WO2018001464A1

WO2018001464A1 - Converter cell arrangement with cooling system

Info

Publication number: WO2018001464A1
Application number: PCT/EP2016/065009
Authority: WO
Inventors: Jaroslav Hemrle; Lilian Kaufmann; Oleksandr SOLOGUBENKO; Rebei Bel Fdhila; Thomas R. ERIKSSON
Original assignee: Abb Schweiz Ag
Priority date: 2016-06-28
Filing date: 2016-06-28
Publication date: 2018-01-04
Also published as: CN109417859A; CN109417859B

Abstract

A converter valve arrangement is disclosed, including a converter cell, a first and second semiconductor switch element, a container and a two-phase cooling system including at least one evaporator arranged in thermal contact with the semiconductor switch elements and to at least partly evaporate a dielectric working fluid. The two-phase cooling system and the converter cell are arranged within the container, and a fluid return line is arranged to receive a two-phase flow from the at least one evaporator. During operation, the converter valve arrangement may include an electrically critical region between the container and the converter cell, and the two-phase cooling system is arranged to generate a single-phase flow including only a gas phase or a liquid phase of the dielectric working fluid from the two-phase flow received by the fluid return line, and to convey the single-phase flow through the electrically critical region.

Description

CONVERTER CELL ARRANGEMENT WITH COOLING SYSTEM

TECHNICAL FIELD

The present disclosure relates to the field of power electronic converters. In particular, the present disclosure relates to a two-phase cooling system for such converters.

TECHNICAL BACKGROUND

A Modular Multilevel power Converter (MMC), also known as Chain- Link Converter (CLC), comprises a plurality of converter cells, or converter sub-modules, serially connected in converter branches, or phase legs, that in turn may be arranged in a wye/star, delta, and/or indirect converter topology. Each converter cell may comprise, in the form of a half-bridge or full-bridge circuit, a capacitor for storing energy and power semiconductor switches such as insulated gate bipolar transistor (IGBT) devices, gate-turn-off thyristor (GTO) devices, integrated gate commutated thyristor (IGCT) devices, or MOSFETs for connecting the capacitor to the converter branch with one or two polarities. The voltage per converter cell capacitor may be between 1 kV and 6 kV; whereas the voltage of a converter branch may be in a range from 10 kV to several 100 kV.

An MMC controller with a processor and corresponding software, or a field programmable gate array (FPGA), may be responsible for controlling the converter cells and operating the power semiconductor switches by means of a pulse width modulation scheme. MMCs may be used for a number of applications in electric power transmission and distribution, including High Voltage Direct Current (HVDC) applications as well as Static VAR

Compensators (Statcoms) and/or Flexible AC Transmission System (FACTS) applications. The latter may include devices for static power-factor correction as well as for voltage quality and stability purposes based on production or absorption of reactive power.

In MMC converter cells, power electronic modules comprising the power semiconductor switches represent a major source of power losses, while some heat may also be generated in the cell capacitors. Adequate cooling of both the power electronic modules and cell capacitors may be required for proper operation and long lifetime of the convertor components.

Conventionally, power semiconductor switch elements are cooled by forced water flow, while the cell capacitors are cooled by free convection of air, with capacitor containers being exposed via all surfaces to the

surrounding circulating air. With the capacitor heating intensity per volume being limited, and despite a rather low thermal conductivity of the container filler material, air cooling on all capacitor container surfaces may ensure minimal length of the thermal paths and may prove sufficient for preventing undesirable hot spot temperature levels in the capacitor that may significantly affect the life-cycle of the capacitor. However, with increasing footprint requirements, the heat-generating components are packed more closely, the total heating power density increases, and alternative cooling concepts may have to be envisaged.

The patent EP 2277365 B1 discloses a high power drive stack system for cooling of power silicon devices arranged on a plurality of modules which in turn are stacked in a common support structure. A dielectric fluid cooling system utilizes a vaporizable dielectric refrigerant and comprises a plurality of fluid conduits, a condenser, a pump, and an evaporator positioned on at least one module.

SUMMARY

An object of the present disclosure is therefore to provide for sufficient cooling of converter cells arranged in a compact design. This objective is achieved by a converter valve arrangement according to the independent claim. Other embodiments are defined by the dependent claims.

In a stack or succession of converter cells arranged on top of or next to each other, both the power electronics modules and cell capacitors face corresponding components of adjacent converter cells, which requires adequate electric, dielectric, and short-circuit design measures in-between the cells. With increasing footprint requirements, heat-generating components are packed more closely and the space available next to the cell capacitors, specifically between the power electronic modules and the capacitors, becomes scarce. Accordingly, in a compact converter design, circulation of ambient air in a stacking direction may be impeded or reduced to an extent that precludes cooling of the cell capacitors via capacitor surfaces

perpendicular to the stacking direction. Here, a cooling system that transfers, via dedicated thermal connections, excess cooling power from forced convection boiling cooling of the power electronic modules to the cell capacitors ensures sufficient cooling of the latter.

According to one aspect of the present disclosure, a forced convection boiling two-phase cooling system for a converter cell of a multilevel power converter is provided. The converter cell may include a cell capacitor for storing energy and a first semiconductor switch element and a second semiconductor switch element to connect the cell capacitor to cell terminals. The cooling system may include at least one evaporator arranged in thermal contact with at least one of the semiconductor switch elements and adapted for evaporating a dielectric working or cooling fluid, or refrigerant, with the at least one evaporator being fluidly connected in a closed working fluid circuit including a pump for circulating the fluid and a central condenser. The cooling system may be adapted to thermally connect, or transfer heat from, the cell capacitor to a fluid supply or return line of the working fluid.

In one embodiment, the cooling system may include only one evaporator, and the only one evaporator may be arranged in thermal contact with both the first and second semiconductor switch elements. In other embodiments, the cooling system may include more than one evaporator, e.g. two or three evaporators or more, and the evaporators may be arranged such that each semiconductor switch element is in thermal contact with at least one evaporator. If, for example, multiple evaporators and the semiconductor switch elements are arranged in a stacked fashion, one or more of the multiple evaporators may be in thermal contact with only one of the first and second semiconductor switches. In this or other examples, one or more of the multiple evaporators may be in thermal contact with both the first and the second semiconductor switches. In some examples, no evaporator may be in thermal contact with more than one semiconductor switch element. If more than one evaporator is included in the cooling system, some or all

evaporators may be fluidly connected in parallel in the closed working fluid circuit.

In one embodiment, the cooling system may include a dedicated thermal link or path or other connection with high thermal conductivity, such as a cold finger or a heat pipe, to thermally connect the cell capacitor to a working fluid supply or return line supplying condensed working fluid to and returning evaporated working fluid from the at least one evaporator.

In one embodiment, a capacitor terminal of the cell capacitor may be electrically connected through a capacitor bushing and a capacitor connection to the first semiconductor switch, and the thermal link may connect the capacitor connection to the fluid supply or return line. In this case, the thermal link may be electrically conducting without requiring its own bushing. In an alternative embodiment of the present disclosure, an electrically insulating thermal link may be provided for thermally connecting the fluid supply or return lines to a surface or housing of the cell capacitor.

One aspect of the present disclosure is also directed to a Modular Multilevel power Converter (MMC) converter arrangement for HVDC and FACTS applications, with a converter valve including a plurality of series connected converter cells each including a cell capacitor for energy storing purpose and two semiconductor switch elements to connect the cell capacitor to cell terminals, and with a cooling system. The cooling system may include a closed working fluid circuit including a condenser for condensing

evaporated working fluid and a pump for circulating the fluid. The cooling system may further include, for each converter cell, at least one evaporator arranged in thermal contact with the semiconductor switches of the cell, the at least one evaporator being adapted to receive condensed dielectric working fluid and to return evaporated working fluid. The cooling system may include, for each converter cell, a dedicated thermal link with high thermal conductivity adapted to thermally connect, and transfer heat from, the cell capacitor to a fluid supply or return line supplying condensed working fluid and returning evaporated working fluid from the at least one evaporator. If more than one evaporator is included in the cooling system for a converter cell, the evaporators may be fluidly connected in parallel. In an arrangement as defined here, a converter branch or phase leg may include one or several converter valves, with one independent cooling circuit per converter valve.

In one embodiment, more than one evaporator may be included and the fluid supply lines of the converter arrangement, in the tubing sections connecting to individual evaporators, may have flow-restricting throttle elements to balance a flow of working fluid among the different evaporators. In particular, these throttle elements may account for gravity in case the converter cells are stacked non-horizontally, specifically vertically.

The present disclosure is also directed to a converter valve

arrangement that includes at least one converter cell, a first semiconductor switch element and a second semiconductor switch element, a container and a two-phase cooling system. The two-phase cooling system may include at least one evaporator arranged in thermal contact with at least one of the first semiconductor switch element and the second semiconductor switch element, and adapted for at least partly evaporating a dielectric working fluid. The two- phase cooling system may include a fluid return line for returning dielectric working fluid from the at least one evaporator.

The at least one converter cell and the two-phase cooling system may be arranged within the container. During operation, the converter valve arrangement may include at least one electrically critical region between the container and the at least one converter cell. The at least one electrically critical region may include at least a region that is affected by a potential difference between a wall of the container and the at least one converter cell.

The fluid return line may be arranged to receive a two-phase flow including a gas phase and a liquid phase of the dielectric working fluid. The two-phase flow may be received by the fluid return line from the at least one evaporator. The two-phase flow may result from an incomplete evaporation of the dielectric working fluid. The fluid return line may output, at a first end, at least an intermediate flow that includes at least a part of the two-phase flow. Here, "at least a part of means that the intermediate flow may include at least some of the fluid received from the two-phase flow, although not necessarily in the same phase (liquid or gas), or combination of phases, as received. The two-phase cooling system may be arranged to generate at least one single-phase flow that includes only a gas phase or a liquid phase of the dielectric working fluid from the intermediate flow that is received from the fluid return line. The two-phase cooling system may be arranged to convey the at least one single-phase flow through the at least one electrically critical region.

In one embodiment, the container may include at least one outlet, through which a fluid may pass from the inside of the container to the outside of the container, through a wall of the container. The two-phase cooling system may be arranged to convey the at least one single-phase flow to the at least one outlet through the at least one electrically critical region.

In one embodiment, at least one of the at least one outlet may be a liquid outlet, and at least one of the at least one single-phase flow may be a liquid flow. The two-phase cooling system may be arranged to convey the liquid flow to the liquid outlet through at least one of the at least one electrically critical region.

In one embodiment, at least one of the at least one outlet may be a gas outlet, and at least one of the at least one single-phase flow may be a gas flow. The two-phase cooling system may be arranged to convey the gas flow to the gas outlet through at least one of the at least one electrically critical region.

In one embodiment, the two-phase cooling system may include a phase separator that is arranged to receive the intermediate flow from the fluid return line. The phase separator may be arranged to separate at least a portion of the liquid phase of the dielectric working fluid from the intermediate flow to output a second intermediate flow that includes only a liquid phase of the dielectric working fluid.

In one embodiment, the phase separator may be arranged to separate at least a portion of the gas phase of the dielectric working fluid from the intermediate flow to output the gas flow that is to be conveyed to the gas outlet.

In one embodiment, the diameter of the fluid return line may be larger than or equal to a counter current flow limit diameter to at least partly separate, during operation of the converter valve arrangement, the phases of the received two-phase flow using gravity. The intermediate flow may include more of a gas phase than a liquid phase of the dielectric working fluid, and the fluid return line may be arranged to output, at a second end that is different from the first end of the fluid return line, a third intermediate flow that may include more of a liquid phase than a gas phase of the dielectric working fluid.

In one embodiment, the two-phase cooling system may include a liquid accumulator that is arranged to receive either one of the second intermediate flow or the third intermediate flow.

In one embodiment, the liquid accumulator may be arranged to receive the second intermediate flow from the phase separator.

In one embodiment, the liquid accumulator may be arranged to receive the third intermediate flow from the fluid return line.

In one embodiment, the liquid accumulator may form an integrated part of the fluid return line, e.g. the second end of the fluid return line may include a space for accumulating liquid.

In one embodiment, the two-phase cooling system may include a second phase separator that may be arranged to remove a gas phase of the dielectric working fluid from the third intermediate flow that is output at the second end of the fluid return line. The second phase separator may, in this or other embodiments, be a separate part, or form an integrated part of e.g. the liquid accumulator and/or the fluid return line.

In one embodiment, the fluid return line may be arranged to convey at least a part of the second intermediate flow from the phase separator to the liquid accumulator.

In one embodiment, the two-phase cooling system may include a liquid return line that is arranged to convey at least a part of the second

intermediate flow from the phase separator to the liquid accumulator.

In one embodiment, the fluid return line may include at least one internal deflector near or at a location at which the fluid return line is configured to receive the two-phase flow from the at least one evaporator. In one embodiment, the converter cell may include a cell capacitor, and the two-phase cooling system may be adapted to thermally connect the cell capacitor to the dielectric working fluid.

In one embodiment, the two-phase cooling system may include a thermal link for thermal connection of the cell capacitor to the fluid return line and/or a fluid supply line arranged to supply dielectric working fluid to the at least one evaporator. The thermal link may be arranged to provide thermal connection of the cell capacitor to the fluid return line near or at said location. The fluid supply line may be included in the two-phase cooling system.

In one embodiment, the two-phase cooling system may be arranged such that, with the container in a standing position, the liquid accumulator is located in a lower section of the container and the phase separator is located in an upper section of the container. Here, lower and upper may be taken to mean nearer to and further from the ground respectively when measured along a gravitational direction.

The present disclosure relates to all possible combinations of features mentioned herein, including the ones listed above as well as other features which will be described in what follows with reference to different

embodiments. Any embodiment described herein may be combinable with other embodiments also described herein, and the present disclosure relates also to all such combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure will be explained in more detail in the following illustrative and non-limiting detailed description of embodiments. Reference is made to the appended drawings, in which:

Figurel is a schematic illustration of a cooling system layout according to one or more embodiments of the present disclosure;

Figure 2 is a schematic illustration of thermal links between a capacitor and a cooling circuit according to one or more embodiments of the present disclosure;

Figure 3 is a schematic illustration of a converter valve arrangement according to one or more embodiments of the present disclosure; Figure 4 is a schematic illustration of a converter valve arrangement according to one or more embodiments of the present disclosure;

Figure 5 is a schematic illustration of a converter valve arrangement according to one or more embodiments of the present disclosure; and

Figure 6 is an enlarged view of a schematic illustration of a converter valve arrangement according to one or more embodiments of the present disclosure.

In the drawings, like reference numerals will be used for like elements unless stated otherwise. Unless explicitly stated to the contrary, the drawings show only such elements that are necessary to illustrate the example embodiments, while other elements, in the interest of clarity, may be omitted or merely suggested.

DETAILED DESCRIPTION

Exemplifying embodiments will now be described more fully hereinafter with reference to the accompanying drawings. The drawings show currently preferred embodiments, but 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.

Some embodiments of cooling systems according to the present disclosure will now be described with reference to figures 1 and 2.

Figure 1 shows the layout of a cooling system for a converter valve 1 with four identical converter cells 2 each with a number of constituents as described further below (wherein, for the sake of legibility, corresponding reference numerals 13 to 18 have not been repeated for all instances). The solid lines marked with filled arrows indicate the refrigerant or working fluid circuit. Refrigerant in the liquid state is pumped by pump 3 and directed by the tubing or conduits of supply line 4 to evaporators 15 in the cells. Cooling takes place via predominantly latent heat of liquid-vapour phase transition (boiling). The resulting vapour with highest possible quality leaves the converter valve via the tubing of return line 5, and condenses in condenser 6 by means of heat exchange with some cooling media. The liquid refrigerant is then collected in separator 7, further subcooled in sub-cooler heat exchanger 8 which is fluidly connected to pump 3 closing the flow circuit. Several converter valves can be connected to the same supply line in parallel; the refrigerant flow distribution is regulated by valves 9 and controlled by mass flow meter 10. The system also includes one or more filters/driers 1 1 to remove water from the refrigerant. In exemplary off-shore applications a preferred cooling fluid for heat exchangers 6 and 8 is sea water pumped by pump 12 through tubing denoted by dash-dot lines.

The sub-cooler 8 may be an integrated part of cooler 6, with or without any accumulator and/or separator in between. In some embodiments, the sub-cooler 8 may be optional, e.g. if gravitational head is enough to avoid cavitation in the pump 3.

Each converter cell 2 comprises a capacitor 13 and two semiconductor switch elements 14, specifically two IGBT modules. Two evaporators 15 are fluidly connected in parallel to the liquid refrigerant supply line 4 and the vapour return line 5. The lower evaporator is in good thermal and electrical contact with the collector of the lower IGBT module and with the emitter of the upper IGBT module. The upper evaporator is in good thermal and electrical contact with the collector of the upper IGBT module. In case the tubing is made of electrically conducting materials, electrically insulating sections 16 should be added to both supply line 4 and return line 5 in-between the two evaporators 15. In the tubing connecting evaporators 15 with supply line 4, flow-restricting throttle elements 17 are added in order to ensure the homogeneous distribution of fluid through all evaporators of the system.

Throttle elements 17 may be valves or small cross-section tubing sections. Ultimately, in order to improve cooling of capacitors 13 according to the invention, thermal links 18 with high thermal conductance are thermally connecting the capacitors with tubing 4 and 5.

In this and other embodiments described herein, the cooling system is illustrated as having two evaporators 15 in each converter cell 2. It is, however, envisaged that the cooling system may include for example only one evaporator 15 in each converter cell 2, or that the cooling system includes more than two evaporators 15 in each converter cell 2 (e.g. three, four or more). It is also envisaged that the number of evaporators 15 may differ between different converter cells 2. As an illustrative example, a single evaporator 15 may be arranged between, and in thermal contact with, two semiconductor switch elements 14 in a converter cell 2. As another illustrative example, two or three evaporators 15 may be arranged e.g. in a stacked fashion together with two semiconductor elements 14 in a converter cell 2.

Figure 2 shows, on the left-hand side, a first exemplary embodiment of the thermal link between the capacitor and the two-phase cooling circuit. The capacitor housing 13a encloses a number of capacitor elements connected by a capacitor busbar 13b to a first capacitor terminal 13c. The capacitor terminal in turn is connected, by means of capacitor bushing 13d and first electrical capacitor connection 13e, to a first semiconductor switch element 14a. A branch of the first electrical capacitor connection leads to a

neighbouring converter cell (not depicted), while a second capacitor terminal (not depicted) is connected to the second semiconductor switch element 14b. A first thermal link 18a is provided between the capacitor bushing 13d and an electrically insulating section 16 of the supply line 4.

Figure 2 shows, on the right-hand side, a second exemplary

embodiment of the thermal link between the capacitor and the two-phase cooling circuit. Specifically, a second thermal link 18b is provided between a capacitor housing 13f and the return line 5. The thermal link according to the second embodiment may be alternative or complementary to a thermal link according to the first embodiment, and both thermal links 18a, 18b may use either of the supply line 4 and the return line 5 as a heat sink and preferably end at electrically insulating sections in-between the two evaporators of a cell.

The thermal network between the heat sources within the capacitor housings and the two-phase cooling circuit includes various thermal resistances, in particular relating to conductive heat transfer inside the capacitor container. The thermal resistance for collection and transfer of heat from the capacitor elements towards the capacitor bushing is determined by the design, including geometry and material properties, of the capacitor internal busbar. The design of the busbar may focus exclusively on the electrical properties, and disregard thermal properties, if a capacitor-internal heat collector as a heat transfer facilitator, electrically isolated from yet adjacent to the busbar, is provided.

In any compact converter design the available space for convective air, sensors and control elements is inherently limited. In case of a converter valve immersed in dielectric gas a minimum distance between the cell capacitor and other converter elements is inversely proportional to a dielectric breakdown electric field strength of the dielectric gas. Accordingly, even between an outer surface of the cell capacitor and a dielectric gas enclosure there may not be sufficient space for circulating a cooling medium, in addition to the actual absence of such cooling medium, and alternative cell capacitor cooling concepts as described in this application are all the more beneficial.

The total flow rate of the refrigerant for the system is selected such that the total latent heat of boiling is equal or higher than the total heat losses in the semiconductor switch elements and cell capacitors. The total flow rate should be minimized in order to reduce the pumping power, yet it should be large enough to avoid uncontrolled dry-out in any of evaporators. The equivalent flow distribution between different evaporators connected fluidly in parallel is assured by throttle elements, having each different flow resistances specific for each evaporator and compensating, e.g., for the difference in the elevation-dependent pressure of the fluid. The flow resistance of throttle elements can be either fixed or variably determined in dependence on a temperature measurement at the IGBT modules. The two-phase cooling leads to a very homogeneous distribution of temperature across the IGBT modules of the converter valve.

The non-flammable dielectric refrigerant may be a refrigerant known as R134a, or alternatives with a lower global warming potential, such as

R1234yz or similar. Flow rates may depend on the type of the two-phase refrigerant and may be of the order of 0.01 kg/s per cell, and thus one order of magnitude lower than for conventional water cooling. The lower flow rates of the cooling fluid result in reduced wear of the tubing and, for the case of horizontal converter valve arrangement, a considerable reduction of pump load, as well as in a considerably lower total mass of cooling fluid. In addition, dielectric properties of a number of refrigerant fluids are more favourable for high-voltage applications than those of water which requires de-ionizing equipment to be included in the cooling system.

The condenser and sub-cooler may reject the heat to ambient air, to thermal loads, or to a water bath e.g. on off-shore platforms. For highest cooling performance, the cooler can be also cooled by active refrigeration circuit to suppress the working fluid temperature even below ambient, subject to techno-economic optimization. The connection can be direct (refrigerant to the ambient cooling medium) or indirect (via intermediate loop, e.g. clean water).

With reference to figures 3-5, embodiments of a converter valve arrangement will now be described.

Figure 3 shows a layout of a converter valve arrangement 1 that includes a container 20 in which three identical converter cells 2 are arranged in a stacked configuration. The number of converter cells 2 may, in other embodiments, be larger or smaller than three. Each converter cell 2 has a cell capacitor 13, a first and second switch element 14 (e.g. IGBTs) and a first and second evaporator 15.

During operation of the converter valve arrangement 1 , each

evaporator 15 is supplied with a liquid phase of a dielectric working fluid via a fluid supply line 4. When the fluid passes through the evaporators 15 it may absorb heat generated by e.g. the switch elements 14 or other components in the converter cell 2, a process during which at least a part of the fluid may evaporate such that it changes from a liquid phase to a gas phase.

Depending on e.g. the mass flow rate of the supplied dielectric working fluid, not all of the fluid may evaporate. Instead, at least a part (e.g. 20-40%) of the fluid may remain in a liquid phase and the output from the evaporator 15 may be a two-phase fluid that includes a mix of both a gas phase and a liquid phase of the dielectric working fluid. By allowing for incomplete evaporation of the dielectric working fluid, the heat transfer intensity and stability of the two- phase cooling system may be tailored, and the risk of e.g. an uncontrolled dry-out in certain areas may be avoided. After passing through the evaporator 15, the two-phase flow of dielectric working fluid is collected in a fluid return line 5. The fluid return line 5 may be constructed e.g. from tubing or conduits, and configured to at least receive the two-phase flow from the evaporator 15 at a location 23.

During operation of the converter valve arrangement 1 , large voltage differences between at least some of the components of the converter valve arrangement 1 may be generated. If for example, as indicated in figure 3, the container 20 is grounded, potential differences between e.g. a converter cell 2 and a wall of the container 20 may reach up to several hundred kilovolts or higher (e.g. -400 kV or higher). The possible presence of such potential differences may create dielectrically critical regions, regions in which the dielectric design and dielectric properties of components therein may be of increased importance.

One challenge may be the possibility of presence of more than one phase (e.g. gas and liquid) of a fluid in such a dielectrically critical region. This includes a requirement with respect to electric withstand properties in some or all design regimes, and also with respect to other behaviors such as e.g. partial discharges or decomposition due to such discharges and the properties of the resulting products. In the converter valve arrangement 1 in figure 1 , and in other embodiments in accordance with the present disclosure, the challenge is overcome or at least partially alleviated by arranging the two- phase cooling system such that the different phases of the dielectric working fluid are reliably separated before the fluid enters the dielectrically critical regions (i.e. regions exposed to and/or affected by potential differences higher than few kVs). Generally, current understanding of the dielectric behavior of a two-phase fluid in larger electrical fields may be relatively low. By separating the phases, i.e. such that only liquid or only gas is present in e.g. a certain tube or line in a certain place within the container, more standard design rules corresponding to a gas- or liquid-only insulation situation may be applied.

In figure 3, the direction of flows of liquid and gas phases of the dielectric working fluid are illustrated with filled and empty arrows

respectively. After the fluid return line 5 receives the two-phase flow of the dielectric working fluid, at least a part of the two-phase flow is conveyed by the fluid return line 5 and output as an intermediate flow at a first end of the fluid return line. At this end, in one embodiment, a phase separator 24 is arranged. The phase separator 24 receives the intermediate flow and may separate at least a portion of a liquid phase of the dielectric working fluid from the intermediate flow, and output a second intermediate flow which includes only the separated liquid phase. The phase separator 24 may for example use centrifugal action, heating, condensation and/or other methods to separate the liquid phase.

The second intermediate flow output from the phase separator 24 is conveyed via a liquid return line 27 to a liquid accumulator 25 in which liquid may be temporarily stored. If, as illustrated in figure 3, the converter valve arrangement 1 is arranged such that the container 20 is standing up, the liquid accumulator 25 may be positioned below the phase separator 24, such that gravity may assist the transportation of liquid from the phase separator 24 to the liquid accumulator 25. If such an alignment and/or gravity assisted transportation is not available, it may be envisaged that e.g. one or many pumps are used to assist the transportation of liquid within certain parts of the two-phase cooling system.

Within the container 20, a potential difference between a converter cell 2 and a wall of the container 20 may be large enough to generate a dielectrically critical region. In figure 3, two such dielectrically critical regions 21 a and 21 b are illustrated. In this embodiment, the container 20 includes a liquid outlet 22a, and the liquid accumulator 25 may output a single-phase liquid flow that is conveyed to the liquid outlet 22a through the dielectrically critical region 21 a (by e.g. a tubing or conduit). Within the same dielectrically critical region 21 a, dielectric working fluid in liquid form may, in this or other embodiments, be supplied to the fluid supply line 4 via a liquid inlet 28 and also conveyed through the dielectrically critical region 21 a. As only a single phase (liquid) of the dielectric working fluid is locally present (e.g. in the same tube or conduit) in the dielectrically critical region 21 a, proper insulation may be provided using more standard design rules for liquid-only insulation compared to if both liquid and gas were to be conveyed together (e.g. in the same tube or conduit) through region 21 a and out through e.g. the outlet 22a.

The liquid inlet 28 and the liquid outlet 22a may in turn, in some embodiments, be connected to an external circuit such as described earlier, in which the liquid phase of the dielectric working fluid may e.g. be cooled further before it is re-circulated back into the converter valve arrangement 1 via the liquid inlet 28.

Figure 3 also shows an optional recirculation pump 29 which may be arranged to re-circulate some or all of the single-phase flow output from the liquid accumulator 25 back to the fluid supply line within the container 20 (within the converter valve arrangement 1 ) and through the dielectrically critical region 21 a. As mentioned above, only having a single phase of the dielectric working fluid locally present in the region 21 a may facilitate the insulation procedure.

The pump 29 may be omitted in some embodiments, or be replaced with, or complemented by, e.g. a passive mixing device (such as a liquid ejector). The pump 29 or its alternative/complement may compensate for a pressure drop that may occur throughout the cooling system. In an

embodiment in which all of the single-phase flow is re-circulated, the liquid outlet 22a may be optional.

The phase separator 24 may be arranged to separate at least a portion of the gas phase of the dielectric working fluid from the intermediate flow that it receives from the fluid return line 5. The container 20 may include a gas outlet 22b, and the phase separator 24 may output a gas flow that is conveyed to the gas outlet 22b through the dielectrically critical region 21 b.

As only a single phase (gas) of the dielectric working fluid is locally present in the dielectrically critical region 21 b, proper insulation may be provided using more standard design rules for gas-only insulation compared to if both liquid and gas were to be conveyed together (e.g. in the same tubing or conduit) through region 21 b and out through e.g. the outlet 22b. The presence of only a single phase may also facilitate electrical insulation between the phase separator 24 and the wall of the container 20, especially if combined with filling the container 20 with a dielectric gas and by providing solid insulating material of e.g. a tube or conduit that is used to convey the gas from the phase separator 24 to the gas outlet 22b. The same principles may apply also when electrically insulating e.g. the liquid accumulator 25 from the wall of the container 20.

As the gas (vapor) leaves the phase separator 24 towards the gas outlet 22b, it may be accelerated and the static pressure may slightly drop. In order to assure that such a condition may not lead to any local liquid nucleation, an auxiliary heater (not shown) may be arranged e.g. within the gas outlet in order to evaporate any liquid drops that are created due to this effect.

After the gas flow exits the container 20 through the gas outlet 22b, the gas may be conveyed to an external condenser (such as the condenser 6 described earlier) where it may be condensed back to liquid, or otherwise taken care of.

Figure 4 shows an alternative embodiment of a converter valve arrangement 1 which may be equivalent to the converter valve arrangement 1 described with reference to figure 3 except that the phase separator 24 includes a condenser. The condenser is water cooled by an external supply 30 of cooling water, but it is also envisaged that other types of condensers (e.g. air or electrically cooled) may be used. In the setup in figure 4, all gas that enters the phase separator 24 from the fluid return line 5 may be condensed back to liquid such that only a liquid phase of the dielectric working fluid leaves the phase separator 24 (via the liquid return line 27). In figure 4, the pump 29 re-circulates all (or almost all) of the single-phase liquid flow back to the fluid supply line 4, and no liquid inlets/outlets and gas outlets may therefore be required during normal operation as the two-phase cooling loop may be closed inside of the converter valve arrangement 1 .

Figure 5 shows an alternative embodiment of a converter valve arrangement 1 which may be similar to the converter valve arrangement 1 described with reference to figure 3, except that the fluid return line 5 is designed such that its diameter is sufficiently large so that most (or at least a part of) the gas and liquid phases of the liquid phase of the dielectric working fluid separate within the fluid return line 5 itself, using gravity. If the container 20 is standing up, the two-phase cooling system may be arranged such that liquid entering the fluid return line 5 falls downwards towards the bottom of the container 20 while gas entering the fluid return line 5 rises upwards. To achieve this, the diameter of the fluid return line 5 may be equal to or larger than a counter current flow limit (CCFL) diameter at which the gas (vapor) velocity is sufficiently low and the liquid may drop by gravity without being entrained and forced upwards together with the gas. The CCFL diameter may be calculated based on the required cooling power of the two-phase cooling system, and may be fixed or varied with e.g. height above the container bottom as smaller flow rates may be expected at the lower converter cells.

The fluid return line 5 is arranged to output a third intermediate flow at its second end. By the above principle, the intermediate flow going towards the phase separator 24 may include more of a gas phase than a liquid phase of the dielectric working fluid, while the third intermediate flow may include more of a liquid phase than a gas phase of the dielectric working fluid.

The liquid accumulator 25 is received to return the third intermediate flow output at the second end of the fluid return line 5, and to temporary store this liquid together with liquid received from the phase separator 24 via the liquid return line 27. It may be envisaged that, in one embodiment, the fluid return line 5 is arranged to convey both the second intermediate flow (from the phase separator 24) and the third intermediate flow towards the liquid accumulator 25, in which case the liquid return line 27 may be optional. It may also be envisaged that the liquid accumulator 25 is an integrated part of the fluid return line 5.

In figure 5, a second phase separator 26 is arranged to remove a gas phase of the dielectric working fluid from the third intermediate flow. The phase separator 26 may operate on similar principles as the phase separator 24, and may be used to remove any gas phase that may not be completely separated by the fluid return line 5. It is envisaged that the second phase separator 26 may be an integrated part of the liquid accumulator 25 and/or the fluid return line 5.

In Fig. 6, an enlarged view of an exemplary embodiment of a converter valve arrangement and a two-phase cooling system around the location 23 is shown. At the location 23, the fluid return line 5 receives the two-phase fluid flow from at least one of the first and second evaporators, and the fluid return line 5 is provided with three internal deflectors 31 a, 31 b, and 31 c at this location 23. It is envisaged that more or fewer deflectors may be used, and the deflectors may be arranged at or near the location 23.

A deflector may be e.g. a fin, a flap, a porous material or other structures and/or combinations thereof that may improve a separation of a gas phase and a liquid phase of the dielectric working fluid. The deflectors may use the kinetic energy of the two-phase fluid flow to collect drops of liquid and deflect them downwards.

In for example the converter valve arrangements 1 in figures 3, 4, 5 and 6, a thermal link 18 is provided for thermal connection of the cell capacitor 13 to the fluid supply line 5. The thermal link 18 is arranged such that it attaches to (or at least is in thermal contact with) the fluid return line 5 at this location 23, which may provide for an improved thermal connection between the cell capacitor 13 and the fluid supply line 5.

While the invention has been described in detail in the drawings and foregoing description, such description is to be considered illustrative or exemplary and not restrictive. Variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising 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 or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain elements or steps are recited in distinct claims does not indicate that a combination of these elements or steps cannot be used to advantage, specifically, in addition to the actual claim dependency, any further meaningful claim combination shall be considered disclosed.

Claims

1 . A converter valve arrangement comprising at least one converter cell (2), a first semiconductor switch element (14) and a second

semiconductor switch element (14), a container (20) and a two-phase cooling system comprising:

at least one evaporator (15) arranged in thermal contact with at least one of the first semiconductor switch element (14) and the second

semiconductor switch element, wherein the at least one evaporator is adapted for at least partly evaporating a dielectric working fluid;

a fluid return line (5) arranged to receive a two-phase flow including a gas phase and a liquid phase of the dielectric working fluid from the at least one evaporator and to output, at a first end of the fluid return line, at least an intermediate flow including at least a part of the two-phase flow,

wherein the at least one converter cell and the two-phase cooling system are arranged within the container,

wherein, during operation, the converter valve arrangement comprises at least one electrically critical region (21 a; 21 b) between the container and the at least one converter cell, and

wherein the two-phase cooling system is arranged to generate at least one single-phase flow including only a gas phase or a liquid phase of the dielectric working fluid from the intermediate flow, and to convey the at least one single-phase flow through said at least one electrically critical region.

2. The converter valve arrangement according to claim 1 , wherein the container comprises at least one outlet (22a; 22b), wherein the two-phase cooling system is arranged to convey the at least one single-phase flow to the at least one outlet through said at least one electrically critical region.

3. The converter valve arrangement according to claim 2, wherein at least one of said at least one outlet is a liquid outlet (22a), wherein at least one of said at least one single-phase flow is a liquid flow, and wherein the two-phase cooling system is arranged to convey said liquid flow to the liquid outlet through at least one of said at least one electrically critical region.

4. The converter valve arrangement according to claim 2 or 3, wherein at least one of said at least one outlet is a gas outlet (22b), wherein at least one of said at least one single-phase flow is a gas flow, and wherein the two- phase cooling system is arranged to convey said gas flow to the gas outlet through at least one of said at least one electrically critical region.

5. The converter valve arrangement according to any one of claims 1 -4, wherein the two-phase cooling system comprises a phase separator (24) arranged to receive the intermediate flow from the fluid return line, and to separate at least a portion of the liquid phase of the dielectric working fluid from the intermediate flow to output a second intermediate flow including only a liquid phase of the dielectric working fluid.

6. The converter valve arrangement according to any one of claims 1 -5, wherein a diameter of the fluid return line is larger than or equal to a counter current flow limit diameter to at least partly separate, during operation of the converter valve arrangement, the phases of the received two-phase flow using gravity, wherein the intermediate flow includes more of a gas phase than a liquid phase of the dielectric working fluid, and wherein the fluid return line is arranged to output, at a second end different from the first end, a third intermediate flow including more of a liquid phase than a gas phase of the dielectric working fluid.

7. The converter valve arrangement according to claim 5 or 6, wherein the two-phase cooling system comprises a liquid accumulator (25) arranged to receive either one of the second intermediate flow or the third intermediate flow.

8. The converter valve arrangement according to claim 7, wherein the liquid accumulator forms an integrated part of the fluid return line.

9. The converter valve arrangement according to any one of claims 6-8, wherein the two-phase cooling system comprises a second phase separator

(26) arranged to remove a gas phase of the dielectric working fluid from the third intermediate flow output at the second end of the fluid return line.

10. The converter valve arrangement according to any one of claims 7-9, wherein the two-phase cooling system comprises a liquid return line (27) arranged to convey at least a part of the second intermediate flow from the phase separator to the liquid accumulator.

1 1 . The converter valve arrangement according to any one of claims 1 -10, wherein said at least one electrically critical region includes at least a region affected by a potential difference between a wall of said container and said at least one converter cell.

12. The converter valve arrangement according to any one of claims 1 -1 1 , wherein the converter cell comprises a cell capacitor (13), and wherein the two-phase cooling system is adapted to thermally connect the cell capacitor to the dielectric working fluid.

13. The converter valve arrangement according to claim 12, wherein the two-phase cooling system comprises a thermal link (18) for thermal connection of the cell capacitor to the fluid return line and/or a fluid supply line (4) arranged to supply dielectric working fluid to the least one evaporator.

14. The converter valve arrangement according to any one of claims 1 -13, wherein the fluid return line comprises at least one internal deflector (31 a;

31 b; 31 c) near or at a location (23) at which it is configured to receive the two- phase flow.

15. The converter valve arrangement according to claim 13, wherein the thermal link is arranged to provide thermal connection of the cell capacitor to the fluid supply line.