WO2014071985A1 - Subsea unit comprising a two-phase cooling system and a subsea power system comprising such a subsea unit - Google Patents

Subsea unit comprising a two-phase cooling system and a subsea power system comprising such a subsea unit Download PDF

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Publication number
WO2014071985A1
WO2014071985A1 PCT/EP2012/072211 EP2012072211W WO2014071985A1 WO 2014071985 A1 WO2014071985 A1 WO 2014071985A1 EP 2012072211 W EP2012072211 W EP 2012072211W WO 2014071985 A1 WO2014071985 A1 WO 2014071985A1
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WO
WIPO (PCT)
Prior art keywords
evaporator
cooling fluid
subsea
type
electric component
Prior art date
Application number
PCT/EP2012/072211
Other languages
French (fr)
Inventor
Christian Spindler
Thomas Gradinger
Original Assignee
Abb Technology Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abb Technology Ltd filed Critical Abb Technology Ltd
Priority to PCT/EP2012/072211 priority Critical patent/WO2014071985A1/en
Publication of WO2014071985A1 publication Critical patent/WO2014071985A1/en

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Classifications

    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K7/00Constructional details common to different types of electric apparatus
    • H05K7/20Modifications to facilitate cooling, ventilating, or heating
    • H05K7/2029Modifications to facilitate cooling, ventilating, or heating using a liquid coolant with phase change in electronic enclosures
    • H05K7/20309Evaporators
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D15/00Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies
    • F28D15/02Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes
    • F28D15/0266Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes with separate evaporating and condensing chambers connected by at least one conduit; Loop-type heat pipes; with multiple or common evaporating or condensing chambers
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K7/00Constructional details common to different types of electric apparatus
    • H05K7/14Mounting supporting structure in casing or on frame or rack
    • H05K7/1422Printed circuit boards receptacles, e.g. stacked structures, electronic circuit modules or box like frames
    • H05K7/1427Housings
    • H05K7/1432Housings specially adapted for power drive units or power converters
    • H05K7/14337Housings specially adapted for power drive units or power converters specially adapted for underwater operation
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K7/00Constructional details common to different types of electric apparatus
    • H05K7/20Modifications to facilitate cooling, ventilating, or heating
    • H05K7/20218Modifications to facilitate cooling, ventilating, or heating using a liquid coolant without phase change in electronic enclosures
    • H05K7/20236Modifications to facilitate cooling, ventilating, or heating using a liquid coolant without phase change in electronic enclosures by immersion
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K7/00Constructional details common to different types of electric apparatus
    • H05K7/20Modifications to facilitate cooling, ventilating, or heating
    • H05K7/2089Modifications to facilitate cooling, ventilating, or heating for power electronics, e.g. for inverters for controlling motor
    • H05K7/20936Liquid coolant with phase change

Definitions

  • the present disclosure generally relates to electric subsea installations and in particular to a subsea unit comprising a two-phase cooling system for cooling electric components arranged in the subsea unit, and to a subsea power system comprising such a subsea unit.
  • Oil and gas production subsea employs electric equipment like drilling motors, pumps, and compressors that are currently driven by frequency converters located on topside platforms. Electric power is provided to the subsea machinery by expensive umbilicals. By installing frequency converters and other power electronic equipment subsea, cables and topside installations could be spared and enormous cost savings could be achieved.
  • Power semiconductors are employed as main switching elements in power electronic equipment such as frequency converters. Cooling systems are essential in power electronic devices to transfer the heat generated by the power semiconductors. Frequency converters in the medium voltage and power range drive electric motors by controlling the speed and torque of these machines and are a well proven equipment in the entire onshore and offshore platform based industry.
  • Concept (1) has the advantage that standard electric/electronic components, known from onshore installations, can be used, while disadvantages include thick walls needed for the enclosure to withstand the pressure difference between inside and outside. Thick walls make the equipment heavy and costly. In addition, heat transfer through thick walls is not very efficient and huge, expensive cooling units are required.
  • Concept (2) has the advantage that no thick walls are needed for the enclosure since no pressure difference exists between inside and outside the containment. Cooling is greatly facilitated by thin walls. Disadvantages of concept (2) are that all the components must be free of gas inclusions and compressible voids; otherwise they implode during pressurization and are destroyed.
  • the present disclosure relates to concept (1).
  • a subsea power converter according to the first concept is disclosed in "Controlled subsea electric power distribution with SEPDISTM” by Solvik et al. published in ABB Review
  • the subsea power converter has a converter enclosure in the form of a high-pressure vessel to protect the internal power converter components from high ambient sea-water pressure, and a two-phase cooling system for cooling the power converter components.
  • the two-phase cooling system has a condenser located outside the pressure vessel such that sea-water can be utilised for condensing the cooling fluid from liquid to vapour phase.
  • the heat-generating components cooled by the two-phase cooling system are placed in the cooling liquid, i.e. in the liquid phase of the two- phase cooling fluid.
  • cooling fluids such as refrigerants can be rather expensive, and many refrigerants have a high global warming potential. Therefore it would be desirable to be able to provide efficient cooling with less cooling fluid than has previously been possible.
  • a general object of the present disclosure is to provide a subsea unit that solves or at least mitigates the problems of the prior art.
  • a subsea unit comprising a pressure vessel arranged to withstand ambient sea water pressure at sea ground of at least 50 bar, and to keep up an internal pressure that is closer to atmospheric pressure than sea water pressure at the sea ground; a first type of electric component; and a two-phase cooling system arranged to cool the first type of electric component, which two-phase cooling system comprises: a first cooling fluid, a first evaporator arranged within the pressure vessel, wherein the first evaporator is arranged in thermal connection with the first type of electric component, a condenser arranged outside the pressure vessel to allow the condenser to be in thermal connection with sea water, a vapour conduit arranged to transfer first cooling fluid from the first evaporator towards the condenser, and a liquid conduit arranged to transfer first cooling fluid from the conden
  • An effect which may be obtainable thereby is that a much smaller quantity of two-phase cooling fluid is needed for efficient cooling of the first type of electric component to be maintained, since only conduits of relatively small cross section have to be filled with cooling fluid. Thereby a less expensive and more environmentally friendly subsea unit may be provided.
  • Such components are typically semiconductors, e.g. insulated-gate bipolar transistors (IGBT).
  • IGBT insulated-gate bipolar transistors
  • Other, low-power and passive components typically have a much lower loss density and do generally not require direct cooling by evaporation.
  • the pressure vessel comprises a second type of electric component, and a second cooling fluid outside the closed loop of the two-phase cooling system, forming an auxiliary cooling system in the pressure vessel, which second cooling fluid is arranged to cool the second type of electric component, and which second cooling fluid has a dielectric strength that is sufficiently high to electrically insulate the second type of electric component from its surrounding.
  • the first type of electric component which may comprise a power semiconductor, typically has a high heat flux compared to the second type of electric component, which may be a low-power and passive electric component and does therefore not need as efficient cooling as the first type of electric component.
  • the two-phase cooling system and the second cooling fluid provide two essentially independent cooling systems; the two-phase cooling system which is arranged to cool the first type of electric component which has higher cooling requirements than the second type of electric component, and the second cooling fluid which is arranged to cool the second type of electric component.
  • the second cooling fluid In addition to the cooling of the second type of electric component, the second cooling fluid simultaneously provides electrical insulation of the second type of electric component from its surround.
  • the necessary wall thickness linearly increases with the vessel diameter.
  • a second cooling fluid arranged to cool the second type of electric component, wherein the second cooling fluid has a dielectric strength that is sufficiently high to electrically insulate the second type of electric component from its surrounding, a more compact pressure vessel may be provided.
  • several second types of electric components may be arranged in a more compact manner, the second type of electric component may be arranged closer to the first type of electric component, and the second type of electric component may be arranged closer to the pressure vessel enclosure which typically has another electric potential than the second type of electric component.
  • the two-phase cooling system comprises a second evaporator forming part of the closed loop of the two-phase cooling system, wherein the second evaporator is arranged to receive heat from the second cooling fluid.
  • the second evaporator is arranged to receive heat from the second cooling fluid.
  • One embodiment comprises a heat sink arranged in thermal connection with the second evaporator and in fluid communication with the second cooling fluid to transfer heat from the second cooling fluid to the second evaporator.
  • the effective heat absorption area for absorbing heat from the second cooling fluid may be increased, rendering the cooling of the second type of electric component more efficient.
  • cooling of the second type of electric component is provided by natural convection of the second cooling fluid in the pressure vessel. Thereby simple and robust cooling of the second type of electric component may be provided.
  • both the vapour conduit and the liquid conduit have an insulating section to electrically insulate the first evaporator from the pressure vessel.
  • the first type of electric component e.g. a power semiconductor
  • MMC modular multi-level converter
  • the second cooling fluid is a gas.
  • the second cooling fluid is pressurised to a pressure higher than atmospheric pressure.
  • the pressure is in the range of 3-10 bar. This pressure is still small compared to the ambient sea-water pressure of the subsea unit when installed on the sea ground, and it should not cause damage to any electric components, as a pressure of e.g. 100 or 300 bar would do.
  • a pressure of 3-10 bar is, however, high compared to the atmospheric pressure of 1 bar and therefore allows a significant reduction of clearance distances, approximately 3-10 times smaller clearances, in gas insulation according to the Paschen curve. This enables a significant increase in compactness and reduction in pressure vessel volume.
  • the second cooling fluid may be an oil or ester. The dielectric strength of an oil or ester is significantly higher than that of for example air at one bar pressure.
  • the first type of electric component and the second type of electric component form part of a frequency converter.
  • the first type of electric component is attached to the first evaporator.
  • the subsea unit of the first aspect disclosed herein may beneficially be included in a subsea power system.
  • a subsea power system comprising a subsea unit according to the first aspect.
  • the subsea power system is a transmission system. According to one embodiment the subsea power system is a distribution system.
  • Fig. l is a schematic cross-sectional side view of a first example of a subsea unit for subsea installation.
  • Fig. 2 is a schematic cross-sectional side view of a second example of a subsea unit for subsea installation.
  • Fig. 1 depicts a schematic cross-sectional side view of a first example of a subsea unit for installation on the seabed.
  • Subsea unit 1-1 comprises a pressure vessel 3 arranged to withstand high ambient sea-water pressure when the subsea unit 1-1 is arranged on the sea ground, and to keep up or maintain a pressure that is closer to atmospheric pressure than to sea-water pressure at the sea ground.
  • the pressure vessel 3 has an enclosure 3- 1 which provides the necessary mechanical strength at the intended operational depth. This may be obtained by manufacturing the pressure vessel 3 from suitable material, e.g. steel, with a properly dimensioned wall thickness of the enclosure 3-1.
  • the subsea unit 1-1 is arranged to house electric components, forming an electric device such as a frequency converter. Depending on the type of electric device arranged in subsea unit 1-1, power transmission, power distribution and/or power control can be provided on the sea ground, close to power consuming units such as drilling motors, pumps, and compressors operating on sea ground.
  • subsea unit 1-1 comprises at least one first type of electric component 9, and at least one second type of electric component 11.
  • a first type of electric component 9 has a higher loss density than a second type of electric component 11.
  • Examples of the first type of electric component are power electronic devices such as IGBTs and integrated gate-commutated thyristors (IGCT).
  • Examples of the second type of electric component are a capacitor, an inductor, a bus bar and a printed circuit board assembly (PCBA).
  • the subsea unit 1-1 further comprises a two-phase cooling system 5, a loop thermosyphon, arranged to provide cooling of first type of electric
  • the two-phase cooling system 5 is arranged to circulate a two- phase first cooling fluid F-i, and comprises a vapour conduit 5-1, a liquid conduit 5-2, a first evaporator El arranged in thermal connection with first type of electric components 9, and a condenser C.
  • the vapour conduit 5-1, the liquid conduit 5-2, the first evaporator El and the condenser C together form a closed loop in which the first cooling fluid F-i can circulate to thereby achieve cooling of first type of electric components 9.
  • these can be attached to the first evaporator Ei.
  • First type of electric components 9 can for example be mounted on the first evaporator Ei, either directly or via a thermal interface.
  • the first evaporator El and the condenser C are preferably manufactured from material with good thermal conductivity characteristics.
  • the first evaporator Ei is arranged inside the pressure vessel 3, and the condenser C is arranged on the outside of the pressure vessel 3, i.e. in direct contact with sea water when the subsea unit 1-1- is installed on the sea ground, such that the condenser C can transfer heat absorbed from the first cooling fluid F-i flowing through the condenser C to the surrounding sea water.
  • the vapour conduit 5-1 and the liquid conduit 5-2 hence penetrate the enclosure 3-1 of the pressure vessel 3.
  • the vapour conduit 5-1 and the liquid conduit 5-2 have relatively small dimension in order to structurally weaken the pressure vessel 3, which in use is subjected to high mechanical stress by the sea water, as little as possible.
  • the vapour conduit 5-1, the liquid conduit 5-2, the first evaporator Ei and the condenser C are adapted to maintain an internal pressure, in the closed loop which they form, closer to atmospheric pressure than to sea water pressure at sea ground. Typical internal pressures in the closed loop can for example be in the range of about 1 bar to about 20 bar.
  • the vapour conduit 5-1, the liquid conduit 5-2, the first evaporator El and the condenser C can for example be manufactured from a metallic material such as steel.
  • the condenser C is adapted to withstand the high ambient sea water pressure.
  • the condenser may according to one variation comprise a bundle of conduits with sufficient spacing from each other to avoid or mitigate clogging due to dirt or growth of life.
  • the first cooling fluid F-i in vapour state may be distributed between several condenser conduits for more efficient cooling.
  • the first evaporator Ei has an inlet E1-1 and an outlet E1-2
  • the condenser C has an inlet C-i and an outlet C-2.
  • the vapour conduit 5-1 connects the outlet Ei- 2 of the first evaporator Ei with the inlet C-i of the condenser C
  • the liquid conduit 5-2 connects the outlet C-2 of the condenser C with the inlet Ei-i of the first evaporator El.
  • the vapour conduit, the liquid conduit, the first evaporator and the condenser may either be a single piece closed loop conduit, or the vapour conduit, liquid conduit, the first evaporator and the condenser may alternatively be separate parts that can be assembled to form the closed loop of the two-phase cooling system.
  • the subsea unit l-i may furthermore comprise a second cooling fluid F-2.
  • the second cooling fluid F-2 is provided in an open space in the pressure vessel 3 outside the closed loop of the two-phase cooling system 5 such that the second cooling fluid F-2 can flow freely therein, and forms an auxiliary cooling system in the pressure vessel arranged to cool the second type of electric components 11.
  • the second cooling fluid F-2 moves by natural convection in the open space of the pressure vessel 3.
  • the second cooling fluid may be a gas, oil or an ester.
  • suitable gases are air or nitrogen at 1 bar pressure. The use of nitrogen instead of air can reduce the risk of corrosion.
  • suitable gases are SF 6 and similar gases with very high dielectric strength.
  • the second cooling fluid is gas
  • the second cooling fluid is put under moderate pressure, i.e. a pressure of a few bar.
  • a pressure of a few bar By pressurising the second cooling fluid to a pressure higher than atmospheric pressure, a significant reduction of clearance distance in gas insulation may be obtained. This enables a significant increase in compactness and reduction in pressure vessel volume.
  • the pressure of the second cooling fluid may for example be in the range of 3-10 bar. This pressure is still small compared to the ambient sea water pressure, and it should not cause damage to the second type of electric components or the first type of electric components, as e.g. 100 bar or 300 bar would do.
  • the pressure in the pressure vessel 3 is in the range of about 1 bar to about 10 bars. This pressure range is closer to atmospheric pressure than to sea ground pressure at the depth of intended operation of the subsea unit 1-1, where the hydrostatic pressure level is at least 50 bar.
  • the second cooling fluid is a two-phase fluid to thereby increase the cooling performance. The volatility of the second cooling fluid is beneficially chosen such that in an operating state a moderately high pressure of e.g. 2-10 bar results, in order to obtain increased dielectric strength in the vapour state.
  • the two-phase cooling system 5 is preferably vertically oriented in the subsea unit 1-1 when the subsea unit 1-1 is installed on the seabed to thereby allow gravity-driven circulation of each of the first cooling fluid F-i and the second cooling fluid F-2.
  • the section of the two-phase cooling system 5 provided with the first evaporator El provides a first cooling fluid flow vertically upwards
  • the section of the two-phase cooling system 5 provided with the condenser C provides a cooling fluid flow vertically downwards in the direction of the seabed.
  • the outlet E1-2 of the first evaporator El is arranged in a horizontal plane lower than the horizontal plane in which the inlet C-i of the condenser C is arranged when the subsea unit 1-1 is installed on the seabed.
  • each of the vapour conduit 5-1 and the liquid conduit 5-2 has a respective insulating section 7 arranged inside the pressure vessel 3 to electrically insulate the first evaporator El from the pressure vessel 3.
  • each end of the first evaporator El is connected to an insulating section 7.
  • first type of electric components 9, which are arranged in thermal connection with the first evaporator El may have an electric potential that differs from the electric potential of the enclosure 3-1 of the pressure vessel 3.
  • first type of electric component(s) 9 Since first type of electric component(s) 9 is/are in thermal connection with the first evaporator El, part of the heat emitted by a first type of electric component 9 is absorbed by the first evaporator El.
  • First type of cooling fluid F-i having been cooled by the condenser C, flows in liquid state in the liquid conduit 5-2 towards the first evaporator Ei and absorbs heat from the first evaporator Ei as it passes through the first evaporator Ei. Due to the heat that has been absorbed from the first evaporator El, the first cooling fluid F-i is vaporised and flows upwards in the vapour conduit 5-1. The portion of the first cooling fluid F-i which is in vapour state then flows towards the condenser C arranged on the outside of the pressure vessel 3.
  • the condenser C which is cooled by sea water absorbs heat from the first cooling fluid F-i flowing through the condenser C in vapour phase, and transmits the heat to the sea water which moves by natural convection outside the subsea unit 1-1. Due to the heat transmitted to the condenser C, the first cooling fluid F-i again enters its liquid state and flows in the liquid conduit 5-2 towards the first evaporator El, and the circuit is repeated. Simultaneously, the second cooling fluid F-2 cools second type of electric component(s) 11 in the pressure vessel 3 by means of natural convection.
  • Subsea unit 1-2 of the second example is identical to subsea unit 1-1 of the first example, except that two-phase cooling system 5' of subsea unit 1-2 comprises a second evaporator E2 arranged to receive heat from the second cooling fluid F-2.
  • the second evaporator E2 has an inlet connected to the outlet of the first evaporator Ei and an outlet connected to the inlet of the condenser C.
  • the second evaporator E2 is arranged downstream of the first evaporator El relative the circulation direction of the first cooling fluid F-i.
  • the two-phase cooling system 5' may furthermore according to one variation comprise a heat sink 13 in fluid communication with the second cooling fluid F-2 and in thermal connection with the second evaporator E2 to transfer heat from the second cooling fluid to the second evaporator E2.
  • the heat sink 13 can for example be attached to the second evaporator E2 via a thermal interface, or alternatively the heat sink 13 and second evaporator E2 can be integrated in one piece.
  • the heat sink is a passive component.
  • the heat sink may optionally have fins so as to further increase its heat absorption area and thus its efficiency.
  • the second evaporator E2 and the heat sink 13 are preferably so arranged in the pressure vessel 3 that they can absorb heat from the second cooling fluid F-2 after the second cooling fluid F-2 has absorbed heat from the second type of electric component(s) 11.
  • the second cooling fluid F-2 first flows by the second type of electric component(s) 11 to absorb heat from the second type of electric component(s) 11 and then flows by the heat sink 13 and the second evaporator E2 so as to transfer absorbed heat to the heat sink 13 and the second evaporator E2 to thereby cool the second cooling fluid F-2.
  • the cooling provided by the two-phase cooling system 5' is essentially identical to that provided by two-phase cooling system 5 of the first example.
  • the second cooling fluid F-2 moves by natural convection in the pressure vessel 3, as noted above, the second cooling fluid F-2 first absorbs heat from the first type of electric component(s) and then transmits the absorbed heat to the heat sink 13 and the second evaporator E2.
  • the second evaporator E2 transmits this heat to the first cooling fluid F-i, which is then cooled by the condenser C outside the pressure vessel 3 ⁇
  • components having both high heat flux and low heat flux may be provided in a compact manner.
  • Subsea unit 1-1, 1-2 may beneficially be included in a subsea power system including a plurality of modules/installations such as power transformers, frequency converters and circuit breakers forming part of or forming a complete transmission or distribution network on sea ground.
  • modules/installations such as power transformers, frequency converters and circuit breakers forming part of or forming a complete transmission or distribution network on sea ground.

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  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
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  • General Engineering & Computer Science (AREA)
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Abstract

The present disclosure relates to a subsea unit (1-2) comprising a pressure vessel (3) arranged to withstand ambient sea water pressure at sea ground of at least 50 bar, and to keep up an internal pressure that is closer to atmospheric pressure than sea water pressure at the sea ground; a first type of electric component (9); and a two-phase cooling system (5') arranged to cool the first type of electric component (9), which two-phase cooling system (5') comprises: a first cooling fluid (F-1), a first evaporator (E1) arranged within the pressure vessel (3), wherein the first evaporator (3) is arranged in thermal connection with the first type of electric component (9), a condenser (C) arranged outside the pressure vessel (3) to allow the condenser (C) to be in thermal connection with sea water, a vapour conduit (5-1) arranged to transfer first cooling fluid (F-1) from the first evaporator (E1) towards the condenser (C), and a liquid conduit (5-2) arranged to transfer first cooling fluid (F-1) from the condenser (C) towards the first evaporator (E1), wherein the first evaporator (El), the condenser (C), the vapour conduit (5-1) and the liquid conduit (5-2) form a closed loop for circulating the first cooling fluid (F-1) in the two-phase cooling system (5; 5'). A subsea power system comprising a subsea unit is also presented herein.

Description

SUBSEA UNIT COMPRISING A TWO-PHASE COOLING SYSTEM AND A SUBSEA POWER SYSTEM COMPRISING SUCH A SUBSEA UNIT
TECHNICAL FIELD
The present disclosure generally relates to electric subsea installations and in particular to a subsea unit comprising a two-phase cooling system for cooling electric components arranged in the subsea unit, and to a subsea power system comprising such a subsea unit. BACKGROUND
In recent years, there has been a growing interest in installing electrical installations on the sea floor in depths from a few tens of meters to even kilometres. Oil and gas production subsea employs electric equipment like drilling motors, pumps, and compressors that are currently driven by frequency converters located on topside platforms. Electric power is provided to the subsea machinery by expensive umbilicals. By installing frequency converters and other power electronic equipment subsea, cables and topside installations could be spared and enormous cost savings could be achieved.
Power semiconductors are employed as main switching elements in power electronic equipment such as frequency converters. Cooling systems are essential in power electronic devices to transfer the heat generated by the power semiconductors. Frequency converters in the medium voltage and power range drive electric motors by controlling the speed and torque of these machines and are a well proven equipment in the entire onshore and offshore platform based industry.
In bringing power electronics subsea, two general concepts exist: (1) the equipment stays at or near atmospheric pressure; and (2) the equipment is pressurized to the hydrostatic pressure level on sea ground. The two concepts can be differentiated as follows. Concept (1) has the advantage that standard electric/electronic components, known from onshore installations, can be used, while disadvantages include thick walls needed for the enclosure to withstand the pressure difference between inside and outside. Thick walls make the equipment heavy and costly. In addition, heat transfer through thick walls is not very efficient and huge, expensive cooling units are required. Concept (2) has the advantage that no thick walls are needed for the enclosure since no pressure difference exists between inside and outside the containment. Cooling is greatly facilitated by thin walls. Disadvantages of concept (2) are that all the components must be free of gas inclusions and compressible voids; otherwise they implode during pressurization and are destroyed.
The present disclosure relates to concept (1). A subsea power converter according to the first concept is disclosed in "Controlled subsea electric power distribution with SEPDIS™" by Solvik et al. published in ABB Review
2/2000. The subsea power converter has a converter enclosure in the form of a high-pressure vessel to protect the internal power converter components from high ambient sea-water pressure, and a two-phase cooling system for cooling the power converter components. The two-phase cooling system has a condenser located outside the pressure vessel such that sea-water can be utilised for condensing the cooling fluid from liquid to vapour phase.
Moreover, the heat-generating components cooled by the two-phase cooling system are placed in the cooling liquid, i.e. in the liquid phase of the two- phase cooling fluid.
The problem of cooling power electronics in subsea equipment is however not yet solved satisfyingly. Challenges include inter alia reducing the size and cost of pressure vessels of subsea units.
SUMMARY
In addition to the size and cost issues of current solutions, in immersion cooling where evaporation is obtained by pool boiling, large quantities of fluid are needed to immerse the heat emitting components in the fluid in liquid phase. However, cooling fluids such as refrigerants can be rather expensive, and many refrigerants have a high global warming potential. Therefore it would be desirable to be able to provide efficient cooling with less cooling fluid than has previously been possible.
In view of the above, a general object of the present disclosure is to provide a subsea unit that solves or at least mitigates the problems of the prior art. Hence, according to a first aspect of the present disclosure there is provided a subsea unit comprising a pressure vessel arranged to withstand ambient sea water pressure at sea ground of at least 50 bar, and to keep up an internal pressure that is closer to atmospheric pressure than sea water pressure at the sea ground; a first type of electric component; and a two-phase cooling system arranged to cool the first type of electric component, which two-phase cooling system comprises: a first cooling fluid, a first evaporator arranged within the pressure vessel, wherein the first evaporator is arranged in thermal connection with the first type of electric component, a condenser arranged outside the pressure vessel to allow the condenser to be in thermal connection with sea water, a vapour conduit arranged to transfer first cooling fluid from the first evaporator towards the condenser, and a liquid conduit arranged to transfer first cooling fluid from the condenser towards the first evaporator, wherein the first evaporator, the condenser, the vapour conduit and the liquid conduit form a closed loop for circulating the first cooling fluid in the two-phase cooling system.
An effect which may be obtainable thereby is that a much smaller quantity of two-phase cooling fluid is needed for efficient cooling of the first type of electric component to be maintained, since only conduits of relatively small cross section have to be filled with cooling fluid. Thereby a less expensive and more environmentally friendly subsea unit may be provided. Moreover, it is possible to have a controlled, high-performance cooling of only those electric components that have high loss densities by the thermal connection between such electric components and the first evaporator e.g. by mounting them directly on the first evaporator of the two-phase cooling system. Such components are typically semiconductors, e.g. insulated-gate bipolar transistors (IGBT). Other, low-power and passive components typically have a much lower loss density and do generally not require direct cooling by evaporation.
According to one embodiment the pressure vessel comprises a second type of electric component, and a second cooling fluid outside the closed loop of the two-phase cooling system, forming an auxiliary cooling system in the pressure vessel, which second cooling fluid is arranged to cool the second type of electric component, and which second cooling fluid has a dielectric strength that is sufficiently high to electrically insulate the second type of electric component from its surrounding. Herein the first type of electric component, which may comprise a power semiconductor, typically has a high heat flux compared to the second type of electric component, which may be a low-power and passive electric component and does therefore not need as efficient cooling as the first type of electric component. The two-phase cooling system and the second cooling fluid provide two essentially independent cooling systems; the two-phase cooling system which is arranged to cool the first type of electric component which has higher cooling requirements than the second type of electric component, and the second cooling fluid which is arranged to cool the second type of electric component. In addition to the cooling of the second type of electric component, the second cooling fluid simultaneously provides electrical insulation of the second type of electric component from its surround.
If the admissible compressive stress in the wall material is o, then the necessary wall thickness t for a cylindrical pressure vessel of diameter d to sustain a pressure difference p can be roughly estimated as:
Figure imgf000005_0001
Hence, the necessary wall thickness linearly increases with the vessel diameter. By providing a second cooling fluid arranged to cool the second type of electric component, wherein the second cooling fluid has a dielectric strength that is sufficiently high to electrically insulate the second type of electric component from its surrounding, a more compact pressure vessel may be provided. Especially, due to the electric insulation properties of the second cooling fluid, several second types of electric components may be arranged in a more compact manner, the second type of electric component may be arranged closer to the first type of electric component, and the second type of electric component may be arranged closer to the pressure vessel enclosure which typically has another electric potential than the second type of electric component.
According to one embodiment the two-phase cooling system comprises a second evaporator forming part of the closed loop of the two-phase cooling system, wherein the second evaporator is arranged to receive heat from the second cooling fluid. Thereby at least some of the heat absorbed by the second cooling fluid from the second type of electric component may be transmitted to the first cooling fluid in the two-phase cooling system, improving the cooling efficiency of the second cooling fluid and possibly also of the two-phase cooling system.
One embodiment comprises a heat sink arranged in thermal connection with the second evaporator and in fluid communication with the second cooling fluid to transfer heat from the second cooling fluid to the second evaporator. Thereby, the effective heat absorption area for absorbing heat from the second cooling fluid may be increased, rendering the cooling of the second type of electric component more efficient.
According to one embodiment cooling of the second type of electric component is provided by natural convection of the second cooling fluid in the pressure vessel. Thereby simple and robust cooling of the second type of electric component may be provided.
According to one embodiment, inside the pressure vessel both the vapour conduit and the liquid conduit have an insulating section to electrically insulate the first evaporator from the pressure vessel. Thereby the first type of electric component, e.g. a power semiconductor, can be on a potential different from the pressure vessel, which is electrically on ground. This enables multi-level converter topologies such as modular multi-level converter (MMC).
According to one embodiment the second cooling fluid is a gas.
According to one embodiment the second cooling fluid is pressurised to a pressure higher than atmospheric pressure.
According to one embodiment the pressure is in the range of 3-10 bar. This pressure is still small compared to the ambient sea-water pressure of the subsea unit when installed on the sea ground, and it should not cause damage to any electric components, as a pressure of e.g. 100 or 300 bar would do. A pressure of 3-10 bar is, however, high compared to the atmospheric pressure of 1 bar and therefore allows a significant reduction of clearance distances, approximately 3-10 times smaller clearances, in gas insulation according to the Paschen curve. This enables a significant increase in compactness and reduction in pressure vessel volume. Alternatively, the second cooling fluid may be an oil or ester. The dielectric strength of an oil or ester is significantly higher than that of for example air at one bar pressure.
According to one embodiment the first type of electric component and the second type of electric component form part of a frequency converter. According to one embodiment the first type of electric component is attached to the first evaporator.
The subsea unit of the first aspect disclosed herein may beneficially be included in a subsea power system. Hence, according to a second aspect of the present disclosure there is provided a subsea power system comprising a subsea unit according to the first aspect.
According to one embodiment the subsea power system is a transmission system. According to one embodiment the subsea power system is a distribution system.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise.
BRIEF DESCRIPTION OF THE DRAWINGS
The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:
Fig. l is a schematic cross-sectional side view of a first example of a subsea unit for subsea installation; and
Fig. 2 is a schematic cross-sectional side view of a second example of a subsea unit for subsea installation.
DETAILED DESCRIPTION
The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying
embodiments are shown. The inventive concept 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 be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Fig. 1 depicts a schematic cross-sectional side view of a first example of a subsea unit for installation on the seabed. Subsea unit 1-1 comprises a pressure vessel 3 arranged to withstand high ambient sea-water pressure when the subsea unit 1-1 is arranged on the sea ground, and to keep up or maintain a pressure that is closer to atmospheric pressure than to sea-water pressure at the sea ground. With high ambient pressure is meant a pressure of at least 50 bar, as typical installation depths of subsea unit 1-1 are in the range 1000 metres to 3000 metres. The pressure vessel 3 has an enclosure 3- 1 which provides the necessary mechanical strength at the intended operational depth. This may be obtained by manufacturing the pressure vessel 3 from suitable material, e.g. steel, with a properly dimensioned wall thickness of the enclosure 3-1.
The subsea unit 1-1 is arranged to house electric components, forming an electric device such as a frequency converter. Depending on the type of electric device arranged in subsea unit 1-1, power transmission, power distribution and/or power control can be provided on the sea ground, close to power consuming units such as drilling motors, pumps, and compressors operating on sea ground. To that end subsea unit 1-1 comprises at least one first type of electric component 9, and at least one second type of electric component 11. As defined herein, a first type of electric component 9 has a higher loss density than a second type of electric component 11.
Examples of the first type of electric component are power electronic devices such as IGBTs and integrated gate-commutated thyristors (IGCT). Examples of the second type of electric component are a capacitor, an inductor, a bus bar and a printed circuit board assembly (PCBA).
The subsea unit 1-1 further comprises a two-phase cooling system 5, a loop thermosyphon, arranged to provide cooling of first type of electric
components 9. The two-phase cooling system 5 is arranged to circulate a two- phase first cooling fluid F-i, and comprises a vapour conduit 5-1, a liquid conduit 5-2, a first evaporator El arranged in thermal connection with first type of electric components 9, and a condenser C. The vapour conduit 5-1, the liquid conduit 5-2, the first evaporator El and the condenser C together form a closed loop in which the first cooling fluid F-i can circulate to thereby achieve cooling of first type of electric components 9. In order to obtain a thermal connection between the first evaporator Ei and first type of electric components 9, these can be attached to the first evaporator Ei. First type of electric components 9 can for example be mounted on the first evaporator Ei, either directly or via a thermal interface. The first evaporator El and the condenser C are preferably manufactured from material with good thermal conductivity characteristics. The first evaporator Ei is arranged inside the pressure vessel 3, and the condenser C is arranged on the outside of the pressure vessel 3, i.e. in direct contact with sea water when the subsea unit 1-1- is installed on the sea ground, such that the condenser C can transfer heat absorbed from the first cooling fluid F-i flowing through the condenser C to the surrounding sea water. The vapour conduit 5-1 and the liquid conduit 5-2 hence penetrate the enclosure 3-1 of the pressure vessel 3. Beneficially, the vapour conduit 5-1 and the liquid conduit 5-2 have relatively small dimension in order to structurally weaken the pressure vessel 3, which in use is subjected to high mechanical stress by the sea water, as little as possible. According to one variation, the vapour conduit 5-1, the liquid conduit 5-2, the first evaporator Ei and the condenser C are adapted to maintain an internal pressure, in the closed loop which they form, closer to atmospheric pressure than to sea water pressure at sea ground. Typical internal pressures in the closed loop can for example be in the range of about 1 bar to about 20 bar. Hereto, the vapour conduit 5-1, the liquid conduit 5-2, the first evaporator El and the condenser C can for example be manufactured from a metallic material such as steel. Preferably, the condenser C, like the pressure vessel 3, is adapted to withstand the high ambient sea water pressure. The condenser may according to one variation comprise a bundle of conduits with sufficient spacing from each other to avoid or mitigate clogging due to dirt or growth of life. Thereby, the first cooling fluid F-i in vapour state may be distributed between several condenser conduits for more efficient cooling.
According to one variation of the two-phase cooling system 5, the first evaporator Ei has an inlet E1-1 and an outlet E1-2, and the condenser C has an inlet C-i and an outlet C-2. The vapour conduit 5-1 connects the outlet Ei- 2 of the first evaporator Ei with the inlet C-i of the condenser C and the liquid conduit 5-2 connects the outlet C-2 of the condenser C with the inlet Ei-i of the first evaporator El. The vapour conduit, the liquid conduit, the first evaporator and the condenser may either be a single piece closed loop conduit, or the vapour conduit, liquid conduit, the first evaporator and the condenser may alternatively be separate parts that can be assembled to form the closed loop of the two-phase cooling system.
The subsea unit l-i may furthermore comprise a second cooling fluid F-2. The second cooling fluid F-2 is provided in an open space in the pressure vessel 3 outside the closed loop of the two-phase cooling system 5 such that the second cooling fluid F-2 can flow freely therein, and forms an auxiliary cooling system in the pressure vessel arranged to cool the second type of electric components 11. Preferably, the second cooling fluid F-2 moves by natural convection in the open space of the pressure vessel 3. The second cooling fluid may be a gas, oil or an ester. Examples of suitable gases are air or nitrogen at 1 bar pressure. The use of nitrogen instead of air can reduce the risk of corrosion. Other examples of suitable gases are SF6 and similar gases with very high dielectric strength.
According to one variation, if the second cooling fluid is gas, the second cooling fluid is put under moderate pressure, i.e. a pressure of a few bar. By pressurising the second cooling fluid to a pressure higher than atmospheric pressure, a significant reduction of clearance distance in gas insulation may be obtained. This enables a significant increase in compactness and reduction in pressure vessel volume. The pressure of the second cooling fluid may for example be in the range of 3-10 bar. This pressure is still small compared to the ambient sea water pressure, and it should not cause damage to the second type of electric components or the first type of electric components, as e.g. 100 bar or 300 bar would do. Hence, depending on the specific
implementation, the pressure in the pressure vessel 3 is in the range of about 1 bar to about 10 bars. This pressure range is closer to atmospheric pressure than to sea ground pressure at the depth of intended operation of the subsea unit 1-1, where the hydrostatic pressure level is at least 50 bar. According to one variation, the second cooling fluid is a two-phase fluid to thereby increase the cooling performance. The volatility of the second cooling fluid is beneficially chosen such that in an operating state a moderately high pressure of e.g. 2-10 bar results, in order to obtain increased dielectric strength in the vapour state.
The two-phase cooling system 5 is preferably vertically oriented in the subsea unit 1-1 when the subsea unit 1-1 is installed on the seabed to thereby allow gravity-driven circulation of each of the first cooling fluid F-i and the second cooling fluid F-2. Thus, when the subsea unit 1-1 is installed on the seabed, the section of the two-phase cooling system 5 provided with the first evaporator El provides a first cooling fluid flow vertically upwards, and the section of the two-phase cooling system 5 provided with the condenser C provides a cooling fluid flow vertically downwards in the direction of the seabed. Hence, the outlet E1-2 of the first evaporator El is arranged in a horizontal plane lower than the horizontal plane in which the inlet C-i of the condenser C is arranged when the subsea unit 1-1 is installed on the seabed.
According to one variation, each of the vapour conduit 5-1 and the liquid conduit 5-2 has a respective insulating section 7 arranged inside the pressure vessel 3 to electrically insulate the first evaporator El from the pressure vessel 3. Hence, each end of the first evaporator El is connected to an insulating section 7. Thereby, first type of electric components 9, which are arranged in thermal connection with the first evaporator El, may have an electric potential that differs from the electric potential of the enclosure 3-1 of the pressure vessel 3. The operation of the two-phase cooling system 5 will now be described. When the subsea unit 1-1 has been installed on sea ground and the first type of electric component(s) 9 is/are supplied with power, the first type of electric component(s) 9 emit heat necessitating cooling. Since first type of electric component(s) 9 is/are in thermal connection with the first evaporator El, part of the heat emitted by a first type of electric component 9 is absorbed by the first evaporator El. First type of cooling fluid F-i, having been cooled by the condenser C, flows in liquid state in the liquid conduit 5-2 towards the first evaporator Ei and absorbs heat from the first evaporator Ei as it passes through the first evaporator Ei. Due to the heat that has been absorbed from the first evaporator El, the first cooling fluid F-i is vaporised and flows upwards in the vapour conduit 5-1. The portion of the first cooling fluid F-i which is in vapour state then flows towards the condenser C arranged on the outside of the pressure vessel 3. The condenser C which is cooled by sea water absorbs heat from the first cooling fluid F-i flowing through the condenser C in vapour phase, and transmits the heat to the sea water which moves by natural convection outside the subsea unit 1-1. Due to the heat transmitted to the condenser C, the first cooling fluid F-i again enters its liquid state and flows in the liquid conduit 5-2 towards the first evaporator El, and the circuit is repeated. Simultaneously, the second cooling fluid F-2 cools second type of electric component(s) 11 in the pressure vessel 3 by means of natural convection.
A second example of a subsea unit will now be described with reference to Fig. 2. Subsea unit 1-2 of the second example is identical to subsea unit 1-1 of the first example, except that two-phase cooling system 5' of subsea unit 1-2 comprises a second evaporator E2 arranged to receive heat from the second cooling fluid F-2. The second evaporator E2 has an inlet connected to the outlet of the first evaporator Ei and an outlet connected to the inlet of the condenser C. According to such a variation, the second evaporator E2 is arranged downstream of the first evaporator El relative the circulation direction of the first cooling fluid F-i. The two-phase cooling system 5' may furthermore according to one variation comprise a heat sink 13 in fluid communication with the second cooling fluid F-2 and in thermal connection with the second evaporator E2 to transfer heat from the second cooling fluid to the second evaporator E2. In order to obtain thermal connection between the second evaporator E2 and the heat sink 13, the heat sink 13 can for example be attached to the second evaporator E2 via a thermal interface, or alternatively the heat sink 13 and second evaporator E2 can be integrated in one piece. Typically the heat sink is a passive component. The heat sink may optionally have fins so as to further increase its heat absorption area and thus its efficiency.
The second evaporator E2 and the heat sink 13 are preferably so arranged in the pressure vessel 3 that they can absorb heat from the second cooling fluid F-2 after the second cooling fluid F-2 has absorbed heat from the second type of electric component(s) 11. Thus, in operation the second cooling fluid F-2 first flows by the second type of electric component(s) 11 to absorb heat from the second type of electric component(s) 11 and then flows by the heat sink 13 and the second evaporator E2 so as to transfer absorbed heat to the heat sink 13 and the second evaporator E2 to thereby cool the second cooling fluid F-2. In operation, the cooling provided by the two-phase cooling system 5' is essentially identical to that provided by two-phase cooling system 5 of the first example. However, as the second cooling fluid F-2 moves by natural convection in the pressure vessel 3, as noted above, the second cooling fluid F-2 first absorbs heat from the first type of electric component(s) and then transmits the absorbed heat to the heat sink 13 and the second evaporator E2. The second evaporator E2, in turn, transmits this heat to the first cooling fluid F-i, which is then cooled by the condenser C outside the pressure vessel 3· By means of this disclosure, efficient and robust cooling of electric
components having both high heat flux and low heat flux may be provided in a compact manner.
Subsea unit 1-1, 1-2 may beneficially be included in a subsea power system including a plurality of modules/installations such as power transformers, frequency converters and circuit breakers forming part of or forming a complete transmission or distribution network on sea ground.
It is envisaged that the subsea units presented herein find applications within the oil, gas and petrochemical industry, subsea HVDC/HVAC transmission and distribution systems as well as offshore power generation such as wind energy, tidal energy, wave energy, and ocean current energy. The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims

1. A subsea unit (1-1; 1-2) comprising: a pressure vessel (3) arranged to withstand ambient sea water pressure at sea ground of at least 50 bar, and to keep up an internal pressure that is closer to atmospheric pressure than sea water pressure at the sea ground; a first type of electric component (9); and a two-phase cooling system (5; 5') arranged to cool the first type of electric component (9), which two-phase cooling system (5; 5') comprises: a first cooling fluid (F-i), a first evaporator (Ei) arranged within the pressure vessel (3), wherein the first evaporator (3) is arranged in thermal connection with the first type of electric component (9), a condenser (C) arranged outside the pressure vessel (3) to allow the condenser (C) to be in thermal connection with sea water, a vapour conduit (5-1) arranged to transfer first cooling fluid (F-i) from the first evaporator (El) towards the condenser (C), and a liquid conduit (5-2) arranged to transfer first cooling fluid (F-i) from the condenser (C) towards the first evaporator (El), wherein the first evaporator (El), the condenser (C), the vapour conduit (5-1) and the liquid conduit (5-2) form a closed loop for circulating the first cooling fluid (F-i) in the two-phase cooling system (5; 5').
2. The subsea unit (1-1; 1-2) as claimed in claim 1, wherein the pressure vessel (3) comprises: a second type of electric component (11), and a second cooling fluid (F-2) outside the closed loop of the two-phase cooling system (5; 5'), forming an auxiliary cooling system in the pressure vessel (3), which second cooling fluid (F-2) is arranged to cool the second type of electric component (11), and which second cooling fluid (F-2) has a dielectric strength that is sufficiently high to electrically insulate the second type of electric component (11) from its surrounding.
3. The subsea unit (1-2) as claimed in claim 2, wherein the two-phase cooling system (5') comprises a second evaporator (E2) forming part of the closed loop of the two-phase cooling system (5'), wherein the second evaporator (E2) is arranged to receive heat from the second cooling fluid (F- 2).
4. The subsea unit (1-2) as claimed in claim 3, comprising a heat sink (13) arranged in thermal connection with the second evaporator (E2) and in fluid communication with the second cooling fluid (F-2) to transfer heat from the second cooling fluid (F-2) to the second evaporator (E2).
5. The subsea unit (1-1; 1-2) as claimed in any of claims 2 to 4, wherein cooling of the second type of electric component (11) is provided by natural convection of the second cooling fluid (F-2) in the pressure vessel (3).
6. The subsea unit (1-1; 1-2) as claimed in any of the preceding claims, wherein inside the pressure vessel both the vapour conduit (5-1) and the liquid conduit (5-2) have an insulating section (7) to electrically insulate the first evaporator (El) from the pressure vessel (3).
7. The subsea unit (1-1; 1-2) as claimed in any of claims 2-6, wherein the second cooling fluid (F-2) is a gas.
8. The subsea unit (1-1; 1-2) as claimed in claim 7, wherein the second cooling fluid (F-2) is pressurised to a pressure higher than atmospheric pressure.
9. The subsea unit (1-1; 1-2) as claimed in claim 8, wherein the pressure is in the range of 3-10 bar.
10. The subsea unit (1-1; 1-2) as claimed in any of claims 2-6, wherein the second cooling fluid (F-2) is an oil or ester.
11. The subsea unit (1-1; 1-2) as claimed in any of claims 2-10, wherein the first type of electric component (9) and the second type of electric component (11) form part of a frequency converter.
12. The subsea unit (1-1; 1-2) as claimed in any of the preceding claims, wherein the first type of electric component (9) is attached to the first evaporator (El).
13. A subsea power system comprising a subsea unit (1-1; 1-2) as claimed in any of claims 1-12.
14. The subsea power system as claimed in claim 13, wherein the subsea power system is a transmission system.
15. The subsea power system as claimed in claim 13, wherein the subsea power system is a distribution system.
PCT/EP2012/072211 2012-11-09 2012-11-09 Subsea unit comprising a two-phase cooling system and a subsea power system comprising such a subsea unit WO2014071985A1 (en)

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