EP4657472B1

EP4657472B1 - Method and device for drying insulation materials of high voltage electrotechnical equipment, in particular transformers

Info

Publication number: EP4657472B1
Application number: EP24178386.9A
Authority: EP
Inventors: Martin Carlen
Original assignee: Individual
Current assignee: Individual
Priority date: 2024-05-28
Filing date: 2024-05-28
Publication date: 2026-04-08
Anticipated expiration: 2044-05-28
Also published as: WO2025248375A1; EP4657472A1

Description

Technical Field

The invention pertains to a method and a device for drying insulation materials of high voltage electrotechnical equipment, in particular transformers. The method relates to a process for drying the insulation materials used in coils for transformers, active parts and other components of transformers.

Technical Background

In oil-filled transformers, the insulation materials are mostly made of cellulose-based material. Such materials - being electrical papers, pressboard, or shaped insulating components - can well be impregnated with oil, easily handled, and formed into different shapes, and are belonging to the most cost-effective insulation materials. The main purpose of the insulation is to avoid direct contact between conducting parts being at different electrical potential, to stop discharging events in case of overvoltage, and to mechanically fix components. It enables compact transformer designs.
A drawback of the use of cellulose-based insulation material is its hygroscopic characteristics and absorbance of humidity and water. Under usual ambient conditions, the water content in cellulose-based materials is typically a few percent. The water entering the insulation materials is adsorbed and absorbed from the moisture in the air. Water uptake occurs within hours to days.
Moisture affects the dielectric performance of the solid insulation. It accelerates the aging of cellulose insulation and considerably shortens the lifetime of a transformer. By hydrolysis it can further degrade other than cellulose-based insulation and cause chemical reactions like oxidation of the transformer tank. At high load respectively high temperature, vapor bubbles can form and cause partial discharge, potentially leading to damaged insulation and transformer failures. Moisture also negatively affects the dielectric performance of the transformer oil, e.g. reducing its volume resistivity. At 20°C, saturation of mineral oil with water occurs already at 50 ppm. Water bubbles can emerge in case of higher water content or further reduced temperature, e.g. if the transformer is operated at low load. As a rule of thumb, it is assumed that an increase of the moisture content by 1% reduces the lifetime of a transformer by a factor of two.
For high transformer quality, it is of fundamental importance that the transformer contains as little moisture as possible. A typical requirement is that the water content shall be below 0.5%, but, depending on the type of transformer, the requirement can also be as low as 0.1%. However, removing moisture from cellulose-based insulation takes a lot of energy and time, especially for transformers for higher voltage classes, which require more massive pieces of insulation. While for distribution transformers the typical duration for drying is up to a day, for transformers of the subtransmission voltage range it takes several days and for transmission transformers it is one to several weeks. This may limit the manufacturing throughput, require high investments in equipment for the drying process and significantly adds to the overall time needed for the transformer manufacturing. Application of a most efficient drying process has therefore important implications and constitutes a competitive advantage.
Drying occurs by removing water by evaporation from a solid or from a liquid. There are three main physical parameters which define the dynamics of a drying process, and which can be influenced: temperature, moisture content of the surrounding gas medium and its gas pressure. An increase in temperature increases the evaporation rate, reduces the relative humidity of the ambient medium and increases molecular diffusion and water migration in the cellulose insulation. A low moisture content in the surrounding medium reduces the recondensation of water and increases the amount of vapor that can be taken up. A reduction in gas pressure reduces the boiling point and the vapor pressure.
A combination of increased temperature and reduced pressure constitutes therefore a good system for drying. The increase of temperature requires a heating process. If it is done by a heat transfer medium, heating and reduction of pressure need to be done in cycles, since at low pressure the amount of heat transferred will be very low. Further care must be taken that the temperature is gradually increased and does not exceed a certain maximum temperature, otherwise the insulation may get damaged.
For the drying of transformer components, basically the following systems are presently in industrial use:

Hot air processing: the heat transfer is done by hot air. Since air has low density, the rate of energy transfer is not very high. Evaporation of water can be accelerated by lowering pressure in a next step. Heating and drying may be done in cycles. Hot air drying is insufficient for drying of transformers for higher voltage classes.
Oil spray: hot transformer oil is sprayed on the transformer components and heats them up while vacuum can be pulled. Lighter fractions of transformer oil are removed with the vacuum, changing the viscosity of the oil. Oil impregnation of the insulation reduces the diffusion coefficient and elimination of the moisture becomes more difficult. Scrubbers should be applied to the exhaust of vacuum pumps to eliminate hydrocarbon gases.
Vapor phase drying: the heat transfer is done by a selected hydrocarbon with a relatively low and narrow boiling point (mostly kerosene) which is evaporated and condenses on the transformer components. By releasing the latent heat of evaporation, a high heat transfer to the transformer components is achieved. The moisture is removed in a next phase by pulling vacuum. Also, in this case heating and vacuum phases alternate. Since hydrocarbon and oxygen can constitute explosive mixtures, appropriate caution and equipment must be used and scrubbers should be applied to the exhaust of vacuum pumps.
Low frequency heating (LFH): heating is achieved by applying a low frequency current (typically in the range of 1 Hz) to the primary winding, while the secondary winding is short-circuited. At the same time vacuum is pulled for enhancing the evaporation of the moisture. Since the transformer is not yet filled with oil and the breakdown strength is further reduced due to low air pressure, its dielectric strength is low. For providing sufficient current for heating of the windings only a low frequency can be applied, which requires a low enough supply voltage. LFH gives fast heating of the conductor and the conductor insulation. Insulating pieces not directly connected to a conductor like barriers or clamping rings will require more time to sufficiently heat. A potential handicap of LFH is also that LV and HV windings are heating up differently and local hot spots can occur.

LFH can be combined with oil spraying, providing a more homogenous temperature distribution, but making the system more complex. It can also be combined with hot air, but in that case heating and vacuum phases need to be cycled.
With the above methods it is not possible to selectively heat mainly the insulation of the transformer, but either the whole active part or transformer component is heated, or the conductors are heated to a similar or stronger extend than the insulation.
In US4889965A a method and an apparatus are presented for the drying of paper insulation of high voltage electrotechnical equipment, especially bushings for transformers, by using microwaves for heating. The method requires a central metallic conductor around which insulation paper is wrapped, together with a surrounding metallic and air-tight cylinder, where the central and outer metallic parts constitute a microwave transmission line. Microwaves are propagated through the coaxial transmission line to heat the water in the dielectric insulation. Water vapor is evacuated through a vacuum pump or a cold trap.
In CN111520978A a device for drying solid insulation of transformers is presented, containing a microwave generator, a power supply, a switch, an exhaust fan, a dry air generator and a cabinet made of sealed iron plates. Microwaves are used for heating while dried air is provided to the cabinet and removed by a fan.
In CN113915960A a low-temperature vacuum drying device for a transformer is presented, consisting of a vacuum tank connected to a vacuuming system, a heating system, a condensing system, a microchannel filling system, together with a temperature and a pressure sensor and a PLC controller. The heating system consists of a terahertz wave generator and the inner walls of the tank are made of stainless steel. Water has a very high absorption coefficient for terahertz radiation, what makes its penetration distance very small (<0.1 mm) and deposits the radiation energy into a small volume. The heating power needs to be chosen rather low, resulting in a long process time. Otherwise there is high risk for creating hot spots and damaging of the insulation. There are no requirements for short-circuiting or grounding conductors or other metallic parts.

Summary of the Invention

It is an object of the present invention to provide a method allowing to pass the drying process faster and with lower energy requirements than the known drying methods. Another object of the invention is to present a configuration of devices for performing the drying process.
These objects are achieved by the features of claim 1 and claim 11.
In accordance with an aspect of the present invention according to claim 1 a method for removing moisture from insulation materials used in transformer components is described, comprising a first step wherein the transformer components are loaded into a cabinet with metallic inner surfaces, a second step wherein the metallic parts or the transformer components are grounded to earth potential and/or the windings of the transformer components are short-circuited, a third step wherein the cabinet is closed and made airtight, a fourth step wherein a vacuum pump is launched and the air pressure in the cabinet is reduced to a value less than 10 kPa, a fifth step wherein one of several microwave generators are energized and the transformer components are irradiated by electromagnetic waves at frequencies in the range of 0.5 to 500 GHz. In a further aspect according to claim 2 the intensity of radiation by the one or several microwave generators is variable.
In a further aspect of the method the intensity of radiation by the one or several microwave generators may be variable.
In another aspect of the method the air pressure in the fourth step may be reduced to a value less than 500 Pa.
In another aspect of the method the frequency of the microwave radiation may be in the range 2 to 20 GHz.
In a further aspect of the method the fourth step and the fifth step may be inverted or started in parallel.
In another aspect of the method the one or several microwave generators may be placed outside the cabinet and the electromagnetic radiation may be directed by waveguides from the one or several microwave generators into the cabinet.
In a further aspect of the method in a step after the second step the microwave generators and/or waveguides may be placed and adjusted in the cabinet.
In another aspect according to claim 8 the temperature at different locations in the cabinet are supervised by temperature sensors in order to control the voltage of the microwave generators and therefore the intensity of the electromagnetic radiation.
In a further aspect of the method the vacuum or underpressure in the cabinet may be measured in order to have control over the underpressure and to identify the existence of leaks and wherein the underpressure is generated in order to improve the evaporation of the moisture in the cabinet and the air humidity in the cabinet is measured and the total amount of humidity removed from the transformer components is determined.
In another aspect of the method in a further step it may be decided if the underpressure or vacuum and the humidity are below a target value, and if no, the process is continued and the one or several microwave generators may be further energized.
In a further aspect of the method after sufficiently removing moisture from the transformer components the vacuum pump may be stopped, the cabinet may be ventilated and opened, the earthing connections and the short circuits may be disconnected and the transformer components may be placed into a tank to fill with oil and/or to be further processed.
As a further aspect of the invention according to claim 11 a device for removing moisture from insulation materials used in transformer components is provided, which comprises a cabinet with inner metallic surfaces, having a ceiling, side walls, a floor and a front door, one or several microwave generators placed inside the cabinet or placed outside the cabinet, whereas waveguides are provided to direct the radiation from the one or several microwave generators into the cabinet, and a vacuum pump to reduce the underpressure in the cabinet and to extract the moisture.
In a further aspect of the device the inner metallic surfaces of the cabinet may comprise plates, foils, grids or meshes.
In a further aspect of the device the inner metallic surfaces may comprise steel, aluminum, copper or silver.
In a further aspect of the device the microwave generator may be a magnetron.
In a further aspect of the device the microwave generator may be a solid state microwave generator.
In a further aspect of the device the microwave generator may generate radiation in the frequency range of 0.5 to 500 GHz, preferably 2 to 20 GHz.
The advantages of the present invention over the known methods for removing moisture are apparent and can be retrieved from the following description.

Brief Description of the Figures

In the following, the invention is described in greater detail, by way of example, with reference to the accompanying drawings.

Fig. 1: a flow diagram of the different steps of the present drying method,
Fig. 2: a configuration of the device for executing the method,
Fig. 3: the absorption coefficient of electromagnetic radiation versus frequency by water,
Fig. 4: the phase diagram of water, and
Fig. 5: the Paschen diagram of air.

Detailed description

In Fig. 1 the different steps for drying transformer components containing humid insulation are depicted, which are needed to be dried prior to be inserted into a transformer tank or to be further processed. The method consists in placing one or several transformer components into a cabinet which can be closed airtightly and which is suitable to reduce the cabinet's air pressure by means of a vacuum pump. The cabinet is equipped with at least one or more sources for microwave radiation allowing to radiate the transformer components to be dried. The sources can be microwave generators or similar generators of microwaves which are placed inside the cabinet or waveguides bringing the radiation from microwave generators placed outside the cabinet into the cabinet. The sources are placed in a way that the irradiation of the transformer components is optimized. The inner walls of the cabinet are made of or equipped with microwave reflecting material. In step 1 the transformer components are loaded into a cabinet. In step 2 the metallic parts are grounded to earth potential and the windings are short-circuited. The electrical connection to earth potential of the metallic parts can be made by metallic wires, strips and other means which are clamped or mounted in a different way. The microwave generators and/or waveguides are adjustly placed in the cabinet in step 3. Thereafter, in step 4 the cabinet is closed and made airtight and in step 5 the vacuum pumps are launched and the air pressure in the cabinet is reduced. In step 6 the microwave generators are energized and the transformer components are irradiated. The insulation used in the transformer components starts to become heated. These two steps 5 and 6 can also be inverted or started in parallel. Then the temperature at different locations in the cabinet are supervised by temperature sensors in step 7. Thereupon in step 8 the power of the microwave generators is adjusted according to the measured temperature. In step 9 the vacuum or underpressure in the cabinet is measured and in step 10 the humidity in the cabinet is measured. The condensation of the moisture allows to determine the total amount of humidity removed from the transformer components. Observation of the development of the pressure together with the humidity in the exhaust air allows to identify the existence of leaks in the cabinet and the total amount of water removed from the insulation. In step 11 it is decided if the underpressure or vacuum and the humidity are below target, and if no, steps 7 to 10 are repeated, and if yes, the following steps are executed, i.e. in step 12 the microwave generators are de-energized and in step 13 the vacuum pumps are stopped. Then in step 14 the cabinet is ventilated and in step 15 the cabinet is opened. At the end in step 16 the earthing connections are disconnected and the transformer components removed from the cabinet and further processed.
Active parts are introduced and fixed into the transformer tank, bolts and nuts retightened, necessary electrical connections are made and the cover of the tank is placed. This needs to be quickly executed in order to avoid a new absorption of humidity from the air by the insulation materials. For power transformers a maximum duration of 3-12 hours is allowed for this process, depending on the ambient humidity. A vacuum pump is then connected to the transformer tank and vacuum is drawn to remove the newly absorbed moisture before filling the tank with transformer oil.
After removing dried windings from the cabinet, the windings are placed in a winding press and compacted. The removal of the humidity in the insulation materials made them slightly shrink and loose. The pressing restores their mechanical stability. The windings are then mounted on the transformer core.
In case dried transformer components are not immediately further processed they can also be stored under an ambient of dry air or another dry gas.
In Fig. 2 a configuration for the device executing the method is shown. The configuration consists of a cabinet 20 having a ceiling 21, side walls 22, a floor 23 and a front door 25. The cabinet 20 and the front door 25 are constructed in a way that they are airtight and can support a reduced air pressure compared to the ambient pressure. The transformer components to be dried are placed in the oven. The figure shows a transformer active part or components 27 and two windings 28 which can be placed simultaneously in the cabinet 20. The metallic parts of the transformer components 27 and the windings 28 are grounded electrically by earth connections 29.
The cabinet ceiling 21, side walls 22 and floor 23 are equipped with a number of microwave generators 30 generating the microwave radiation. In a preferred embodiment the microwave generators 30 are placed on first rails 32 so that they can be moved and optimally be placed. The first rails 32 themselves can be place on another set of second rails 34 allowing a movement of the microwave generators 30 which is 90° with respect to the first rails 32.
A vacuum pump 35 is connected to the cabinet. The cabinet 20 contains a device 36 for measuring the air pressure. It also contains a device 37 for measuring the humidity of the air. Several temperature sensors 38 are further placed in the cabinet 20, allowing to supervise and control the temperature of the transformer components.
The microwave generator 30 can be a magnetron or a solid-state microwave generator. The intensity of radiation from the microwave generator 30 can be variable.
Regarding the use of microwave radiation for the heating process of the insulation, the penetration depth of electromagnetic radiation of the frequency range 0.5-500 GHz in water is between about 1000 mm and 0.1 mm as can be seen in Fig. 3. This corresponds to the typical dimensions of transformer components and their insulation materials. The absorption coefficient of water for radiation of this frequency range is typically by a factor of 1000 higher than for most other materials. Preferably the microwave frequency range to be used for the drying process of this invention is between 2 GHz and 20 GHz.
The penetration depth of the microwave radiation in water is in the order of centimetres. Since in the case of moist insulation the water content in the insulation is rather small, the microwave penetration depth is much larger, providing a homogeneous heating of the insulation over its whole thickness.
The use of such radiation allows therefor a targeted and efficient heating of materials used for transformers which contain water or moisture. The by far largest part of the radiation energy is absorbed by the materials to be dried. Due to this dedicated process the heating is fast and requires less energy than other heating methods.
The mass of the active part of a 40 MVA / 110 kV transformer is over 36 tons. It mainly consists of electrical steel, copper conductor and cellulose-based insulation materials, whereas the weight of the insulation is 2-7% of the mass of the active part. For heating the active part from 20°C to 120°C and evaporating a humidity content of 5% of the insulation, an energy of about 450 kWh is required. When using the microwave drying process the microwave energy required for heating is about 300 kWh. The conventional drying method requires therefore about 50% more energy. In both cases the latent heat of evaporation is included in the calculation. The heating of the cabinet and heat losses through the cabinet are not considered in this calculation but are higher in case the heating of the insulation does not occur in a targeted way as in case of the microwave heating.
Selection of a specific radiation frequency allows to adjust the heating process of the insulation to a given transformer design. In case of transformers for lower volt-age classes, using thinner insulation thickness and small distances between parts at different electrical potential, higher frequencies can be used in order to easier penetrate the transformer component structure and to deposit the radiation energy in thin insulation. In case of transformers of higher voltage classes, the use of lower radiation frequencies allows to heat up quite uniformly larger pieces of cellulose-based materials. By varying the radiation frequency during the heating process, it is possible to reach different locations, to achieve a more uniform heating and a uniform temperature and to accelerate the heating process.
The use of electromagnetic radiation of the specified frequency range has further advantages by offering quasi self-adjusting processes. Since the radiation is mainly absorbed by water, with drying out of the insulation, less energy will be absorbed by the already dried insulation, allowing the radiation to reach regions which still contain a higher content of water. The second effect is that the dielectric loss of water reduces with increasing temperature, leading to reduced heating of warmer insulation and increased heating of colder insulation.
The absorbed heat will of course also transfer from the moisture containing insulation materials to the other materials. To keep the temperature at a certain level, after the initial heating, further radiation needs to be supplied. Since the thermal conductivity of the insulation is rather low, it does not quickly cool down. From the moist insulation on the conductors the heat will flow to the metallic conductors, which have high thermal conductivity and distribute the heat homogenously over the whole winding. Therefore, also winding parts are heated up where no direct access of radiation is possible and the risk for local overheating is low.
In a transformer, the magnetic core consisting of grain oriented electrical steel is the component with the highest mass. It contains no or very little cellulose-based insulation. By using the drying methods based on hot air, oil spray or vapor phase, the core is also heated. This is unnecessary for the drying process, requires a lot of energy and slows down the process. By using the specified electromagnetic radiation, the core is only indirectly heated and only slightly warmed up, saving a lot of energy and allowing a faster cooling and further processing after the drying process. By using low frequency heating, the core is also not directly heated. But also insulating parts which are not directly connected to the windings, are only indirectly heated and require a lot of time to get to an adequate temperature. Industrially LFH is therefore mainly used for distribution transformers of not too high ratings, whereas the method disclosed herein can be used for all types of oil-filled transformers.
Electromagnetic waves induce electrical fields and currents in conductors and metallic parts. Such fields can yield to electric discharges and breakdowns between turns in windings or between metallic parts. As in the case of drying by low frequency heating, since the transformer components are not immersed in insulating oil, the dielectric strength is low, especially if the surrounding pressure is reduced. By short-circuiting the windings and earthing them, as well as earthing of other metallic parts, discharges are avoided. Earthing of windings is a key factor for avoiding discharges and attaining a high-quality drying process. With earthed components, no electric field between parts will establish, in contrary to the LFH method.
Application of the electromagnetic radiation to the transformer components has to be done in an enclosure or cabinet 20 which prevents the exit of radiation. This is for safety reasons to avoid that people or sensitive equipment in the surroundings are irradiated or that the radiation creates electromagnetic disturbances. It can easily be realized by cabinet 20 using a Faraday cage type or a metallic box. Metals are mainly reflecting the electromagnetic radiation so that it stays within the enclosure or cabinet 20. This can also be realized by mounting a metallic mesh with small enough mesh size with respect to the applied radiation wavelength, metallic plates, or a metallic foil on the inner surface of an enclosure. The enclosure or cabinet 20 itself can in that case also be made of non-metallic materials. In a preferred embodiment of the invention, the reflecting inner walls of the enclosure or cabinet 20 consist, at least partly, of metals with especially high electric conductivity like aluminum, copper or silver.
The one or several sources of the electromagnetic radiation or microwave generators 30 can be placed at any appropriate location within the cabinet 20. Due to the reflection of radiation on the walls all parts of the transformer components 27 will be reached. Within the coils and between the different inner and outer windings the radiation which is not absorbed is also reflected and further distributed within the coils. The metallic magnetic core also reflects most of the radiation. Placement of the radiation sources is preferentially done above or below transformer coils.
The one or several generators of the electromagnetic radiation or microwave generators 30 can also be placed outside the cabinet 20, either directly mounted on the cabinet 20 or placed further off. One or several windows transparent for the electromagnetic radiation need to allow entering of the radiation into the cabinet 20. The one or several generators 30 can also be placed further off the cabinet 20, while the radiation is guided to the cabinet 20 by waveguides suitable for the respective frequency.
The microwave generators 30 and power supplies may need to be cooled for being able to provide sufficient radiation power for an efficient heating of the transformer components 27. Cooling by a water circuit with metallic pipes can be an efficient cooling method, also if part of the circuit is placed in the cabinet 20 since the cooling water is shielded by the metallic pipes. Alternatively cooling by an oil circuit can be used, where the oil absorbs the electromagnetic radiation much less.
A reduction of the surrounding gas pressure allows to reduce the boiling point of water to a lower temperature and to reduce the vapor pressure as can be seen in the phase diagram of Fig. 4. A reduction of the atmospheric pressure of around 100 kPa to 10 kPa reduces the boiling point from 100°C to below 50°C. Evaporation of water and drying is therefore much accelerated. For an efficient drying process, the encasing has to be airtight. It needs to be constructed in a way that it can withstand the forces caused by applying vacuum. Reduced pressure is achieved by connecting and operating a vacuum pump to the enclosure. The application of reduced pressure has the additional advantage that convective heat transfer from the transformer components to the encasing is reduced or eliminated, resulting in energy savings. The air pressure in the encasing should be reduced to below 10 kPa. In a preferential embodiment the pressure is reduced to below 500 Pa.
As showed in Fig. 5 (see W. Carey et al, "Characterization of Paschen curve anomalies at high p*d values", Proc. IEEE Pulsed Power Conf., Chigago, 19-23 June 2011) the dielectric strength of air decreases with reduced pressure according to the Paschen curve to a minimum of about 330 V at p*d = 0.1 kPa*cm, where p is the pressure in Pascal and d the gap distance in cm. For a given distance of 1 cm, the dielectric strength decreases by about a factor of 100 if the ambient pressure is reduced from 100 kPa to 100 Pa. A correct earthing of metallic parts is therefore even more important.
As described, the present method allows to apply reduced pressure while at the same time heating the insulation of the transformer components. Since no alternation of these two steps is needed, the duration for drying is considerably reduced. Due to the earthing of metallic parts, electrical discharges which could damage the insulation or the equipment are avoided so that high microwave radiation intensity can be used and a high heating rate is applied. While the typical drying time for a 40 MVA / 110 kV transformer is 3-4 days, the present method allows to reduce the drying time to about 2 days. This allows to carry out 3 instead of 2 drying processes per week, a 50% increase of throughput.
As already mentioned above, waveguides can be used to bring the electromagnetic radiation to a specific location. This can be 1) to bring the radiation to and into the cabinet; 2) to bring it to specific locations of the transformer components which are otherwise only slightly heated, 3) to concentrate the radiation to specific locations. Waveguides can have a straight or bended geometry. The use of waveguides allows to optimize and homogenise the heating process. Waveguides are hollow metal pipes. The microwaves are imagined as travelling down the guide by being repeatedly almost fully reflected by the walls. The cross section of the waveguides is selected to the according to the microwave frequency with standardized sizes being commercially available.
Cellulose-based transformer insulation should not be heated above 120°C during the drying process, otherwise depolymerization and degradation of the insulation occurs. It is therefore advisable to observe the temperature of the insulation during the application of electromagnetic waves and use the readings to control the input power of the radiation.
This is done by using at least one temperature sensor, which is placed in the encasing. In a preferred embodiment, the temperature sensor is an IR camera which allows to observe a large part of the transformer components. In case standard transformer components are dried for which the heating behaviour is known and a heating rate is predefined, the process can also be run without a temperature control, based on a standard heating profile.
The energy put into the cabinet and the insulation can be controlled by either turning the generator on and off, or by using a generator with controllable radiation intensity. The later offers fast heating at the beginning and the possibility to keep the temperature smooth and constant once the set-temperature is achieved. However, a well controllable generator will be more expensive.

Claims

Method for removing moisture from insulation materials in transformer components with metallic parts and/or windings comprising the following order of steps: a first step (1) wherein the transformer components are loaded into a cabinet with metallic inner surfaces, a second step (2) wherein the metallic parts of the transformer components are grounded to earth potential and/or the windings of the transformer components are short-circuited, a third step (4) wherein the cabinet is closed and made airtight, a fourth step (5) wherein a vacuum pump is launched and the air pressure in the cabinet is reduced to a value less than 10 kPa, a fifth step (6) wherein one or several microwave generators are energized and the transformer components are irradiated at frequencies in the range of 0.5 to 500 GHz; wherein the fifth step (6) is started after, before or in parallel with the fourth step (5).
Method according to claim 1, wherein the intensity of radiation by the one or several microwave generators is variable.
Method according to claim 1 or 2, wherein the air pressure in the fourth step is reduced to a value less than 500 Pa.
Method according to claim 1 to 3, wherein the frequency of the microwave radiation is in the range 2 to 20 GHz.
Method according to one of claims 1 to 4, wherein the one or several microwave generators are placed outside the cabinet and the electromagnetic radiation is directed by waveguides from the one or several microwave generators into the cabinet.
Method according to one claims 1 to 5, wherein in a step (3) after the second step (2) the microwave generators and/or waveguides are placed and adjusted in the cabinet.
Method according to one of claims 1 to 6, wherein the temperature at different locations in the cabinet are supervised by temperature sensors in order to control the voltage of the microwave generators and therefore the intensity of the electromagnetic radiation.
Method according to one of claims 1 to 7, wherein the vacuum or underpressure in the cabinet is measured in order to have control over the underpressure and to identify the existence of leaks and wherein the underpressure is generated in order to improve the evaporation of the moisture in the cabinet and the air humidity in the cabinet is measured and the total amount of humidity removed from the transformer components is determined.
Method according to one of claims 1 to 8, wherein in a further step (11) it is decided if the underpressure or vacuum and the humidity are below a target value, and if no, the process is continued and the one or several microwave generators may be further energized.
Method according to one of claims 1 to 9, wherein after sufficiently removing moisture from the transformer components the vacuum pump is stopped, the cabinet is ventilated and opened, the earthing connections and the short circuits are disconnected and the transformer components are placed into a tank to fill with oil and/or to be further processed.
Device for performing the method of one of the proceeding claims comprising a cabinet (20) with inner metallic surfaces, having a ceiling (21), side walls (22), a floor (23) and a front door (25), one or several microwave generators (30) placed inside the cabinet (20) or placed outside the cabinet (20), whereas waveguides are provided to direct the radiation from the one or several microwave generators (30) into the cabinet (20), and a vacuum pump (35) to reduce the air pressure in the cabinet (20) and to generate underpressure and to extract moisture.
Device according to claim 11, wherein the inner metallic surfaces of the cabinet (20) comprising plates, foils, grids or meshes and/or the inner metallic surfaces comprising steel, aluminum, copper or silver.
Device according to one of claims 11 to 12, wherein the microwave generator is a magnetron or a solid-state microwave generator.
Device according to one of the claims 11 to 13 wherein the microwave generator generates radiation in the frequency range of 0.5 to 500 GHz, preferably 2 to 20 GHz.