Aspects of the invention relate to a Gas-Insulated Switchgear (GIS) comprising a conductor arrangement insulated by a dielectric insulation gas. In particular, aspects of the invention relate to Gas-Insulated Switchgear having a cooling system for cooling a conductor section of the conductor arrangement by a spray comprising liquid droplets of a cooling liquid suspended in the dielectric insulation gas. Further aspects relate to a method of cooling in the gas-insulated switchgear by the cooling system, and to a use of the cooling system.

Technical background:

Gas-Insulated Switchgear such as circuit breakers, busbars, and gas-insulated conducting lines have a conductor arrangement encapsulated by a housing containing a dielectric gas such as SF ₆ . This dielectric gas surrounds the conductor arrangement and allows medium voltage to be applied to the conductor arrangement without dielectric breakdown between the conductor arrangement and grounded parts such as the casing. A typical rated voltage of a medium voltage gas-insulated switchgear is at least 1 kV and at most 52 kV.

There is a need for GIS having larger power ratings and smaller footprints. Due to increased requirements both on voltage levels - increasing the insulation performance requirements - and on current ratings - increasing the resistive heat and therefore the cooling requirements - the demands on the used cooling and insulation media are becoming more severe.

A well-known highly effective cooling method is evaporative two-phase cooling. In this concept, a refrigerant boils in contact with the part to be cooled, thereby removing heat at a high rate per refrigerant mass due to the high latent heat of evaporation. However, the industry has been reluctant to use evaporative cooling in gas-insulated switchgear because of its unknown and potentially negative influence on electric breakdown strength. There has been a concern that evaporation cooling may negatively affect the dielectric performance. In particular, for evaporative cooling it would be necessary to bring a liquid phase of the refrigerant close to the conductor arrangement. However, the dielectric strength of the liquid phase is different from that of the gas phase, and the dielectric strength of a mixture of the two is poorly known and potentially insufficient. In addition, heat pipes are considered to be effective for cooling of hot spots, but heat pipes present additional components and assembly costs. Furthermore, heat pipe elements are typically electrically conductive and therefore have been considered to be incompatible with the dielectric needs of gas-insulated switchgear. Therefore, such two-phase cooling methods have not been in use in GIS.

Another recent trend in GIS has been a push to replace SF ₆ by alternative dielectric gases. SF ₆ has very good electric insulation properties and is also a good medium for gas cooling. However, its very high global warming potential has been driving the search for more eco-friendly alternatives. Therefore, for example, EP 2443632, US 8704095 and US 2013/0277334 describe gas-insulated switchgear having a dielectric insulation medium comprising fluoroketone as one gas component.

In particular, US 2013/0277334 addresses the problem that the insulation performance of the fluoroketone may be limited due to the relatively high boiling points of the fluoroketone, especially in a low temperature environment. In this case, only a relatively low saturated vapour pressure of the fluoroketone can be maintained without fluoroketone becoming liquefied. Therefore, US 2013/0277334 suggests a fluid management system comprising a heater and/or vaporizer in order to control the vapour pressure of the insulation medium components. This fluid management system is not a cooling system, but to the contrary is most useful for counter- acting the adverse effect of loss of vapor pressure in low temperature environments down to about -20° C, and works best in combination with a heater. The spraying nozzles are to be used in cold conditions where no cooling is necessary, not in hot conditions where the problem of loss of vapor pressure does not occur in the first place. In addition, the spraying nozzles described in US 2013/0277334 do not generate a spray directed at a conductor section, but are generally spraying an insulation medium into the apparatus, but not directed specifically at any element within the apparatus. Thus, this document does not address any problem related to cooling.

Thus, there exists a need in medium voltage gas-insulated switchgear for effective cooling of at least parts of the conductor assembly, of improving the electric insulation, and of using an eco-friendly insulation gas.

Summary of the invention

An object of the present invention is to address at least some of the above-mentioned issues at least partially. In view of the above, a gas-insulated switchgear according to claim 1, a method according to claim 21, and a use according to claim 25 are provided. Embodiments are evident from dependent claims and claim combinations.

An advantage is that by generating a spray of a cooling liquid, evaporative cooling of the conductor section is enabled due to evaporation of the liquid droplets contained in the spray. This cooling is particularly effective, because the spray is directed at or towards or immediately towards the conductor section. At the same time, a spray is suspended in the dielectric insulation gas as a two-phase mixture. Therefore, the dielectric strength is not affected negatively. Therefore, in an embodiment, the cooling system fulfils both thermal (cooling) and dielectric (insulation) functions at the same time. In other words, the cooling system providing liquid droplets to be deposited on the to-be-cooled conductor or conductor section and to be evaporated thereon is compatible and cooperative with the dielectric insulation gas present in the gas-insulated switchgear for the purpose of providing dielectric insulation. In yet other words, the liquid droplets are for cooling whereas the dielectric insulation gas is for dielectric insulation, and thus cooling droplets and dielectric insulation gas are functionally complementary with each other.

Further advantages, features, aspects and details that can be combined with embodiments described herein are evident from the dependent claims, claim combinations, the description and the drawings.

In particular, the invention relates to a method of claim 21, with embodiments as claimed in claims 22-24 or corresponding to any of the embodiments of the gas-insulated switchgear. Brief description of the Figures:

The details will be described in the following with reference to the figures, wherein

Fig. 1 is a schematic cross-sectional view of a GIS according to a first embodiment of the invention;

Fig. 2 is a schematic cross-sectional view of a GIS according to a second embodiment of the invention;

Fig. 3 is a graph showing a measured temperature difference between a hot part and the cooling fluid as a function of heater power, for various cooling methods;

Fig. 4 is a graph showing a measured heat transfer coefficient as a function of the total heat removed, for various cooling methods;

Fig. 5a - Fig. 5d are schematic cross-sectional views of busbars arranged at various angles with respect to the cooling system according to embodiments of the invention;

Fig. 6 is a graph showing a measured temperature difference between heater and bulk fluid as a function of the angle between a nozzle spray direction and a heater surface; and

Fig. 7 is a graph showing a measured dielectric strength of the insulation gas including suspended droplets of cooling liquid, as a function of a local heating power.

Detailed description of the Figures and of embodiments:

Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with any other embodiment to yield yet a further embodiment. It is intended that the present disclosure includes such modifications and variations.

Within the following description of the drawings, the same reference numbers refer to the same or to similar components. Generally, only the differences with respect to the individual embodiments are described. Unless specified otherwise, the description of a part or aspect in one embodiment can be applied to a corresponding part or aspect in another embodiment as well.

Fig. 1 is a schematic cross-sectional view of a GIS according to a first embodiment of the invention. The GIS 1 comprises a housing 2 defining a housing volume 4. The GIS 1 further comprises conductor arrangement 10 (here: a busbar; also switch, in particular disconnector or earthing switch or interrupter) arranged within the housing volume 4 and extending in a length direction orthogonal to the image plane of Fig. 1.

For dielectrically insulating the conductor arrangement 10, the housing volume 4 is filled with a dielectric insulation gas. The pressure of the dielectric insulation gas depends on the gas and on the dielectric requirements; typical pressures range from 1 bar to 7 bar. The conductor arrangement 10 develops heat due to the electric current carried by the conductor arrangement 10 during operation. Therefore, at least a portion of the conductor arrangement 10 is cooled. The portion to be cooled, which may be the entire conductor arrangement 10 or a sub-part of it, is herein referred to as the conductor section 12.

For cooling the conductor section 12, the GIS 1 comprises a cooling system 20. The cooling system 20 has a spray generator 22 with a spray nozzle 24. The spray nozzle 24 generates a spray 30 of cooling liquid directed at the conductor section 12. The spray 30, described in further detail below, is constituted from liquid droplets of the cooling liquid suspended in the dielectric insulation gas as a two-phase mixture.

The cooling system further comprises a cooling liquid reservoir 32 at the bottom of the housing 2, and a cooling liquid supply line 26 connecting the cooling liquid reservoir 32 to the spray nozzle 24. Further, a pump 28 and a valve system 27 are arranged in the cooling liquid supply line 26. The valve system 27 may include a separate valve for each nozzle 24 as shown in Fig. 1, or a single valve for several or all nozzles as shown in Fig. 2.

The cooling liquid provided from the cooling liquid reservoir 32 is pressed along the cooling liquid supply line 26 by the pump 28 towards the nozzles 24. The nozzle 24, described in further detail below, generates from the cooling liquid a fine spray cone, the spray having droplets of the cooling liquid, preferably with a diameter of 1 μιη to 1 mm.

The nozzle 24 is directed such that the spray cone of the spray 30 hits the conductor section 12. The liquid droplets evaporate on the hot conductor section 12 and thereby absorb heat from the conductor section 12 in an efficient manner. After the cooling liquid has evaporated, the vaporized cooling fluid flows to the housing walls by natural convection and re-condenses there. The condensed liquid flows along the compartment walls to the cooling liquid reservoir 32 at the bottom of the housing 2. Thereby, the cycle for the cooling liquid is completed.

The cooling liquid reservoir 32 at the bottom of the housing 2 may have a volume of about 3 1 (liters), for example. The bottom wall may be slightly tilted towards the liquid collection.

The cooling system 20 shown in Fig. 1 thus allows the conductor sections 12 to be cooled by evaporating spray droplets created by the nozzles 24. The heat is absorbed very efficiently due to evaporation of the liquid droplets, and is then transported to the compartment walls by a flow pattern established within the volume.

Typically, at the conductor 10 there will be locations either locally generating especially high heat fluxes (e.g. contact resistances) or that are thermally more insulated than other components (e.g. bushings cast in insulating material). Such configurations may result in hot spots. Therefore, preferably, such hot spots are designated as the conductor section 12 to be cooled, and in embodiments the spray generator 22 is especially directed to such hot spots.

In embodiments, further equipment includes a temperature sensor 62, a pressure sensor 64, a cooling liquid level sensor 66, and a dielectric gas supply valve 6. A control system (not shown) may operate the supply of the cooling liquid (via operation of the valves 27 and pump 28) and/or the supply of the dielectric gas (via supply valve 6) in dependence of the readout data obtained from the temperature sensor 62, the pressure sensor 64, and/or the cooling liquid level sensor 66. Alternatively, the valves and their control can be omitted, e.g. in case the nozzles 24 are in constant operation. The cooling liquid may be the liquid phase of the dielectric insulation gas or of a dielectric insulation gas component. The vapor and liquid phase of the cooling liquid may thus mix with other (in particular non-condensing) dielectric insulation gas components that ensure sufficient insulation at a wide range of temperatures, and in particular at low temperature conditions, when most of the cooling fluid may be condensed. Examples for the dielectric gas and the cooling fluid are discussed further below.

Fig. 2 shows another embodiment of a GIS 1 being a busbar compartment. Only differences with respect to Fig. 1 are described. In the busbar compartment 1 of Fig. 2, thee conductors (busbars) 10, one for each phase, are arranged within the housing volume 4. For each busbar 10, the cooling system 20 has a respective spray generator 22 with a spray nozzle 24 directed to a conductor section (hot part) 12 of the respective busbar 10.

Fig. 2 shows that each of the three spray generators is arranged at a height above the height of the conductor section and is directed downwardly towards the conductor section. In particular, the spray generator is arranged such that droplets or all droplets contained in the spray are directed onto the conductor section by combination of the outlet velocity from the spray generator 22 and gravity.

In embodiments of Fig. 2, the middle spray generator is arranged vertically above the middle conductor section, whereas the other two spray generators are horizontally shifted with respect to the respective conductor section, but are still directed downwardly towards that conductor section (such the respective conductor sections are within the spray cone of the spray generators).

In complete generality for the gas-insulated switchgear 1 as described herein, the spray generator 22 for generating the spray 30 is preferably arranged to emit liquid droplets directly towards the conductor section 12; and/or liquid droplets, in particular all liquid droplets or the whole spray 30, fall or falls onto the conductor section 12 for being at least partly, in particular completely, evaporated thereon under operating conditions of the gas-insulated switchgear 1.

Thus in the gas-insulated switchgear as described herein, the spray generator 22 is designed to emit liquid droplets of a suitable droplet diameter range and in a suitable droplet emission pressure range and is arranged in a suitable distance range from the conductor section 12, such that liquid droplets, in particular all liquid droplets or the whole spray 30, fall or falls onto the conductor section 12 for being evaporated thereon under operating conditions of the gas- insulated switchgear 1.

In the gas-insulated switchgear 1, droplets of the cooling liquid that are suspended in the dielectric insulation gas can contribute to the dielectric insulation inside the gas-insulated switchgear 1, whereas droplets that aggregate on the conductor section 12 do not contribute to the dielectric insulation inside the gas-insulated switchgear.

The GIS 1 of Fig. 2 further comprises a condenser system 40 for improving thermal takeout (dissipation) through the housing walls. The condenser system 40 has a cooler adapted for cooling the insulation gas to a temperature at which at least the evaporated cooling fluid (which is a gas component of the dielectric gas) is cooled so that it condenses into a liquid phase.

The cooler of the condenser system 40 may, for example, include passive elements such as a finned cooling surface at the top of the compartment, having fins at the inside and/or outside of the housing walls, as indicated by dashed lines on the top of the housing 2. Further, an active cooling element, e.g. for generating a forced airflow outside the housing 2 along the condenser system 40, may also be provided (not shown). In addition or alternatively, the condenser system 40 may include additional elements, such as water-cooling pads 44 or a two-phase thermosyphon arranged, e.g., on the side walls of the housing 2. The vaporized cooling fluid may flow, by natural convection (as indicated by arrows 42) to the condenser system 40 and re- condense there. The condenser system 40 may further comprise vertical natural convection channels, possibly enhanced by vertical fins, arranged on the side walls. The condensed liquid then flows along the compartment walls to the cooling liquid reservoir 32 at the bottom of the housing 2.

The cooling system 20 of Figs. 1 and 2 allows evaporation-condensation heat transfer from the busbars or other heat sources with much higher thermal efficiency than the previously known, merely convective cooling of the conductors. Additionally, while in natural gas cooling the heat rejection from the compartment is taking place primarily in the upper area of the compartment, efficient condensation can take place at any surface of the compartment, as long as the temperature of the compartment walls is below the saturation conditions in the compartment, for example also on the side walls or at the bottom of the compartment.

In the following, we describe further possible general aspects, embodiments and variations according to the invention. These aspects / embodiments are optional. Each of these aspects or embodiments can be implemented in combination with any other embodiment, and/or in combination with any other aspect described herein.

Background gas and cooling fluid

First, general aspects regarding the background gas and the cooling fluid are described.

According to an aspect, the cooling fluid is at least a gas component of the background gas. The background gas may be the same as the cooling fluid or it may have additional gas components. The additional gas components may include at least one of air, one or more air components such as C0 ₂ , O2 and/or N ₂ , SF ₆ , and a fluoroketone.

According to an aspect of this invention, the GIS may be filled with a fluoroketone being the background gas. The fluoroketone may be C ₆ Fi20, also referred to as "C6" herein. Other gases, such as air or CO2, can further be added to the background gas to increase the total pressure and thereby the dielectric performance. A typical commercially available background gas or background gas component is Novec 649.

The C6 (Novec 649) may also be used as the cooling fluid, so that the spray generator sprays a spray of C6 onto the conductor section. C6 spray is characterized by small liquid droplets which are suspended in the background gas (that may also contain gaseous C6). C6 offers an excellent balance between operating pressure, dielectric and heat transfer performance.

Cooling by two-phase C6 spray in a C6-containing background gas has the following advantages: C6 is very eco-friendly with a global warming potential of 1. C6 droplets evaporate in direct contact with hot surfaces, or while approaching them, which results in a very efficient cooling of the hot parts. The heat transfer coefficient is about an order of magnitude larger than the one of C6 gas without spray, or the one of SF ₆ . This results in significantly lower temperatures of the parts to be cooled, such as busbars. The dielectric performance of this configuration with cooling by spray is superior to the one of similar gas mixtures without spray, over a large range of operational parameters.

Other dielectric fluids can be also considered in addition to or instead of C6, both regarding the background gas and the cooling fluid. For example, the background gas and/or the cooling fluid may contain C5-type fluoroketone (i.e. fluoroketone containing 5 carbon atoms). Generally a cooling fluid is preferred whose saturation curve is such that within the full range of operating temperature of the switchgear a reasonable total operating pressure of the GIS is obtained. A reasonable total operating pressure is, generally, in the range from a few mbar to a few bar, for example 5 mbar to 10 bar. The total pressure results from the partial pressure of the cooling fluid together with the partial pressure of background gas.

Thus, according to an aspect of the invention, the insulation gas has a global warming potential lower than the one of SF6 over an interval of 100 years, and optionally comprises at least one gas component selected from the group consisting of C02, 02, N2, H2, air, N20, a hydrocarbon, in particular CH4, a perfluorinated or partially hydrogenated organof uorine compound, and mixtures thereof, such as a fluoroketone for example having at least 4 C atoms and at most 6 C atoms.

According to a further aspect, the cooling liquid comprises at least one gas component of the insulation gas in liquid phase.

According to a further aspect, the insulation gas comprises at least one further gas component, which is not condensable within an operating temperature range.

According to a further aspect, the cooling liquid has a boiling temperature below a maximum operational temperature of the conductor section, so that at least some of the liquid droplets evaporate upon contact with the conductor section. Spray generator

Next, general aspects regarding the spray generator are discussed.

The spray generator generates a spray of cooling liquid directed at the conductor section. The spray is constituted from liquid droplets of the cooling liquid suspended in the dielectric insulation gas as a two-phase mixture, and is characterized in more detail further below.

The spray generator may be a spray nozzle. An exemplary embodiment of a spray nozzle is a PJ type low flow nozzles, manufactured by BETE Fog Nozzle, Inc. This type of nozzle is being used in industrial processes, fire extinguishing applications, and the like. The nozzle can contain a plate and a liquid exit line, shaped as a U and having a small exit opening directed against the plate. By directing the cooling liquid through the small exit opening against the plate, the nozzle generates from the cooling liquid a fine spray cone, the spray having droplets of the cooling liquid with a diameter of 1 μιη to 1 mm. The diameter can be adjusted by adjusting the size and orientation of the exit opening relative to the plate, as well as the pressure of the cooling liquid. Other nozzle designs are available, as well. According to further embodiments, a distance of the spray generator from the conductor section to be cooled is chosen between 1 cm and 50 cm, preferably between 2 cm and 20 cm, more preferably between 5 cm and 10 cm. This distance ensures an adequate cooling and dielectric breakdown strength. To maximize the dielectric strength, the spray generator is designed to avoid sharp edges. The spray generator may also be recessed behind a dielectric shielding structure, for example by being embedded into the compartment wall surface. Also, the spray generator may be arranged so that it does not protrude further into the volume than other geometrical features of the compartment, such as cooling fins. In other words, at least one of the spray generators 22 is recessed behind a dielectric shielding structure, and/or at least one of the spray generators 22 is embedded into a wall surface of the housing 2, and/or at least one of the spray generators 22 does not protrude further into the housing volume 4 than other geometrical features of the housing 2, such as cooling fins (see e.g. Fig. 2).

Instead of a nozzle, the spray generator may also be realized by any other device generating small liquid suspended droplets, such as an nebulizer, in particular an ultrasonic nebulizer. According to an aspect, a plurality of spray generators may be provided. For example, the number of spray generators may be at least one, at least two, at least three or at least six spray generators. An upper limit may be 20 or 10 spray generators. The spray generators may be arranged in an array- like manner, e.g. as an impinging jet array.

Alternatively, there might be only one single spray generator, e.g. for thermally controlling a single hot spot.

The spray generators may be all the same or may be different, e.g. for producing different sizes and/or concentrations of droplets. For example, a first spray generator may be adapted for a larger droplet size optimized for hot-spot cooling, while a second spray generator may be adapted for a smaller droplet size optimized for improving insulation in an electrically critical area.

According to an alternative aspect, the spray generator can be a nebulizer, such as an ultrasonic (e.g. piezo-electrically actuated) nebulizer adapted to create the two-phase mixture. Optionally, a fan may be provided for distributing the spray (nebulized two-phase mixture) to the conductor section to be cooled.

In case of the spray generator being a nozzle, a pump is provided for pressurizing the cooling liquid supplied to the nozzle. The pump may pressurize the cooling liquid to a pressure of, for example, 1.1 bar to 15 bar, preferably 3 bar to 10 bar. The pumping requirement for supply of the dielectric fluid is proportional to the product of the pressure before the nozzle and the volume flow rate. For a nozzle pressure of 7 bars, the power is about 1 W per each 0.1 liter/minute. A system with six such nozzles and a small membrane pump with total efficiency of 50% would thus require a power of about 12 W during operation.

Generally, the spray generator may include additional devices for generating, distributing and/or transporting the spray to the conductor section. For example, the spray generator may comprise a fan and/or a flow deflector for directing the flow of the spray. Spray generator arrangement

Next, general aspects regarding the positioning of the spray generator are discussed.

According to an aspect, the spray generator defines a spray cone, i.e. a region emanating from the spray generator, the region being defined by the spray generator aperture and direction as the region within which the spray is directed. The spray cone does not need to be exactly cone shaped but may for example have the shape of an (elliptic) cone. The spray cone generally diverges from the spray generator in an angular manner. According to an aspect, the spray generator is directed such that the conductor section is within the spray cone (intersects with the spray cone).

According to a further aspect, the spray generator defines a center axis, such that the spray generator directs the spray on average along the center axis. The center axis may be the axis of the spray cone mentioned above. According to an aspect, the center axis is directed towards the conductor section (intersects with the conductor section). According to a further aspect, the center axis is directed towards the conductor section within a spatial angle of 10°, preferably of 5° (i.e. a virtual cone about the center axis, defined by the above maximum angle from the center axis, intersects with the conductor section).

According to a further aspect, the spray generator is arranged at a distance of at most 50 cm, preferably at most 30 cm, more preferably at most 20 cm, and more preferably at most 15 cm from the conductor section, for obtaining an improved cooling efficiency. According to a further aspect, the spray generator is arranged at a distance of at least 1 cm, preferably at least 2 cm from the conductor section, for obtaining a sufficient dielectric strength.

According to a further aspect, the spray generator is directed to the conductor section from such a distance that at least 20 wt%, preferably at least 30 wt% of the spray flow rate reaches the region of the conductor section in a liquid state so that it is evaporated by the heat of the conductor section and contributes to cooling the conductor section. According to an aspect, at least 10 wt%, preferably at least 20 wt% of the spray flow rate impinge in a liquid state on the conductor section to be evaporated from the conductor section. According to a further aspect, the spray generator is directed to the conductor section from such a distance that the spray impinges in a liquid state on the conductor section and generates a liquid film on at least a part of the conductor section, in particular a continuous liquid film covering the conductor section, during operation of the gas-insulated switchgear, in particular when the spray generator is operative.

According to a further aspect, the spray generator is arranged at a height above the height of the conductor section. Preferably, the spray generator is arranged vertically above the conductor section. According to an aspect, the spray generator is directed downwardly towards the conductor section.

According to a further aspect, the spray generator is recessed behind a dielectric shielding structure.

In case of a plurality of spray generators, the aspects described herein may be applicable for at least one of the spray generators, and preferably for each of the spray generators, and for the conductor section or a respective portion thereof. Controller:

According to a further aspect, the switchgear has a controller configured for sensing a temperature (e.g. indicative of the conductor section temperature), and for activating the cooling system, if the temperature exceeds a predetermined threshold. The threshold may depend on further parameters such as pressure, transmitted power, or the like.

Operation parameters:

Next, operation parameters for the cooling system are discussed. Typical conditions in medium voltage switchgear result in thermal losses generated by different hot elements (e.g. busbars) at densities of about 1 kW/m ³ to 10 kW/m ³ . The cooling system 20 and the condenser system 40 are adapted for transferring this amount of heat flux from the conductor section 12 to and across the housing walls of the GIS.

According to an embodiment, the amount of cooling liquid supplied to the spray generator can be from about 0.02 liter/minute to about 5 liter/minute per spray generator (nozzle). Typical jet nozzles such as the PJ type low flow nozzles deliver these amounts at liquid pumping pressures ranging from 2 bar to 10 bar, or even higher flow rates at higher pressure.

In an exemplary configuration, C6-fluorketorne (Novec 649) is used as the cooling fluid. This cooling fluid has a density of about 1.600 kg/m ³ ; and a heat of vaporization of about 88 kJ/kg. Considering these properties, an evaporation of cooling liquid supplied at 0.1 liter/minute absorbs up to 235 W. For a compartment with total internal losses of 1.5 kW, an advantageous flow rate is then about 0.64 1/min, or about 0.6 1/min. Using the jet nozzles as described above, the flow of about 0.6 1/min may for example be delivered by 6 nozzles, each delivering 0.1 1/min of cooling liquid.

The position of the nozzles may be in the compartment such as to face the conductor sections to be cooled, for example, above the busbars and one nozzle near each of the six bushings of the busbars.

Using C5-fluorketorne as the cooling fluid (density 1550 kg/m ³ , latent heat 96 kJ/kg), one obtains for the same cooling configuration a slightly lower mass flow rate, still in the ballpark of 0.6 1/min. Other fluids with similar properties, such as HFE-7100, can be considered as well and give similar flow rates.

In practice, the flow rate may be chosen a little higher than the above estimates, as not all the spray may intercept the cooled parts, so that some of the liquid will not evaporate and will pass through the system in liquid form only. Alternatively, lower flow rates than what would correspond to the total compartment cooling requirements can also be applied, in particular in cases e.g. in which the target of the spray cooling is a singular spatially limited critical hot spot.

Thus, according to an embodiment, a cooling power of 100 W to 300 W per spray generator is provided. According to another embodiment, the cooling liquid is supplied to the spray generator at a flow rate of 0.02 1/min to 1 1/min per spray generator, preferably at a flow rate of at least 0.05 1/min and/or at most 0.5 1/min per spray generator. The piping to the spray generator is dimensioned such that the required flow can easily pass at moderate speed and pressure drop. For the flow rates stated above, a distribution tubing of 5 mm inner diameter is suitable to supply cooling liquid at the above-mentioned rates to 6 nozzles. In a comparative example, heat is removed by passive heat transfer across the housing wall (by passive natural gas cooling on both sides of the wall). For this heat removal, a typical overall heat transfer coefficient is 20-50 W/m ² K. For cooling through the top of the housing having a surface area of 0.5 m ² , with fins effectively tripling the effective heat exchanger surface area to 1.5 m ² , removal of 1500 W thus requires a large temperature difference across the housing wall of about 20-40 K.

According to an embodiment of the invention, the heat transfer can be improved by condensation of the cooling fluid at the inner side of the compartment. Due to the condensation, the heat transfer coefficient is increased to about 200-500 W/m ² K (the lower limit depending on the amount of non-condensable gas which limits the cooling efficiency). Thus, removal of 1500 W is possible for a temperature difference across the housing wall of only a few K, even using smooth internal surface without fins. Because also the side walls are available for heat transfer, even without fins an effective condensation area of 1-3 m ² can be assumed. The removal of the heat from the outside of the housing would usually present a larger thermal resistance similar to the comparative example. Therefore, according to an embodiment, the outside surface of the housing is enlarged, e.g. by fins, vertical walls or similar passive cooling structures, resulting in lower thermal resistance. As no particular dielectric requirements are imposed on the outside of the walls, there are no practical dielectric limitations to such cooling structures. According to a further aspect, an active or passive cooling device such as a fan or a thermosyphon are provided to the outside surface of the housing, for removing heat even more efficiently.

According to an aspect, the cooling system may be controlled, such as to be activated only at high thermal load situations, e.g. in response to a current passing through the conductor section exceeding a predetermined threshold, and/or a temperature measured by a temperature sensor exceeding a predetermined threshold.

The operating temperature range of this design is from -15° C to about 60° C, depending on the fluid selection and the pressure limits of the application. The temperatures reached at normal operation are generally lowered by the highly improved cooling. Characterization of the spray

In the following, general aspects characterizing the spray are discussed. The spray may also be referred to as mist, fog, or droplet suspension, all of these terms denoting a mixture of liquid droplets suspended in the gas phase, and being used interchangeably herein. The spray is constituted from liquid droplets of the cooling liquid suspended in the dielectric insulation gas as a two-phase mixture.

The spray may be created by flow rates of cooling liquid of about 0.01 liter/min to 1 liter/min per spray generator, preferably of at least 0.02 liter/min or 0.05 liter/min per spray generator. The spray rate is preferably at most 0.5 liter/min or 0.3 liter/min per spray generator.

The liquid droplets of the spray have in embodiments an average droplet diameter of at least 1 μιη and of at most 1 mm. In particular, the minimum average droplet diameter is at least 3 μιη, preferably at least 5 μιη, and more preferably at least 10 μιη. Further in particular, the maximum average droplet diameter is at most 500 μιη, preferably at most 300 μιη, more preferably at most 100 μιη, and most preferably at most 50 μιη. The average droplet diameter can be determined, for example, by using a Phase Doppler Particle Analyzer (PDPA) or similar system. Specifically, the droplet diameter may be measured by a PDPA at a laser wavelength of 532 nm, at a distance of 1 cm from the spray generator's orifice.

As additional tradeoff, the size of droplets affects the balance between dielectric and thermal performance. Larger generated droplets intercept the cooled surface with higher kinetic energy and are therefore more readily brought into direct contact with the cooled surface. On the other hand, smaller droplets are typically more favorable from the dielectric enhancement perspective, but due to their low mass they typically evaporate before reaching the surface and their flow is more difficult to control. In practice, droplet sizes of the order of tens of micrometers, with a maximum performance at 40 μιη, were found suitable from the perspectives of cooling performance, flow control and dielectric properties.

According to a further aspect, the spray has an electrical breakdown strength of at least 4 kV/mm, and in particular at least 5 kV/mm and more preferably at least 6 kV/mm. Unless stated otherwise, all properties of the spray are defined at 10 °C.

Aspects related to the cooling liquid circulation and cooling liquid reservoir

According to an aspect, the cooling system comprises a cooling liquid reservoir, e.g. at the bottom of the housing, and a cooling liquid supply line connecting the cooling liquid reservoir to the spray generator. The cooling system may further comprise a condenser system having a cooler adapted for cooling the dielectric gas to a temperature, at which the evaporated cooling liquid re-condenses to a liquid phase. In particular, recondensation occurs in a lower part of bottom part of the gas-insulated switchgear.

In embodiments, the cooler may have an enhanced cooling structure allowing for enhanced heat transfer when compared to the housing wall of the housing. The enhanced cooling structure may comprise passive cooling elements, such as cooling fins, a thermosiphon and/or a heat exchanger, or an active cooler element. For example, the inner housing wall surface of the housing may comprise a finned cooling surface and/or a convection channel for the cooling liquid.

In embodiments, the spray 30 comprises at least one component, which is essentially liquid within an operating temperature range of the gas-insulated switchgear, or which is essentially liquid within an operating temperature range of the dielectric insulation gas inside the gas- insulated switchgear under operating conditions of the gas-insulated switchgear. Essentially liquid shall mean that the component is liquid with a low vapour pressure (of e.g. some mbar or some 10 mbar) at operating conditions, in particular at operating temperatures, of the gas- insulated switchgear. Exemplary components are: fluoroketones having more than 6 carbon atoms, such as, e.g., C7-FK or C8-FK or other; or HFE 7500. Liquid components provide cooling by the liquid having a higher heat capacity than the dielectric insulation gas surrounding the conductor arrangement 10 or conductor section 12. Aspects related to the takeout of heat from the housing by the cooling system

According to embodiments, an inner housing wall surface, e.g. at the top and/or the side of the housing, is adapted for the cooling fluid to condense upon contact with the inner wall. The inner housing wall surface may comprise a condensation-enhancing structure, such as a finned cooling surface and/or a convection channel. The inner housing wall surface may comprise a condensate channel for guiding the condensate towards a cooling liquid reservoir located at a bottom portion of the housing.

According to embodiments, an outer housing wall surface, e.g. at the top and/or the side of the housing, has a cooling structure for enhancing heat removal (relative to an ordinary smooth outer wall surface). The cooling structure may comprise surface-enlarging elements, such as fins or projecting walls. According to further embodiments, the cooling structure may comprise an external two-phase cooling system, such as a thermosyphon. According to further embodiments, the cooling structure may comprise an active cooling device, such as a device for generating a forced coolant flow, e.g. a fan or a water-pipe cooling system.

Measurements relating to the cooling efficiency

Fig. 3 is a graph showing a measured difference between a temperature Tb of a hot part and the cooling fluid temperature 7 , as a function of heater power of a heater heating the hot part, for various cooling methods. A lower temperature difference thus indicates that the cooling method is more efficient.

More specifically, the following dielectric gases and cooling methods are shown: a mixture of C6 and air with additional spray cooling using C6 as cooling fluid according to the invention (circles connected with dashed lines), and the following comparative examples: a mixture of C6 and air without spray cooling (rectangles connected by solid lines); SF ₆ without spray cooling (circles connected by solid lines); and air without spray cooling (triangles connected by solid lines). For each dielectric gas and cooling method, two measurements were conducted using two different pressures or partial air pressures, as indicated in the legend of Fig. 3.

As can be seen in Fig. 3, the embodiment of the invention, with spray cooling using C6 (dashed lines) show a very low temperature rise Tb - 7/ of the heated part relative to the cooling fluid, approximately 10 times lower than the comparative examples (solid lines). This low temperature rise indicates the superior cooling efficiency of the spray cooling relative to the comparative examples without spray cooling.

Fig. 4 is a graph showing a measured heat transfer coefficient as a function of the total heat removed, for various cooling methods. For the measurement of Figure 4, the conductor portion to be cooled was a vertically oriented busbar- shaped metal piece. Heating power was applied to the conductor portion in order to simulate the effect of the conductor being heated by an electrical current. The conductor portion was cooled by a nozzle situated above the conductor portion and operated with a flow rate of cooling liquid of about 0.1 1/min. Here, the solid circles show a comparative example without spray cooling ("nozzle off), whereas the other measurement points show measurements with spray cooling according to the present invention, for various background gas pressures.

As can be seen in example of Figure 4, the heat transfer coefficient by spray cooling at a given flow rate drops with increasing local heat generated at the conductor section. This is because the delivered amount of cooling liquid was kept constant (at about 0.1 1/min); and this fixed amount of cooling liquid is constraining the heat transfer performance for cooling the conductor section. Thus, at some point the spray evaporates completely, and additional heat needs to be removed by other, less efficient cooling mechanisms (convective gas cooling and radiation). The proportion of these less efficient cooling mechanisms increases with increasing heating power. For such higher heating powers, additional spray generators may be provided, so that no dry-outs occur.

Fig. 5a - Fig. 5d are schematic cross-sectional views of busbars (conductor sections 12 to be cooled), and with spray generators 24 of a cooling system according to an embodiment of the invention, the spray generators 24 being arranged at various angles with respect to the surface normal of the conductor section 12. Fig. 5a illustrates an arrangement having an angle of 0°, Fig. 5b of 45°, Fig, 6c of 60°, and Fig. 5d of 90°.

Fig. 6 is a graph showing a measured temperature difference between the conductor section 12 and the cooling fluid, as a function of the angle between the spray generator with respect to the surface normal of the conductor section 12 (i.e. the angle illustrated in Fig. 5a to 6d). Herein, the conductor section 12 is heated with a standard heating power, the background dielectric gas is a C6-air mixture, and the cooling liquid is C6. As discussed with respect to Fig. 3, the temperature difference is a measure of the cooling efficiency, a low temperature indicating a high cooling efficiency.

As seen in Fig. 6, most favorable is a horizontal arrangement of the cooled surface (angle of 0° as illustrated in Fig. 5a), because in this case most of the spray directly intercepts the surface to be cooled. For a maximum cooling effect such orientation of the busbars, or placement of the nozzle at such relative angle so that the busbar surface is most exposed to the nozzle, is preferable, as illustrated in Figure 5 a.

Thus, according to general embodiments, the spray generator faces a (main) surface of the conductor section, at an angle deviating from the surface normal by at most 30°, preferably by at most 10°. According to a further general embodiment, the spray generator is oriented such that the spray is directed towards the conductor section.

Measurements and aspects relating to the dielectric strength

A remarkable advantage of embodiments of the invention is that the spray of the cooling liquid does not necessarily diminish the dielectric strength in any significant manner, and may even improve the dielectric strength.

Several measurements of the dielectric strength (breakdown strength) of a setup as shown in Fig. 1 have been conducted, with a background dielectric gas being a C6-air mixture, and the cooling liquid being C6. Here, for a setup in which the spray generator 24 was switched off, an average dielectric strength (with "nozzle OFF") of 10.0 kV/mm was obtained (herein, the dielectric breakdown voltage is always defined as the peak breakdown voltage kVpeak, unless specified otherwise). In contrast, when the spray generator 24 was switched on so that a cooling spray was generated by the spray generator 24, the average dielectric strength (with "nozzle ON") was increased to 11.8 kV/mm. Thus, a dielectric strength increase of more than 10% is obtained due to the presence of the cooling liquid droplets.

Fig. 7 is a graph showing the dependence of the measured dielectric strength (of the insulation gas including the spray of cooling liquid), as a function of a local heating power used for heating the conductor section 12. With increasing heating (increasing local heat flux), the dielectric strength is reduced. The reduction of the dielectric strength is believed to be caused by a reduction in spray density, as more of the spray evaporates with increased heating power. (The evaporation of the spray and its effect on the heat transfer coefficient was already discussed in the context of Figure 4). This reduction can be counteracted by providing more spray generators 24 and/or by increasing the supply of cooling liquid provided to each spray generator, in particular at the critical locations.

Therefore, at given local heating similar flow rates are recommendable from the dielectric and the cooling perspective that guarantee sufficient local supply of the spray without excessive drying out of the spray. However, a tradeoff exists between thermal management, which benefits from delivering most of the spray directly onto the surface (e.g. even film cooling) versus the dielectric benefit which is best obtained by spray distributed in the bulk. In an optimal arrangement, the nozzle produces a solid cone of at least 30° and at most 160° opening angle, preferably at least 45° and/or at most 135° opening angle, and most preferably about 90° opening angle. These opening angle ranges allow for a simultaneous wetting of the surface and filling the dielectric spacing with spray.