Vacuum drying method for transformer body

Abstract

The present application belongs to transformer body drying, and particularly discloses a vacuum drying method for transformer body, which comprises the following steps: S1: placing the transformer body into a vacuum drying room and heating the transformer body through low-frequency heating; S2: evaporating water vapor in the vacuum drying room through steam heating; and S3: reducing the pressure in the vacuum drying room and discharging the evaporated water vapor out of the vacuum drying room. The present application combines low-frequency heating, high-temperature heating, multiple intermediate pressure reduction, low-vacuum pressure reduction and high-vacuum pressure reduction, fully heats and reduces the pressure, and reaches the lowest humidity, so as to guarantee the dryness of the transformer body. The present application effectively avoids the condensation of water vapor on the surface of the transformer body and causes the rust of the iron core, and also guarantees the physical and chemical properties of Midel7131 synthetic ester oil injected into the transformer, and further improves the product quality.

Description

Technical Field

[0001] This invention belongs to the field of transformer body drying technology, specifically relating to a vacuum drying method for transformer bodies. Background Technology

[0002] With the rapid development of AC / DC and new energy power systems in my country, the capacity of single transformers is increasing, and their operational reliability is of great concern to power users. To ensure manufacturing quality, power users are demanding increasingly lower partial discharge levels in transformers. The dryness of the transformer body is a crucial factor affecting the level of partial discharge.

[0003] Transformers often exhibit high partial discharge levels. Comparative analysis reveals that even transformers manufactured using the same blueprints at different times and with different batches of raw materials can show significant variations in partial discharge. This is particularly true for large-capacity transformers with core diameters exceeding 900mm, where extensive rust easily forms on the upper surface and sides of the upper yoke after the transformer body has dried. Rust, being a metallic oxide, can easily penetrate the transformer body along with Midel 7131 synthetic ester oil if not properly cleaned, affecting the insulation strength and severely impacting the physicochemical properties of the oil, posing a significant threat to product testing and operational reliability. Rust formation on the upper yoke surface is generally caused by uneven temperature distribution between the core and insulation surfaces during the transformer body drying process.

[0004] In existing technologies, the transformer body is usually dried by directly placing it into a vacuum drying chamber and then reducing the pressure inside the chamber. However, when a lot of water evaporates from the surface of the transformer body, if it encounters a nearby iron core with a lower temperature, the water vapor will condense and adhere to the surface of the iron core, which can lead to problems such as rusting of the upper yoke. Summary of the Invention

[0005] This invention provides a vacuum drying method for transformer bodies to solve the problem of poor drying effect in current transformer bodies.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is: a vacuum drying method for a transformer body, comprising the following steps:

[0007] S1: Place the transformer body in a vacuum drying chamber and heat the transformer body by low-frequency heating;

[0008] S2: Steam heating is used to evaporate water vapor in the vacuum drying chamber;

[0009] S3: Reduce the pressure inside the vacuum drying chamber to expel the evaporated water vapor from the vacuum drying chamber.

[0010] In a preferred embodiment of the present invention, in step S1, the frequency of low-frequency heating is obtained according to formula 1:

[0011]

[0012] Where f is the low-frequency power supply frequency; e t denoted as , where is the rated turn potential of the transformer at 50Hz; k is the withstand voltage of the turn insulation of the transformer body under non-oil immersion conditions; B is the rated magnetic flux density of the core column at 50Hz; and S is the effective cross-sectional area of ​​the core column.

[0013] In a preferred embodiment of the present invention, the low-frequency heating duration is 3.5 to 4.5 hours.

[0014] In a preferred embodiment of the present invention, in step S2, the steam temperature for steam heating is 110-140°C.

[0015] In a preferred embodiment of the present invention, in step S2, the temperature of the evaporation medium introduced during steam heating increases in a stepwise manner.

[0016] In a preferred embodiment of the present invention, step S2, in which the temperature of the steam increases in a stepwise manner, includes:

[0017] The evaporation medium is filled with evaporation medium at a temperature of 113-117℃ for 6 hours, then filled with evaporation medium at a temperature of 118-122℃ for 4 hours, and finally filled with evaporation medium at a temperature of 133-137℃ until the pressure in the vacuum drying chamber reaches 150mbar to complete this step.

[0018] In a preferred embodiment of the present invention, in step S3, the steam pressure in the vacuum drying chamber is adjusted from 145-150 mbar to 15-25 mbar.

[0019] In a preferred embodiment of the present invention, the pressure reduction in step S3 specifically includes multiple intermediate pressure reductions and vacuum pressure reductions, wherein the number of intermediate pressure reductions is obtained by Equation 2:

[0020]

[0021] Where: N represents the number of intermediate blood pressure reductions; S N U is the rated capacity of the transformer; U is the voltage level of the transformer; k is the thickness correction factor for the insulation components used in the transformer.

[0022] In a preferred embodiment of the present invention, the pressure reduction in step S3 specifically includes intermediate pressure reduction, low vacuum pressure reduction, and high vacuum pressure reduction, wherein the extraction power during high vacuum pressure reduction is at least 10,000 times that during low vacuum pressure reduction.

[0023] In a preferred embodiment of the present invention, the high vacuum depressurization in step S3 specifically includes:

[0024] When the lower viewing window of the main condenser in the vacuum drying chamber does not display condensate, the chamber pressure is checked. If the chamber pressure is greater than 20 mbar, low vacuum depressurization continues; if the chamber pressure is not greater than 20 mbar, high vacuum depressurization begins.

[0025] The high vacuum depressurization process ends when the pressure inside the room is less than 0.13 mbar.

[0026] The technical solution provided by this invention has the following advantages compared with the prior art:

[0027] This invention employs low-frequency heating followed by vacuum drying of the transformer body. Low-frequency heating allows the magnetic field to pass through the entire cross-section of the conductor, enabling rapid heating and high efficiency. Compared to traditional heating equipment, it is convenient and quick, eliminating the need for ovens, heaters, or other equipment, and directly heating the workpiece itself. Since the heat is generated internally, it is less affected by external conditions, significantly improving heating quality. Following low-frequency heating, kerosene steam heating is applied for more thorough heating. Multiple intermediate pressure reductions remove most of the water vapor, followed by a gradual process of low-vacuum and high-vacuum pressure reductions. This combination of low-frequency heating, high-temperature heating, multiple intermediate pressure reductions, low-vacuum pressure reductions, and high-vacuum pressure reductions ensures sufficient heating and pressure reduction to achieve the lowest possible humidity, guaranteeing the dryness of the transformer body. This invention improves the transformer core drying process, ensuring that the temperature of metal components near the transformer core insulation, such as the core and clamps, is controlled above 40°C before drying. This effectively prevents water vapor condensation on their surfaces, thus avoiding core rusting. Simultaneously, it preserves the physicochemical properties of the Midel 7131 synthetic ester oil injected into the transformer, thereby improving product quality. Therefore, this invention provides a convenient, practical, and effective vacuum drying method for Midel 7131 synthetic ester oil transformers with low-frequency heating, effectively preventing core rusting during the drying process of large transformers. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained from these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of a vacuum drying device for a transformer body according to an embodiment of the present invention.

[0030] As shown in the figure:

[0031] 10 - Body; 20 - Vacuum drying chamber; 30 - Steam generator; 40 - Low frequency power supply; 50 - Power cord; 60 - Quick cable plug-in terminal. Detailed Implementation

[0032] For ease of understanding, the following embodiments illustrate a vacuum drying method for a transformer body. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0033] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation and positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0034] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0035] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0036] like Figure 1 As shown, a vacuum drying device for a Midel 7131 synthetic ester oil transformer with low-frequency heating is disclosed. The device includes a transformer core 10, a vacuum drying chamber 20, a steam generator 30, a variable-frequency low-frequency power supply 40, a power cord 50, and a quick-connect cable terminal 60. Specifically, before the transformer core is heated for insulation drying, the transformer core is preheated, effectively preventing water vapor generated during the core drying process from condensing on the core surface.

[0037] The specific steps include:

[0038] S1: Place the transformer body in a vacuum drying chamber and heat the transformer body at low frequency.

[0039] Based on the transformer's technical parameters, select the frequency and output method of the low-frequency power supply.

[0040] Pressing the "Ready" button initiates the preparation phase. Except for the two fore-amplifier pumps VP127 and VP130, which require manual operation, all other equipment will be automatically controlled by the PLC. The PLC will automatically display the equipment's operating status and process parameters, and can also manually print process parameters at any time. Therefore, the equipment can only be considered to be in normal working condition once it enters the preparation phase. Taking the ODFS-400000 / 500 single-phase three-winding autotransformer as an example, its voltage combination is... The magnetic flux density of the core column at 50Hz is 1.696T; the no-load loss is P0 = 94.9kW; and the core cross-sectional area is 11531.554cm². 2 The core-column potential at 50Hz is 434.21V / turn.

[0041] Based on the technical parameters of the product to be dried, the frequency of the variable frequency low-frequency power supply output is determined by Equation 1:

[0042]

[0043] Where: f -- low-frequency power supply frequency, Hz;

[0044] e t --Rated turn potential of transformer at 50Hz, V / turn;

[0045] k--The dielectric strength of the turns insulation under non-oil immersion conditions is usually controlled at ≤15%;

[0046] B -- Rated magnetic flux density of the iron core column at 50Hz, in tons (T);

[0047] S -- Effective cross-sectional area of ​​the iron core column, in cm² 2 .

[0048] Since this product is a single-phase transformer, a single-phase output method is adopted when selecting the low-frequency power supply output for the frequency converter. To avoid excessive noise from the transformer core during the low-frequency heating process, which could affect the analysis and judgment of other possible fault types, and to ensure sufficient heat generation from the core, the magnetic flux density in the transformer core column is typically controlled between 0.7 and 0.85B; the transformer no-load loss is ≥80%P0.

[0049] Secondly, the low-frequency power supply and the transformer are connected via a power cable, with the variable-frequency power supply connected to the low-voltage coil of the transformer body being dried. The low-frequency power supply is located outside the vacuum drying chamber; the transformer body is placed inside the vacuum drying chamber; the connecting cable between the low-frequency power supply and the transformer body, which serves as the power line, needs to be connected via a quick-connect cable terminal arranged on the wall of the vacuum drying chamber.

[0050] The low-frequency power supply heating stage involves the transformer body undergoing low-frequency heating for 3.5 to 4.5 hours under normal pressure after the vacuum drying oven is closed. Then, the low-frequency power supply is turned off, and the steam heating stage begins.

[0051] Here, before heating the transformer body with kerosene and steam, the transformer core is first energized under no-load for 4 hours using a variable frequency low-frequency power supply to ensure that the surface temperature of the transformer core is not lower than 40°C. This ensures that the core is heated sufficiently and that the interior of the transformer body is heated evenly. It also improves drying efficiency and avoids prolonged heating, which would otherwise lead to resource waste.

[0052] The transformer body is then heated by a kerosene steam generator. The heating temperature of the evaporating medium in the evaporator exceeds 100 degrees Celsius, preferably 110-130 degrees Celsius, and increases in a stepwise manner.

[0053] The evaporation medium temperature is set above 100 degrees Celsius primarily to accelerate water vapor sublimation. The temperature rises in a stepwise manner rather than reaching the target temperature all at once. This is mainly to prevent damage to the transformer's insulating components. Excessive or rapid heating can cause the surface fibers of the insulating components to dry and shrink, while internal moisture begins to evaporate. This can ultimately create an excessive pressure difference between the inside and outside of the insulating components, leading to surface cracking. This ensures the stable drying process of the transformer.

[0054] The process involves filling the vacuum drying chamber with an evaporating medium at 113-117°C for 6 hours, followed by filling with an evaporating medium at 118-122°C for 4 hours, and finally filling with an evaporating medium at 133-137°C until the pressure inside the chamber reaches 150 mbar. Specifically, the heating temperature of the evaporating medium is controlled at 115°C, adjusted to 120°C after 6 hours, and then adjusted to 135°C after another 4 hours.

[0055] S3: Reduce the pressure inside the vacuum drying chamber.

[0056] As the heat carrier steam is introduced, the pressure inside the chamber rises to 150 mbar. To accelerate the dehumidification of the cellulose, during heating, depending on the size of the transformer, the inlet valve can be closed to achieve intermediate pressure reduction. Before pressure reduction, the steam valve and the inlet valve of the chamber should be closed to condense the carrier stored in the main insulation, reducing the steam pressure from 150 mbar to approximately 20 mbar. This lowers the pressure inside the chamber, lowering the sublimation point of water. In other words, at lower pressure, water can sublimate at a lower temperature, resulting in higher drying efficiency.

[0057] 1. When the temperature of the reactor body exceeds 95 degrees, multiple intermediate pressure reductions will be initiated.

[0058] The intermediate pressure reduction process is a crucial step in the heating phase of the drying equipment. It accelerates the heating rate of the drying chamber, shortens the overall processing time, and speeds up the evaporation of moisture from the insulating material. This is because as the temperature rises during heating, the moisture content inside the vacuum chamber increases, and the pressure gradually rises as well. The pressure is reduced to approximately 120-150 mbar.

[0059] When the transformer body temperature exceeds 95 degrees Celsius, intermediate voltage reduction is performed. The number of such reductions depends on factors such as the transformer's capacity, voltage level, and the size, specifications, and weight of the insulation materials used. Therefore, based on years of production experience, a calculation formula has been derived to guide production practice. The calculation method is Equation 2:

[0060]

[0061] In the formula: N represents the number of voltage reduction cycles, rounded up; S N U is the rated capacity of the transformer (MVA); U is the voltage level of the transformer (kV); k is the thickness correction factor for the insulation components used in the transformer. When the thickness of the insulation components is 50mm, take 1.0; when the thickness is 60mm, take 1.05; when the thickness is 70mm, take 1.1; when the thickness is 80mm, take 1.2.

[0062] Taking the above product as an example, the calculation result of its pressure reduction times is given by formula a:

[0063]

[0064] The 500kV transformer requires six intermediate voltage reductions. Each intermediate voltage reduction lasts for 1 hour, with a 5-hour interval between each reduction. After completing the intermediate voltage reductions, the condensate in the collection room must not increase for 5 consecutive hours, the transformer insulation temperature must be greater than 112 degrees Celsius, and the heating time must be no less than 40 hours.

[0065] 2. When the pressure inside the room drops to 140 mbar, turn off the intermediate pressure reduction and turn on the low vacuum pressure reduction.

[0066] When the pressure inside the chamber is approximately 140 mbar, and the chamber contains a certain amount of water vapor and kerosene vapor, with the temperature of the chamber walls and the chamber body around 125°C, a low-vacuum depressurization stage is adopted to create suitable conditions for high vacuum evacuation and to prevent a large amount of water and kerosene vapor from entering the main vacuum system. The low-vacuum time must not be less than 2 hours, and the vacuum is evacuated until the pressure inside the vacuum chamber is less than 2.0 kPa (20 millibars).

[0067] 3. When there is no condensate in the lower viewing window of the main condenser, check the pressure inside the chamber. When the pressure inside the chamber drops to 20 mbar, activate the high vacuum depressurization function.

[0068] The extraction power during high vacuum depressurization is at least 10,000 times that during low vacuum depressurization.

[0069] For single products of 500kV or above, the high vacuum time is more than 48 hours; for combined furnace products, the high vacuum time is more than 60 hours, and the pressure inside the vacuum chamber indicated by the resistance vacuum gauge is less than 13Pa.

[0070] Vacuum drying is carried out in the high vacuum stage until the vacuum level inside the chamber is ≤0.2mbar, the core temperature is ≤130℃ and the insulation material temperature is ≤130℃, and then it stops.

[0071] Measure the water yield of the insulation material three times consecutively (within 1 hour). For a 500kV transformer, the water yield should be ≤5g / h. If the filling process needs to be started in the shortest possible time, the furnace door may be opened ahead of schedule after verifying all data and obtaining written consent from the workshop supervisor, provided that all other requirements are met.

[0072] After the reactor body is dried by kerosene vapor phase drying, when the drying temperature reaches 120-125℃, the vacuum degree reaches 13Pa, the water content of the insulation material is <10ml / t·h (t is the total weight of the insulation material in tons, and h is the time in hours), and the water content is 0.05-0.15%, the reactor body drying treatment is considered qualified.

[0073] Finally, shut off the pressure reduction, end the drying process, and remove the furnace. Use a dry air generator to release the vacuum, open the vacuum chamber, and immediately lift the furnace body to the assembly area.

[0074] This invention employs low-frequency heating followed by vacuum drying of the transformer body. Low-frequency heating allows the magnetic field to pass through the entire cross-section of the conductor, enabling rapid heating and high efficiency. Compared to traditional heating equipment, it is convenient and quick, eliminating the need for ovens, heaters, or other equipment, and directly heating the workpiece itself. Since the heat is generated internally, it is less affected by external conditions, significantly improving heating quality. Following low-frequency heating, kerosene steam heating is applied for more thorough heating. Multiple intermediate pressure reductions remove most of the water vapor, followed by a gradual process of low-vacuum and high-vacuum pressure reductions. This combination of low-frequency heating, high-temperature heating, multiple intermediate pressure reductions, low-vacuum pressure reductions, and high-vacuum pressure reductions ensures sufficient heating and pressure reduction to achieve the lowest possible humidity, guaranteeing the dryness of the transformer body.

[0075] This invention improves the transformer core drying process, ensuring that the temperature of metal components near the transformer core insulation, such as the core and clamps, is controlled above 40°C before drying. This effectively prevents water vapor condensation on their surfaces, thus avoiding core rusting. Simultaneously, it preserves the physicochemical properties of the Midel 7131 synthetic ester oil injected into the transformer, thereby improving product quality. Therefore, this invention provides a convenient, practical, and effective vacuum drying method for Midel 7131 synthetic ester oil transformers with low-frequency heating, effectively preventing core rusting during the drying process of large transformers.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A vacuum drying method for transformer body, characterized in that, Includes the following steps: S1: Place the transformer body in a vacuum drying chamber and heat the transformer body by low-frequency heating; S2: Steam heating is used to evaporate water vapor in the vacuum drying chamber; S3: Reduce the pressure inside the vacuum drying chamber to expel the evaporated water vapor from the vacuum drying chamber; In step S1, the frequency of low-frequency heating is obtained according to equation 1: Where f is the low-frequency power supply frequency; e t denoted as , where is the rated turn potential of the transformer at 50Hz; k is the withstand voltage of the turn insulation of the transformer body under non-oil immersion conditions; B is the rated magnetic flux density of the core column at 50Hz; and S is the effective cross-sectional area of ​​the core column.

2. The vacuum drying method for a transformer body according to claim 1, characterized in that, The low-frequency heating duration is 3.5 to 4.5 hours.

3. The vacuum drying method for a transformer body according to claim 1, characterized in that, In step S2, the steam temperature for steam heating is 110-140℃.

4. The vacuum drying method for a transformer body according to claim 1, characterized in that, In step S2, during the steam heating, the temperature of the evaporation medium increases in a stepwise manner.

5. The vacuum drying method for a transformer body according to claim 4, characterized in that, In step S2, the temperature of the steam increases in a stepwise manner, including: The evaporation medium is filled with evaporation medium at a temperature of 113-117℃ for 6 hours, then filled with evaporation medium at a temperature of 118-122℃ for 4 hours, and finally filled with evaporation medium at a temperature of 133-137℃ until the pressure in the vacuum drying chamber reaches 150mbar to complete this step.

6. The vacuum drying method for a transformer body according to claim 1, characterized in that, In step S3, the steam pressure in the vacuum drying chamber is adjusted from 145-150 mbar to 15-25 mbar.

7. The vacuum drying method for a transformer body according to claim 1, characterized in that, The pressure reduction in step S3 specifically includes multiple intermediate pressure reductions and vacuum pressure reductions. The number of intermediate pressure reductions is obtained using Equation 2: Where: N represents the number of intermediate blood pressure reductions; S N U is the rated capacity of the transformer; U is the voltage level of the transformer; k is the thickness correction factor for the insulation components used in the transformer.

8. The vacuum drying method for a transformer body according to claim 1, characterized in that, The pressure reduction in step S3 specifically includes intermediate pressure reduction, low vacuum pressure reduction, and high vacuum pressure reduction, wherein the extraction power during high vacuum pressure reduction is at least 10,000 times that during low vacuum pressure reduction.

9. A vacuum drying method for a transformer body according to claim 8, characterized in that, The high-vacuum depressurization process in step S3 specifically includes: When the lower viewing window of the main condenser in the vacuum drying chamber does not display condensate, the chamber pressure is checked. If the chamber pressure is greater than 20 mbar, low vacuum depressurization continues; if the chamber pressure is not greater than 20 mbar, high vacuum depressurization begins. The high vacuum depressurization process ends when the pressure inside the room is less than 0.13 mbar.

Images