EP0374062B1 - Process and apparatus for drying the paper insulation of a high-tension electrotechnical device, and microwave energy applicator for this purpose - Google Patents

Abstract

The water and microwave permeable dielectric insulation in an electrotechnical high voltage equipment is dried by means of microwaves. The electrotechnical equipment has a central conductor on which the insulation is wrapped, and it is disposed within an electrically conducting hollow cylinder with its central conductor coaxial with the latter cylinder to form a coaxial, microwave transmission line. Microwaves are propagated through the coaxial transmission line to heat the water in the dielectric insulation, to transform it into water vapor, and to thereby dry the insulation. The microwave transmission line is air-tight and the water vapor is evacuated through a vacuum pump or cold trap.

Description

The present invention relates to a method of drying, by microwave, a multi-layer paper insulator, forming part of a high-voltage electrotechnical device, such as a transformer bushing. The invention also relates to a microwave energy applicator and a dryer containing such an applicator, the latter as well as the dryer being intended for carrying out the method.

In high-voltage transformers, bushings pass through the metal casing, in order to allow a connection, from the outside, with the winding terminals. One such crossing includes a conductor wrapped in multi-layer paper insulation. The paper insulation, as well as part of the conductor, are housed inside a cylindrical ceramic envelope, hermetically sealed and filled with oil. The paper insulation is therefore moistened with this oil.

In use, moisture infiltrates inside the bushing through cracks and joints in the ceramic casing. Water therefore crosses the paper and destroys the dielectric insulation. The crossing must then be repaired.

To do this, it is known to remove the conductor with its paper insulator from the ceramic envelope to put them in a conventional oven. The disadvantage of conventional ovens is that the drying is very long and can go up to a month. It follows that such drying is so costly that the bushings are discarded and replaced, the replacement of the bushings being more advantageous than their restoration by drying the paper insulation.

The main object of the present invention is therefore to provide a quick and inexpensive means of drying the paper insulation of the bushings, of the type mentioned above, by means of microwaves.

• monter ledit dispositif électrotechnique dans un élément tubulaire creux fait d'un matériau conducteur d'électricité, le conducteur électrique étant coaxial avec ledit élément tubulaire, pour ainsi former une ligne coaxiale de transmission des micro-ondes,
• transmettre des micro-ondes à travers la ligne coaxiale de transmission pour chauffer l'eau infiltrée dans l'isolant diélectrique et la transformer en vapeur d'eau,
• et évacuer la vapeur d'eau à l'extérieur de ladite ligne coaxiale de transmission.

More specifically, according to the present invention, it is proposed a method for drying a dielectric insulator, permeable to water and microwaves, forming part of an electrotechnical device comprising an electrical conductor, internal and elongated, wrapped in the aforementioned insulator infiltrated with water, the method consists in :

• mounting said electrotechnical device in a hollow tubular element made of an electrically conductive material, the electrical conductor being coaxial with said tubular element, so as to form a coaxial microwave transmission line,
• transmit microwaves through the coaxial transmission line to heat the water infiltrated into the dielectric insulation and transform it into water vapor,
• and discharging the water vapor outside said coaxial transmission line.

The transformation of water into water vapor and the evacuation of this vapor outside the transmission line produces a drying of the dielectric insulator.

• un élément tubulaire creux fait d'un matériau conducteur d'électricité,
• des moyens de montage dudit dispositif électrotechnique dans cet élément tubulaire, ledit conducteur électrique étant coaxial avec l'élément tubulaire, de façon à former une ligne coaxiale de transmission des micro-ondes apte à propager les micro-ondes à travers l'isolant diélectrique pour ainsi appliquer l'énergie des micro-ondes à l'eau infiltrée.

The invention also provides a microwave energy applicator for applying microwave energy to water infiltrated into a dielectric insulator, permeable to water and microwaves, forming part of the invention. an electrotechnical device comprising an elongated internal electrical conductor, wrapped in the aforementioned insulation, said applicator comprising:

• a hollow tubular element made of an electrically conductive material,
• means for mounting said electrotechnical device in this tubular element, said electrical conductor being coaxial with the tubular element, so as to form a coaxial microwave transmission line capable of propagating the microwaves through the dielectric insulator for thus applying the microwave energy to the infiltrated water.

• un élément tubulaire creux fait d'un matériau conducteur d'électricité,
• des moyens de montage dudit dispositif électrotechnique dans cet élément tubulaire, ledit conducteur électrique étant coaxial avec ledit élément tubulaire, de façon à former une ligne coaxiale de transmission des micro-ondes,
• des moyens de propagation des micro-ondes à travers la ligne coaxiale de transmission, et donc à travers l'isolant diélectrique, pour chauffer ladite eau infiltrée dans l'isolant diélectrique et la transformer en vapeur d'eau,
• et des moyens pour évacuer la vapeur d'eau à l'extérieur de ladite ligne coaxiale de transmission des micro-ondes.

The present invention further relates to a device for drying a dielectric insulator, permeable to water and microwaves, forming part of an electrotechnical device comprising an elongated internal electrical conductor, wrapped in the aforementioned insulator infiltrated with water, this device comprising:

• a hollow tubular element made of an electrically conductive material,
• means for mounting said electrotechnical device in this tubular element, said electrical conductor being coaxial with said tubular element, so as to form a coaxial microwave transmission line,
• means for propagating microwaves through the coaxial transmission line, and therefore through the dielectric insulator, to heat said water infiltrated into the dielectric insulator and transform it into water vapor,
• and means for discharging water vapor outside said coaxial microwave transmission line.

Thus, the transformation of water into water vapor and the evacuation of the vapor out of the transmission line produces a drying of the dielectric insulator.

Microwave drying of the insulation of an electrotechnical device, of the kind mentioned above, is quick and inexpensive, so that it becomes more advantageous to recondition the electrotechnical device by drying its dielectric insulator , rather than replacing it.

• La fig. 1 représente, sous la forme d'un diagramme synoptique, un premier mode de réalisation d'un appareil de séchage d'un isolant de papier selon l'invention, incluant un applicateur d'énergie des micro-ondes.
• La fig. 2A est une vue en coupe longitudinale de l'applicateur d'énergie des micro-ondes faisant partie de l'appareil de séchage de la fig. 1, comportant deux fenêtres à micro-ondes disposées, respectivement, à ses extrémités, cette vue étant prise suivant la ligne C-C de la fig. 2B.
• La fig. 2B est une vue en coupe transversale de l'applicateur d'énergie à micro-ondes de la fig. 2A prise suivant la ligne A-A de cette figure.
• La fig. 2C est une autre vue en coupe transversale de l'applicateur d'énergie à micro-ondes de la fig. 2A prise suivant la ligne B-B de cette dernière figure.
• La fig. 3, apparaissant sur la même planche de dessins que la fig. 1, représente, sous la forme d'un diagramme synoptique, un second mode de réalisation d'un appareil de séchage d'un isolant de papier selon la présente invention.

The drying efficiency of the insulation can be improved by closing the two ends of the microwave coaxial transmission line using two microwave windows, thereby obtaining an airtight enclosure. The water vapor that forms inside the transmission line can be removed using a vacuum pump or a cryogenic trap. The objects, advantages and other characteristics of the present invention will appear more clearly on reading the following nonlimiting description of preferred embodiments with reference to the accompanying drawings.

• Fig. 1 shows , in the form of a block diagram, a first embodiment of a device for drying a paper insulator according to the invention, including a microwave energy applicator.
• Fig. 2A is a view in longitudinal section of the microwave energy applicator forming part of the drying apparatus of FIG. 1 , comprising two microwave windows arranged, respectively, at its ends, this view being taken along the line CC of FIG. 2B .
• Fig. 2B is a cross-sectional view of the microwave energy applicator of FIG. 2A taken along line AA in this figure.
• Fig. 2C is another cross-sectional view of the microwave energy applicator of FIG. 2A taken along line BB of this last figure.
• Fig. 3 , appearing on the same drawing board as FIG. 1 shows , in the form of a block diagram, a second embodiment of a device for drying a paper insulator according to the present invention.

In the various figures of the drawings, the corresponding elements are designated by the same reference signs.

The two apparatuses for drying a paper insulator, illustrated in figs. 1 and 3 , each have a microwave energy applicator 1 . The applicator 1 has a first end which is connected to a first microwave window 2 and a second end which is connected to a second microwave window 3 .

The structure of the applicator 1 and of the windows 2 and 3 appears in detail in FIGS. 2A, 2B and 2C .

More specifically, the applicator 1 first comprises a tubular metallic element 4 , electrically conductive, namely a circular cylinder in cross section, in which is mounted the bushing whose insulator is to be dried.

In the example illustrated, the bushing 5 is designed to be used in a high voltage transformer. Such a crossing 5 comprises a metallic conductor 6 , namely a hollow elongated tube whose ends are threaded externally to allow an electrical connection. The conductor 6 is straight and cylindrical, that is to say circular in cross section. It is, on the other hand, insulated by means of a dielectric insulator 7 of multi-layer paper, enveloping said conductor 6 . As can be seen in fig. 2A , one end of the insulator 7 of multi-layer paper is perpendicular to the conductor 6 , while the other end of the insulator is conical. Of course, the bushing 5 has been removed from its ceramic envelope and, as said above, its paper insulation is impregnated with oil, but also contains water which has got there. infiltrated.

It is important that the cylinder 4 and the conductor 6 are coaxial as precisely as possible. For this purpose, the bushing 5 is held in position inside the cylinder 4 by means of two annular end supports 8 and 9 , interposed between the paper insulation 7 and the internal surface of the cylinder 4 . The two supports 8 and 9 are made of a dielectric material permeable to microwaves and are provided with passages, such as 9 ′ in FIG. 2B , passing right through each support when the supports 8 and 9 are inserted between the cylinder 4 and the paper insulator 7 of the bushing.

It follows that the metal cylinder 4 and the conductor 6 form a coaxial microwave transmission line, capable of propagating the microwaves. More precisely, the conductor 6 constitutes the internal conductor of the line, the cylinder 4 , the external conductor of the same line and the paper insulator 7 acts as a dielectric in the coaxial transmission line.

The cylinder 4 is provided with an upper and vertical exhaust duct 10 , two lower and vertical exhaust ducts 11 and 12 and as well as two collar end fittings 13 and 14 .

The microwave energy applicator 1 comprises, in in addition, two sets of coaxial-to-coaxial transition 15 and 16 provided at their respective ends. Each of the transition assemblies 15 , 16 comprises a tapered waveguide section 17 , 18 (external conductor), of circular cross section and a conical internal conductor 19 , 20 coaxial with the waveguide section 17 , 18 and, also, of circular cross section.

The waveguide section 17 is provided with a collar connection 21 at its close end and another collar connection 26 at its remote end, the two connections being conventional. Likewise, the waveguide section 18 is provided with a flange connection 23 at its close end and with another flange connection 24 at its remote end, the two connections being conventional. The collar fittings 13 and 21 are fixed together to form an airtight seal between them. An airtight seal is also obtained between the collar connections 14 and 23 of the cylinder 4 and of the waveguide section 18 . The production of such seals is well known to those skilled in the art and it is therefore not necessary to specify further.

As shown in fig. 2A , the internal conductor 19 is provided with a pin 19 ′, at its close end, this pin 19 ′ being inserted under pressure into the corresponding end of the hollow tube 6 . Likewise, the internal conductor 20 is provided with a pin 20 ′, at its close end, which pin 20 ′ is also inserted under pressure into the other end of the hollow tube 6 . Thus the internal conductors 19 and 20 are mounted on the hollow tube 6 , which conducts electricity.

The microwave window 2 comprises a section 27 of standard 50 Ω waveguide, of circular cross section which comprises, at its close end, a standard collar connector 28 , fixed to the collar connector 26 , to make a airtight seal between fittings 26 and 28 . The waveguide section 27 further includes a standard flange connection 29 at its remote end. In the same way, the microwave window 3 comprises a section 30 of standard circular waveguide of 50 Ω having a standard collar connection 31 , at its close end, attached to collar fitting 24 . Also, an airtight seal is formed between the collar fittings 24 and 31 . The waveguide section 30 obviously has a standard collar connection 32 at its remote end.

Each of the two microwave windows 2 , 3 also has an internal central conductor 33 , 34 coaxial with the section 27 , 30 of waveguide and of circular and constant cross section. The hollow end, brought closer to each conductor 33 , 34 , is forced inside a coaxial hole dug in the end remote from the internal conical conductors 19 , 20 , so that the conductor 33 , 34 is fixed to the conductor 19 , 20 .

Each window 2 , 3 comprises, in addition, an annular stop 35 , 36 , made of a dielectric material permeable to microwaves. Each stop 35 , 36 can be slid longitudinally inside the waveguide section 27 , 30 and thus around the internal conductor 33 , 34 . A first O-ring 37 , 38 seals the seal between the stop 35 , 36 and the bore of the waveguide section 27 , 30 , while a second O-ring 39 , 40 seals the seal between the stop 35 , 36 and the internal conductor 33 , 34 . The stop 35 , 36 keeps the conductor 33 , 34 coaxial with the waveguide section 27 , 30 and its position inside the waveguide section is adjusted so as to obtain an optimal reflection coefficient which is as low as possible.

As we can judge, the window 2 , the transition assembly 15 , the cylinder 4 , the conductor 6 , the transition assembly 16 and the window 3 constitute, overall, a coaxial microwave transmission line, capable to propagate microwaves whose external conductor is constituted by the coaxial sections of waveguide 27 and 17 , the cylinder 4 and the sections 18 and 30 of waveguide and whose internal conductor is constituted by conductors 33 , 19 , 6 , 20 and 34 aligned on a common axis.

The function of the transition assembly 15 is to adapt the impedance of window 2 to the impedance of the coaxial line of transition formed by cylinder 4 and conductor 6 . For this purpose, the internal diameter of the waveguide section 17 gradually increases from the internal diameter of the waveguide section 27 to the internal diameter of the cylinder 4 , while the internal conical conductor 19 sees its external diameter increase gradually, from that of driver 33 to that of driver 6 .

The transition assembly 16 likewise adapts the impedance of the coaxial transmission line formed by the cylinder 4 and the conductor 6 to that of the microwave window 3 . For this purpose, the internal diameter of the waveguide section 18 decreases gradually from the internal diameter of the cylinder 4 to the internal diameter of the waveguide section 30 , while the internal conical conductor 20 also sees its external diameter gradually decrease, from that of conductor 6 to that of conductor 34 .

Impedance matching, using transition assemblies 15 and 16 , prevents the formation of a standing wave in the coaxial microwave transmission line that includes window 2 , transition assembly 15 , cylinder 4 , the conductor 6 , the transition assembly 16 and the window 3 , due to the reflections that would cause the absence of such an adaptation of impedances. A standing wave of this nature has maximums and minimums and would cause uneven heating of the water in the paper insulation.

Thus, the windows 2 and 3 , the transition assemblies 15 and 16 , the cylinder 4 and the conductor 6 define an airtight annular enclosure.

As can be seen in fig. 1 , the paper insulation drying apparatus comprises a high power source 41 of microwaves, advantageously constituted by a magnetron generator. The microwaves, coming from the source 41 , are transmitted to the window 2 by means of a conventional waveguide-to-coaxial transition device (not illustrated), connected to the standard collar connection 29 of the guide section. wave 27 ( fig. 2A ). The microwaves, coming from the source 41 , pass through the window 2 , the applicator 1 and the window 3 and are transmitted to a suitable load 42 through a conventional coaxial-to-waveguide transition device (not illustrated). ) attached to standard fitting 32 ( fig. 2A ). The load 42 being adapted, there is no reflection, since the load then absorbs all the energy of the microwaves which propagate through the windows 2 and 3 and through the applicator 1 , without being absorbed by the water itself. It follows that a standing wave does not form in the applicator 1 , so that the water is heated uniformly throughout the paper insulator 7 , due to a uniform distribution of the electric field in the transmission line.

In fig. 3 , the high power microwave source 41 transmits microwaves to window 2 , through a circulator 45 connected to window 2 , by means of a waveguide-to-coaxial transition device ( not shown) which is fixed to the standard flange connection 29 ( fig. 2A ). The microwaves propagate through the window 2 , the applicator 1 and the window 3 and are transmitted to an adjustable short circuit 47 connected to the collar connection 32 ( FIG. 2A ) of the waveguide section 30 , using a conventional coaxial-to-waveguide transition device (not shown). The microwaves which reach the short circuit 47 are reflected and transmitted to a suitable load 46 through the window 3 , the applicator 1 , the window 2 and the circulator 45 . A standing wave with maximums and minimums is therefore produced inside the coaxial line formed by the cylinder 4 and the conductor 6 . The short circuit 47 is displaced to move the minimums and maximums of the standing wave in the applicator 1 , in order to obtain a heating of the water which is uniform in the insulator 7 of paper. Of course, the energy of the microwaves, reaching the appropriate charge 46 , is absorbed by the latter, without significant reflection of the microwaves.

The microwaves, which propagate through the coaxial transmission line formed by the cylinder 4 and the conductor 6 , cause the water molecules to oscillate in the paper insulator 7 . The water is thus heated and transformed into water vapor. In each of the embodiments of FIGS. 1 and 3 , a vacuum pump 43 sucks the water vapor thus produced through the upper exhaust duct 10 of FIG. 2A , in order to improve and speed up the drying process, by forcing the evacuation of water vapor from the airtight enclosure. It is therefore important that the O-rings 37 , 38 , 39 and 40 and the pairs of collar fittings 26 , 28 ; 13 , 21 ; 14 , 23 and 24 , 31 create airtight seals, in order to obtain an airtight enclosure which is discussed above. The oil, which permeates the paper insulator 7 , is also heated by microwaves and by steam, so that its fluidity increases and that it flows by gravity towards an oil trap 44 , by means of two lower exhaust pipes 11 and 12 , this being true for the two embodiments of FIGS. 1 and 3 .

The passages, such as 9 ′ , made through the supports 8 and 9 , allow the flow of water vapor and oil from the transition assemblies 15 and 16 to the respective exhaust ducts 10 , 11 and 12 .

The vacuum pump 43 can be replaced by a cryogenic trap using liquid nitrogen ( N₂ ) or carbon dioxide ( CO₂ ), such a cryogenic trap being, of course, well known in the art.

In each of the embodiments of FIGS. 1 and 3 , the paper insulation 7 is dried quickly by microwaves, the drying time being counted in hours rather than days, as is the case for drying in conventional ovens. The drying of the insulator 7 becomes economical and the restoration at low cost of the crossbeams of high voltage transformers is therefore possible.

The microwave transmission line formed by the cylinder 4 and the conductor 6 being coaxial, the dominant TEM mode of microwave propagation is particularly effective in terms of uniformly drying the paper insulation, since the microwaves -waves are distributed uniformly in the annular space between the conductor 6 and the cylinder 4 , so that the water is heated uniformly in the insulation 7 .

The frequency of the microwaves is important in that their wavelength must be close to the internal diameter of the cylinder 4 , so as to obtain a propagation according to the dominant TEM mode.

It often happens that a cylindrical metallic shield 48 , illustrated in broken lines in FIG. 2A , either provided in the paper insulator 7 . In this case, two coaxial and parallel transmission lines are formed, the first being constituted by the conductor 6 and the shield 48 , while the second is constituted by the shield 48 and the cylinder 4 . The present invention can also be used to dry the multi-layer paper insulation, but an appropriate adaptation of the impedances must then be made using the transition assemblies 15 and 16 and, optionally, adjusting screws.

The present invention can be used to dry the multi-layer paper insulation from high-voltage transformer bushings or any other high-voltage electrotechnical device, provided that they have a central internal conductor around which the paper insulation multilayer is wound and that such a conductor can be used as an internal conductor of a coaxial microwave transmission line. The central conductor of the electrotechnical device need not be straight. In fact, it may be somewhat curved, provided that the tubular member 4 is similarly curved to form the required coaxial microwave transmission line. The central conductor of the device may also exhibit discontinuities, provided that an appropriate impedance matching is carried out.

The present invention can also be used for a dielectric insulator other than paper, provided that such an insulator is permeable to water and microwaves.

Claims (18)

1. A method of drying water-permeable and microwavepermeable dielectric insulation in an electrotechnical equipment (5) including an inner and elongated electric conductor (6) wrapped in said insulation infiltrated with water, characterized in that it consists in:

- mounting said electrotechnical equipment (5) within a hollow tubular element (4) made of electrically conducting material, the electric conductor (6) being coaxial with said tubular element (4) to thereby form a coaxial, microwave transmission line (15);

- propagating microwaves through the coaxial transmission line to heat the water infiltrated within the dielectric insulation (7) and transform it into water vapor; and

- evacuating the water vapor outside of said coaxial transmission line (15), the water being thus transformed into water vapor and evacuated outside of the transmission line causing drying of the dielectric insulation.

2. A dielectric insulation drying method according to claim 1, characterized in that said water vapor evacuating step comprises a step consisting in sucking the water vapor outside of the coaxial, microwave transmission line.

3. A dielectric insulation drying method according to claim 1, characterized in that said microwave propagating step consists in:

- producing within said coaxial transmission line a standing wave with maximums and minimums, and

- moving said maximums and minimums of the standing wave along the transmission line to produce a uniform heating of the water throughout said dielectric insulation (7).

4. A dielectric insulation drying method according to claim 1, wherein the dielectric insulation is impregnated with oil, characterized in that it further comprises a step of evacuating oil from said dielectric insulation (7) outside of the transmission line.

5. A microwave energy applicator for applying microwave energy to water infiltrated within a water-permeable and microwave-permeable dielectric insulation of an electrotechnical equipment including an inner and elongated electric conductor wrapped in said dielectric insulation, characterized in that it comprises:

- a hollow tubular element (4) made of electrically conducting material; and

- means for mounting the electrotechnical equipment (5) within the tubular element in such a manner that said electric conductor (6) be coaxial with the tubular element (4) to form a coaxial, microwave transmission line capable of propagating microwaves through the dielectric insulation (7), to thereby apply microwave energy to the infiltrated water.

6. A microwave energy applicator according to claim 5, characterized in that said means for mounting said equipment comprises annular support members (8, 9) inserted between the tubular element (4) in the electrotechnical equipment (5), said support members being made of a dielectric material transparent to microwaves and presenting passages (9′) from one side to the other side of said support members when inserted between the equipment (5) and the tubular element (4).

7. A microwave energy applicator according to claim 5, characterized in that the coaxial transmission line formed by the elongated conductor (6) in the tubular element (4) has first and second ends, and in that it further comprises:

- a first coaxial-to-coaxial transition (15) having a first end and having a second end connected to the first end of the transmission line; and

- a second coaxial-to-coaxial transition (16) having a first end and having a second end connected to the second end of the coaxial, microwave transmission line.

8. A microwave energy applicator according to claim 7, characterized in that the first end of the first transition (15) is closed by means of a first microwave window (2) and the first end of the second transition (16) is closed by means of a second microwave window (3), the coaxial, microwave transmission line including the elongated electric conductor (6), the tubular element (4), the first and second transitions (15, 16) and the first and second windows (2, 3) defining together an air-tight enclosure.

9. A microwave energy applicator according to claim 8, characterized in that:

- the tubular element (4) is circular in cross section and has a given inner diameter;

- the electric conductor (6) of the electrotechnical equipment is also circular in cross section and has a given outer diameter;

- each of the windows (2, 3) comprises:

. a section of circular waveguide (27, 30) having a given inner diameter;

. an elongated, central conductor (33, 34) circular in cross section, coaxial with the circular waveguide section and having a given outer diameter; and

. an annular stopper (35, 36) slidable in the sectional circular waveguide (27, 30) and on the central conductor of the window, this stopper being made of a dielectric material transparent to microwaves; and

- each of the transitions (15, 16) comprises:

- a frusto-conical waveguide section circular in cross section, interposed between the waveguide section of the corresponding window and the hollow tubular element, and having an inner diameter gradually varying from the inner diameter of the tubular element to the inner diameter of said circular waveguide section of the window; et

- an inner, taper electric conductor (19, 20) circular in cross section, interposed between the electric conductor of the electrotechnical equipment and the central conductor of the corresponding window, and having an outer diameter gradually varying from the outer diameter of the conductor of the electrotechnical equipment to the outer diameter of the central conductor of the microwave window.

10. A microwave energy applicator according to claim 9, characterized in that each of the microwave windows (2, 3) comprises a first O-ring (37, 38) forming a sealing joint between its stopper (35, 36) and its circular waveguide section (27, 30), and a second O-ring (39, 40) forming a sealing joint between its annular stopper (35, 36) and its central conductor (33, 34).

11. A microwave energy applicator according to claim 5, characterized in that the coaxial transmission line comprises first and second ends closed by means of first and second microwave windows, respectively.

12. A microwave energy applicator according to claim 11, characterized in that the tubular element (4) comprises an outlet conduit (10) for evacuating water vapor outside of the coaxial, microwave transmission line, the infiltrated water being transformed into water vapor upon application of microwave energy.

13. A microwave energy applicator according to claim 5, characterized in that the dielectric insulation (7) is impregnated with oil, and in that the tubular element (4) comprises at least one bottom outlet conduit through which oil of increased fluidity, from the insulation, is evacuated by gravity toward an oil trap (44).

14. An apparatus for drying water-permeable and microwave-permeable dielectric insulation in an electrotechnical equipment including an inner and elongated electric conductor wrapped in the insulation infiltrated with water, characterized in that it comprises:

- a hollow tubular element (4) made of electrically conducting material;

- means for mounting the electrotechnical equipment (5) within the tubular element, the electric conductor (6) being coaxial with the tubular element (4) thereby forming a coaxial, microwave transmission line;

- means for propagating microwaves through the coaxial transmission line and accordingly through the dielectric insulation to heat said water infiltrated within the dielectric insulation (7) and transform it into water vapor; and

- means (10, 43) for evacuating the water vapor outside of the coaxial, microwave transmission line, a transformation of water into water vapor and an evacuation of said water vapor outside of the transmission line causing drying of the dielectric insulation.

15. A dielectric insulation drying apparatus according to claim 14, characterized in that the microwave propagating means comprises:

- a high power microwave source (41) generating microwaves and connected to a first end of the coaxial, microwave transmiss ion line; and

- a matched load (42) connected to a second end of said coaxial line, whereby, in operation, the microwaves are transmitted from the high power microwave source to the matched load through the coaxial transmission line, the energy of the microwaves which is not absorbed by the water infiltrated within the dielectric insulation being absorbed by the matched load to prevent reflection of microwaves at the level of said load and production of a standing wave within the coaxial, microwave transmission line.

16. A dielectric insulation drying apparatus according to claim 14, characterized in that the means for propagating microwaves through the coaxial transmission line comprises:

- a high power microwave source (41) for generating microwaves;

- a microwave circulator (45) interposed between a first end of the transmission line and said microwave source;

- a waveguide short circuit (47) connected to a second end of the coaxial, microwave transmission line; and

- a matched load (46) connected to the circulator (45), whereby, in operation, the microwaves are transmitted to the short circuit from the high power microwave source through the circulator and the coaxial transmission line, said microwaves are reflected by the waveguide short circuit and transmitted to the matched load through the coaxial transmission line and the circulator, the energy of the microwaves reaching the matched load being absorbed by said matched load, and a standing wave with maximums and minimums being produced inside of the coaxial, microwave transmission line as the microwaves are reflected by the short circuit.

17. A dielectric insulation drying apparatus according to claim 16, characterized in that said waveguide short circuit (47) is an adjustable short circuit, which is moved to change the position of the maximums and minimums of the standing wave in the coaxial transmission line to provide for uniform heating of the infiltrated water throughout the dielectric insulation.

18. A dielectric insulation drying apparatus according to claim 14, characterized in that the water vapor evacuating means (10, 43) comprises means (43) for sucking said water vapor.

Images