A staged casting process for epoxy resin coils with fillers

Abstract

This invention relates to the field of dry-type power transformer manufacturing technology, and in particular to a staged casting process technology for epoxy resin coils with fillers. The process includes coil drying, epoxy resin preparation, vacuum casting, semi-gelling, secondary vacuum casting, curing, and demolding. The coil drying process involves: epoxy resin preparation; vacuum casting: the vacuum level in the casting tank reaches 3-4 mbar, and casting is performed at a temperature of (Tn-10)℃ using intermittent stroke casting with an interval of 1.5-2 minutes between each stroke; and a pause of 8 minutes after approximately 30 minutes of casting, followed by another 30 minutes of casting and an 8-minute pause. By improving the traditional vacuum casting process and introducing two or more stages of vacuum casting and semi-gelling for large-capacity coils, this effectively promotes bubble removal, reduces filler settling, decreases shrinkage stress, and lowers the risk of cracking. This significantly improves coil quality and ensures the long-term safe operation of the transformer.

Description

Technical Field

[0001] This invention relates to the field of dry-type power transformer manufacturing technology, and in particular to a graded casting process for epoxy resin coils with filler. Background Technology

[0002] In the manufacturing of dry-type transformer coils, the vacuum casting process is a crucial factor affecting product quality. With the development of power technology in recent years, high-voltage, large-capacity power supply is increasingly common, such as offshore wind power generation, which is now moving towards 66kV and capacities of 10MVA and above. High-voltage, large-capacity dry-type transformer coils are characterized by their height and large volume. Traditional one-time vacuum casting processes require a large amount of resin, sometimes necessitating two batching processes. Secondly, the high pressure at the bottom of the coil hinders bubble removal. Thirdly, the long casting time leads to more severe filler settling, resulting in uneven resin properties between the upper and lower parts, increasing the risk of cracking. Fourthly, the cumulative effect of resin curing shrinkage is amplified, often requiring large amounts of replenishment during curing due to shrinkage, and the increased internal stress caused by shrinkage also contributes to cracking. Therefore, improving the casting process for large-capacity, large-volume dry-type transformer coils is particularly necessary. Summary of the Invention

[0003] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0004] In view of the problems existing in the above-mentioned vacuum casting process of epoxy resin with filler for large-capacity dry transformer coils, this invention is proposed.

[0005] Therefore, the purpose of this invention is to provide a staged casting process for epoxy resin coils with fillers.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a staged casting process for epoxy resin coils with fillers, comprising the following 7 steps in sequence:

[0007] Drying the coil;

[0008] Epoxy resin preparation;

[0009] Vacuum casting: The vacuum degree of the casting tank reaches 3-4 mbar, and the temperature is (Tn-10)℃ for casting. Intermittent stroke casting is used, with an interval of 1.5-2 minutes between each stroke. After about 30 minutes of casting, the casting is paused for 8 minutes, then cast for another 30 minutes and paused for 8 minutes. This process is repeated, and the total casting time is controlled within 1.5 hours.

[0010] Semi-gel: The temperature of semi-gel is Tn℃, and the curing time is long. When the epoxy resin is cured to a viscosity of about 28 Pa·s to 33 Pa·s, the semi-gel process is completed.

[0011] Secondary vacuum casting;

[0012] Curing: The curing process is a stepped curing process under normal pressure; the curing oven is heated to the gel temperature Tn℃ in 40 minutes and cured for 7 hours; the temperature is increased to (Tn+15)℃ in 30 minutes and cured for 3 hours; the temperature is increased to (Tn+35)℃ in 30 minutes and cured for 3 hours; the temperature is then increased to 135℃ in 30 minutes and cured for ≥12 hours; the temperature is then cooled down with the curing oven to (Tg+20)℃ and cooled down for ≥3 hours;

[0013] Demolding: The demolding sequence is as follows: terminal mold → panel → outer mold → core mold → air channel bar. After demolding, the coil is moved back to the curing oven and slowly cooled to room temperature.

[0014] In a preferred embodiment of the graded casting process for the filled epoxy resin coil of the present invention, Tg is the glass transition temperature.

[0015] As a preferred embodiment of the graded casting process for the filled epoxy resin coil of the present invention, the vacuum casting process is as follows: after the vacuum casting is completed, the vacuum degree is adjusted to 1-2 mbar and maintained for ≥40 minutes, then the vacuum is broken, and the pressure is increased to 2 standard atmospheres and maintained for 30 minutes; then the atmospheric pressure is broken again.

[0016] As a preferred embodiment of the graded casting process for the filled epoxy resin coil described in this invention, the coil is first pre-dried under normal atmospheric pressure during the drying process, and then vacuum dried. The pre-drying temperature is 105℃, the heating rate is 1 hour, and the temperature is maintained at 105℃ for ≥8 hours. The coil is then cooled in the furnace for 2 hours to (Tn-10)℃ and maintained at (Tn-10)℃ for ≥8 hours. The vacuum drying temperature is (Tn-10)℃~(Tn-5)℃ and maintained for ≥2 hours.

[0017] As a preferred embodiment of the graded casting process for the filled epoxy resin coil described in this invention, the epoxy resin preparation includes: pre-drying of silica powder, preparation of A / B tanks, mixing, and degassing. The silica powder is pre-dried at 105°C for ≥10 hours and cooled to (Tn-15)°C in the furnace for later use. The A tank (resin tank) and B tank (curing agent tank) of the casting system are respectively mixed, stirred, and degassed according to the required proportions. The vacuum degree of tank A is 2-3 mbar, the temperature is (Tn-10)°C, and the degassing time is ≥4 hours; the vacuum degree of tank B is 3-4 mbar, the temperature is (Tn-15)°C, and the degassing time is ≥4 hours. The mixing tank is a static mixing tank with a vacuum degree of 2 mbar and a temperature of (Tn-12)°C. Dynamic mixing requires stirring for 45-60 minutes until no more bubbles overflow, completing the mixing process.

[0018] As a preferred embodiment of the graded casting process for the filled epoxy resin coil described in this invention, this process is suitable for casting coils of dry-type transformers with larger capacity and higher voltage levels, such as coils with a capacity of 6300kVA and above, and a voltage level of 35kV and above.

[0019] As a preferred embodiment of the graded casting process for the filled epoxy resin coil described in this invention, this graded casting process is applicable to different types and formulations of filled epoxy resins used for casting all dry transformer coils. The specific temperature value of the curing gel temperature "Tn" varies depending on the type and formulation of the epoxy resin and is recommended data from the epoxy resin supplier.

[0020] As a preferred embodiment of the graded casting process of the filled epoxy resin coil described in this invention, in the semi-gel process, the actual deviation of the semi-gel time at Tn℃ temperature is only allowed to be within 10 minutes. The semi-gel time should include the time for subsequent secondary vacuum casting and other transfer connection time. Vacuuming can be started 30 minutes before the required time deadline.

[0021] As a preferred embodiment of the graded casting process for the filled epoxy resin coil of the present invention, the graded casting process can be divided into two stages or multiple stages of casting and gelation. Specifically, the number of casting and gelation stages is preferably such that the amount of filled resin used in the first stage is 250-350 kg.

[0022] As a preferred embodiment of the graded casting process for the filled epoxy resin coil described in this invention, when the room temperature is below 15°C, the outer coil is immediately wrapped with a thin cotton blanket after the outer mold is removed; each time a section of the air duct bar is pulled out, the part that has been pulled out is also immediately covered with a thin cotton blanket to slow down the cooling rate. The demolded coil is quickly moved back to the curing oven and slowly cooled to room temperature with the oven.

[0023] The beneficial effects of this invention are as follows: This invention involves two or more stages of epoxy resin casting and gelation for large-capacity dry-type transformer coils. Because the amount of casting material used in each stage is small and the casting time is short, it facilitates defoaming; filler sedimentation is limited to a certain extent, resulting in more uniform resin properties in different parts of the coil and reducing the risk of cracking caused by differences in resin thermal conductivity and toughness; it better differentiates the cumulative effect caused by curing shrinkage, reducing internal stress caused by curing shrinkage; and it does not affect the multiple batching and mixing of epoxy resin, solving the problem of insufficient one-time batching. The large-capacity coils cast by this invention have a smooth surface, minimal indentation, and low local discharge, ensuring quality. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0025] Figure 1 This is a schematic diagram of the staged casting effect (three-stage casting) of an epoxy resin coil with filler.

[0026] Figure 2 This is a schematic diagram of the process flow for a staged casting process of an epoxy resin coil with filler. Detailed Implementation

[0027] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0028] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0029] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0030] Secondly, the present invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of the present invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not according to the usual scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. In addition, actual fabrication should include three-dimensional spatial dimensions of length, width, and depth.

[0031] This process is suitable for casting coils of dry-type transformers with larger capacity and higher voltage levels, such as coils with a capacity of 6300kVA and above, and a voltage level of 35kV and above.

[0032] The special symbols used in the following description are:

[0033] Tn – Curing gel temperature (unit: °C). This should be recommended by the epoxy resin supplier. The specific temperature value of "Tn" varies depending on the type and formulation of the epoxy resin. (Tn-10) indicates a curing gel temperature 10 °C lower than the recommended temperature, and (Tn+10) indicates a curing gel temperature 10 °C higher than the recommended temperature.

[0034] Tg – Glass transition temperature (unit: °C). The specific temperature value of “Tg” varies depending on the type and formulation of the epoxy resin and is obtained by testing by the epoxy resin supplier.

[0035] Example 1, referring to Figures 1-2 This is the first embodiment of the present invention, which provides a graded casting process for a filled epoxy resin coil, including 7 steps: coil drying, epoxy resin preparation, vacuum casting, semi-gelling, secondary vacuum casting, curing, and demolding.

[0036] Specifically, step 1 – drying the coil;

[0037] Further, coil pre-drying: The wound coil semi-finished product is assembled with the outer mold and panel, the bolts are tightened and sealed, and then the whole thing is hoisted into the oven or drying chamber. The drying parameters of the oven are set as follows: air pressure is normal atmospheric pressure, temperature is 105℃, heating rate is 1 hour; 105℃ is held for ≥8 hours; the temperature is cooled to (Tn-10)℃ in the oven for 2 hours, cooling rate is 2 hours, and (Tn-10)℃ is held for ≥8 hours.

[0038] Further, the coil undergoes vacuum drying: The coil with mold is quickly removed from the oven and transferred to the casting tank. The casting feed pipe is connected, and the tank door is closed. The vacuum pump is started to create a vacuum, with the temperature set to (Tn-10)℃~(Tn-5)℃, the vacuum degree <0.5mbar, and the vacuuming time is 2 hours, in preparation for casting.

[0039] Further, step 2 – epoxy resin preparation;

[0040] Further, the pre-drying of silicon micropowder: place the silicon micropowder in an oven and set the drying parameters of the oven: the air pressure is normal atmospheric pressure, the drying temperature is 105℃, the heating rate is 1 hour; maintain 105℃ for ≥10 hours; cool down to (Tn-15)℃, the cooling rate is 2 hours, and maintain (Tn-15)℃ for ≥8 hours. The pre-drying of silicon micropowder and the pre-drying of coils in step 1 are carried out simultaneously, and the timing is planned.

[0041] Further, preparation of materials in tank A: In tank A (resin tank) of the casting system, resin, silica powder and pigment are mixed in the required proportion, the stirring and degassing valves are opened, the vacuum degree of tank A (resin tank) is set to 2-3 mbar, the temperature is (Tn-10)℃, and the stirring and degassing time is ≥4 hours. At this time, there should be no bubbles overflowing from the tank.

[0042] Furthermore, the preparation of tank B: In the B tank (curing agent tank) of the casting system, the curing agent and silicon powder are mixed in the required proportion, the stirring and degassing valves are opened, the vacuum degree of tank B (curing agent tank) is 3-4 mbar, and the temperature is (Tn-15)℃; the stirring and degassing time is ≥4 hours, and there should be no bubbles overflowing in the tank at this time. The preparation procedures of tanks A and B are carried out simultaneously.

[0043] Further, mixing and degassing: The required proportions of materials A and B are pumped into a static mixing tank through a casting system. The vacuum degree and temperature of the mixing tank are maintained at 2 mbar, and the temperature is maintained at (Tn-12)℃. If the casting system is a dynamic mixing tank, the vacuum degree and temperature are maintained at 2 mbar, and the temperature is maintained at (Tn-12)℃. Mixing and stirring are required for 45 to 60 minutes. At this time, no air bubbles overflow from the mixing tank, and the mixing and degassing are completed.

[0044] Further, step 3 – vacuum casting;

[0045] Further, the pouring process: Adjust the vacuum level of the pouring tank to 3-4 mbar and the temperature to (Tn-10)℃, open the material conveying valve to pour, and use intermittent stroke pouring with an interval of 1.5-2 minutes between each stroke; after pouring for about 30 minutes, pause pouring for 8 minutes, then pour for another 30 minutes and pause for 8 minutes, and repeat this process. Generally, the total pouring time should be controlled within 1.5 hours.

[0046] Furthermore, maintain vacuum: After casting, adjust the vacuum level to 1-2 mbar and maintain it for ≥40 minutes, then break the vacuum, pressurize to 2 standard atmospheres and maintain it for 30 minutes; then break the atmosphere again.

[0047] Further, in step 4 – semi-gel: the temperature is raised to the curing gel temperature Tn℃ to begin the first stage of gel curing. The curing time varies depending on the type and formulation of the epoxy resin. Specifically, the "semi-gel" process is completed when the epoxy resin reaches a viscosity of approximately 28 Pa·s to 33 Pa·s.

[0048] Further, in step 5—secondary vacuum casting: the method for "secondary vacuum casting" is the same as in step 3, vacuum casting. The temperature of the casting tank is adjusted to (Tn-10)℃, and the vacuum degree is 3-4 mbar. The material delivery valve is then opened for casting. Intermittent stroke casting is used, with each stroke spaced 1.5-2 minutes apart; and after approximately 30 minutes of casting, casting is paused for 8 minutes, then again for 30 minutes, followed by an 8-minute pause, and so on. Generally, the total casting time is controlled within 1.5 hours.

[0049] Further, step 6—curing: The coil strip mold that has completed the secondary vacuum casting is transferred from the casting tank to the curing oven for curing. The curing process is step-by-step and is carried out under normal pressure.

[0050] In the first stage, the curing oven is heated to the gel curing temperature Tn℃ over 40 minutes, and the curing time is 7 hours. In the second stage, the temperature is heated to (Tn+15)℃ over 30 minutes and maintained at this temperature for 3 hours.

[0051] In the third stage, the temperature is raised to (Tn+35)℃ in 30 minutes and maintained at this temperature for 3 hours to cure.

[0052] The fourth stage involves heating for 30 minutes to 135°C and maintaining this temperature for curing for ≥12 hours. The fifth stage involves cooling the resin in the curing oven to (Tg+20)°C, where Tg is the glass transition temperature of the resin, and the cooling time is ≥3 hours.

[0053] Further, in step 7—demolding: the cooled coil mold is moved from the curing oven to the demolding table to begin demolding. The demolding sequence is as follows: terminal mold → panel → outer mold → core mold → air passage bar. Then, a utility knife is used to preliminarily cut off the larger resin sharp corners and edges. Finally, the coil is moved back to the curing oven and slowly cooled to room temperature with the oven.

[0054] Example 2, refer to Figures 1-2 This is the second embodiment of the present invention. The difference between this embodiment and the first embodiment is that this embodiment is a refinement of process 4-7 in embodiment 1.

[0055] Specifically, step 4 – semi-gel;

[0056] Under normal atmospheric pressure, the temperature is raised to the gelation temperature Tn℃ to begin the first stage of gel curing. The semi-gel curing time varies depending on the type and formulation of the epoxy resin. Specifically, the "semi-gel" process ends when the epoxy resin reaches a viscosity of approximately 28 Pa·s to 33 Pa·s. The curing time required to reach this viscosity at Tn℃ must be determined and measured in advance using specialized testing instruments.

[0057] In particular, since the viscosity of epoxy resin increases rapidly in the later stage of the crosslinking reaction, the deviation of the semi-gel time should be controlled within 10 minutes. If the gel is stopped before the required curing time, the effect of curing shrinkage stress will be poor; if the required curing time is exceeded, the crosslinking reaction between the primary and secondary resins will be poor, resulting in delamination.

[0058] Furthermore, the semi-gel curing time should include the time required for subsequent secondary vacuum casting and vacuuming. Preferably, vacuuming should begin 30 minutes in advance.

[0059] Specifically, the semi-gel process is preferably performed in a casting tank to facilitate the connection with subsequent processes. If this process needs to be transferred to another curing oven, the curing time of the semi-gel must take into account the time required to transfer the coil from the curing oven back to the casting tank, as well as the time required to evacuate the casting tank.

[0060] Further, step 5 – secondary vacuum casting;

[0061] Furthermore, the method for "secondary vacuum casting" is described in Section (3) Vacuum Casting. The temperature of the casting tank is adjusted to (Tn-10)℃, and the vacuum level is 3-4 mbar. The material delivery valve is then opened for casting. An intermittent stroke casting method is used, with each stroke interval of 1.5-2 minutes; and after approximately 30 minutes of casting, casting is paused for 8 minutes, then cast for another 30 minutes and paused for 8 minutes, repeating this process. Generally, the total casting time is controlled within 1.5 hours.

[0062] Furthermore, maintain vacuum: After casting, adjust the vacuum level to 1-2 mbar and maintain it for ≥40 minutes. Then break the vacuum, pressurize to 2 standard atmospheres, maintain it for 30 minutes, and then break the atmospheric pressure.

[0063] Furthermore, replenishment: During the vacuum period, if the resin level drops significantly and becomes too low, the resin casting material should be replenished promptly to the required level.

[0064] Further, step 6—curing;

[0065] The coil strip mold, after completing the secondary vacuum casting, is transferred from the casting tank to the curing oven for curing. The curing process is step-by-step and is carried out under normal pressure.

[0066] In the first stage, the curing oven is heated to the curing gel temperature Tn℃ in 40 minutes, and the curing time is 7 hours.

[0067] In the second stage, the temperature is raised to (Tn+15)℃ in 30 minutes and maintained at this temperature for 3 hours to cure.

[0068] In the third stage, the temperature is raised to (Tn+35)℃ in 30 minutes and maintained at this temperature for 3 hours to cure.

[0069] The fourth stage involves heating for 30 minutes to 135°C and maintaining that temperature for curing for ≥12 hours.

[0070] In the fifth stage, the temperature is lowered to (Tg+20)℃ as the curing oven cools down, where Tg is the glass transition temperature of the resin, and the cooling time is ≥3 hours.

[0071]

[0072] Further, step 7—demolding;

[0073] Remove the cooled coil mold from the curing oven to the demolding table. First, remove the mold for the umbrella skirt terminals, then remove the panel and outer mold. Next, lift out the core mold and remove the air duct rods one by one. Then, use a utility knife to preliminarily trim any large resin sharp corners and edges. Finally, move the coil back to the curing oven and let it cool slowly to room temperature.

[0074] Furthermore, when the room temperature is below 15°C, immediately wrap the outer coil with a thin cotton blanket after removing the outer mold; after each section of the airway bar is pulled out, cover the part that has been pulled out with a thin cotton blanket to slow down the cooling rate.

[0075] The remaining steps are the same as in Example 1.

[0076] Example 3, referring to Figures 1-2 This is the third embodiment of the present invention, which differs from the previous embodiments in that the above process describes the epoxy resin casting and gelation process for two stages of the transformer coil. In fact, depending on the size of the transformer coil, more stages of epoxy resin casting and gelation can also be selected.

[0077] Specifically, after the second vacuum casting, the semi-gel process in the fourth step is carried out, followed by a third vacuum casting.

[0078] Furthermore, the specific casting and gelation stages should ideally be based on a resin content of 250–300 kg for the first stage. For example, coils with a resin content of 500–600 kg should be cast and gelled in two stages; coils with a resin content of 600–900 kg should be cast and gelled in three stages; and coils with a resin content of over 900 kg should be cast and gelled in four stages.

[0079] Furthermore, regardless of the brand, model, or ratio of filled epoxy resin used, the above process can be followed to perform appropriate staged casting and curing, thereby improving the quality of coil casting.

[0080] The remaining steps are the same as in Example 2.

[0081] 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A staged casting process for epoxy resin coils with fillers, characterized in that: The process consists of the following 7 steps: Drying the coil; Epoxy resin preparation includes silica powder pre-drying, A / B tank preparation, mixing and degassing, where A tank is a resin tank and B tank is a curing agent. Vacuum casting: The vacuum degree of the casting tank reaches 3-4 mbar, and the temperature is (Tn-10)℃ for casting. Intermittent stroke casting is used, with an interval of 1.5-2 minutes between each stroke. After every 30 minutes of casting, the casting is paused for 8 minutes, then cast for another 30 minutes and paused for 8 minutes. This process is repeated, and the total casting time is controlled within 1.5 hours. Semi-gel: The temperature for semi-gel treatment is Tn℃, and the curing time is when the epoxy resin solidifies to a viscosity of 28 Pa·s to 33 Pa·s. The "semi-gel" process is then completed. "Tn" is the curing gel temperature, and the specific temperature value of "Tn" varies depending on the type and formulation of the epoxy resin. In the secondary vacuum casting process, the temperature of the casting tank is adjusted to (Tn-10)℃, and the vacuum degree is 3-4 mbar. The material conveying valve is opened for casting, and intermittent stroke casting is used. The interval between each stroke is 1.5-2 minutes. After every 30 minutes of casting, the casting is paused for 8 minutes, and then the casting is paused for 8 minutes after another 30 minutes. This process is repeated. Curing: The curing process is a stepped curing process under normal pressure; the curing oven is heated to the gel temperature Tn℃ in 40 minutes and cured for 7 hours; the temperature is increased to (Tn+15)℃ in 30 minutes and cured for 3 hours; the temperature is increased to (Tn+35)℃ in 30 minutes and cured for 3 hours; the temperature is then increased to 135℃ in 30 minutes and cured for ≥12 hours; the temperature is then cooled down with the curing oven to (Tg+20)℃ and cooled down for ≥3 hours; Demolding: The demolding sequence is as follows: terminal mold → panel → outer mold → core mold → air channel bar. After demolding, the coil is moved back to the curing oven and slowly cooled to room temperature.

2. The staged casting process for the filled epoxy resin coil as described in claim 1, characterized in that: Tg is the glass transition temperature.

3. The staged casting process for the filled epoxy resin coil as described in claim 2, characterized in that: After the vacuum casting process is completed, the vacuum level is adjusted to 1-2 mbar and maintained for ≥40 minutes. Then the vacuum is broken, and the pressure is increased to 2 standard atmospheres and maintained for 30 minutes. Then the pressure is broken again.

4. The staged casting process for the filled epoxy resin coil as described in claim 3, characterized in that: In the drying process of the coil, the coil with mold is first pre-dried under normal atmospheric pressure, and then vacuum dried. The pre-drying temperature is 105℃, the heating rate is 1 hour, and it is maintained at 105℃ for ≥8 hours. Then, it is cooled in the furnace for 2 hours to (Tn-10)℃ and maintained at (Tn-10)℃ for ≥8 hours. The vacuum degree of vacuum drying is <0.5mbar, the temperature is (Tn-10)℃~(Tn-5)℃, and it is maintained for ≥2 hours.

5. The staged casting process for the filled epoxy resin coil as described in claim 4, characterized in that: The silicon micropowder is pre-dried at 105℃ for ≥10 hours and then cooled to (Tn-15)℃ in the furnace for later use. Tanks A and B of the casting system are respectively used for batching, stirring, and degassing according to the required proportions. Tank A has a vacuum of 2-3 mbar and a temperature of (Tn-10)℃, with a degassing time ≥4 hours; Tank B has a vacuum of 3-4 mbar and a temperature of (Tn-15)℃, with a degassing time ≥4 hours. The mixing tank is a static mixing tank with a vacuum of 2 mbar and a temperature of (Tn-12)℃. Dynamic mixing requires stirring for 45-60 minutes until no more bubbles overflow, completing the mixing process.

6. The staged casting process for the filled epoxy resin coil as described in claim 5, characterized in that: This process is suitable for casting coils of dry-type transformers with a capacity of 6300kVA and above and a voltage level of 35kV and above.

7. The staged casting process for the filled epoxy resin coil as described in claim 6, characterized in that: This graded casting process is applicable to different types and formulations of filled epoxy resin used for casting all dry transformer coils.

8. The staged casting process for the filled epoxy resin coil as described in claim 7, characterized in that: In the semi-gel process, the actual duration deviation of the semi-gel at Tn℃ is only allowed within 10 minutes. The semi-gel duration should include the time for subsequent secondary vacuum casting and vacuuming, as well as other transfer and connection times.

9. The staged casting process for the filled epoxy resin coil as described in claim 8, characterized in that: The graded casting process consists of two or more stages of casting and gelation. The specific number of casting and gelation stages is preferably 250-350 kg of filler resin in the first stage.

10. The staged casting process for the filled epoxy resin coil as described in claim 9, characterized in that: When the room temperature is below 15℃, immediately wrap the outer coil with a thin cotton blanket after removing the outer mold; after each part of the air channel bar is pulled out, cover the part that has been pulled out with a thin cotton blanket to slow down the cooling rate. After demolding, the coil is quickly moved back to the curing oven and slowly cooled to room temperature with the oven.

Images