The present disclosure relates generally to the field of high-voltage/high-power converters. The present disclosure relates in particular to a valve unit for such power converters.

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.

In one configuration, a building block or a valve unit of a power converter may comprise a plurality of converter cells connected in series. A general challenge in the present technical field is to provide more compact (HVDC) power converter stations, for e.g. offshore applications, in order to facilitate installation and transport of the power converter stations. Further, required air clearance in valve hall design often makes the valve hall very large. There is a general need of reducing the volume of the valve halls.

SUMMARY

An object of at least some embodiments of the present disclosure is to wholly or at least partly address the above mentioned issues.

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 of a power converter station. The valve unit comprises at least one compartment delimiting an interior space and at least part of at least one power converter arranged within the interior space. A wall of the at least one compartment comprises an at least partially electrically-conductive inner layer facing the interior space and an at least partially electrically-conductive outer layer. An insulating material is arranged between the outer layer and the inner layer. Further, the at least one compartment is at least partially filled with a gas including air and includes at least one port for access to the interior space.

The present disclosure provides a valve unit including a compartment in which at least part of a power converter (in some embodiments the whole power converter or even several power converters) may be installed. The valve unit according to embodiments of the above defined general aspect is based on hybrid insulation in that insulation of the power converter (or part of it) is obtained by insulation via the wall of the compartment and via air present within the compartment. In the present embodiments, the compartment is at least partially filled with a gas including air. Insulation between cells or components of the power converter (or part of it) is achieved by air. In particular, the compartment is unsealed (i.e. not hermitically closed) and is open to air.

By a gas including air is meant that the compartment may be at least partially filled with a gas such as ambient air, dry air, air with reduced oxygen content, air with increased nitrogen content (N ₂ ), air with increased carbon dioxide content (C0 ₂ ) or any other kinds of technical air in the sense that the temperature, humidity, partial content or contamination of the air is controlled. For example, the increased amount of CO2 or N2 may be obtained by flushing in these gases in the compartment. The compartment may also be filled with any other gas mixtures comprising air. Air may be ambient air or any other kind of air (as defined above) located at the outside of the compartment and entering the compartment via at least one opened port of the compartment, such as e.g. the port for access to the interior space (like a maintenance port). Further, as compared to valve units in which the power converter and/or any other components of the valve unit are all submerged in an insulating material, the valve unit according to the above embodiment is advantageous in that the insulating material is enclosed (or sealed) within a wall of the compartment between the inner layer and the outer layer of the wall. In such a valve unit, the insulating material is prevented from being in direct contact with any active component of the power converter (or part of it). The electrical components (and any other active components) of the power converter are instead subject to air, which solves the problem of material compatibility between the electronic components of the power converter and the insulating material.

Embodiments of the present disclosure are also advantageous in that the amount of insulating material, such as e.g. oil, required for the insulation is reduced as only the wall(s) of the compartment may be filled by the insulating material.

Further, providing an air-based insulation of the power converter and enclosing the insulating material within the wall of the compartment reduce the risk of fire as compared to prior art systems based on e.g. oil-filled tanks and the need of safety precautions for gas leakage (as air is used instead of other gases being less friendly to the environment).

The valve unit is also provided with a port (or opening) for access to the interior space, and thereby to the power converter (or part of it) arranged within the interior space. The port may be a maintenance port such as a manhole provided at one side of the compartment for access to the interior space by a human, a robot or a robot arm. Providing an opening at a wall of the compartment facilitates maintenance operations.

Generally, at least some embodiments of the present disclosure provide for a more compact valve unit and thereby more compact power station since it provides an improved use of air as insulating medium and also an improved design for performing maintenance operations within the valve unit. In particular, the insulation distance between power converters (or components of a power converter) within a valve unit may be reduced.

By the term compartment is meant a structure delimiting an area or a region in space. It will be understood in particular that the compartment does not necessarily need to be an enclosed space. The compartment may be opened to air (e.g. ambient air) in the sense that air may circulate in and out of the compartment (via at least one

port/opening). The compartment may for example delimit an interior space by a floor and walls without the need of any roof or top part. Similarly, a side or wall of the compartment other than the top part may be missing (or removed), yet still defining (or delimiting) an area, or region in space, by means of the extension of the other walls. The compartment may therefore delimit an opened inner space (or interior space opened to air). The wall(s) or any other surface(s) of the compartment determine the boundaries of the inner space.

The compartment may also be referred to as a receptacle (or tank or container) adapted to receive a power converter (or part of it) to form a valve unit of a power converter station. A wall of the compartment includes an at least partially electrically- conductive inner layer facing the interior space delimited by the compartment. The shape of the inner layer thereby defines the shape of the interior space of the compartment. Further, a wall of the compartment includes an at least partially electrically-conductive outer layer. The shape of the outer layer defines an outside surface of the compartment. By the term interior space or inner space is meant the space delimited by the wall(s) of the compartment. It is also meant that the power converter (or part of it) may be installed (or located) within the area delimited by the wall(s) of the compartment. Thus, the interior space or inner space may also be referred to as an installation space. It will be appreciated that, although the valve unit may in the following be described with reference to only one compartment or receptacle, the valve unit may comprise a plurality of compartments. Further, although it may be referred to only one power converter arranged in the compartment, a plurality of power converters may be arranged within a single compartment or only part of a power converter may be arranged in a single compartment (in other words, a single power converter may be installed in several compartments, which may be connected through a bus bar between the compartments).

According to an embodiment, the at least partially electrically-conductive outer layer may form an outer container and the at least partially electrically-conductive inner layer may form an inner container. The insulating material is then arranged between the inner container and the outer container. In the present embodiment, the container defined by the inner layer of the compartment is placed inside the outer container defined by the outer layer. It will be appreciated that the shape of the inner layer defines the shape of the interior space and the shape of the outer layer defines the shape of the outside surface of the compartment.

For exemplifying purposes only, the inner container and the outer container may be two cylinders wherein a cylinder of smaller diameter is placed within a cylinder of larger diameter. In this example, the compartment may extend along an axial direction (the axial direction of the cylinders) and includes mainly a single cylindrical wall and two end surfaces (a base and a top) extending in planes intersecting the axial direction. In some configurations, each of the inner layer and the outer layer may be shaped to form the cylindrical wall and the two end surfaces. The port (or opening) may be provided at one of the two end surfaces of the first and second cylinders, the port extending through both the inner layer and the outer layer (i.e. through the inner container and the outer container).

According to an embodiment, the compartment may comprise a plurality of walls and/or either one of the inner layer and the outer layer may comprise a plurality of sub-elements. The compartment may have different shapes and the compartment may be obtained by assembling a plurality of walls. According to some embodiment, the walls of the compartment may be detachable for assembly and/or disassembly of the compartment. Generally, the valve unit may be manufactured in a modularized manner in order to facilitate assembly and

disassembly. Referring again to the example wherein the compartment is made of a cylindrical inner container within a cylindrical outer container with an insulating layer or material arranged between the two cylindrical containers, each of the inner cylindrical container, the outer cylindrical container and the insulating layer may be made of several separate pieces. These pieces may be assembled together to form the containers. Further, for each of the inner and outer containers, the cylindrical wall may be a single piece (or several pieces) distinct from the two end surfaces (base and top) of the container. In other examples, the containers may not necessarily be cylindrical but e.g. have a box-shape wherein each wall of the box may be a separate piece. Accordingly, in some embodiments, the at least one compartment may be box-shaped or cylindrical. For example, the box-shaped compartment may have a rectangular or square cross-section within a plane intersecting an axial direction along which the compartment extends. In some embodiments, an outside surface of the compartment may comprise at least one rounded corner, and in particular the compartment may be cylindrical. Although it may be envisaged to provide compartment having a rectangular structure with sharp corners, it will be appreciated that, for the purpose of an HVDC converter station, a cylindrical shape or, more generally, 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, which provides the advantage that space can be more efficiently used, thereby reducing the size of the power station.

According to an embodiment, the insulating material may have a larger electrical breakdown withstand than air.

Generally, the main part of the voltage drop is intended to be placed over the insulation provided in the wall(s) of the compartment. The insulating material arranged within the wall(s) of the compartment is therefore selected to have a large electrical breakdown withstand, at least larger than air, such that the total thickness of the required insulating material (and thereby the thickness of the wall(s) of the compartment) can remain small, or at least be reduced.

In some embodiments, the insulation material may be a solid insulation material such as for example a polymer or rubber. The solid insulation material may be a thermoplastic or a thermoset such as for example epoxy. In some other embodiments, the insulation material may be a liquid-based insulation material, a gas-based insulation material or a gel-based insulation material. The insulating material may for example be oil, such as mineral oil, silicon oil, natural esters, synthesis esters and/or vegetable oil as examples. Alternatively, the insulating material may include a combination of cellulose material and a dielectric liquid. As yet another alternative, the insulating material may include a combination of sand and a dielectric liquid. According to further examples, the insulating material may include synthetic fiber materials that could be impregnated by a liquid. In particular, the dielectric liquid may be an ester liquid since it is less flammable than e.g. mineral oil. It will be appreciated that other examples may be envisaged.

According to an embodiment, the compartment may be totally opened at one side, thereby providing the maintenance port. As mentioned above, the compartment may delimit an interior space even if a wall or side of the compartment is removed, like a box or cylinder (or similar three-dimensional structure) opened at one side. In such embodiments, the opened side (or missing wall) of the compartment provide the maintenance port or opening for access to the interior space.

In particular, at the opened side of the compartment, the valve unit may further comprise a structure with solid insulating material to extend the wall(s) of the at least one compartment such that the at least one compartment is isolated from any neighboring (or adjacent) compartment of the valve unit. In embodiments based on liquid-based insulating material, such as oil, a structure with solid insulation material arranged at the wall edges defining the opened side increases creepage distance between inner layers of neighboring compartments (i.e. between neighboring inner containers). According to an embodiment, a portion of a wall of the compartment may be opened to provide the maintenance port. In some embodiments, the valve unit may further comprise at least one removable cover to seal the maintenance port. The cover may comprise an at least partially electrically-conductive inner layer facing the inner space of the compartment and an at least partially electrically-conductive outer layer. An insulating material may be arranged between the inner layer and the outer layer.

In particular for embodiments based on liquid-based insulation, the cover may comprise a first cover corresponding to the inner layer (i.e. a cover made of the material of the inner layer of the compartment) and a second cover for the outer layer (i.e. a cover made of the material of the outer layer of the compartment). Access to the installation space or inner space of the compartment may be possible by at least partially draining out the insulating liquid located between the inner layer and the outer layer, removing the second cover and removing the first cover. Generally, instead of providing an overall sealed compartment with insulation material in it, embodiments of the present disclosure provide an opening at one side of the compartment (or container), thereby allowing people, a robot or a robot arm to have easy access to the valve stack for maintenance operation. Insulation is achieved by air and by the insulating material enclosed within the wall(s) of the compartment.

It will be appreciated that, in these embodiments, the compartment may comprise at least one port for opening (or connecting) the interior space of the compartment to outside air. According to an embodiment, the maintenance port (or opening) may be located at a top side of the compartment. According to an embodiment, the compartment may be arranged in a room of a building, the room having a floor and walls. In the present embodiment, the outer layer of a wall of the compartment may include at least part of some of the walls of the room and at least part of the floor of the room on a condition that the floor and the walls of the room are electrically connected to ground.

In some embodiments, the inner layer and the outer layer may include a metal.

Further, the outer layer may be electrically connected to ground (or a ground potential or a certain voltage potential having a large voltage difference from the inner layer).

According to an embodiment, the power converter may include at least one stack of power converter cells. As mentioned above, insulation is provided by air within the compartment and via the insulating material enclosed in the wall(s) of the

compartment. In the present embodiment, the internal voltage difference within the valve stack (or stack of power converter cells) is insulated by air such that the voltage drop within the valve stack is only a fraction of voltage upon the full system, i.e. upon the whole HVDC converter which may be composed of several compartments each having a single or several valve stacks. The HVDC converter may be constructed such that the valve stacks are connected in series. In at least some of the embodiments of the present disclosure, the air clearance distance for each valve stack may be substantially shorter than the air clearance that is required in a conventional air insulated valve design since the air clearance distance for a conventional valve design is determined by a maximum HVDC valve stack voltage to ground, which for example could be a few hundred kV. In the present embodiments, the air clearance distance is determined by the maximum valve stack voltage to the inner layer (or inner container), which may just be a few tens of kV.

According to an embodiment, at least one of the power converter cells in the stack may be electrically connected to the inner layer. In particular, the inner layer of the wall(s) of the compartment may be electrically connected to an intermediate voltage or intermediate cell of the stack. More specifically, the inner layer (or inner container in some embodiments) may be connected to a cell located in a center position or close to a center position within the stack of power converter cells. This ensures that the maximal voltage drop over air is not larger than the largest voltage difference within the whole valve stack.

According to some embodiments, the valve unit may further comprise a bushing (or connector) for connection of the power converter to a transmission line (e.g. a bus bar). The connector may be arranged at the maintenance port or another port (or opening) or flange of the compartment.

According to some embodiments, a power converter station comprising at least one valve unit as defined in any one of the preceding embodiments is provided.

The present disclosure is applicable for high voltage power equipments with various voltage levels in which it is desired to improve space management. 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. 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 three-dimensional view of a compartment for a valve unit of a power converter station in accordance with some embodiments;

Figure 2A shows a schematic three-dimensional view of a valve unit of a power converter station in accordance with some embodiments;

Figure 2B shows a schematic cross-sectional view of a valve unit of a power converter station in accordance with some embodiments; Figure 3 shows a schematic view illustrating covers of an opening of a valve unit in accordance with an embodiment;

Figure 4 shows a schematic view of a power converter station according to some embodiments;

Figures 5A and 5B show a schematic view of a plurality of valve units in accordance with some embodiments; and

Figures 6A and 6B show a schematic view of a plurality of valve units in accordance with some other embodiments. 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 compartment of a valve unit 100 according to an embodiment is described.

Figure 1 shows a three-dimensional cross-sectional view of a compartment (or receptacle) 100 delimiting an interior space or inner space 150. The compartment 100 includes a wall 105 comprising an inner layer 110 and an outer layer 120. An insulating material 130 is arranged between the inner layer 110 and the outer layer 120. The outer insulating material 130 is sandwiched between the inner layer 110 and the outer layer 120. The inner layer 110 faces the inside of the compartment 100, i.e. faces the interior space 150. The outer layer 120 corresponds to the outside of the compartment 100. Both the outer layer 120 and the inner layer 110 may be made of at least partially electrically-conductive material (in some embodiments, wholly electrically-conductive material) such as a metal.

The interior space 150 is adapted to receive at least part of a power converter (or a whole power converter or several power converters), e.g. in the form of a valve stack (not shown in Figure 1 but in Figure 2). The compartment 100, and more specifically the interior space 150, is at least partially filled with a gas including air. Insulation of the components of any power converter arranged within the compartment 100 is therefore provided by air and by means of the insulating material 130 enclosed within the wall 105.

The gas in the compartment may be ambient air, dry air, air with reduced oxygen content, air with an increased content of e.g. N ₂ or C0 ₂ or any other kinds of technical air in the sense that the temperature, humidity, partial content or

contamination of the air is controlled.

The compartment 100 includes also a maintenance port or opening 160 for access to the interior space 150. The maintenance port 160 may be a manhole (with a diameter in the meter-range) such that a person or operator may enter the compartment 100.

In the present embodiment, the compartment 100 may also be described as comprising an outer container (or outer tank) 120 and an inner container (or inner tank) 110 arranged within the outer container 120. An insulating material 130 is then arranged between the outer container 120 and the inner container 110.

In Figure 1, the compartment 100 (or the containers 110, 120) has a cylindrical shape. Although a cylindrical shape provides some advantages, other shapes may be envisaged such as a box-shape. In Figure 1, the outer container 120 extends along an axial direction and comprises a cylindrical side wall 105 and two end surfaces 121, 126 intersecting the axial direction.

The opening 160 is provided at an end surface 121 provided at the top of the cylindrical container 120 in Figure 1. It will however be appreciated that the opening 160 may be placed at any other side or wall of the container 120. The opening 160 is a through hole which extends through the outer container 120, the insulating material 130 and the inner container 110. Figure 1 also shows that the outer container 120 may include a plurality of pieces or sub-elements 121-126. This plurality of pieces 121-126 forms the outer container 120 when assembled together. In particular, Figure 1 shows that the outer container 120 may include a first end surface 121, a second end surface 126, and a cylindrical side surface extending from the first end surface 121 to the second end surface 126. The cylindrical side surface includes a plurality of pieces 122-125 and extends generally along the axial direction of the compartment 100. The pieces 121-125 of the outer container 120 may be detachable in order to facilitate assembly and/or disassembly of the compartment 100. Although it is herein described that the outer container (or outer layer) 120 may be made of several pieces, it will be appreciated that the inner container (or inner layer) 110 may also be made of several pieces, as illustrated in Figure 1. In other embodiments, either one or both of the outer container 120 and the inner container 110 may be made of a single (continuous) layer.

With reference to Figures 2A and 2B, valve units in accordance with some embodiments are described.

Figure 2A shows a three-dimensional cross-sectional view of a valve unit 200 comprising a compartment such as described with reference to Figure 1.

Figure 2A shows also that the valve unit comprises a power converter (or at least part of it), such as a valve stack, 280 arranged within the interior space 150 of the compartment 200. The valve stack 280 may include a plurality of power converter cells disposed on top of each other.

In the embodiment shown in Figure 2A, the power converter cells have the shape of annular rings (only half of the valve stack is shown, i.e. halves of circular converter cells are shown, in Figure 2A since it is a cross-sectional view) such that the valve stack is cylindrical. A power converter cell may include a capacitor unit, such as an annular capacitor unit, within which switching devices (such as e.g. IGBTs) are arranged. The serial connection of a plurality of power converter cells in the valve stack results in the power converter 280. The number of power converter cells within a valve stack may vary depending on the desired voltage achievable by the valve stack. Insulation between successive power converter cells of the valve stack within the valve unit is obtained by air present in the receptacle. The present disclosure is however not limited to such a specific type of power converter and other power converters, based on other types of valve stack or other types of designs, may be envisaged. In particular, the present disclosure is not only limited to power converters. The present disclosure, and more specifically the enclosure or compartment such as described with reference to Figure 1, may be in particular applicable for other types of high- voltage equipments (such as circuit breakers or inductors) for which electrical insulation is also needed.

As illustrated in Figure 2, the outer layer (or outer container) 120 of the valve unit 200 may be electrically connected to ground (or a certain voltage potential having a large voltage difference from the inner container). The outer layer 120 may include a metal or a material comprising metal.

Although not depicted in Figure 2A, at least one part of the valve stack (or other installed equipment) 280 may be electrically connected to the inner layer 110. In particular, a power converter cell located at a center position within the valve stack 280 may be connected to the inner layer 110. The inner layer 120 may include a metal (or a material including metal).

Further, a portion of a wall of the compartment may be opened to provide the opening 160. Referring to both Figures 1 and 2A, the opening 160 may be placed at an end surface 121 of the cylindrically shaped compartment 100 (or valve unit 200). The opening 160 is sufficiently large to allow access to the interior space 150 by a person or operator 270. In Figures 1 and 2A, the opening 160 is shown to be arranged at a top side 121 of the compartment 100. Should maintenance operations be needed, the operator 270 can enter the valve unit 200 via the opening 160 and go down to the valve stack to perform maintenance operations. The valve unit 200 may also comprise a port insulator 290 with shed for electrically separating the voltage potential between the inner tank 110 and outer tank 120. In particular, the port insulator 290 may be arranged at the opening 160. The port insulator 290 may be tubular such that access to the interior space 150 by an operator 270 is possible (within the tube). As illustrated in Figure 2A, an operator may enter the interior space 150 via the hollow port insulator 290.

Figure 2B shows a schematic cross-sectional view of a valve unit of a power converter station in accordance with some embodiments. Figure 2B shows a cross-section of a valve unit which is almost equivalent to the valve unit shown in Figure 2A except that the compartment has another design and that electrical connections or bushings are illustrated. In both Figures 2A and 2B, the maintenance port 160 is shown to remain opened. Figure 2B shows a valve unit 235 with a compartment comprising an outer tank 120 and an inner tank 110 having a more rounded bottom as compared to the

compartment of the valve unit 200 shown in Figure 2A. The compartment of the valve unit 235 has the general shape of a bottle. The top part of the compartment of the valve unit 235 has the shape of some kind of bottle neck as defined by the shape of the inner tank 110 and the outer tank 130.

Figure 2B illustrates also that an inner tank separator (or separating means) may be arranged between the inner tank 110 and the outer tank 120 to form the gap in which the insulating material 130 may be arranged.

The valve unit 235 comprises also a port insulator 290 arranged at the top part, or opening 160, of the bottle-shaped compartment. The opening 160 of the valve unit 235 defines the maintenance port. The port insulator 290 is provided with shed to increase the creepage distance.

Figure 2B also illustrates that the valve unit 235 may be equipped with bushings or connectors 226, 227 at flanges 228, 229 for connection to other valve units of the power converter station. More specifically, the valve unit comprises a first flange 228 and first bushing 226 in an upper portion of the compartment, i.e. close to an upper converter cell of the power converter 280 and a second flange 229 and bushing 227 in a lower portion of the compartment, i.e. close to a lower converter cell of the power converter 280 (i.e. at a converter cell located at an opposite side of the valve stack). The valve unit 235 may also comprise additional flanges such as the flange 236 located at the outside surface of the upper portion of the compartment as illustrated in Figure 2B. Figure 2B illustrates also the connection 134 of one of the power converter cells of the power converter 280 to the inner layer 110 of the compartment.

Figure 3 shows a schematic view illustrating covers of a maintenance port of a valve unit in accordance with some other embodiments. The embodiment shown in Figure 3 differs from the embodiment shown in Figures 2A and 2B in that the maintenance port may be sealed. It will however be appreciated that the present embodiment may still be combinable with the embodiments shown in Figures 2A and 2B as the valve unit may be equipped with more than one port. Figure 3 shows that the valve unit may include more than one opening (or port). The valve unit 300 may comprise a sealable maintenance port, as will be further described in the following, and an additional port 340 which may remain open.

The additional port 340 and the maintenance port may have different sizes. The additional port 340 may have a smaller size and may be adapted to allow connection of a control fiber (or a water cooling pipe, or an air flow tube). In this design, the maintenance port may not always be opened but other smaller ports, such as the additional port 340, may remain opened such that the inner space 150 of the valve unit 300 is connected to the outside air.

Figure 3 shows at 300 only a part of a valve unit or compartment. In particular, Figure 3 shows a wall including an inner layer 110 facing the interior space denoted 150. The wall includes also an outer layer 120 and an insulating material 130 (or insulating medium) is intended to be enclosed (or sandwiched) between the inner layer 110 and the outer layer 130. Figure 3 shows that the wall includes an opening or through hole in which the person (or operator such as a robot or robot arm) 270 may enter to access the interior space 150. Figure 3 shows that the inner layer 110 comprises a first hole 311 and that the outer layer 120 comprises a second hole 321. The valve unit 300 may then comprise two covers, a first cover 312 to cover (or close) the first hole 311 of the inner layer 110 and a second cover 322 to cover (or close) the second hole 321 of the outer layer 120.

According to one procedure, if the insulating material 130 is for instance oil (or in general liquid-based), then the person may enter the interior space 150 once the insulating layer has been partly drained, i.e. removed from the inside of the wall of the compartment, and once the first cover 312 and the second cover 322 displaced to free out the openings or holes 311, 321.

Figure 4 shows at 400 a schematic view of at least part of a power converter station according to some embodiments. The power converter station 400 includes a plurality of valve units 401-404. Although only four valve units are denoted 401-404 in Figure 4, the example shows a section including twelve valve units. It will be appreciated that the power converter station, or a section of it, is not limited to this number and that any other number of valve units may be envisaged, depending on the application and the desired power and voltage. Figure 4 shows also a person (or operator) 270 which may enter the valve units 401- 404 via their respective holes located on top of their respective compartments in order to perform maintenance operations. The valve units 401-404 may be equivalent to any one of the valve units 100-300 described in the above embodiments with reference to Figures 1-3.

With reference to Figures 5 A and 5B, valve units in accordance with other embodiments are described.

Figures 5A and 5B schematically show at 500 a cross-sectional view and a top view, respectively, of a plurality of valve units 501-504. The valve units 501-504 may be equivalent to each other, such as represented in Figure 5. Thus, only one valve unit 501 is described in more detail in the following.

Figures 5 A and 5B show a valve unit 501 with a compartment made of a plurality of walls. The walls are connected together to form an inner space 550 at which a plurality of power converters or valve stacks 580, 581, 582 are arranged. In Figure 5B, it is shown that each valve unit or compartment comprises six valve stacks.

However, it will be appreciated that any other number of valve stacks may be arranged within a compartment to form a valve unit.

Figures 5A and 5B show a configuration in which the compartment of the valve unit 501 is rectangular. Although Figures 5 A and 5B show such rectangular compartments with sharp corners, it may be envisaged to provide similar compartments with rounded corners. Other geometries may be envisaged.

Further, although four walls and a bottom plate (or floor) are shown in Figures 5A and 5B to form a compartment, any other number of walls may be envisaged to obtain a desired geometry.

A wall 505 of the valve unit 501 may include an electrically conductive outer layer 520 and an electrically conductive inner layer 510. In the embodiment shown in Figures 5 A and 5B, it is envisaged that a solid insulating material is arranged (or sandwiched) between the outer layer 520 and the inner layer 510.

In the embodiment shown in Figures 5 A and 5B, the valve unit is totally opened at one side 560 for providing access to the interior space 550, thereby enabling maintenance operation to be performed by an operator or robot 270. In general, it will be appreciated that the maintenance port in its opened state provides air convection for ambient cooling, releases pressure in case of explosion caused by a component failure and provides refresh air to avoid accumulation of particles or humidity in the compartment. The opening may also be used for inserting a control cable like an optical fiber, a cooling water pipe (in and out), and/or a power cable (in and out).

In this embodiment, the four walls of the valve unit and its floor (or bottom part) define the inner space 550. As can be seen in Figure 5B, a plurality of valve units 501-504 may be arranged close to each other to form a power converter station 500 (or at least part of it). In Figure 5B, twelve compartments or valve units, each comprising six valve stacks, are shown.

Still referring to Figures 5 A and 5B, it will be appreciated that the compartments (or the valve units) may be arranged in a room of a building, the room having a floor and walls such that the outer layer of a wall of the compartment corresponds to a wall of the room or its floor. In other words, the outer container 520 may be the wall(s) and floor of the building. The floor and the walls of the room may then be electrically connected to ground.

With reference to Figures 6 A and 6B, valve units in accordance with other embodiments are described.

Figures 6A and 6B schematically show at 600 a cross-sectional view and a top view, respectively, of a plurality of valve units 601-604. The valve units 601-604 may be equivalent to each other, such as represented in Figure 6. The valve units 601-604 shown in Figures 6A and 6B, including an inner space 650 and a number of valve stacks 680, 681, 682, are equivalent to the valve units 501-504 described with reference to Figure 5 except that, in a wall 605 of a valve unit 601, the insulating material 630 is not a solid insulating material but a liquid-based insulating material.

The wall 605 of the valve unit 601 includes an electrically conductive outer layer 620 and an electrically conductive inner layer 610 with a liquid-based insulating material 630 arranged in between.

The insulating material may be oil (such as mineral oil or silicon oil). Alternatively, the insulating material may be a combination of cellulose material and a dielectric liquid or a combination of sand and a dielectric liquid. Alternatively, the insulating material may be a gas, such as SF ₆ , C0 ₂ or N ₂ .In particular, the gas may have a pressure of one bar or higher. Further, the gas may have an electrical withstand which is higher than air. Figure 6A also shows that an additional structure 666 may be provided at the compartment edges (or container edges) at the top of the compartment, i.e. at the opened side of the compartment, in order to increase the creepage distance between neighboring inner containers (or inner layers of neighboring compartments). In particular, this additional structure may include solid insulating material that extend the walls of the compartment and encapsulate the liquid-based insulating material.

In the embodiment shown in Figures 6 A and 6B, the valve unit is totally opened at one side 660 for providing access to the interior space 650, thereby enabling maintenance operation to be performed by an operator or robot 670. In this embodiment, the four walls of the valve unit and its floor (or bottom part) define the inner space 650. As can be seen in Figure 6B, a plurality of valve units may be arranged close to each other to form a power converter station 600 (or at least part of it). Generally, a difference between the embodiments shown in Figures 5-6 and the embodiments shown in e.g. Figures 2A-2B is that a valve unit shares the same outer layer (or outer wall) as the outer layer (or outer wall) of a neighboring valve unit. While in Figures 2A-2B, each valve unit may be installed separately, the valve units shown in Figures 5-6 are constructed together with an larger outer tank in which a plurality of inner tanks are arranged.

The present disclosure may find applications in e.g. power converter stations for high power transmission, with very high converter voltage and power (in the Giga Watt range). In such applications, each electrical phase may have its own converter arm, i.e. each phase may be obtained by the serial connection of a plurality of valve units.

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.

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.