WO2017063665A1

WO2017063665A1 - Energy absorbing converter cell for hvdc applications

Info

Publication number: WO2017063665A1
Application number: PCT/EP2015/073546
Authority: WO
Inventors: Erik Persson; Georgios Demetriades; Ming Li; Nan Chen; Rebei Bel Fdhila
Original assignee: Abb Schweiz Ag
Priority date: 2015-10-12
Filing date: 2015-10-12
Publication date: 2017-04-20

Abstract

The present disclosure relates to a converter cell including a capacitor unit, at least one switching device and an electrical arrangement. The capacitor unit may extend along an axial direction and may include a plurality of pieces. A piece may form a section of the capacitor unit and at least one of the pieces may include at least one capacitor element. The at least one switching device may be arranged at an inner space delimited by the arrangement of the pieces of the capacitor unit such that the capacitor unit surrounds said at least one switching device. The electrical arrangement may be provided for electrically connecting the at least one switching device to at least one piece of the capacitor unit. The electrical arrangement may include at least one element movable along a radial direction upon explosion or failure of the at least one switching device so as to induce a motion of the at least one piece of the capacitor unit along the radial direction. The present disclosure relates also to a valve unit comprising a plurality of such converter cells. The valve unit may form part of a converter in a high voltage direct current converter station.

Description

ENERGY ABSORBING CONVERTER CELL FOR HVDC APPLICATIONS

TECHNICAL FIELD

The present disclosure relates generally to the field of power converters which may include a plurality of power convert cells. The present disclosure relates in particular to a converter cell including a mechanism for reducing damage of the power converter upon explosion of one or more switching devices located within a capacitor unit of the converter cell.

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, such as an HVDC power converter, may comprise a plurality of converter cells (or converter subunits) connected in series. In a converter cell (or subunit), a plurality of solid-state semiconductor switching devices, such as thyristors or transistors like IGBTs, may be associated with a capacitor adapted to store energy for the converter cells.

A general challenge in the present technical field is to provide a more compact converter station, for e.g. offshore HVDC applications, in order to facilitate installation and transport of the converter station. Another challenge, however, is also to reduce damage of the converter station, or elements of the converter station, upon explosion or failure of one or more of the solid-state semiconductor switching devices because of e.g. a fault current. SUMMARY

An object of at least some embodiments of the present disclosure is to wholly or at least partly address one or more of the above mentioned issues. This and other objects are achieved by means of a converter cell as defined in the appended independent claim. Other embodiments are defined by the dependent claims.

According to some embodiments, there is provided a converter cell including a capacitor unit, at least one switching device and an electrical arrangement. The capacitor unit may extend along an axial direction and may include a plurality of pieces. A piece may form a section of the capacitor unit and at least one of the pieces may include at least one capacitor element. The at least one switching device may be arranged at an inner space delimited by the arrangement of the pieces of the capacitor unit such that the capacitor unit surrounds the at least one switching device. The electrical arrangement may be provided for electrically connecting the at least one switching device to at least one piece of the capacitor unit. The electrical arrangement may include at least one element movable along a radial direction upon explosion or failure of the at least one switching device so as to induce a motion of the at least one piece of the capacitor unit along the radial direction.

In the present embodiments, an electrical arrangement with a movable element is provided for connection of at least one piece of the capacitor unit to the switching device. Upon explosion and/or failure of the switching device, a force will be applied on the movable element of the electrical arrangement and, as a result, the piece of the capacitor unit connected to the switching device via the electrical arrangement will be moved along the radial direction. The piece in question, and possibly the whole converter cell, may then be displaced in a radial direction upon explosion of the switching device. In a vertical arrangement, or a vertical stack of a plurality of converter cells, such a converter cell may then be displaced laterally, which then avoids, or at least reduces, degradation of the neighboring cells located above and/or under the displaced converter cell. In other words, with converter cells according to the present embodiments, kinetic energy resulting from the explosion of the switching devices may be absorbed by letting at least one piece of the capacitor unit, or the whole converter cell, "pop out" from its position. In the present embodiments, the kinetic energy is absorbed by a motion of an element of the electrical connection between the switching device and (a piece of) the capacitor unit. This, in turn, applies a radial force on the piece of the capacitor unit. In the present embodiments, a capacitor unit may include a plurality of pieces or sections (in particular capacitive pieces). The capacitor unit may 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 may be more easily 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).

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 capacitor unit may form a body including an inner space or cavity, e.g. a through hole. In the present context, the body may be formed of a plurality of pieces which, when assembled, delimits the inner space within which the electric components (the switching devices) may be arranged. It will be appreciated that in some embodiments the pieces forming the capacitor unit 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 capacitor unit may in some other

embodiments be arranged close to each other, yet with a gap between two successive pieces. Thus, the capacitor unit 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.

It will also be appreciated that the dimensions of the capacitor unit may determine the properties, and in particular the possible capacitance and voltage, of the capacitor unit for a particular selection of materials and number of capacitor elements arranged in each of the pieces. Further, the height of the capacitor unit along the axial direction may be determined by the height of the necessary capacitor unit for achieving a desired capacitance or desired voltage.

It will also be appreciated that a piece of the capacitor unit 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). The capacitor unit of the present disclosure is also 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. 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.

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 capacitor shape, e.g. to reduce hot spot temperatures and to increase the long-term reliability of the capacitor.

It will also 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 listed herein may be arranged within the hollow center of the capacitor unit. Still, one of these electric components, such as the switching device, may explode.

As mentioned above, the electrical arrangement may include at least one element movable along a radial direction such that a piece of the capacitor unit connected to the switching device via the electrical arrangement is movable along the radial direction. It will be appreciated that the movable element may be translated in a plane intersecting the axial direction along which the capacitor unit extends. The electrical arrangement may include a busbar connecting at least one piece of the capacitor unit to the switching device. The electrical arrangement may also include an electrical connector attached to the at least one piece of the capacitor unit for connection of the piece in question to the busbar. The electrical arrangement may also include other electrical leads or connectors between the switching device and the piece of the capacitor unit. Some implementations of the movable element in one of the busbar, the leads and/or one of the electrical connectors will be described in the following.

In general, the movable element may be an element having certain elasticity, for example a material or a component storing a potential mechanical energy which may be released upon a distortion, compression and/or extension of the material or component. The mechanical energy of the movable element obtained upon explosion of a switching device connected to the movable element may then be used to accelerate the piece of the capacitor unit connected to the movable element.

According to an embodiment, the movable element may be at least a portion of a busbar extending at least partially in the radial direction between the at least one switching device and the capacitor unit.

In a particular embodiment, the portion of the busbar may have elastic properties. In this embodiment, a portion of the busbar is used to move (expand) upon explosion of the switching device. An extension of the elastic portion of the busbar in response to the explosion of the switching device being connected at one end of the busbar will cause a motion of the piece of the capacitor unit to which the busbar is attached at its other end. As a result, the busbar will transmit a force to the piece of the capacitor unit.

In an embodiment, the portion of the busbar may include a spring (i.e. may act as a spring). A portion of the busbar may for example be folded (corrugated) as a spring.

In other embodiments, the portion of the busbar may include at least two parts, one part being movable relative to the other part along the radial direction. Energy from the explosion of the switching device may then be transferred further by motion of one part relative to another. A first part of the busbar may be fixed while a second part may slide (glide) relative to the first part along the radial direction. In an exemplifying embodiment, the at least two parts may be connected by a mechanical attachment configured to let the two parts move relative to each other upon appliance of a certain force resulting from the explosion or failure of the at least one switching device. In one example, the two parts may be attached to each other by friction and be immobile (i.e. fixed relative to each other) until a sufficiently high force (resulting from the explosion of the switching device) is applied to at least one of the two parts. In another example, one of the two parts may be attached by a locking bolt tighten with a certain torque allowing the busbar to slide in the other part (acting for instance as a track) at a specified force (e.g. a force corresponding to the explosion of the switching device).

According to an embodiment, the movable element may be a part of at least one electrical connector attached to the at least one piece of the capacitor unit for electrical connection of the at least one piece. In this embodiment, the movable element may be integrated in one of the electrical connectors (or bushings) of the capacitor unit used for connection to at least one switching device (for example via a busbar).

According to a particular embodiment, the movable element may be an elastic part of the electrical connector (or bushing). It will be appreciated that the electrical connector may be so constructed that at least a part of it has elastic properties and may expand upon explosion of the switching device. In other embodiments, the whole electrical connector (or at least its enclosure) may be made of an elastic material expandable upon explosion of the switching device. The electrical connector may be arranged at a wall of the at least one piece of the capacitor unit, which wall may be facing the inner space defined by the capacitor unit. It will be appreciated that in these embodiments, the connection of a piece of the capacitor unit to any switching device is realized within the inner space (or hollow center) of the capacitor unit, thereby providing a more compact converter cell. According to an embodiment, two adjacent pieces of the capacitor unit may be attached to each other via a mechanical contact arranged to break upon the explosion or failure (of the switching device). The mechanical contact, or attaching device may be used for assembling (or joining) the plurality of pieces together in order to form the capacitor unit.

With the present embodiment, upon appliance of a certain force on the piece connected to the switching device when it explodes, the mechanical contact between this piece and a neighboring piece may break such that only one piece of the converter cell is displaced (or ejected, at least temporarily), instead of displacing the whole converter cell. It may then be possible to repair the interior of the converter cell (by for instance installing a new switching device and providing new connections) and replace the piece of the capacitor unit that has been displaced.

Several types of mechanical contacts or attaching devices (or attaching means or fasteners) may be envisaged such as screws or clips. For this purpose, the surfaces of the pieces forming the outside of the capacitor unit may include shallow indentations for inserting/lodging screws connecting two neighboring pieces of the capacitor unit. An additional layer of shielding for smoothing the surface of the capacitor unit may be provided after the screws have been tightened and the pieces assembled.

According to another alternative, two neighboring or successive pieces may be assembled and disassembled using gaps between the two successive capacitor units of a stack of capacitor units. Some kind of specially designed tool may be inserted in such gaps to access fasteners located within the internal space defined by the arrangement of the pieces of the capacitor unit.

According to yet another alternative, the pieces of the capacitor unit may be assembled or disassembled using an attaching system based on a 'plug and play' principle. For example, the bushing outlet of capacitor units may be designed with an electromechanically locking structure, wherein each capacitor unit may be pushed in and have automatic connection. As mentioned above, the mechanical contact may be arranged to break upon appliance of a force corresponding to the explosion of the switching device, thereby disassembling the pieces of the capacitor unit.

According to an embodiment, the pieces may be distributed around the axial direction. The pieces may be arranged (or extend) in a plane intersecting the axial direction.

It will also be appreciated that the pieces may then be arranged adjacent to, or at least next to, each other to form a loop around the axial direction of the capacitor unit. Although the pieces may in some embodiments be attached one to another in a tight manner to form a closed loop (or fully closed loop), in some other embodiments, a gap may be formed between two successive pieces of the loop, thereby providing a more loose arrangement. As an example, the capacitor unit may include two to ten pieces. When joining these two to ten pieces, the capacitive body of the capacitor unit may be formed. The pieces may be shaped such that a cavity (or through hole) is formed in a central portion of the capacitive body such that a capacitive body with a hollow center is obtained. According to an embodiment, each of the pieces may define a section of a ring such that, when the pieces are assembled together, the capacitor unit is ring-shaped. In other words, the pieces may be angularly distributed around the axial direction, i.e. the capacitor unit may have a circular cross-section along a plane intersecting the axial direction. With the present embodiment, an annular capacitor is obtained, which provides a smooth outside surface with the advantage of reducing the requirement on insulation and thereby available space. In other embodiments of the present disclosure, however, it may be envisaged that the cross section (along a plane intersecting the axial direction) of the capacitor unit body is not necessarily round or circular but instead is triangular, rectangular, or square with rounded corners, depending on the number of piece used to form the capacitor unit.

According to an embodiment, a valve unit may be provided. The valve unit may comprise a plurality of converter cells as defined in any one of the preceding embodiments. The plurality of converter cells may be arranged as a stack. In particular, the converter cells may be arranged such that the capacitor units of two adjacent converter cells extend along a common axial direction. Using circular capacitor units, such piling of the converter cells results in a cylindrical valve unit, which is advantageous for space management. Reliability of the valve unit is improved since if the switching device(s) of one converter cell of the valve unit explodes, then the converter cell in question will "pop out" from the stack and leave the other converter cells undamaged or with at least reduced damage. It would then be possible to operate the valve unit without the damaged converter cell and/or to only replace the damaged converter cell for repairing the valve unit.

Further, maintenance operations in such a valve unit are generally improved since the capacitor units of the converter cells are divided in sections. Thus, if a converter cell located within the stack, e.g. in the middle of the stack, is deficient, then it is possible to detach (e.g. unscrew) a piece (i.e. a section) of the capacitor unit and then perform maintenance. Any deficient electronic switch or other device arranged in the center portion of this converter cell could then be repaired or replaced. Similarly, the detached piece may also be repaired or replaced by another detachable piece. Building a valve unit with converter cells based on capacitor units according to the present disclosure facilitates access to the various parts and components of the valve unit, thereby facilitating maintenance. With the capacitor units of the present disclosure, maintenance may be performed without the need of disassembling the various converter cells of the valve unit, which would require space. Providing a valve unit based on capacitor units as described in any one of the preceding embodiments is therefore also advantageous in that a more compact power converter station may be realized.

According to an embodiment, a high voltage direct current (HVDC) converter station may be provided. The HVDC converter station may comprise at least two valve units as defined in any one of the preceding embodiments. 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 and also reduce damage of the equipment or station upon explosion of a device of a converter cell. The present disclosure is advantageous in any applications wherein a stack 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 present disclosure achieves this by introducing a movable element in the electrical arrangement connecting a switching device of a converter cell to a piece o the capacitor unit. Further, maintenance operations may be improved via a piecewise capacitor unit (i.e. a capacitor unit divided in several pieces). 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 perspective view of a capacitor unit of a converter cell in accordance with an embodiment;

Figure 2 shows a schematic top view of capacitor units of converter cells in accordance with some embodiments;

Figure 3 shows a schematic view of a converter cell in accordance with an embodiment;

Figure 4A shows a schematic view of a piece of a capacitor unit of a converter cell with its electrical connectors in accordance with an embodiment;

Figure 4B shows a schematic view of a busbar according to an embodiment;

Figure 4C shows a schematic view of a busbar according to another embodiment;

Figure 5A shows a schematic perspective view and Figure 5B shows a schematic side view of a power converter cell in accordance with an embodiment;

Figure 6 shows a circuit diagram illustrating the electrical connections of a converter cell in accordance with an embodiment; and

Figure 7 shows a schematic view of a valve unit of an HVDC converter in accordance with an 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 capacitor unit 100 of a converter cell according to an embodiment is described. Figure 1 shows a schematic perspective view of the capacitor unit 100. The capacitor unit 100 comprises four pieces 101-104. When assembled together, the four pieces 101-104 form a body extending along an axial direction 108.

The pieces 101- 104 delimit a space or area 120, also referred to as inner space in the following, which corresponds to a hollow center of the capacitor unit 100. Each one of the pieces 101-104 forms a section of the capacitor unit.

The capacitor unit 100 may have different shapes. In some embodiments, an outside surface 106 of the capacitor unit 100 may be circular, such as represented in Figure 1, but it may be envisaged that the outside surface of the body (or the capacitor unit) is elliptic and/or rectangular or square or any other form. It may however be appreciated that the outside surface of the capacitor unit 100 may advantageously comprise rounded corners. According to a particular embodiment, each of the pieces 101- 104 may define a section of a ring such that the capacitor unit 100 is ring-shaped, thereby forming an annular capacitor, such as shown in Figure 1.

Still referring to Figure 1, the pieces 101-104 may be distributed around the axial direction 108. The pieces 101-104 extend in a plane intersecting the axial direction 108. In particular, Figure 1 shows that the pieces 101-104 are arranged in a plane which is perpendicular to the axial direction 108.

Although four pieces 101-104 form the capacitor unit 100 in the example shown in Figure 1, it will be appreciated that the capacitor unit 100 may be divided in another number of pieces. The capacitor unit 100 may be divided in at least two pieces In some particular embodiments, at least one piece may be detachable from the capacitor unit such that it can 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 104 may be removable from the capacitor unit 100 and may be put back in place. As a result, at least one piece may individually be removed and replaced without disturbing the surrounding pieces of the capacitor unit. This improves the accessibility to the inner space (or interior space) delimited by the capacitor unit, at which inner space electronic components (such as switching semiconductor devices) may be arranged. By removing one piece of the capacitor unit, any components located in the inner space may be tested, taken out and possibly replaced or repaired. Such a design of a capacitor unit facilitates maintenance operation and reduces the space requirement for maintenance, which in turn may result in a more compact power station. Further, although only one of the pieces 101- 104 is shown to be detachable from the capacitor unit 100 in Figure 1, i.e. the piece denoted 104 in the present example, it will be appreciated that each of the pieces 101-104 may be detachable from the capacitor unit 100. With reference to Figures 2A and 2B, capacitor units in accordance with some embodiments are described.

Figures 2A and 2B show schematic top views of two different capacitor units 100 and 200 in accordance with some embodiments.

Figure 2A shows a top view of a capacitor unit 100 which may be equivalent to the capacitor unit 100 described with reference to Figure 1. In particular, the capacitor unit 100 includes a capacitive body delimiting an area or inner space 120 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 arrangement of the pieces 101-104. In other words, the inner space 120 corresponds to the (hollow) center of the capacitor unit 100. Figure 2B shows also a top view of another capacitor unit 200 which may be equivalent to the capacitor unit described with reference to Figure 1 except that the area or inner space 220 defined by the arrangement of the pieces of the capacitor unit 200 is circular. The capacitor unit 200 comprises also three pieces 201-203 only to form the capacitor unit. Although Figures 2A and 2B show two examples of possible shapes of inner spaces, other shapes may be envisaged. For example, the inner space may also be elliptic or rectangular.

In general, the pieces of the capacitor unit may define an inner space (or cavity) having 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 capacitor unit includes rounded corners and is circular, the inner space (or internal space) delimited by the pieces of the capacitor unit 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 pieces of the capacitor unit may be a square, which may provide an improved filling factor of the devices installed in it. With reference to Figure 3, a converter cell in accordance with an embodiment is described.

Figure 3 shows a schematic view of a converter cell 300 including a capacitor unit which may be equivalent to any one of the capacitor units described with reference to Figures 1 and 2A and 2B. Figure 3 illustrates at least one alternative for attaching the pieces of the capacitor unit together.

Figure 3 shows a capacitor unit 300 comprising four pieces 301-304 which may be equivalent to the pieces 101-104 described with reference to Figure 1. The pieces 301-304 are joined together by means of attaching devices 331-334 to form the capacitor unit of the converter cell 300, thereby defining the inner space (or hollow center) 320 delimited by the pieces 301-304 of the capacitor unit. In the present example, the attaching devices used for assembling the plurality of pieces 301-304 together are screws.

More specifically, the capacitor unit 300 may comprise a first screw 331 for attaching a first piece 301 with a second piece 302, a second screw 332 for attaching the second piece 302 with a third piece 303, a third screw 333 for attaching the third piece 303 with a fourth piece 304, and a fourth screw 334 for attaching the fourth piece 304 with the first piece 301. As a result, a body with a hollow center is obtained. It will be appreciated that the screws may advantageously be loosely mounted such that gaps are formed between two successive pieces. In some other embodiments however, the screws may be tightened such that the four pieces 301-304 are in physical contact with each other, thereby resulting in a closed loop. The screws may be inserted in enclosures (or boxes) of the pieces 301-304. The enclosures may be made of electrically conductive (e.g. metallic) material or non-electrically conductive material.

At least one switching device 350 is arranged within the inner space 320 of the capacitor unit defined by the pieces 301-304. The switching device 350 may be arranged in the capacitor unit 300 such that the pieces of the capacitor unit surround the entire switching device 350.

The switching device 350 may be connected to the pieces of the capacitor unit by electrical arrangements (or connections) 380 and 390. A first electrical arrangement (or connection) 380 may be established by means of an electrical connector 382 attached to the third piece 303 of the capacitor unit and a busbar 384 electrically connecting the electrical connector 382 to the switching device 350. It will be appreciated that further electrical connectors or leads (not shown) may form the electrical arrangement 380. For example, an additional electrical connector (not shown) may be arranged at the switching device 350 for connecting the switching device 350 to the busbar 384.

The electrical arrangement 380 may include one movable element, which may for example be (part of) the electrical connector 382 or a portion of the busbar 384 such that, upon explosion of the switching device 350 because of a failure (e.g. a fault current), the energy produced by the explosion causes the third piece 382 to be displaced (or ejected) as indicated by the arrow on the left hand side. As a result, the pressure from the explosion is dissipated in a radial direction and not in an axial direction. The movable element may be configured to place the displaced back to its initial position.

An example of the electrical connector 382 will be described in more detail with reference to Figure 4A while an example of the busbar 384 will be described in more detail with reference to Figure 4B.

Similarly, the converter cell 300 may include a second electrical arrangement 390 for connecting the first piece 301 of the capacitor unit to the switching device 350. This second electrical arrangement (or connection) 390 may be established by means of an electrical connector 392 attached to the first piece 301 of the capacitor unit and a busbar 394 electrically connecting the electrical connector 392 to the switching device 350. It will be appreciated that further electrical connectors or leads (not shown) may form the electrical arrangement 390. For example, an additional electrical connector (not shown) may be arranged at the switching device 350 for connecting of the switching device 350 to the busbar 394.

The electrical arrangement 390 may include one movable element, which may for example be (part of) the electrical connector 392 or a portion of the busbar 394 such that, upon explosion of the switching device 350 because of a failure (e.g. a fault current), the energy produced by the explosion causes the third piece 392 to be displaced (or ejected), as indicated by the arrow on the right hand side. As a result, the pressure from the explosion is dissipated in a radial direction and not in an axial direction. The movable element may be configured to place the displaced back to its initial position. An example of the electrical connector 392 will be described in more detail with reference to Figure 4A while an example of the busbar 394 will be described in more detail with reference to Figure 4C. While the switching device 350 is fixedly mounted within the inner space delimited by the pieces 301-304 of the capacitor unit (for instance at a center position), the force along a radial direction resulting from an explosion of the switching device 350 causes either one of the pieces 301 or 304 to be displaced in the radial direction because of the movable element (the electrical connector 382 or the busbar 384 for the electrical arrangement 380 or the electrical connector 392 or the busbar 394 for the electrical arrangement 390).

Figure 4A shows a schematic view of a piece of a capacitor unit with its electrical connectors according to an embodiment.

Figure 4A shows an enlarged view of a piece 400 of a capacitor unit such as e.g. the capacitor unit 100 described with reference to Figure 1. The piece 400 may therefore correspond to any one of the pieces 101-104.

Figure 4A shows a piece 400 having the shape of a trapezoidal block with one curved face 446. More specifically, the piece 400 comprises a first surface 446 defining a portion of the outside surface of the capacitor unit and a second surface 442 defining a portion of the inner space defined by the capacitor unit. The piece 400 comprises also two side surfaces 444, 448, 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 452 (or bottom surface) and a top surface 450.

In the piece 400, the two side surfaces form walls extending in planes intersecting the first (curved) surface 446 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 442 forming a portion of the inner space of the capacitor unit 400. The base surface 452 and the top surface 450 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 400. It will be appreciated that, although Figure 4 shows a piece having a trapezoidal shape, other geometries may be envisaged. For example, the two side surfaces 444 and 448 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 4.

Independently of the geometry used for the piece of the capacitor unit, Figure 4A shows also that the piece 400 may comprise electrical connectors 460 arranged at the second surface 442 defining a portion of the inner space of the capacitor unit. In other words, the electrical connectors 460 are arranged at the wall facing the inner space defined by the capacitor unit. The electrical connectors 460 may be used for connection to at least one switching device or power converter circuitry arranged within the inner space of the capacitor unit, e.g. via a busbar, as shown in Figure 3.

The electrical connectors 460 may include an elastic material or a movable element, which may expand upon appliance of a force on it. In particular, the movable element of the electrical connectors may include a spring. The electrical connectors may for example be pressed against the inner surface 442 when the piece 400 is assembled with other pieces and the switching device to form a converter cell. If the switching device explodes, the energy stored in the elastic part of one of the electrical connectors will cause the piece 400 to be displaced (ejected) along a radial direction (the direction of the force applied by the elastic part of the electrical connector, which may correspond to the direction of compression of the electrical connector).

Generally, the pieces of a capacitor unit, such as the piece 400 shown in Figure 4A, 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.

As mentioned above, in some embodiments, the capacitor unit may be defined by an outside surface which may be elliptic, circular and/or which comprises at least one rounded corner. In particular, the capacitor unit 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 capacitor unit 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.

A circular shaped capacitor unit provides a smooth converter cell profile, which reduces requirement on insulation design and provides other benefits such as a lower stray inductance in current commutation loop.

Figure 4B shows a schematic view of a busbar according to an embodiment, which may for example correspond to the busbar 384 illustrated in Figure 3.

The busbar 384 may extend along a first direction 381, which in Figure 3 may correspond to a radial direction (as the direction indicated by the arrow on the left hand side of Figure 3). The busbar 384 may comprise a main part or lead 383 and a movable element 385 connecting a first portion of the main lead 383 to a second portion of it. The movable element 385 is represented as a spring, which may be a lead (or conductor) folded such that it has a saw-tooth profile. The spring 385 may be compressed or may expand, depending on the force applied on it.

For example, the spring 385 of the busbar 384 may be compressed when it is connected to a piece of a capacitor unit (such as e.g. the piece 400 shown in Figure 4) at its first extremity (or terminal) 387 and to a switching device at its second

(opposite) extremity (or terminal) 388. The spring 385 may then expand if the switching device explodes such that the first extremity 387 of the busbar 384 applies a force along the radial direction 381 that causes the piece connected to this extremity to be displaced.

It will be appreciated that the movable element 385 of the busbar 384 may be implemented differently than with a spring. The portion 385 of the busbar 384 may for example include a material having elastic properties while the main lead (or other portions) may include a rigid material. Expressed differently, the portion 385 may be more elastic than the (main) lead 383.

Figure 4C shows a schematic view of a busbar according to another embodiment, which may for example correspond to the busbar 394 illustrated in Figure 3.

The busbar 394 may extend along a first direction 391, which in Figure 3 may correspond to a radial direction (as indicated by the arrow on the right hand side of Figure 3). The busbar 394 may comprise a first part 393 and a second part 395 which may be movable relative to each other. In this embodiment, the second part 395 of the busbar 394 is attached to the first part 391 by means of a locking bolt 399. The second part 395 includes an opening or track 396 within which the bolt 399 may glide such that the second part 395 may be displaced relative to the first part 393 if a force is applied at an extremity 398 of the second part 395 (e.g. because of an explosion of the switching device attached to it). The portion of the busbar 394 may therefore be in an expanded state, as represented in Figure 4C, or in a compressed state when the second part is moved such that the bolt 399 glides to the other side of the track 396.

As for the example described with reference to Figure 4B, the busbar 394 may be in its compressed state when it is connected to a piece of a capacitor unit (such as e.g. the piece 400 shown in Figure 4) at its first extremity (or terminal) 397 and to a switching device at its second (opposite) extremity (or terminal) 398. The relative motion of the first and second parts may then cause the motion of the piece of the capacitor unit along the radial direction 391 if the switching device explodes. It will be appreciated that the mechanical arrangement between the first part 393 of the busbar 394 and its second part 395 may be implemented differently than with a locking bolt 399 and a track 396 as shown in Figure 4C. For example, the two parts 393 and 395 may be mounted such that their contacting surfaces have a certain friction. Upon appliance of a force being larger than the friction (as a result of an explosion of the switching device), the two parts may move relative to each other. The two surfaces of the two parts may include structures providing the friction. Figure 5A shows a schematic perspective view and Figure 5B shows a schematic side view of a power converter cell in accordance with an embodiment.

Figures 5A and 5B show a converter cell 500 comprising a capacitor unit including a plurality of pieces 501-503 equivalents to the pieces of the capacitor unit 100 described with reference to e.g. Figure 1. It will be appreciated that, in Figures 5A and 5B, the fourth piece of the capacitor unit is not shown for the purpose of illustrating the components arranged within the capacitor unit of the converter cell 500, i.e. the components arranged at the inner space delimited by the pieces 501-503 of the capacitor unit.

Figures 5A and 5B also show electrical connectors 562, 564 for connection of the power converter circuit (i.e. the switching devices) to the pieces 501-503 via their electrical connectors 560 (which may correspond to the electrical connectors 460 described with reference to Figure 4). The electrical connector denoted 562 may represent a positive connection from the capacitor unit while the electrical connector denoted 564 may represent a negative connection from the capacitor unit. In this respect, the bushings or electrical connectors 560 for connection between the semiconductor-based device (or power converter circuit) 552 and the capacitor unit may be arranged at an upper part of an inner wall of the capacitor unit in order to reduce the effect of electromagnetic field around the location where the

semiconductor-based device 552is installed, such as in the embodiments depicted in Figures 4 and 5A-B. Figures 5A and 5B also illustrate that the semiconductor-based device, or the components of the power converter circuit 552, may be arranged in the inner space delimited by the hollow body of the capacitor unit formed by the pieces 501-503 (and the fourth piece, not represented in Figures 5A-B) such that the capacitor unit surrounds these components.

In the embodiment shown in Figures 5A and 5B, the electrical connection between the power converter circuit 552 and a piece of the capacitor unit is established by means of, at one end (at the power converter circuit or switching device), the electrical connectors 562, 564 and, at the other end (at the pieces of the capacitor unit), by the electrical connectors 560. At least one of these electrical connectors may include a movable element (or movable part or elastic material) such that, upon explosion of one of the component of the power converter circuit 552, the energy produced by the explosion causes a piece of the capacitor unit to be displaced, thereby absorbing the energy.

Figure 6 shows a circuit diagram illustrating the electrical connections of a converter cell in accordance with an embodiment. Figure 6 shows a circuit diagram 600 illustrating the electrical connections of the switching devices (e.g. transistors) and the capacitor of the converter cell 500 described with reference to Figure 5. In the circuit diagram 600, the upper part shows the electrical connections in a first power converter cell 500 while the lower part shows the electrical connections in a second power converter cell 650.

In the first power converter cell 500, the switching devices 654 and 652 may for example be IGBTs but may also be other types of semiconductor-based switching devices. As mentioned above, in some embodiments, the types of switching devices may for example be MOSFETs, IGBTs, IGCTs, GTOs, HEMTs or HBTs and the types of semiconductors may for example be silicon, silicon carbide, Gallium Nitride or Gallium Arsenide. A switching device 652 or 654 may comprise two

semiconductor chips in the form of a transistor and a diode connected in parallel to the transistor, such as represented in Figure 6. In some embodiments, however, the switching device may be a single-chip component adapted to replace the two semiconductor chips.

In the present embodiment, the two switching devices 652, 654 are connected in series in a branch itself connected in parallel with the capacitor 610. A first electrode 610a of the capacitor 610 of the first power convert cell 500 is connected to a first port or terminal (e.g. the drain) of a first switching device (or transistor) 654 while a second electrode 610b of the capacitor 610 is connected to a second port or terminal (e.g. the source) of a second switching device or transistor 652. A third port or terminal (e.g. the source) of the first switching device 654 is connected to a fourth port or terminal (e.g. the drain) of the second switching device 652.

In the first power converter cell 500 shown in Figure 6, the first electrode 610a of the capacitor 610 provides a positive bias (DC+) to the first switching device 654 while the second electrode 610b of the capacitor 610 provides a negative bias (DC-) to the second switching device 652.

In the power converter circuit 600, the second power converter cell 650 may be identical to the power converter cell 500. The first and second power converter cells 500 and 650 may be arranged as a stack to form a valve unit, as will be further described with reference to Figure 7.

The power converter circuit 600 comprises also an electrical connection 675 between the first power converter cell 500 and the second power converter cell 650. In particular, the positive bias line (DC+) of the second power converter cell 650 is connected to an electrical node arranged between the two switching devices 652, 654 of the first power converter cell 500 (i.e. at the node connecting the drain of the transistor of the second switching device 652 and the source of the transistor of the first switching device 654 in the first power converter cell 500). The electrical connection 675 may be a conductor, e.g. in the form of a bus bar.

It will also be appreciated that a power converter circuit of a valve unit may comprise more electrical connections than those shown in the circuit diagram 600 of Figure 6. Figure 7 shows a schematic view of a valve unit 700 of a power converter, such as for example an HVDC power converter, in accordance with some embodiments. The valve unit 700 comprises a plurality of converter cells 771-780, i.e. ten converter cells in the present example, arranged as a stack. However, the valve unit 700 may comprise any number of power converter cells, depending on the application and consequently on the desired voltage or desired power. The valve unit 700 may also comprise high voltage capacitor shields arranged between two adjacent (or successive) power converter cells.

As illustrated in Figure 7, upon explosion of the switching device a power converter cell 775 in the valve unit 700, the converter cell 775 or at least a piece of the capacitor unit of the converter cell 755 will "pop out" (i.e. be displaced or ejected, at least temporarily) from the valve unit 700 so as to release pressure in a radial direction. The present embodiments are advantageous as otherwise the pressure may be directed in an axial direction, which may affect other converter cells arranged in proximity to (in particular above and below) the converter cell at which the explosion occurs. It will be appreciated that after the explosion, the spring or the movable part of the electrical arrangement may be arranged to pull the piece of the capacitor unit back to its initial location. The power converter cell 775 may be equivalent to any one of the power converter cells described in the preceding embodiments.

The power converter cells shown in Figure 7 include capacitor units having discshaped 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.

Further, the power converter circuits of the power converter cells 771-780 may be electrically connected in series for increasing the input and/or output voltage of the valve unit 700. It will be appreciated that an HV power station may be formed by connecting a plurality of (at least two) valve units together, such as two stacks of power converter cells as shown in Figure 7. 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. Thus, in order to achieve the high-required voltage, hundreds of converter cells may have to be stacked in series.

The embodiments of the present disclosure are however not limited to such high power applications and it is also envisaged to apply the capacitor units and converter cells described herein in low voltage and medium voltage equipments. For example, embodiments of the present disclosure may be used for motor drives.

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.

Claims

1. A converter cell comprising:

a capacitor unit extending along an axial direction and including a plurality of pieces, a piece forming a section of the capacitor unit and at least one of the pieces including at least one capacitor element;

at least one switching device arranged at an inner space delimited by the arrangement of the pieces of the capacitor unit such that the capacitor unit surrounds said at least one switching device; and

an electrical arrangement for electrically connecting said at least one switching device to at least one piece of the capacitor unit;

wherein the electrical arrangement includes at least one element movable along a radial direction upon explosion or failure of said at least one switching device so as to induce a motion of said at least one piece of the capacitor unit along the radial direction.

2. The converter cell of claim 1, wherein the movable element is at least a portion of a busbar extending at least partially in the radial direction between said at least one switching device and said capacitor unit.

3. The converter cell of claim 2, wherein said at least a portion of the busbar has elastic properties.

4. The converter cell of claim 2 or 3, wherein said at least a portion of the busbar includes a spring.

5. The converter cell of claim 2, wherein said at least a portion of the busbar includes at least two parts, one part being movable relative to the other part along the radial direction.

6. The converter cell of claim 5, wherein said at least two parts are connected by a mechanical attachment configured to let the two parts move relative to each other upon appliance of a certain force resulting from the explosion or failure of said at least one switching device.

7. The converter cell of any one of the preceding claims, wherein the movable element is a part of at least one electrical connector attached to said at least one piece of the capacitor unit for electrical connection of said at least one piece.

8. The converter cell of claim 7, wherein the movable element is an elastic part of said at least one electrical connector.

9. The converter cell of any one of claims 7-8, wherein said at least one electrical connector is arranged at a wall of said at least one piece of the capacitor unit, said wall facing the inner space defined by the capacitor unit.

10. The converter cell of any one of the preceding claims, wherein two adjacent pieces of the capacitor unit are attached to each other via a mechanical contact arranged to break upon said explosion or failure.

11. The converter cell of any one of the preceding claims, wherein the pieces are distributed around said axial direction, the pieces being arranged in a plane intersecting said axial direction.

12. The converter cell of any one of the preceding claims, wherein each of the pieces defines a section of a ring such that the capacitor unit is ring-shaped.

13. A valve unit comprising a plurality of converter cells as defined in any one of the preceding claims, said plurality of converter cells being arranged as a stack.

14. A high voltage direct current converter station, comprising at least two valve units as defined in claim 13.