EP4593041A1

EP4593041A1 - Dielectric heating method for moisture extraction from an active part of a transformer

Info

Publication number: EP4593041A1
Application number: EP24153935.2A
Authority: EP
Inventors: Pritam MUKHERJEE; Ulf Sand; Rebei Bel Fdhila; Dierk Bormann
Original assignee: Hitachi Energy Ltd
Current assignee: Hitachi Energy Ltd
Priority date: 2024-01-25
Filing date: 2024-01-25
Publication date: 2025-07-30
Also published as: WO2025157441A1

Abstract

A dielectric heating method for moisture extraction from an active part (10) of a transformer (1) is provided. The method comprises a step of configuring a system including at least one an apparatus (2) configured to generate electric fields alternating at radio frequencies, wherein the electric fields penetrate at least part of regions between freely selectable electrodes (3). The method further comprises a step of operating the at least one apparatus (2) for drying at least one moist region (7) of the active part (10) of the transformer (1), wherein the at least one moist region (7) is located at least partially between the freely selectable electrodes (3), so that the at least one moist region (7) is exposed to the generated alternating electric fields, resulting in heat generation within the at least one moist region (7).

Description

The present disclosure relates to a dielectric heating method for moisture extraction from an active part of a transformer. This dielectric heating method deals with high frequency electromagnetic heating for moisture extraction from a dielectric material for instance during the production or assembly of a transformer or during the production of an active part of a transformer. It is also possible to apply such method for the moisture extraction from used transformers.
Conventionally, the transformer's active part containing dielectric or insulation material is subjected to a heating process inside an oven multiple times during the production of the transformer. The heating process can be carried out to remove the moisture from the cellulose insulation so that the winding/s and supporting structure/s of the transformer can be shaped to the final dimensions and the clamping pressure/s can be adjusted. Conventionally, the heating process to finally dry for instance the cellulose based dielectric materials is mediated by kerosene vapor. These heating processes, however, are time-intensive and energy consuming.
Embodiments of the present disclosure address the above shortcomings in whole or in part. The embodiments of the method for moisture extraction for instance from an active part of the transformer are subject matters of the independent and dependent claims.
The present disclosure proposes a new drying technique that targets the moisture inside the active part of the transformer. The active part of the transformer can be the working part of the transformer without the tank and oil. The active part of the transformer may be seen as an assembly comprising for instance a core, a plurality of windings and insulation regions within the transformer. The active part can comprise a press-ring and/or other integral parts. For instance, cellulose-based materials may be used in the insulation regions. The insulation regions may be located around the core and/or the winding/s, between neighboring windings, between the core and one winding and/or between the core and several windings of the transformer. The insulation regions may comprise the press-ring.
The idea of the new drying technique utilizes the heat produced from interactions between a high-frequency alternating electric field and water molecules, to conduct the drying process in a significantly faster and more energy-efficient manner. The electric field is alternating at typical radio frequencies, but this method does not use radiation in the form of radio-waves or microwaves.
Internal and/or external electrodes, which are freely selectable, can be utilized as electrodes when the electric field is generated and alternated at high frequencies, for instance at radio frequencies. Integral part/s of the transformer, for instance integral part/s of the active part of the transformer, can be utilized as freely selectable electrode/s during the process of generating heat within the insulation regions, for example, within the cellulose-based insulation regions.
According to an embodiment of a dielectric heating method for moisture extraction from an active part of a transformer, the method comprises a step of configuring a system including at least one apparatus which is configured to generate electric fields alternating at radio frequencies, wherein the electric fields penetrate at least part of regions between freely selectable electrodes. The apparatus is operated for drying at least one moist region of the active part of the transformer, wherein the at least one moist region is located at least partially or entirely between the freely selectable electrodes, so that the at least one moist region is exposed to the generated alternating electric fields, resulting in heat generation within the at least one moist region.
In contrast to conventional heating method using kerosene vapor and/or hot air, the dielectric heating generates heat directly inside the moist region which is made for instance from a dielectric material. Here, the electric energy is converted into heat due to dielectric losses in the dielectric material in the moist region. The dielectric heating thus depends on interactions between molecules in the moist region and the alternating electric field which oscillates at high frequencies, for instance at radio frequencies.
Such a dielectric heating method takes advantages of the use of high-frequency electromagnetic drying which is useful for fast and targeted heating. Such heating techniques are very useful for drying since the heat is directly generated within the bulk of the material in the moist region due to the interaction the alternating electric field for instance with dipolar water molecules. This method further provides distinct advantages over traditional heating processes, since heat is directly generated within the bulk of material in the moist region as opposed to the heat flowing from outside the moist region into the moist region.
Using this method, a reverse temperature gradient can be created, wherein the reverse temperature gradient may have highest temperature in the middle of the bulk, for instance in the center of the moist region, and wherein the temperature decreases from the center towards outer surfaces of the moist region. This is in sharp contrast compared to the case of conventional heating, where the temperature gradient is the exact opposite and shows highest temperature at outer surfaces and gradually decreases towards the center of the moist region. Such temperature gradient makes it difficult to dry the center for instance of thick blocks of pressboard material. This problem can be overcome by using the dielectric heating method which can be self-regulating. This means that more heat is generated if more moisture is present. Thus, it is possible to prevent overheating and save time and energy.
Using the dielectric heating method, useful spatial and temporal variation of heat generation during the drying process can be realized due to the heat distribution described above. For instance, the spatial variation occurs due to higher heat generation at moist regions compared to drier ones. Moreover, the temporal variation is due to less heat generation as the heating process progresses, since there would be less water molecules in the moist region or in the bulk. Thus, compared to conventional heating processes using only kerosene and/or hot air, the dielectric heating process is significantly faster and more energy-efficient.
According to a further embodiment of the dielectric heating method, the dielectric heating method is used for moisture extraction from the active part of the transformer, wherein the moist region of the transformer is an integral dielectric part of the active part of the transformer. The integral dielectric part/s of the active part can be adjacent to electrically conducting integral parts of the transformer. The moist region or the integral dielectric part can be located between the electrically conductive integral parts of the active part of the transformer. During the process of dielectric heating, the electrically conductive integral part/s can be utilized as internal electrode/s to generate heat within the moist regions or within the dielectric part/s of the active part or of the transformer in a targeted manner. The electrically conductive integral part/s of the active part of the transformer can comprise a core, one part of the core, and different windings. The moist region or the integral dielectric part of the active part of the transformer can be any insulation regions between the windings, located between the core and the windings, around the core or around the windings.
According to a further embodiment of the dielectric heating method, the method is used for moisture extraction from a plurality of different moist regions in the active part of the transformer. Different integral parts of the active part of the transformer may be used as different freely selectable electrodes for the purpose of drying the different moist regions in the active part of the transformer, in a targeted manner.
The integral parts being used as different freely selectable electrodes are electrically conductive and can be made from one metallic material or from several metallic materials. The electrically conductive integral part can be a core, one part of the core or a winding of the transformer. The moist regions can be insulation regions which can be made from dielectric materials, for instance based on cellulose. The moist region can be free of any electrically conductive material. It is also possible that the freely selectable electrodes are chosen in such a way, that there is no electrically conductive material located between the freely selectable electrodes.
Conventionally, during the process of manufacturing a transformer, the active part of the transformer shall be heated at least twice during the manufacturing process: (i) while shaping the winding to adjust the clamping pressure; and (ii) during a vapor phase drying to dry the cellulose materials within the active part of the transformer. The transformer may comprise thick blocks of pressboard, such as press-rings, which are difficult to dry at their center. Hence, it may take a long time and considerable energy consumption to reach the desired level of dryness, for example at 1 % or 0.5 % relative humidity. Moreover, it is practically quite difficult to determine whether the material or the insulation region has reached 1 % or 0.5 % relative humidity at its center. By using the dielectric heating method described here, the process of drying the moist regions for instance within the active part of the transformer can be simplified, not only during the process of manufacturing a new transformer but also during the process of extracting moisture from a used transformer.
According to a further embodiment of the dielectric heating method, the moist region is a dielectric region of the transformer and is at least one of: a press-ring of the transformer; an insulation region between two neighboring windings of the transformer; and an insulation region between one winding and a core of the transformer.
Depending on which dielectric region of the transformer shall be dried, internal and/or external electrodes can be used as the freely selectable electrodes during the process of extracting moisture from the dielectric region in question. It is possible that only external electrodes, only internal electrodes, or a combination of external and internal electrodes are used. The to be dried dielectric region, i.e. the moist region, is located at least partially or entirely between the freely selectable electrodes.
According to a further embodiment of the dielectric heating method, the apparatus is configured to generate electric fields, alternating at frequencies in a range from 1 MHz to 50 MHz, for instance in a range from 1 MHz to 41 MHz, from 3 MHz to 41 MHz, from 3 MHz to 30 MHz, from 4 MHz to 41 MHz, from 5 MHz to 41 MHz, or from 13 MHz to 41 MHz.
The potential, i.e. the voltage, between the freely selectable electrodes can be chosen in a range from 0.5 kV to 100 kV, for instance from 0.5 kV to 90 kV, from 0.5 kV to 80 kV, from 0.5 kV to 70 kV, from 5 kV to 100 kV, from 10 kV to 100 kV, from 15 kV to 100 kV or from 1 kV to 60 kV, from 5 kV to 50 kV, from 10 kV to 50 kV or from 15 kV to 45 kV.
According to a further embodiment of the dielectric heating method, at least one of the freely selectable electrodes comprises one integral part of the transformer being used as one internal electrode. The integral part of the transformer can be a core or any winding of the transformer. The other freely selectable electrode can be an external electrode or another integral part of the active part or of the transformer. The other integral part can be the core or another winding.
According to a further embodiment of the dielectric heating method, one integral part of the transformer being used as one internal electrode of the freely selectable electrodes is a winding of the transformer. The winding can be a low-voltage or a high-voltage winding of the transformer. The other freely selectable electrode can be an external electrode or the core or another winding of the transformer.
According to a further embodiment of the dielectric heating method, one integral part of the transformer being used as one internal electrode of the freely selectable electrodes is the core of the transformer. Further freely selectable electrode/s can be external electrodes or further winding/s of the transformer. The moist region can be located between the core and the further freely selectable electrode/s.
According to a further embodiment of the dielectric heating method, the freely selectable electrodes comprise two different integral parts of the transformer being used as two different internal electrodes. The two different integral parts can be two neighboring windings of the transformer. It is also possible that the two different integral parts are the core and one winding of the transformer.
According to a further embodiment of the dielectric heating method, the two different integral parts of the transformer being used as the two different internal electrodes are two different windings of the transformer. It is possible that the two different windings are arranged next to each other and do not surround each other. It is also possible that the two different windings are arranged next to each other, wherein one of the windings is surrounded by another winding. The moist region can be located between the two different windings used as the freely selectable electrodes.
For example, the two different integral parts of the transformer being used as the freely selectable electrodes are a low-voltage winding and a high-voltage winding of the transformer. Furthermore, it is also possible that the two different integral parts being used as the freely selectable electrodes are two neighboring high-voltage windings or two neighboring low-voltage windings.
According to a further embodiment of the dielectric heating method, the two different integral parts of the transformer being used as the two different internal electrodes comprise a core and at least one winding of the transformer. In this case the moist region is located between the core and the one winding.
According to a further embodiment of the dielectric heating method, at least one of the freely selectable electrodes is an external electrode which is not an integral part of the transformer or of the active part of the transformer. For instance, the external electrode is located outside a press-ring. It is possible to use more than one external electrodes. Furthermore, it is also possible to use a combination of one or more external electrodes and one or more internal electrodes. The internal electrode can comprise the core, one part of the core, one winding or several windings of the transformer. The moist region can be the press-ring or an outer block or ring of the transformer.
According to a further embodiment of the dielectric heating method, at least one of the freely selectable electrodes comprises one integral part of the transformer being used as one internal electrode. At least one of the freely selectable electrodes can be an external electrode which is not an integral part of the transformer. For instance, the core or one part of the core of the active part forms one freely selectable electrode. It is also possible to use a combination of two or more external electrodes and the core as the freely selectable electrodes.
According to a further embodiment of the dielectric heating method, the freely selectable electrodes comprise external electrodes which are not integral parts of the transformer. The number of external electrodes can be at least two, three or four. For example, the active part of the transformer is partially or completely surrounded by the external electrode/s.
In all embodiments, the apparatus configured to generate the electric field alternating for instance at radio frequencies can comprise at least a source unit, for instance a voltage supply unit. The apparatus is thus not configured to generate electromagnetic radiation in the narrow sense. The apparatus is configured to generate alternating electric fields oscillating for instance at radio frequencies. The external electrodes can be integral parts of the apparatus or not be integral parts of the apparatus. The internal electrodes are not integral parts of the apparatus but are integral parts of the transformer, for instance of the active part of the transformer. It is possible to use electrical connection/s to electrically connect the internal electrode/s to the voltage supply unit. The electrical connection/s can be parts of the apparatus or of the system comprising the apparatus. The electrical connection/s can be used for grounding purposes.
The method described here can be realized by using external electrode/s and/or internal electrode/s such as the windings and the core for high-frequency electromagnetic heating. Different terminals configurations of the freely selectable electrodes, for instance of the windings and the core, could be used to emphasize heating in strategic areas. It is also possible to place electrodes inside a drying chamber and place the cellulose-based insulation between these. This method may fully or partially replace the conventional heating. For instance, the method may be utilized either with or without use of Kerosene.
The present disclosure comprises several embodiments of the dielectric heating method. Every feature described with respect to one of the embodiments is also disclosed herein with respect to the other embodiments, even if the respective feature is not explicitly mentioned in the context of the specific embodiment.
The accompanying figures are included to provide a further understanding. In the figures, elements of the same structure and/or functionality may be referenced by the same reference signs. It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
While the disclosure is amenable to various modifications and alternative forms, specifics thereof are shown by way of example in the figures and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular described embodiments and examples.
The accompanying figures are included to provide a further understanding of the heating method. In the figures, elements of the same structure and/or functionality may be assigned to the same reference signs. It is to be understood that the examples shown in the figures are illustrative representations and are not necessarily true to scale.

Figure 1 shows a general concept of a dielectric heating method for moisture extraction from a moist region.
Figure 2 shows an example of the dielectric heating method for moisture extraction from a moist region of the active part of the transformer.
Figures 3A, 3B, 4A, 4B, 5A and 5B show further examples of the dielectric heating method for moisture extraction from different moist regions of the active part of the transformer.

Figure 1 schematically shows an active part of a transformer comprising a region 7 which is for instance a moist region 7. The region 7 can be partially or entirely electrically insulating. The region 7 comprises for instance one dielectric material or several dielectric materials. The dielectric material can be a cellulose based dielectric material.
For moisture extraction from the region 7, an apparatus 2 can be used to generate electric fields alternating at high frequencies, for instance at radio frequencies. The electric fields penetrate regions between freely selectable electrodes 3. In Figure 1, two freely selectable electrodes 3 are shown, wherein the moist region 7 is located between the two selectable electrodes 3. Thus, during the operation of the apparatus 2, the moist region 7 is exposed to the alternating electric field/s. Since water molecules are electric dipoles, they move or rotate as they try to align themselves with the alternating electric field. These molecular movements of the water molecules create heat by friction as the rotating molecules collide with other molecules in the moist region 7, for instance with the molecules of the dielectric material, and force these other molecules into motion, resulting in generation of heat within the moist region 7.
This kind of heating is self-regulating, since more heat will be generated if there are more water molecules present, i.e. more moisture. Possible overheating can thus be prevented. Moreover, the self-regulating heating process is reflected by the temporal and spatial variations of the heat generation, since more heat is generated at humid regions compared to drier ones. The temperature gradient may have highest temperature in the center of the moist region having the highest humidity, wherein the temperature decreases from the center towards outer regions having lower humidity. Thus more heat is generated at the most humid region resulting in realizing a targeted and efficient heating method.
The targeted heating method is also reflected by the fact that the electrodes 3 are freely selectable. Hence, depending on the arrangements or configurations of the electrodes 3, individual moist regions 7 can be dried in a targeted manner. The electrodes 3 shown in Figure 1 can be external electrodes, internal electrodes or a combination of at least one internal electrode and one external electrode.
An external electrode can be understood to mean an electrically conductive structure which is not an integral part of the transformer 1 or of the active part 10 of the transformer 1. The external electrode can be part of the apparatus 2. An internal electrode can be understood to mean an electrically conductive structure which is an integral part of the transformer 1 or of the active part 10 of the transformer 1. The integral part of the transformer 1 or of the active part 10 can be an electrically conductive structure, for example, a winding structure or a core structure of the transformer 1.
The apparatus 2 comprises a source unit for generating an alternating electric field. For instance, the apparatus 2 can comprise a voltage supply unit. It is also possible that the apparatus 2 comprises electrical connection/s which can be used to electrically connect the voltage supply unit with the freely selectable electrodes 3. The electrical connection/s can also be used for grounding purposes as schematically shown in Figure 1.
The dielectric heating method for moisture extraction from an active part of a transformer thus comprises a step of configuring a system including at least one apparatus which is configured to generate electric fields alternating at radio frequencies. The system can further include the freely selectable electrodes 3. The freely selectable electrodes 3 may be integral parts or not integral parts of the transformer 1. It is possible that the system includes further electrical connections.
Figure 2 shows a concrete example of the dielectric heating method for moisture extraction from a transformer 1, for instance from the active part 10 of the transformer 1.
The active part 10 comprises a press-ring 70. The press-ring 70 can be a pressboard formed from a dielectric material. The dielectric material can be based on cellulose. It is possible that the press-ring 70 form an outermost layer of the active part 10.
The active part 10 comprises a core 4 and winding/s 5 around the core 4 or around some parts of the core 4. The winding 5 can be separated from the core 4 by an insulation region 73. The insulation region 73 can be made from a dielectric material which can be based on cellulose. As schematically shown in Figure 2, the moist region 7 can comprise the press-ring 70 and/or the insulation region 73.
The press-ring 70 and/or the insulation region 73 can be subjected to electric field alternating at high frequencies, for instance at radio frequency as shown in Figure 2. Here the core 4 or part of the core 4 could be used as one freely selectable electrode 3 which is an internal electrode 30. For instance, the core 4 can be grounded. As shown in Figure 2, further external electrode/s 31 can be used as further freely selectable electrode/s 3. The external electrode/s 31 can be electrically connected to a voltage source which is used to generate the alternating electric fields.
In Figure 2, there are two external electrodes 31. The external electrodes 31 are arranged outside the active part 10 of the transformer 1. The geometries of the external electrodes 31 can be adapted to the geometries of the press-ring 70, the insulation region 73 and/or of the core 4. As shown in Figure 2, the external electrodes 31 can have a curved shape or can be of a ring-shaped structure. It is possible that only one external electrode 31 is used. The only one external electrode 31 can be curved or ring-shaped. Furthermore, it is possible that two or more external electrodes 31 are used simultaneously. It is also possible that only the external electrodes 31, i.e no internal electrodes, are used. In the latter case, none of the integral parts of the active part 10 of the transformer is used as a freely selectable electrode.
Since the press-ring 70 and/or the insulation region 73 are exposed to the alternating electric field, water molecules within in the press-ring 70 and/or the insulation region 73 are excited or forced to move or rotate. The water molecules will collide with further molecules of the dielectric material/s in the press-ring 70 and/or the insulation region 73 resulting in generation of heat within the press-ring 70 and/or within the insulation region 73.
According to Figures 3A to 5B, integral parts of the active part 10 of the transformer 10 can be used as electrodes 3, namely as internal electrodes 30. Thus, the dielectric heating method or the electromagnetic drying can be facilitated without resorting to external electrodes 31. According to Figures 3A to 5B, basically two galvanically isolated metallic parts in the active part 10 of the transformer 1 can be utilized as the freely selectable electrodes 3, in particular as internal electrodes 30. The excitation can be connected either from line-end terminals, neutral-end terminals, or tap-winding terminals.
Figure 3A shows a further concrete example of the dielectric heating method for moisture extraction from the active part 10 of the transformer 1. The active part 10 comprises a core 4 and a plurality of windings 5. The core 4 can comprise lateral parts 41 and vertical parts 42. The lateral parts 41 and vertical parts 42 can be connected directly to each other. The plurality of windings 5 comprise first windings 51 and second windings 52. The second windings 52 can be high-voltage windings. The first windings 51 can be low-voltage windings. The first winding 51 is for instance an inner winding 51 surrounding one vertical part 42 of the core 4. The second winding 52 is for instance an outer winding surrounding both the inner winding 51 and the one vertical part 42 of the core 4.
For instance, the active part 10 of the transformer 1 comprises a plurality of pairs of windings 5, wherein each pair of windings 5 comprises one first winding 51 and one second winding 52. As shown in Figure 3A, the first winding 51 and the second winding 52 of the same pair are arranged around one common vertical part 42 of the core 4. In other words, both the first winding 51 and the second winding 52 laterally surround one common vertical part 42 of the core 4. The first winding 51 is located between the vertical part 42 of the core 4 and the second winding 52. The first winding 51 can enclose the associated vertical part 42 of the core 4. The second winding 52 can enclose both the associated first winding 51 and the associated vertical part 42 of the core 4.
As shown in Figure 3A, the active part 10 can comprise a first insulation region 71 located between the first winding 51 and the second winding 52 of the same pair of windings 5. The active part 10 can comprise a second insulation region 72 located between the two neighboring second windings 52 of two neighboring pairs of windings 5. The active part 10 can comprise a third insulation region 73 located between the first winding 51 and its associated vertical part 42 of the core 4. Any of the vertical part 42 of the core 4 can be laterally surrounded by the first insulation region 71 and/or by the third insulation region 73. At least some of vertical parts 42, for instance inner vertical parts 42 of the core 4 can be laterally surrounded by the second insulation region 72.
In Figure 3A, three vertical parts 42 of the core 4 and three pairs of windings 5 are shown. It is, however, possible that the active part 10 of the transformer 1 comprises more than three vertical parts 42 of the core 4 and thus more than three pairs of windings 5. Any of the first insulation region 71, the second insulation region 72 and the third insulation region 73 can be formed from a dielectric material, for instance from a material based on cellulose. Any of the first insulation region 71, the second insulation region 72 and the third insulation region 73 can be a moist region 7 of the active part 10 of the transformer 1. The core 4, a part of the core 4 and/or any of the windings 5 can be used as electrode/s 3, for instance as internal electrode/s 30 for performing the dielectric heating method for moisture extraction from any of the first insulation region 71, second insulation region 72 and third insulation region 73 in a targeted manner.
According to Figure 3A, the first winding 51 and the second winding 52 of the same pair of windings 5 are used as the internal electrodes 30 for extracting moisture from the first insulation region 71. Using a first connection 61, the first winding 51 can be grounded. The second winding 52 can be electrically connected to a source unit 20, for example via a second connection 62. The source unit 20 is part of the apparatus 2. For example, the source unit 20 comprises a voltage supply unit configured for generating an electric field alternating at radio frequencies. The source unit 20 can also be grounded, for example, via the second connection 62 .
In Figure 3A, only the windings 51 and 52 of one left pair of windings 5 are used as internal electrodes 30 for heating the first insulation region 71 located between the first winding 51 and the second winding 52. In this way, moisture can be extracted from the first insulation region 71 being the moist region 7 in a targeted manner. Any pair of the windings 5 can be used as internal electrodes 30 for extracting the moisture from the associated first insulation region 71.
The dielectric heating method for moisture extraction shown in Figure 3B is basically identical to dielectric heating method shown in Figure 3A, except that the second winding 52 is grounded for instance via the second connection 62. The first winding 51 can be electrically connected to the source unit 20 via the first connection 61.
The dielectric heating method for moisture extraction shown in Figure 4A is basically identical to dielectric heating method shown in Figure 3A, except that two neighboring second windings 52 are used as internal electrodes 30. In this way, the dielectric heating method is configured for moisture extraction from the second insulation region 72 between the two neighboring second windings 52.
In Figures 3A and 3B, the two different windings 5 used as internal electrodes 30, namely one first winding 51 and one second winding 52, are arranged next to each other, wherein the first winding 51 is surrounded by the second winding 52. In Figure 4A, however, the two different windings 5 used as internal electrodes 30, namely one second winding 52 and one further second winding 52, are arranged next to each other and do not surround each other. The two different second windings 5 in Figure 4A enclose different vertical parts 42 of the core 4. If the second insulation region 72 between these two neighboring second windings 52 is a moist region 7, the moist region 7 can be heated for moisture extraction from this second insulation region 72.
As shown in Figure 4A, an inner second winding 52, i.e. the middle second winding 52, being an internal electrode 30 is grounded, for instance via the first connection 61. An outer second winding 52, i.e. the left second winding 52, is another internal electrode 30 and can be electrically connected to the source unit 20 via the second connection 62. Using appropriate second windings 52 as internal electrodes 30, any of the second insulation regions 72 between two neighboring second windings 52 can be heated for moisture extraction from that second insulation region 72.
The dielectric heating method for moisture extraction shown in Figure 4B is basically identical to dielectric heating method shown in Figure 4A, except that the outer second winding 52 being used as an internal electrode 30 is grounded, for instance via the second connection 62. The inner second winding 52, i.e. the middle second winding 52, is another internal electrode 30 and can be electrically connected to the source unit 20 via the first connection 61.
The dielectric heating method for moisture extraction shown in Figure 5A is basically identical to dielectric heating method shown in Figure 3B, except that the core 4 or part of the core is used as a further internal electrode 30. In this way, the dielectric heating method is configured for moisture extraction from the third insulation region 73 between the first winding 51 and the core 4 or between the first winding 51 and one vertical part 42 of the core 4.
As shown in Figure 5A, the core 4 being an internal electrode 30 is grounded, for instance via the second connection 62. The left first winding 51 is another internal electrode 30 and is electrically connected to the source unit 20 via the first connection 61. Using appropriate first windings 51 as internal electrodes 30, any of the third insulation regions 73 being a moist region 7 between the core 4 and any of the first windings 51 can be heated for moisture extraction from that third insulation region 73 in a targeted manner.
The dielectric heating method for moisture extraction shown in Figure 5B is basically identical to dielectric heating method shown in Figure 5A, except that the first winding 51 being an internal electrode 30 is grounded, for instance via the first connection 61. The core 4 is another internal electrode 30 and is electrically connected to the source unit 20 via the second connection 62. For instance, the second connection 62 is electrically connected to a lateral part 41 of the core 4. It is, however, possible that the second connection 62 is electrically connected any part 41 or 42 of the core 4.
Thus, different combinations of the freely selectable electrodes 3 can be utilized to target specific moist regions 7 between the freely selectable electrodes 3 for drying. The freely selectable electrodes 3 can comprise external electrode/s 31 and/or internal electrode/s 30, wherein the internal electrode 30 can be any winding 5 or the core 4 of the active part 10 of the transformer 1. This results in a heating process which is targeted, significantly fast, efficient, and less time-consuming. Such a method can be applied not only during the production of new transformers, for example of active parts of new transformers, but also for the moisture extraction from used transformers, for instance from active parts of used transformers.
The examples shown in the figures as stated represent examples of the dielectric heating method for moisture extraction from a transformer, for instance from an active part of the transformer; therefore, they do not constitute a complete list of all examples according to the improved heating method. Actual arrangements or implementation of the method may vary from the examples described above.

Reference signs

1: transformer
10: active part of the transformer

2: apparatus
20: source unit/ voltage supply unit

3: electrode
30: internal electrode
31: external electrode

4: core
41: lateral part of the core
42: vertical part of the core

5: winding
51: first winding/ low-voltage winding
52: second winding/ high-voltage winding

61: first connection
62: second connection

7: moist region

70: press-ring
71: first insulation region between two windings
72: second insulation region between two windings
73: insulation region between winding and core

Claims

A dielectric heating method for moisture extraction from an active part (10) of a transformer (1), comprising following steps:
- configuring a system including at least one apparatus (2) configured to generate electric fields alternating at radio frequencies, wherein the electric fields penetrate at least part of regions between freely selectable electrodes (3); and

- operating the at least one apparatus (2) for drying at least one moist region (7) of the active part (10) of the transformer (1), wherein the at least one moist region (7) is located at least partially between the freely selectable electrodes (3), so that the at least one moist region (7) is exposed to the generated alternating electric fields, resulting in heat generation within the at least one moist region (7).
The dielectric heating method according to claim 1 for moisture extraction from a plurality of different moist regions (7) in the active part (10) of the transformer (1), wherein different integral parts of the active part (10) of the transformer (1) are used as different freely selectable electrodes (3) for the purpose of drying the different moist regions (7) in the active part (10) of the transformer (1) in a targeted manner.
The dielectric heating method according to any of preceding claims, wherein the at least one moist region (7) is a dielectric region of the transformer (1) and is at least one of:
- a press-ring (70) of the transformer (1),

- an insulation region (71, 72) between two neighboring windings (5) of the transformer (1), and

- an insulation region (73) between one winding (5) and a core (4) of the transformer (1).
The dielectric heating method according to any of preceding claims, wherein the at least one apparatus (2) is configured to generate electric fields alternating at frequencies in a range from 1 MHz to 50 MHz.
The dielectric heating method according to any of preceding claims, wherein at least one of the freely selectable electrodes (3) comprises one integral part of the transformer (1) being used as one internal electrode (30).
The dielectric heating method according to any of the preceding claims, wherein one integral part of the transformer (1) being used as one internal electrode (30) of the freely selectable electrodes (3) is a winding (5, 51, 52) of the transformer (1).
The dielectric heating method according to any of the preceding claims, wherein one integral part of the transformer (1) being used as one internal electrode (30) of the freely selectable electrodes (3) is a core (4) of the transformer (1).
The dielectric heating method according to any of the preceding claims, wherein the freely selectable electrodes (3) comprise two different integral parts of the transformer (1) being used as two different internal electrodes (30).
The dielectric heating method according to claim 8, wherein the two different integral parts of the transformer (1) being used as the two different internal electrodes (30) are two different windings (5, 51, 52) of the transformer (1).
The dielectric heating method according to claim 9, wherein the two different windings (5, 51, 52) are arranged next to each other and do not surround each other.
The dielectric heating method according to claim 9, wherein the two different windings (5, 51, 52) are arranged next to each other, and wherein one of the windings (5, 51) is surrounded by another winding (5, 52).
The dielectric heating method according to claim 8, wherein the two different integral parts of the transformer (1) being used as the two different internal electrodes (30) comprise a core (4) and at least one winding (5, 51, 52) of the transformer (1).
The dielectric heating method according to any of claims 1 to 7, wherein at least one of the freely selectable electrodes (3) is an external electrode (31) which is not an integral part of the transformer (1).
The dielectric heating method according to any of the claims 1 to 7, wherein
- at least one of the freely selectable electrodes (3) comprises one integral part of the transformer (1) being used as one internal electrode (30), and

- at least one of the freely selectable electrodes (3) is an external electrode (31) which is not an integral part of the transformer (1).
The dielectric heating method according to any of the claims 1 to 7, wherein the freely selectable electrodes (3) comprise external electrodes (31) which are not integral parts of the transformer (1).