WO2016066187A1

WO2016066187A1 - Power converter assembly with insulating material-covered electrodes

Info

Publication number: WO2016066187A1
Application number: PCT/EP2014/073069
Authority: WO
Inventors: Ming Li; Erik Persson; Dong Wu; Mats Hyttinen; Liliana AREVALO; Nan Chen; Olof Hjortstam
Original assignee: Abb Technology Ltd
Priority date: 2014-10-28
Filing date: 2014-10-28
Publication date: 2016-05-06

Abstract

A power converter assembly (100) is disclosed wherein an insulating material is provided e.g. in the form of a layer, cover or coating on a surface of energized elements (200, 250) of the power converter assembly (100). For energized elements (200, 250) of a power converter assembly (100), e.g. a busbar (200) and/or a corona shield (250), the DC corona inception voltage and impulse breakdown strength in air (and/or another fluid) gaps (d_1, d_2) between the energized elements (200, 250) and any object (110) in the surroundings of the power converter assembly (100), such as a wall, floor or ceiling within a building in which the power converter assembly (100) is arranged, may be increased significantly by means of providing an insulating material on a surface of the energized elements (200, 250) compared to employing bare energized elements (200, 250).

Description

POWER CONVERTER ASSEMBLY WITH INSULATING MATERIAL-COVERED

ELECTRODES

TECHNICAL FIELD

The present invention generally relates to the field of power systems such as electrical power distribution or transmission systems, e.g. High Voltage Direct Current (HVDC) power transmission systems. Specifically, the present invention relates to a power converter assembly having an electrically conducting member or electrode on which there is provided an insulating material, which power converter assembly may be used in a power system.

BACKGROUND

Power systems such as electrical power distribution or transmission systems are used to supply, transmit and use electric power. High Voltage Direct Current (HVDC) power transmission is becoming increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems.

An HVDC converter station is a type of station configured to convert between high voltage direct current (DC) and alternating current (AC). An HVDC converter station may comprise a plurality of elements such as a converter or a plurality of converters connected in series or in parallel. Converters may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized as line-commutated converters, using e.g. thyristors as switches, or voltage source converters, using transistors such as insulated gate bipolar transistors (IGBTs) as switches (or switching devices). A plurality of solid-state semiconductor devices such as thyristors or IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter, which may also be referred to as an HVDC converter valve. During normal operation of e.g. an HVDC power transmission system or an HVDC grid including the HVDC converter, the solid-state semiconductor devices in the HVDC converter may at times be in a conducting mode in which they are conducting current and at other times be in a blocking mode, in order to attain a desired or required wave form of the current, as known in the art.

It may be desired or even required to shield components in a power system (such as a HVDC power system) which are operated at relatively high DC voltages in order to reduce or eliminate the risk of partial discharges, arcing or flashovers occurring between the component and for example a wall, floor or ceiling within a building in which the component is arranged. For example, an HVDC converter is often arranged in a purpose-built building, which may be referred to as a valve hall or converter hall, for accommodating the HVDC converter.

SUMMARY

There is a demand for increase in the rated voltage for example in HVDC power systems. Occurrence of DC corona discharge in HVDC power systems with relatively high operating voltages may pose a challenge for the electrical insulation design for components for example in converter stations due to increase in strength of the electric field to which shield electrodes are subjected. Such increase in electrical field strength is a result e.g. of high voltage levels and also the desire or need for a compact design of the components. In order to mitigate or eliminate DC corona discharge in HVDC power systems with relatively high operating voltages, new designs of corona shields may be required.

For example in offshore applications, a desire or need for more compact designed DC converters may pose another challenge for the electrical insulation design. In this context, an increase in switching impulse (SI) and lightning impulse (LI) breakdown strength in air (and/or another fluid) gaps between high voltage components and objects in their surroundings, such as, for example, a wall, floor or ceiling may be required. To that end, an improvement of the construction of the DC converter may be needed.

In designing a converter hall, several considerations may have to be taken into account. For example, the air clearance between a converter and the walls, floor and ceiling of the converter hall should conform to security requirements. The dimensions of the converter hall may depend at least in part on the required or intended operating voltage of the electrical power distribution or transmission system. In general, the higher the operating voltage or rated voltage, the larger the air clearance that is required. The dimensions of the converter hall may among other things also depend on presence of any buildings adjacent the converter hall. At the same time, there is however also a desire for the converter hall to be as small as possible. This is for example due to that available space often is scarce and/or expensive, and the size of the converter hall may directly affect the costs for constructing and configuring the converter hall. This may in particular be the case in offshore applications.

Converter hall size and/or dimension(s) may depend directly on the air clearances required between earthed walls, ceilings and/or floors of the converter hall and the different energized elements accommodated within the converter hall, e.g. corona shields for shielding converter assemblies and/or busbars electrically connecting different energized elements. As known in the art, corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor which is electrically energized. Corona discharge may occur when the gradient of the electric field around the conductor is sufficiently high so as to form a conductive region. The lowest voltage at which continuous corona of specified amplitude may occur as the applied voltage is gradually increased may be referred to as the corona inception voltage. For a power converter assembly, corona inception voltage may depend on the configuration of any corona shield arranged so as to shield energized elements of the power converter assembly. Thus, appropriate design and/or construction of the corona shield may facilitate or even enable reducing the required air clearances, and reduce or even eliminate pre-discharge during use of the power converter assembly.

In view of the above, a concern of the present invention is to facilitate or enable reducing the required air clearance between a power converter assembly for use in a power system and objects in the surroundings of the power converter assembly, such as a wall, floor or ceiling within a building in which the power converter assembly is arranged.

A further concern of the present invention is to facilitate or enable reducing a size or dimension of a building in which a power converter assembly for use in a power system is arranged.

To address at least one of these concerns and other concerns, a power converter assembly in accordance with the independent claim is provided. Preferred embodiments are defined by the dependent claims.

It has been found that in order to increase corona inception voltage for example in power converter assembly applications, an insulating material may be provided e.g. in the form of a layer, cover or coating on a surface of energized elements of the power converter assembly. For energized elements of a power converter assembly, the corona inception voltage may be increased significantly by means of providing an insulating material on or coupled to a surface of the energized elements of the power converter assembly, compared to if the energized elements are 'bare'. The insulating material may for example include dielectric material such as epoxy and/or silicone. Insulating material, e.g. in the form of a coating or layer, may be provided e.g. on an outer surface of components such as for example corona shields and busbars of power converter assemblies. It is contemplated that insulating material may in addition or in alternative be provided on other energized components of the power converter assembly. The energized components of the power converter assembly which may be provided with insulating material, e.g. on an outer surface thereof, may be

components that are at least in part constituting a 'boundary' with respect to the surroundings of the power converter assembly, i.e. 'outer' components of the power converter assembly. However, in alternative or in addition, it is contemplated to provide 'inner' components (i.e. components that are located within or internally with respect to the power converter assembly) with insulating material e.g. on an outer surface thereof.

According to a first aspect, there is provided a power converter assembly for converting between alternating current power and direct current power. The power converter assembly comprises a converter cell and an electrically conducting member electrically connected to the converter cell. An insulating material is provided on at least an outer surface of the electrically conducting member, for increasing corona inception voltage for corona discharge at the at least an outer surface of the electrically conducting member.

By increasing the corona inception voltage for corona discharge at the at least an outer surface of the electrically conducting member, the required air clearance (e.g. for the rated, required or desired operating DC voltage of the power system) between the power converter assembly or the electrically conducting member and other objects in the

surroundings of the power converter assembly, such as earthed walls, ceilings and/or floors in a building in which the power converter assembly is arranged or located, may be decreased.

In some applications it may be required or desired to withstand switching impulse (SI) and lighting impulse (LI) overvoltages which may occur between the power converter assembly or the electrically conducting member and other objects in the

surroundings of the power converter assembly, such as earthed walls, ceilings and/or floors in a building in which the power converter assembly is arranged or located. In particular in power systems with a relatively high rated or required voltage, a positive SI voltage may be an important dimensioning parameter in designing and/or constructing a power converter assembly and possibly the converter hall. In order to attain higher insulation strength in air (and/or another fluid) gaps under positive LI and SI voltages, an appropriate design of the corona shield may be required. Thereby, the required air clearance between the power converter assembly or the electrically conducting member and other objects in the

surroundings of the power converter assembly may be reduced.

In addition to increasing corona inception voltage for corona discharge at the at least an outer surface of the electrically conducting member, it has been found that by means of the insulating material provided on at least an outer surface of the electrically conducting member, the air clearance which may be required for positive LI and SI voltages between the power converter assembly or the electrically conducting member and other objects in the surroundings of the power converter assembly may be decreased. In other words, the positive impulse insulation strength of air gaps may be significantly increased by means of the insulating material provided on at least an outer surface of the electrically conducting member, compared to if the electrically conducting member would be 'bare'.

Thus, by means of the insulating material provided on at least an outer surface of the electrically conducting member, the power converter assembly may be arranged closer to other objects in the surroundings of the power converter assembly. For example, two power converter assemblies may be arranged relatively close to each other in the converter hall.

Hence, a more compact arrangement of power converter assemblies in the converter hall may be achieved. In turn, a reduction of a size or dimension of the converter hall may be facilitated or enabled. Although reference is made herein to "air" clearance, or "air" gap, it is to be understood that the power converter assembly may be surrounded by a fluid other than air. By means of providing an insulating material on at least an outer surface of the electrically conducting member, corona inception voltage for corona discharge brought on by the ionization of the fluid surrounding the power converter assembly or the electrically conducting member may be increased.

Without being bound by any theory, it is proposed that the increase in corona inception voltage when employing coating and/or layer of dielectric material with a relatively large thickness, e.g. between 1 mm to 3 mm, may be related to reduction of electric field stress on the electrically conducting member due to the coating, cover and/or layer of dielectric material, and/or presence of space charge. Also, it is proposed that increase in corona inception voltage when employing coating, cover and/or layer of dielectric material with a relatively small thickness, e.g. below 1 mm, may be related to suppression of field emission, i.e. emission of electrons from the surface of the electrically conducting member when subjected to a relatively high electric field.

The above-described beneficial effects of providing insulating material on a surface of an energized component of a power converter assembly have been verified by means of experimental studies of corona inception voltage and breakdown voltage in air gaps of varying size between electrodes and an earthed surface. One experimental arrangement was a high voltage rod electrode with a hemispherical end having a diameter of about 50 mm. Corona inception voltage and breakdown voltage were measured both with a bare electrode and with the electrode covered with a silicone rubber coating having a thickness of about 3 mm and a dielectric constant of about 3.0. The silicone rubber coating was coupled to the electrode via a semiconductor layer such that the semiconductor layer was arranged between the silicone rubber coating and the electrode. The air gap was varied in the range from 0 mm to 150 mm. Another experimental arrangement used a high voltage rod electrode, both bare and covered with an epoxy coating having a thickness of about 2.5 mm and air gaps up to about 200 mm.

It is to be understood that the power converter assembly may comprise a plurality of converter cells, and a plurality of electrically conducting members electrically connected to the plurality of converter cells.

By an electrically conducting member being electrically connected to the converter cell it may be meant that the electrically conducting member is directly electrically connected to the converter cell. However, the electrically conducting member may be indirectly electrically connected (e.g. via one or more intermediate components) to the converter cell.

The power converter assembly may be for use in a power system, such as a HVDC power system. However, the power converter assembly is not limited to use in HVDC power systems. Nevertheless, according to an example the power converter assembly may be included in, or be constituted by, a HVDC converter.

The electrically conducting member may for example comprise a corona shield arranged in relation to the converter cell so as to shield the converter cell.

In alternative or in addition, the electrically conducting member may comprise an electrical conductor such as a busbar, which electrical conductor is configured to electrically connect the power converter assembly to another component included in the power system.

The electrically conducting member, such as a corona shield and/or an electrical conductor such as a busbar, may be made of a material for example including a metallic material, e.g. including one or more metals, e.g. Al, or metal alloys. However, in principle any material that has a sufficiently high electrical conductivity may be employed.

The insulating material may for example comprise a dielectric material.

The insulating material may for example be provided on the at least an outer surface of the electrically conducting member by way of a coating, cover(ing) and/or a layer of the insulating material. At least a portion of the coating, cover and/or layer may have a thickness between about 1 mm and 3 mm. According to an example, the thickness of the coating, cover and/or layer may be between about 1 mm and 3 mm over the whole coating, cover and/or layer. The coating, cover and/or layer may have a smooth, regular surface, or a rough, irregular surface. The coating, cover and/or layer may include portions having different degrees of surface roughness and/or smoothness. According to one example, substantially the entire surface of the coating, covering and/or layer is smooth.

For a relatively thick insulation layer, with a thickness between approximately 1 mm and 3 mm, an air gap flashover has been found to in most cases not puncture the insulation layer. However, in case an air gap flashover would puncture the insulation layer during operation of the power converter assembly, any puncture point(s) or region(s) of the insulation layer can be filled with new insulating material e.g. during maintenance.

The insulating material may for example include at least one material selected from a group comprising epoxy and silicone, or silicone rubber. However, these are merely examples and other insulating materials may in alternative or in addition be employed.

The coating, cover and/or layer of the insulating material may be coupled to the at least an outer surface of the electrically conducting member via a semiconductor layer, such that the semiconductor layer is arranged between the coating, cover and/or layer of the insulating material and the at least an outer surface of the electrically conducting member. The semiconductor layer may for example be sandwiched or bonded between the coating, cover and/or layer of the insulating material and the at least an outer surface of the electrically conducting member. The semiconductor layer may facilitate coupling of the insulating material to the at least an outer surface of the electrically conducting member. The semiconductor layer may alleviate differences in resistance between the insulating material and the electrically conducting member. The insulating material may hence be indirectly connected or coupled, i.e. via at least one intermediate component, to the at least an outer surface of the electrically conducting member. However, according to an example, the insulating material may be directly connected or coupled to the at least an outer surface of the electrically conducting member, i.e. without any intermediate component therebetween.

According to a second aspect, there is provided a power system comprising a power converter assembly according to the first aspect.

According to a third aspect, there is provided an electrically conducting member for use in a power converter assembly according to the first aspect, wherein an insulating material is provided on at least an outer surface of the electrically conducting member for increasing corona inception voltage for corona discharge at the at least an outer surface of the electrically conducting member.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments. It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the description herein. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 is schematic block diagram of a power converter assembly according to an embodiment of the present invention.

Figure 2 is a schematic sectional side view of a power converter assembly according to another embodiment of the present invention.

Figure 3 is a schematic sectional side view of a portion of a corona shield in accordance with an embodiment of the present invention.

Figure 4 is a schematic sectional side view of a busbar in accordance with an embodiment of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested. DETAILED DESCRIPTION

The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present 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 by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

Figure 1 is schematic block diagram of a power converter assembly 100 according to an embodiment of the present invention for use in a power system. The power converter assembly 100 comprises a cell or valve 150 electrically connected to an electrical conductor 200 such as a busbar configured to electrically connect the power converter assembly 100 to another component which may be included in the power system. The power converter assembly 100 further comprises a corona shield 250 electrically connected to the cell 150. The cell 150 may for example include a plurality of solid-state semiconductor devices such as thyristors or IGBTs which may be connected together, for instance in series, to form a building block of a power converter. Although only a single cell 150 is illustrated in Figure 1, the power converter assembly 100 may include a plurality of cells 150, e.g. tens or hundreds of electrically connected cells.

As illustrated in Figure 1, the power converter assembly 100, or the corona shield 250, is arranged at a distance d_\ from an object 110, which for example may be a wall, a ceiling or a floor in a building in which the power converter assembly 100 is arranged. As further illustrated in Figure 1 , the electrical conductor 200 is arranged at a distance d_2 from the object 110.

One or both of the electrical conductor 200 and the-corona shield 250 may be provided with an insulating material provided on an outer surface of the electrical conductor 200 and the corona shield 250, respectively, for increasing corona inception voltage for corona discharge at the outer surface of the electrical conductor 200 and the electromagnetic shield 250, respectively. The electrical conductor 200 and/or the corona shield 250 may be made of a material for example including a metallic material, e.g. including one or more metals, e.g. Al, or metal alloys. However, the electrical conductor 200 and/or the-corona shield 250 may in principle be made of any material that has a sufficiently high electrical conductivity. The insulating material, which for example may include a dielectric material, may for example be provided on an outer surface of the electrical conductor 200 and the corona shield 250, respectively, by way of a coating, cover and/or a layer of the insulating material, which for example may comprise a dielectric material, possibly with a thickness between about 1 mm and 3 mm. The insulating material, which for example may comprise a dielectric material, may for example include epoxy and/or silicone or silicone rubber.

However, other types of insulating materials may in alternative or in addition be used. The shape of the corona shield 250 as illustrated in Figure 1 is according to an example. Other shapes are possible and are within the scope of embodiments of the present invention.

Figure 2 is a schematic sectional side view of a power converter assembly 100 according to an embodiment of the present invention. According to the embodiment illustrated in Figure 2, the power converter assembly 100 comprises a plurality of cells 150, or valves, arranged in two stacks, each stack including several cells 150. The stacks of cells 150 are suspended from a ceiling 120 of a building (not shown in Figure 2) in which the power converter assembly 100 is arranged or located, by means of insulators 130 which extend through central holes in the cells 150. The cells 150 are arranged on top of each other and are electrically connected. Within the building in which the power converter assembly 100 is arranged or located, there may be other object(s) in the surroundings of the power converter assembly 100, such as a wall or floor. Between such other objects (not shown in Figure 2) there may be an air gap.

It is to be understood that the arrangement of the cells 150 in stacks and the number stacks which are illustrated in Figure 2 are according to examples. The power converter assembly 100 may in principle comprise any number of stacks of cells 150, e.g. a single stack of cells 150. Further, the cells 150 must not necessarily be arranged in stacks, and other arrangements are possible and are within the scope of embodiments of the present invention.

The power converter assembly 100 comprises a top corona shield 260 and bottom corona shield 270, arranged at the top and bottom of the stacks of cells 150, respectively. As used herein, the terms "top" and "bottom" refer to a longitudinal direction of the stacks. The top corona shield 260 and/or the bottom corona shield 270 may be provided with through-holes for allowing passage of the insulators 130 therethrough and for effecting coupling or connection of the top-corona shield 260 and/or the bottom corona shield 270 to the stack(s) of cells 150.

As illustrated in Figure 2, the power converter assembly 100 may comprise corona shields 280 arranged around the cells 150. As indicated in Figure 2, there may be a separation between adjacent corona shields 280, and also between the top corona shield 260 and the corona shields 280 adjacent to the top corona shield 260, and between the bottom corona shield 270 and the corona shields 280 adjacent to the bottom corona shield 270. It is to be understood that only some of the cells 150 and some of the corona shields 280 arranged around the cells 150 are indicated by reference numerals in Figure 2.

The power converter assembly 100 comprises electrical conductors 160 between cells 150 and the corona shields 280, and electrical conductors 1 between one of the cells 150 and the top corona shield 260 and between another one of the cells 150 and the bottom corona shield 270, respectively. Only a few of the electrical conductors 160 between cells 150 and the corona shields 280 are indicated by reference numerals 160 in Figure 2.

At least one of the corona shields 260, 270, 280 illustrated in Figure 2 may be provided with an insulating material provided on an outer surface of the respective corona shield 260, 270, 280, for increasing corona inception voltage for corona discharge at the outer surface of the respective corona shield 260, 270, 280. Each of the corona shields 260, 270, 280 illustrated in Figure 2 may be made of a material for example including a metallic material, e.g. including one or more metals, e.g. Al, or metal alloys. However, each of the corona shields 260, 270, 280 illustrated in Figure 2 may in principle be made of any material that has a sufficiently high electrical conductivity. The insulating material, which for example may include a dielectric material, may for example be provided on an outer surface of the respective corona shield 260, 270, 280 by way of a coating, cover and/or layer of the insulating material, which for example may comprise a dielectric material, possibly with a thickness between about 1 mm and 3 mm. The insulating material, which for example may comprise a dielectric material, may for example include epoxy and/or silicone or silicone rubber. However, other types of insulating materials may in alternative or in addition be used.

The shape of each of the corona shields 260, 270, 280 illustrated in Figure 2 is according to an example. Other shapes are possible and are within the scope of embodiments of the present invention.

Further, it is to be understood that various components which are not illustrated in Figure 2 may be included in the power converter assembly 100. Such components, which thus are not shown in Figure 2, may for example include electrical conductors which electrically interconnect various components of the power converter assembly 100, such as the cells 150, etc. For example, any electrical conductors between the stacks of cells 150 that may be included in the power converter assembly 100 are not shown in Figure 2.

Figure 3 is a schematic sectional side view of a portion of a corona shield 290 in accordance with an embodiment of the present invention, for use in a power converter assembly. The corona shield 290 is provided with an insulating material in the form of a layer, cover or coating 292 coupled to an outer surface 294 of the corona shield 290. The layer, cover or coating 292 of the insulating material is coupled to the outer surface 294 of the corona shield 290 via a semiconductor layer 296, such that the semiconductor layer 296 is arranged between the layer, cover or coating 292 of the insulating material and the outer surface 294 of the corona shield 290. The semiconductor layer 296 may for example be sandwiched or bonded between the layer, cover or coating 292 of the insulating material and the outer surface 294 of the corona shield 290. The semiconductor layer 296 may facilitate coupling of the insulating material to the outer surface 294 of the corona shield 290. The semiconductor layer 296 may alleviate differences in resistance between the insulating material and the corona shield 290. The corona shield 290, a portion of which is illustrated in Figure 3, may for example be a corona shield 280 arranged around a cell 150 in a power converter assembly 100 such as described with reference to Figure 2, or a top corona shield 260 or a bottom corona shield 270 in a power converter assembly 100 such as described with reference to Figure 2.

Figure 4 is a schematic sectional side view of a busbar 300 in accordance with an embodiment of the present invention, for use in a power converter assembly. Figure 4 illustrates a cross section of the busbar 300 along a longitudinal direction thereof. The busbar 300 is provided with an insulating material in the form of a layer, cover or coating 302 coupled to an outer surface 304 of the busbar 300. The layer, cover or coating 302 of the insulating material is coupled to the outer surface 304 of the busbar 300 via a semiconductor layer 306, such that the semiconductor layer 306 is arranged between the layer, cover or coating 302 of the insulating material and the outer surface 304 of the busbar 300. The semiconductor layer 306 may for example be sandwiched or bonded between the layer, cover or coating 302 of the insulating material and the outer surface 304 of the busbar 300. The semiconductor layer 306 may facilitate coupling of the insulating material to the outer surface 304 of the busbar 300. The semiconductor layer 306 may alleviate differences in resistance between the insulating material and the busbar 300.

The shape of the corona shield 290 as illustrated in Figure 3 and the shape of the busbar 300 as illustrated in Figure 4 are according to examples. Other shapes are possible and are within the scope of embodiments of the present invention. For example, the corona shield 290 must not necessarily have a curved shape but may for example exhibit a substantially flat, plate like configuration. Also, for example, the busbar 300 is not limited to a circular cross section but may for example have an oval cross section. Different portions of the busbar 300 may have cross sections of different form.

The insulating material provided on the corona shield 290 and/or the busbar

300 may for example comprise a dielectric material. The insulating material may for example include epoxy and/or silicone.

Although Figures 3 and 4 illustrate certain components of a power converter assembly, it is to be understood that according to one or more embodiments of the present invention an insulating material, e.g. a dielectric material, may be provided on another type of electrically conducting member of the power converter assembly, e.g. electrically connected to a converter cell of the power converter assembly.

While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A power converter assembly (100) for converting between alternating current power and direct current power, the power converter assembly comprising:

a converter cell (150);

an electrically conducting member (200, 250, 260, 270, 280, 290, 300) electrically connected to the converter cell;

wherein an insulating material is provided on at least an outer surface (292, 302) of the electrically conducting member for increasing corona inception voltage for corona discharge at the at least an outer surface of the electrically conducting member.

2. A power converter assembly according to claim 1, wherein the electrically conducting member comprises a corona shield (250, 260, 270, 280, 290) arranged in relation to the converter cell so as to shield the converter cell.

3. A power converter assembly according to any claim 1 or 2, wherein the power converter assembly is for use in a power system and the electrically conducting member comprises an electrical conductor (200, 300) configured to electrically connect the power converter assembly to another component included in the power system.

4. A power converter assembly according to any one of claims 1-3, wherein the insulating material comprises a dielectric material.

5. A power converter assembly according to any one of claims 1-4, wherein the insulating material includes at least one material selected from a group comprising epoxy and silicone.

6. A power converter assembly according to any one of claims 1-5, wherein the insulating material is provided by way of a coating, cover and/or a layer (292, 302) of the insulating material.

7. A power converter assembly according to claim 6, wherein at least a portion of the coating, cover and/or layer has a thickness between 1 mm and 3 mm.

8. A power converter assembly according to claim 6 or 7, wherein the coating, cover and/or layer of the insulating material is coupled to the at least an outer surface of the electrically conducting member via a semiconductor layer (296, 306) such that the semiconductor layer is arranged between the coating, cover and/or layer of the insulating material and the at least an outer surface of the electrically conducting member.

9. A power system comprising a power converter assembly (100) according to any one of claims 1-8.

10. An electrically conducting member (200, 250, 260, 270, 280, 290, 300) for use in a power converter assembly (100) according to any one of claims 1-8, wherein an insulating material is provided on at least an outer surface (292, 302) of the electrically conducting member for increasing corona inception voltage for corona discharge at the at least an outer surface of the electrically conducting member.