the capacitor unit 500 and may be put back in place. Further, although only one of the pieces 501-504 is shown to be detached from the capacitor unit 500 in Figure 5, i.e. the piece denoted 504 in the present example, it will be appreciated that all the pieces 501-504 may be detachable from the capacitor unit 500. In general, at least one of the pieces may for example be a detachable section of the body. As a result, at least one piece may individually be removed and replaced without disturbing the surrounding pieces of the body. This improves also the accessibility to the inner space (or interior space) delimited by the body (or capacitor unit) of the converter cell, at which inner space electronic components (such as switching semiconductor devices) may be arranged. By removing one piece of the capacitive body, any components located in the inner space may be tested, taken out and possibly replaced or repaired. Such a design of the body of the converter cell facilitates maintenance operation and reduces the space requirement for maintenance, which in turn may result in a more compact power station. It will be appreciated that, in some embodiments, each of the pieces of the body may be detachable (i.e. form detachable sections of the body). With reference to Figure 6, capacitor units in accordance with some embodiments are described.

Figure 6 shows a schematic top view of two different capacitor units 500 and 600 in accordance with some embodiments.

Figure 6A shows a top view of a capacitor unit 500 which may be equivalent to the capacitor unit 500 described with reference to Figure 5. In particular, the capacitor unit 500 includes a capacitive body delimiting an area or inner space 570 which is a square. By inner space is meant the space or area which is located within the closed loop defined by the body formed by the pieces 501-504. In other words, the inner space 570 corresponds to the central portion of the capacitor unit 500.

Figure 6B shows also a top view of another capacitor unit 600 which may be equivalent to the capacitor unit described with reference to Figure 5 except that the area or inner space 670 defined by the capacitive body of the capacitor unit 600 is circular. The capacitor unit 600 comprises also three pieces 601-603 only to form the capacitive body. Although Figures 6A and 6B show two examples of possible shapes of inner spaces defined by the hollow bodies of two capacitor units, other shapes may be envisaged. For example, the inner space may also be elliptic or rectangular.

Figure 6B illustrates also that the body of the capacitor unit 600 (and thereby the resulting converter cell once a switching device is arranged within the capacitor unit) extends between an outer diameter 660 and an inner diameter 662. Figure 7 shows a schematic view of a piece of a capacitor unit according to an embodiment. Figure 7 shows an enlarged view of a piece 700 of a capacitor unit such as e.g. the capacitor unit 500 described with reference to Figure 5. The piece 700 may therefore correspond to any one of the pieces 501-504. Figure 7 shows a piece 700 having the shape of a trapezoidal block with one curved face 746. More specifically, the piece 700 comprises a first surface 746 defining a portion of the outside surface of the hollow body and a second surface 742 defining a portion of the inner space defined by the hollow body. The piece 700 comprises also two side surfaces 744, 748, each of which is to be arranged in contact with, or facing (closely to), a neighboring piece when assembled in a capacitor unit. The piece comprises also a base surface 752 (or bottom surface) and a top surface 750.

In the piece 700, the two side surfaces form walls extending in planes intersecting the first (curved) surface 746 forming a portion of the outside of the capacitor unit at an angle which is less than 90 degrees. The two side surfaces are linked by the second surface 742 forming a portion of the inner space of the capacitor unit 700. The base surface 752 and the top surface 750 extend in planes which perpendicularly intersect the two side surfaces and the first and second surfaces. The surfaces of the piece form a closed box in which an insulating material or in which a plurality of capacitive sub- elements may be arranged to provide the capacitive functionality of the piece 700.

It will be appreciated that, although Figure 7 shows a piece having a trapezoidal shape, other geometries may be envisaged. For example, the two side surfaces 744 and 748 may perpendicularly intersect the first surface and the second surface, thereby resulting in a more cubic shaped piece or section of the capacitor unit.

Further, the second surface defining a portion of the inner space of the capacitor unit may be curved, thereby defining a more circular inner space, rather than a square inner space such as obtained with the piece shown in Figure 7. Figure 7 also shows that the piece 700 comprises electrical connectors 760 arranged at the second surface 742 defining a portion of the inner space of the capacitor unit. In other words, the electrical connectors are arranged at the wall facing the inner space defined by the body. The electrical connectors 760 may be used for connection to at least one switching device or power converter circuitry arranged in the hollow center of the capacitor unit.

Generally, the pieces of a capacitor unit, such as the piece 700 shown in Figure 7, form an enclosure or container in which at least one capacitor element may be arranged. The capacitor element may include metal plates and a dielectric material arranged between the metal plates. The capacitor element may for example be a wound-film capacitor. The enclosure or container defined by a piece may be made of electrically conductive material, such as a metal, but may also be made of a non- conductive material. Further, depending on whether the enclosure is to be used for shielding, i.e. depending on the application, the enclosure or container may also be coated by a non-conductive painting. Assembling the plurality of pieces may result in a cylindrical capacitor. Figure 8 A shows a valve unit 1100, or at least part of it (the column of converter cells with its internal supporting structure), in which a piecewise capacitor unit (or piecewise capacitive body) is used to form the converter cells. It may be considered that Figure 8A shows part of a valve unit, such as e.g. the valve unit 1200 which will be described with reference to Figure 9, under assembly.

The valve unit 1100 includes a supporting structure 100, which may be equivalent to the supporting structure 100 described with reference to Figure 1. In Figure 8 A, a supporting structure 100 with two holding plates 121 and 122 and four connecting elements as rods 141-144 are shown.

The first holding plate 121 is arranged to receive a first converter cell 131 while a second holding plate 122 is arranged to receive a second converter cell 132. The first and second converter cells 131, 132 or the main bodies of these converter cells may be equivalent to any one of the bodies or converter cells 500 and 600described with reference to Figures 5 and 6. Although only one converter cell 131 is shown to be arranged on the first holding plate denoted 151, another converter cell may be inserted between the first holding plate 121 and the second holding plate 122 such that the first holding plate 121 hold two converter cells. The first holding plate has a through-hole 151 via which electrical connections between successive converter cells may be established.

Figure 8 A illustrates also that the rods 141-144 extend within the inner space defined by the pieces of the bodies (or capacitor units) of the converter cells 131, 132. With the present configuration, the converter cells and the holding plates do not need to be successively mounted on the rods 141-144. The holding structure may first be realized as a whole, i.e. by assembly of the holding plates 121, 122 and the connecting elements 141-144. The converter cells may then be mounted on the supporting structure in a piecewise manner, i.e. by first assembling the four pieces of a converter cell together with its associated switching device and any other auxiliary devices and then assembling another converter cell.

Generally, in these embodiments, the intersection of a plurality of connecting elements with a holding element defines a number of compartments at the holding element corresponding to the number of pieces of the body arranged at the holding element, wherein a piece of the body is arranged at one compartment. The connecting elements extending between a first holding element and a second holding element may for example be fixed at the first holding element between two adjacent pieces of the body arranged at said first holding element. In such embodiments, there may be some gap between two adjacent pieces of the body of the converter cell such that the connecting elements may be inserted or arranged between these two adjacent pieces. In these embodiments, the connecting elements are arranged within as space delimited by the outer perimeter of the converter cell and an inner perimeter of the converter cell as defined by the assembly of the pieces.

Figure 9 shows a schematic view of a valve unit 1200 of a power converter, such as for example an HVDC power converter, in accordance with some embodiments. The valve unit 1200 comprises a plurality of converter cells 1271-1280, i.e. ten converter cells in the present example, arranged as a stack by means of a holding structure 100. However, the valve unit 1200 may comprise any number of power converter cells, depending on the application and consequently on the desired voltage or desired power.

The valve unit 1200 may also comprise high voltage capacitor shields arranged between two adjacent (or successive) power converter cells.

As illustrated in the enlarged view of a portion of the valve unit shown in Figure 9, a power converter cell 1275 in the valve unit 700 may comprise a capacitor unit and power converter circuit of the type described with reference to any one of Figures 5- 7. The power converter cells shown in Figure 9 may include capacitor units having disc-shaped enclosures. Other shapes may be envisaged, such as enclosures with circular, elliptical or rectangular cross-sections. In an embodiment, the capacitor units (and thereby the power converter cells) may have the form of rings surrounding the power converter circuits. As already illustrated in Figure 9, the power converter circuits of the power converter cells 1271-1280 may for example be electrically connected in series for increasing the input and/or output voltage of the valve unit 1200.

As illustrated in Figure 9, the use of a converter cell having a body divided in a plurality of pieces is beneficial since detachment of one of the pieces facilitates the accessibility to any components of the convert cells (i.e. within the inner space defined by the capacitor unit of the converter cell), thereby facilitating maintenance operations and reducing the space requirement for such operations.

Figure 8B shows a schematic view of a valve unit in accordance with another embodiment. Figure 8B shows a valve unit 1300 which is equivalent to the valve unit 1100 described with reference to Figure 8A except that the connecting elements are displaced. In Figure 8B, the supporting structure 1310 includes a plurality of connecting elements 1341-1344 which do not extend within the inner space 151 formed by the converter cells of the stack but extend between the outer diameter of the converter cell denoted 1331 and its inner diameter . Similarly, the connecting elements 1341-1344 extend between the outer diameter of the converter cell denoted 1332 and its inner diameter. A gap may be provided between adjacent pieces of a converter cell such that the connecting elements may be fixed at the holding plates 121, 122. In these embodiments, the connecting elements 1341-1344 are arranged between two pieces arranged side by side.

With reference to Figure 10, a valve unit 1400 according to an embodiment is described.

Figure 10 shows a cross- sectional view of a valve unit 1400 comprising a container (or enclosure) 280 and a plurality of cells arranged as a stack within the enclosure 280 using a supporting structure including a plurality of rods 141-143 and holding elements 120-124 such as described with reference to Figures 1 and 2.

The valve unit 1400 shown in Figure 10 may be equivalent to the valve unit 200 described with reference to Figure 2 except that it also shows a first connector 1460 for connection of one end of the stack of the valve unit to another valve unit and a second connector 1470 for connection of another end of the stack of the valve unit to yet another valve unit.

The container 280 including the stack of converter cells may be closed or sealed. For example, a spacing element may be arranged at one of the ends or base surfaces of the enclosure. As depicted in Figure 10, a first spacing element 1416 may be arranged at one end 288 of the container 280 while a second spacing element 1414 may be arranged at an opposite end 289 of the container 280. More specifically, the first spacing element 1416 may be arranged at the junction between the first connector 1460 and the top end 288 of the container 280 while the second spacing element 1414 may be arranged at the junction between the second connector 1470 and the bottom end 289 of the container 280.

The first spacing element 1416 and the second spacing element 1414 may for example be made of an insulating material.

A spacing element may be arranged at one end of the container for separating the container from a container of another valve unit to which said valve unit is connected. The first connector 1460 includes a busbar element 1461 arranged to connect the stack of converter cells (and more particularly an upper cell of the stack) to the stack of an adjacent valve unit (not shown in Figure 10). The first connector 1460 may include a busbar element made of a single piece bent so as to connect two cells located on top of the stacks of two adjacent valve units. Alternatively, as depicted in Figure 10, the busbar element of the first connector 1460 may include a plurality of segments. For example, the first connector may have a nodal element 1462 for connecting a first segment of the busbar element to another segment of the busbar element extending along the axial direction 110 for electrically connecting the converter cells in series. In the particular embodiment shown in Figure 10, the nodal element 1462 is configured to direct a segment of the busbar element 1461 in a direction intersecting (e.g. orthogonal to) the axial direction 110 along which the container 280 extends so as to reach an adjacent valve unit.

The second connector 1470 includes a busbar element 1471 arranged to connect the stack of converter cells (and more particularly a lower cell of the stack) to the stack of an adjacent valve unit (not shown in Figure 1). The second connector 1470 may include a busbar element made of a single piece bent so as to connect two cells located at the bottom of the stacks of two adjacent valve units. Alternatively, as depicted in Figure 10, the busbar element of the second connector 1470 may include a plurality of segments. For example, the second connector may have a nodal element 1472 for connecting a first segment of the busbar element to another segment of the busbar element extending along the axial direction 110 for electrically connecting the converter cells in series. In the particular embodiment shown in Figure 1, the nodal element 1472 is configured to direct a segment of the busbar element 1471 in a direction intersecting (e.g. orthogonal to) the axial direction 110 along which the container 2801 extends so as to reach an adjacent valve unit.

The first connector 1460 and the second connector 1470 may be configured to direct a busbar element in opposite directions so as to reach the stack of two different valve units, thereby enabling a serial connection of the valve units to form a larger converter.

In the embodiment shown in Figure 10, the stack of cells and its supporting structure may be inserted or removed from the enclosure 280 by removing the first spacing element or spacer 1416, the first connector 1460 mounted at the top end of the cylinder-like container 280, thereby opening the container 280, and by optionally removing the second spacing element 1414 and disassembling a top part of the container 280.

Optionally, the valve unit 1400 may also be equipped with a top (or first) capacitor shield 1411 at the top end 288 of the container 280 and with a bottom (or second) capacitor shield 1413 at the bottom end 289 of the container 280 at which the supporting structure, and in particular the rods 141-143, may be attached in order to support the stack of converter cells. The electrical shields 1411 and 1413 may be removable parts of the container for facilitating insertion of the column of converter cells in the container.

Expressed more generally, the valve unit may be connected to another valve unit via its first connector or may be connected to an AC or DC transmitting line via a plug-in cable termination. The first connector may for example include a first busbar element for electrical connection of the cell in question to a cell of another valve unit. The first connector may for example be arranged to electrically connect an upper cell of the column of a first valve unit to an upper cell of a column of a second valve unit. The first connector may be insulated by an electrically insulating gas. As shown in the example of Figure 10, it will be appreciated that a valve unit may be equipped with a second connector at another end of the container for connection of another cell of the column to another valve unit. According to some embodiments, the first connector and the second connector may be located at opposite ends of the container. In particular, the first connector may be configured to electrically connect an upper cell of the column (e.g. the one located on top of the stack) to an upper cell of a column of the other valve unit. Analogously, the second connector may be configured to electrically connect a lower cell of the column to a lower cell of the column (e.g. the lowest one in the valve stack) of a third valve unit. For example, in the case of a cylinder-like container (or enclosure), the first connector may be arranged at a first base of the cylinder- like container and the second connector may be arranged at a second base of the cylinder-like container opposite to the first base.

Further, or alternatively, it will be appreciated that the valve unit may be equipped with a plug-in cable termination or a gas-insulted busbar for connection to an alternating current (AC) transmission line. Similarly, the valve unit may be equipped with another plug-in cable termination or a gas-insulated busbar for connection to a direct current (DC) transmission line. Whether the valve unit is equipped with one or more connectors or a plug-in cable termination (or gas-insulated busbar) depends on the position of the valve unit in question in an arrangement of a plurality of valve units used to build a power station. In a configuration with only two valve units, the first valve unit may include a plug-in cable termination or a gas-insulated busbar for connection to an AC transmission line at one end of its container while at its opposite end the first valve unit is connected to the second valve unit via a first connector electrically insulated by gas. A cell at an opposite end of the second valve unit may then be connected to a DC transmission line via a plug-in cable termination or a gas- insulated busbar.

With reference to Figure 11, a valve unit 1500 in accordance with yet another embodiment is described. The valve unit 1500 includes a container 1580 including at least four main parts. The container 1580 may be cylindrical and its center part may be made of two tubular-like parts 1582 and 1583 which may comprise flanges for assembly of these two parts. The container 1580 may also comprise two end portions which may be shaped as bottle-necks. A container made of several portions may facilitate the assembly of the valve unit and in particular the insertion of the converter cells and the supporting structure inside the container.

As shown in Figure 11, the valve unit 1500 may include a supporting structure including a plurality of holding elements 1520 (although only one is shown in Figure 15 for not obscuring too much the figure) and a plurality of connecting elements (or rods) 1540 mechanically connecting the holding elements (or holding plates) together. The connecting elements 1540 are arranged within the outer perimeter of the converter cell 1530 arranged on the holding element 1520.

According to an embodiment, a power converter station is provided. The power converter station may comprise at least two valve units as defined in any one of the preceding embodiments. The power converter station may also be referred to as a high voltage direct current (HVDC) converter station.

It will also be appreciated that the plurality of converter cells of the valve unit may only constitute part of a larger converter. In particular, according to some

embodiments, a converter arm (or converter branch) may comprise at least two valve units as defined in any one of the preceding embodiments. In these embodiments, the converter cells of a first valve unit may be electrically connected in series to the converter cells of a second valve unit, as described above.

The HVDC converter station may also be built by assembling at least two valve arms wherein one arm comprises a number of valve units electrically connected in series. The HVDC converter station may for example include three arms to provide a three- phase converter. As mentioned above, the outer surface of the container may include an electrically conductive material or layer such as a metal, which may then be used for grounding. In a valve arm or power converter station, all the containers of the valve units may include a metal (or an electrically conductive outer surface) with electrical potential to ground.

The present disclosure is applicable for power equipments with various voltage levels such as e.g. a high voltage power converter station but also medium voltage equipments, in which it is desired to improve space management. The embodiments of the present disclosure are advantageous in any applications wherein a column of converter cells may be used. For exemplifying purposes only, embodiments of the present disclosure may be beneficial to achieve converters such as a static

synchronous compensator (STATCOM) for flexible AC transmission systems (FACTS) applications, motor drives, sub-sea power converters and DC-DC converters for DC grid. Other applications may however be envisaged. The embodiments of the present disclosure improve space management by means of gas insulation in the container and by arranging the supporting structure within the outer diameter of the column of converter cells, i.e. via an internal supporting structure. The present disclosure is generally advantageous for applications in which a more compact power equipment is desired, such as in applications where space for installation of the electric power equipment is limited and/or for offshore wind farm applications. In the context of the present disclosure, the term valve unit may be interchangeably replaced with the terms converter valve stack, block unit or apparatus (of a power converter).

Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. In particular, although the examples shown in Figures 4 and 6 include five valve units, it will be appreciated that any other number of valve units may be used to form a power converter station (or at least part of it). Further, reference to a first, second, third, fourth and fifth valve units may be interchanged such as for instance the third valve unit is considered to be the first valve unit and vice versa.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word

"comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage.