Graded pouring process technology for epoxy resin coil with filler

Abstract

The invention relates to the technical field of dry-type power transformer manufacturing, in particular to a graded pouring process technology for an epoxy resin coil with filler, which comprises the steps of coil drying, epoxy resin preparation, vacuum pouring, semi-gelling, secondary vacuum pouring, curing and demolding. Drying the coil; preparing epoxy resin; vacuum pouring is conducted, specifically, pouring is conducted when the vacuum degree of a pouring tank reaches 3-4 mbar and the temperature is (Tn-10) DEG C, intermittent stroke pouring is conducted, and the interval time of each stroke is 1.5-2 minutes; and the pouring is paused for 8 minutes after each about 30 minutes of pouring, and then the pouring is paused for 8 minutes after 30 minutes of pouring, so that a traditional vacuum pouring process is improved, and two-stage or multi-stage vacuum pouring and semi-gel procedures are introduced to a high-capacity coil, so that bubble discharge is effectively promoted, filler sedimentation is reduced, shrinkage stress is reduced, and the cracking risk is reduced; and great benefits are provided for improving coil quality and guaranteeing long-term safe operation of the transformer.

Description

Technical Field

[0001] The invention relates to the technical field of dry-type power transformer manufacturing, in particular to a graded casting process technology of a filler epoxy resin coil. Background Art

[0002] In the manufacture of dry-type transformer coils, the vacuum casting process of the coils is a key link related to product quality. With the development of power technology in recent years, high-voltage and large-capacity power supply and consumption have gradually increased. For example, offshore wind power generation has begun to develop towards 66kV and a capacity of more than 10MVA. The coils of high-voltage and large-capacity dry-type transformers have the obvious characteristics of being long in height, large in volume, and high in temperature. In the traditional one-time vacuum casting process, a lot of resin is used, and sometimes two batchings are required; secondly, when the coil is high, the bottom pressure is high and the bubbles cannot be discharged; secondly, the casting takes a long time and the filler settles more seriously, and the resin performance of the upper / lower parts is uneven, causing cracking hazards; thirdly, the cumulative effect of resin curing shrinkage is amplified, and there is often a problem of a large amount of material replenishment during curing due to shrinkage, and the increase in internal stress caused by shrinkage is also a cracking hazard. Therefore, it is particularly necessary to improve the casting process of dry-type transformer coils with larger capacity and larger volume. Summary of the invention

[0003] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

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

[0005] Therefore, the object of the present invention is to provide a graded casting process technology for a filler epoxy resin coil.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions: A graded casting process technology of epoxy resin coil with filler: comprising the following 7 steps in sequence:

[0007] Drying of coils;

[0008] Epoxy resin preparation;

[0009] Vacuum pouring: The vacuum degree of the pouring pot reaches 3-4mbar, and the pouring is carried out at a temperature of (Tn-10)℃. The pouring is intermittent, and the interval between each stroke is 1.5-2 minutes; and the pouring is paused for 8 minutes after about 30 minutes of pouring, and then paused for 8 minutes after pouring for another 30 minutes, and the total pouring 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 solidifies to a viscosity of about 28pa.s to 33pa.s, the "semi-gel" process is completed;

[0011] Secondary vacuum pouring;

[0012] Curing: The curing process is step-by-step and curing under normal pressure; the curing furnace is heated to the curing gel temperature Tn℃ for 40 minutes, and the curing time is 7 hours; the temperature is raised to (Tn+15)℃ for 30 minutes, and the curing time is 3 hours; the temperature is raised to (Tn+35)℃ for 30 minutes, and the curing time is 3 hours; the temperature is raised to 135℃ for 30 minutes, and the curing time is ≥12 hours; the temperature is lowered to (Tg+20)℃ as the curing furnace cools, and the cooling time is ≥3 hours;

[0013] Demolding: The demolding sequence is: terminal mold → panel → outer mold → core mold → airway rod. After demolding, the coil is moved back to the curing furnace and slowly cooled to room temperature in the furnace.

[0014] As a preferred solution of the graded casting process technology of the epoxy resin coil with filler described in the present invention, wherein: the Tg is the glass transition temperature.

[0015] As a preferred solution of the graded casting process technology of the filled epoxy resin coil described in the present invention, 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 then pressurized to 2 standard atmospheric pressures and maintained for 30 minutes; and then the atmospheric pressure is broken.

[0016] As a preferred solution of the graded casting process technology of the filled epoxy resin coil described in the present invention, wherein: in the drying of the coil, the coil with the mold is first pre-dried under normal atmospheric pressure, and then vacuum dried, the pre-drying temperature is 105°C, the heating rate is 1 hour, and 105°C is maintained for ≥8 hours, and the temperature is cooled to (Tn-10)°C with the furnace for 2 hours, and (Tn-10)°C is maintained for ≥8 hours, the vacuum degree of vacuum drying is <0.5mbar, and the temperature is (Tn-10)°C~(Tn-5)°C, and it is maintained for ≥2 hours.

[0017] As a preferred solution of the graded casting process technology of the epoxy resin coil with filler described in the present invention, wherein: the epoxy resin preparation includes silicon micropowder pre-drying, A / B tank preparation, mixing and degassing, the silicon micropowder pre-drying temperature is 105°C, the drying time is ≥10 hours, and it is cooled to (Tn-15)°C with the furnace for standby use. The A tank (resin tank) and B tank (curing agent tank) of the casting system are respectively batched, stirred and degassed according to the required proportion; the vacuum degree of tank A is 2-3mbar, the temperature is (Tn-10)°C, and the degassing time is ≥4 hours; the vacuum degree of tank B is 3-4mbar, the temperature is (Tn-15)°C, and the degassing time is ≥4 hours. The mixing tank is a static mixing tank, the vacuum degree of the mixing tank is 2mbar, the temperature is (Tn-12)°C, and the mixing tank is a dynamic mixing tank, which needs to be stirred for 45-60 minutes, and the mixing is completed after no bubbles overflow.

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

[0019] As a preferred solution of the graded casting process technology of the filled epoxy resin coil described in the present invention, the graded casting process is applicable to all filled epoxy resins of different models and formulations for casting all dry transformer coils, wherein the specific temperature value of the curing gel temperature "Tn" varies according to the model and formulation of the epoxy resin, and is the recommended data of the epoxy resin provider.

[0020] As a preferred solution of the graded casting process technology of the filled epoxy resin coil described in the present invention, in which: in the semi-gel process, the actual time deviation of the semi-gel at the temperature of Tn°C is only allowed to be within 10 minutes, and the semi-gel time should include the time for vacuuming of the subsequent secondary vacuum casting and other transportation connection time. Vacuuming can be started 30 minutes in advance before the end of the required time.

[0021] As a preferred solution of the graded casting process technology of the filled epoxy resin coil described in the present invention, the graded casting process can be divided into two stages or multiple stages of casting and gelling process, and the specific casting and gelling stages are preferably 250-350 kg of the filled resin in the first stage.

[0022] As a preferred solution of the graded casting process technology of the filler epoxy resin coil of the present invention, when the room temperature is lower than 15°C, a thin quilt is used to wrap the outer ring of the coil immediately after the outer mold is removed; each time a part of the airway rod is pulled out, a thin quilt is also used to cover the pulled out part to slow down the cooling rate. The demoulded coil is quickly moved back to the curing furnace and slowly cooled to room temperature in the furnace.

[0023] Beneficial effects of the present invention: The present invention performs two-stage or even multi-stage epoxy resin pouring and gelling on large-capacity dry-type transformer coils. Because the single (each stage) pouring material is small and the pouring time is short, it is conducive to degassing; the filler sedimentation is limited to a certain extent, the performance of the resin in each part of the coil is more uniform, and the cracking risk caused by the difference in resin thermal conductivity, toughness and strength is reduced; the cumulative effect caused by curing shrinkage is better differentiated, and the internal stress of curing shrinkage is reduced; it does not affect the multiple batching and mixing of epoxy resin, and solves the problem of insufficient dosage of one-time batching. The large-capacity coil cast by the present invention has the quality assurance of smooth surface, no obvious depression, and low partial discharge. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without any creative labor.

[0025] Figure 1 This is a schematic diagram of the graded casting effect of a filled epoxy resin coil (three-stage casting).

[0026] Figure 2 The figure is a process flow diagram of a graded casting process technology for epoxy resin coils with fillers. DETAILED DESCRIPTION

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0028] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present 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 term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0030] Secondly, the present invention is described in detail with reference to the schematic diagram. When describing the embodiments of the present invention in detail, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

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

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

[0033] Tn——curing gel temperature (unit: °C). It needs to be recommended by the epoxy resin supplier. The specific temperature value of "Tn" varies according to the type and formula of the epoxy resin. (Tn-10) means 10°C lower than the recommended curing gel temperature, and (Tn+10) means 10°C higher than the recommended curing gel temperature.

[0034] Tg - Glass transition temperature (unit: °C). The specific temperature value of "Tg" varies according to the type and formula of epoxy resin, and is obtained by testing by the epoxy resin supplier.

[0035] Example 1, reference Figure 1-2 , which is the first embodiment of the present invention, provides a graded casting process technology for epoxy resin coils with fillers, including 7 steps: coil drying, epoxy resin preparation, vacuum casting, semi-gel, secondary vacuum casting, curing, and demoulding.

[0036] Specifically, process 1 - drying of coils;

[0037] Further, coil pre-drying: assemble the wound coil semi-finished product with the outer mold and panel, tighten the bolts and seal it, and then hoist it into the oven or drying box as a whole. Set the drying parameters of the oven: the air pressure is normal atmospheric pressure, the temperature is 105℃, the heating rate is 1 hour; the 105℃ is maintained for ≥8 hours; cool down with the furnace for 2 hours to (Tn-10)℃, the cooling rate is 2 hours, and (Tn-10)℃ is maintained for ≥8 hours.

[0038] Further, vacuum drying of the coil: quickly remove the coil with mold from the oven and transfer it to the pouring tank, connect the pouring feed pipe, and close the tank door. Start the vacuum pump to evacuate, set the temperature to (Tn-10)℃~(Tn-5)℃, the vacuum degree <0.5mbar, and the vacuum time is 2 hours. Prepare for pouring.

[0039] Further, process 2 - epoxy resin preparation;

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

[0041] Furthermore, the A tank is prepared: the A tank (resin tank) of the casting system mixes the resin, silicon powder and color paste according to the required proportions, opens the stirring and degassing valves, sets the vacuum degree of the A tank (resin tank) to 2-3 mbar, the temperature to (Tn-10) ° C, and the stirring and degassing time to ≥4 hours. At this time, there should be no bubbles overflowing from the tank.

[0042] Furthermore, preparation of materials in tank B: the tank B (curing agent tank) of the casting system mixes the curing agent and silicon powder in the required proportion, opens the stirring and degassing valves, the vacuum degree of tank B (curing agent tank) is 3-4 mbar, and the temperature is (Tn-15) °C; the stirring and degassing time is ≥4 hours, at which time there should be no bubbles overflowing from the tank, and the preparation procedures of tanks A / B are carried out simultaneously.

[0043] Further, mixing and degassing: the required ratio of A / B material is poured into the static mixing tank through the pouring system, the vacuum degree and temperature of the mixing tank are maintained at 2mbar, and the temperature is maintained at (Tn-12)°C. If the pouring system is a dynamic mixing tank, the vacuum degree and temperature are 2mbar, and the temperature is maintained at (Tn-12)°C. The mixture needs to be stirred for 45 to 60 minutes. At this time, there is no bubble overflowing from the mixing tank, and the mixture is degassed.

[0044] Further, process 3 - vacuum casting;

[0045] Further, pouring: adjust the vacuum degree of the pouring tank to 3-4 mbar, adjust the temperature to (Tn-10)°C, open the feed valve for pouring, and adopt intermittent stroke pouring, with an interval of 1.5-2 minutes between each stroke; and pause pouring for 8 minutes after pouring for about 30 minutes, and then pause for 8 minutes after pouring for 30 minutes, and repeat this process. Generally, the total pouring time is controlled within 1.5 hours.

[0046] Furthermore, vacuum is maintained: after pouring is completed, the vacuum degree is adjusted to 1-2 mbar and maintained for ≥40 minutes, then the vacuum is broken and pressurized to 2 standard atmospheric pressures and maintained for 30 minutes; then the atmospheric pressure is broken again.

[0047] Further, process 4 - semi-gel: the temperature is raised to the curing gel temperature Tn°C, and the first stage of gel curing begins. The curing time varies depending on the type and formula of the epoxy resin. Specifically, when the epoxy resin is gelled to a viscosity of about 28pa.s to 33pa.s, the "semi-gel" process is completed.

[0048] Further, process 5 - secondary vacuum pouring: The method of "secondary vacuum pouring" refers to the vacuum pouring in process 3. The temperature of the pouring tank is adjusted to (Tn-10)℃, and the feeding valve is opened for pouring when the vacuum degree is 3-4mbar. Intermittent stroke pouring, each stroke interval time is 1.5-2 minutes; and after pouring for about 30 minutes, the pouring is paused for 8 minutes, and then the pouring is paused for 30 minutes and 8 minutes, and so on. Generally, the total pouring 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 furnace for curing. The curing process is step-by-step and is cured under normal pressure.

[0050] In the first stage, the curing furnace was heated to the curing gel temperature Tn°C for 40 minutes, and the curing time was 7 hours;

[0051] In the second stage, the temperature is raised to (Tn+15)°C in 30 minutes and maintained at this temperature for 3 hours;

[0052] In the third stage, the temperature is raised to (Tn+35)°C in 30 minutes and maintained at this temperature for 3 hours;

[0053] The fourth stage is then heated to 135°C for 30 minutes and maintained at this temperature for curing for ≥12 hours;

[0054] In the fifth stage, the temperature is lowered to (Tg+20)°C as the curing furnace cools down, where Tg is the glass transition temperature of the resin, and the cooling time is ≥3 hours.

[0055] Further, process 7 - demoulding: the cooled coil mold is moved out of the curing furnace to the demoulding table and demoulding begins. The demoulding sequence is: terminal mold → panel → outer mold → core mold → airway rod, and then use a utility knife to make a preliminary cut off of the larger resin corners, and finally move the coil back to the curing furnace and slowly cool to room temperature in the furnace.

[0056] Example 2, reference Figure 1-2 , which is the second embodiment of the present invention. This embodiment is different from the first embodiment in that: this embodiment is a refinement of steps 4 to 7 in embodiment 1.

[0057] Specifically, process 4 - semi-gel;

[0058] The temperature is raised to the curing gel temperature Tn℃ under normal atmospheric pressure to start the first stage of gel curing. The length of semi-gel curing time varies depending on the type and formula of the epoxy resin. Specifically, when the epoxy resin gels to a viscosity of about 28pa.s to 33pa.s, the "semi-gel" process is completed. The curing time required to reach this viscosity at Tn℃ needs to be determined and measured in advance using a special testing instrument.

[0059] In particular, since the viscosity increases very quickly in the later stage of the epoxy resin cross-linking reaction, the time deviation of the semi-gel is controlled within 10 minutes. If the gel is terminated before the required curing time, the effect of the curing shrinkage stress will be poor; if the required curing time is exceeded, the cross-linking reaction between the primary and secondary resins will be poor, resulting in stratification.

[0060] Furthermore, the semi-gel curing time should include the time required for vacuuming during the subsequent secondary vacuum pouring. Preferably, vacuuming should be started 30 minutes in advance.

[0061] In particular, the semi-gel process is preferably carried out in a pouring tank to facilitate the connection with subsequent processes. If this process needs to be transferred to another curing furnace, the semi-gel curing time must take into account the time required for the coil to be transferred from the curing furnace back to the pouring tank and the time required for the pouring tank to be evacuated.

[0062] Further, process 5 - secondary vacuum pouring;

[0063] Further, the method of "secondary vacuum pouring" refers to the vacuum pouring in Article (3). The temperature of the pouring tank is adjusted to (Tn-10)℃, and the feed valve is opened for pouring when the vacuum degree is 3-4mbar. An intermittent stroke pouring method is adopted, with an interval of 1.5-2 minutes between each stroke; and the pouring is paused for 8 minutes after about 30 minutes of pouring, and then paused for 8 minutes after pouring for 30 minutes, and so on. Generally, the total pouring time is controlled within 1.5 hours.

[0064] Furthermore, vacuum is maintained: after pouring is completed, the vacuum degree is adjusted to 1-2 mbar, maintained for ≥40 minutes, and then the vacuum is broken and pressurized to 2 standard atmospheric pressures, maintained for 30 minutes before breaking the atmospheric pressure.

[0065] Furthermore, replenishing material: during the vacuum maintenance period, if the resin drops too much and the liquid level is low, the resin casting material needs to be replenished to the required liquid level in time.

[0066] Further, process 6—curing;

[0067] The coil strip mold that has completed the secondary vacuum pouring is transferred from the pouring tank to the curing furnace for curing. The curing process is step-by-step and is cured under normal pressure.

[0068] In the first stage, the curing furnace was heated to the curing gel temperature Tn°C for 40 minutes, and the curing time was 7 hours;

[0069] In the second stage, the temperature is raised to (Tn+15)°C in 30 minutes and maintained at this temperature for 3 hours;

[0070] In the third stage, the temperature is raised to (Tn+35)°C in 30 minutes and maintained at this temperature for 3 hours;

[0071] The fourth stage is then heated to 135°C for 30 minutes and maintained at this temperature for curing for ≥12 hours;

[0072] In the fifth stage, the temperature is lowered to (Tg+20)°C as the curing furnace cools down, where Tg is the glass transition temperature of the resin, and the cooling time is ≥3 hours.

[0073]

[0074]

[0075] Further, process 7—demolding;

[0076] Move the cooled coil mold out of the curing oven to the demolding table, first remove the mold of the shed terminal, then remove the panel and outer mold, then lift out the core mold, and then take out the airway rods one by one. Then use a utility knife to make a preliminary cut off of the larger sharp corners of the resin, and finally move the coil back to the curing oven to slowly cool down to room temperature.

[0077] Furthermore, when the room temperature is lower than 15°C, a thin blanket is used to wrap the outer ring of the coil immediately after the outer mold is removed; each time a part of the airway stick is pulled out, a thin blanket is used to cover the pulled-out part to slow down the cooling rate.

[0078] The remaining steps are the same as those in Example 1.

[0079] Example 3, reference Figure 1-2 , which is the third embodiment of the present invention. This embodiment is different from the above embodiments in that the above process expresses the epoxy resin pouring and gelling process of the transformer coil at two levels. In fact, according to the size of the transformer coil, more levels of epoxy resin pouring and gelling schemes can also be selected.

[0080] Specifically, after the second vacuum casting, the semi-gel process in the fourth process is performed, and then the vacuum casting is performed three times.

[0081] Furthermore, the specific pouring and gelling stages are preferably 250-300 kg of the first-stage filler resin dosage. For example, a coil with a filler resin dosage of 500-600 kg is poured and gelled in two stages; a coil with a filler resin dosage of 600-900 kg is poured and gelled in three stages; a coil with a filler resin dosage of more than 900 kg is poured and gelled in four stages.

[0082] Furthermore, no matter what brand, model, or ratio of epoxy resin with filler is used, the corresponding graded pouring and curing can be carried out according to the above process to achieve the purpose of improving the casting quality of the coil. The remaining steps are the same as in Example 2.

[0083] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A graded casting process technology for epoxy resin coils with fillers, characterized in that: It includes the following 7 steps in sequence: Drying of coils; Epoxy resin preparation; Vacuum pouring: The vacuum degree of the pouring pot reaches 3-4mbar, and the pouring is carried out at a temperature of (Tn-10)℃. The pouring is intermittent, and the interval between each stroke is 1.5-2 minutes; and the pouring is paused for 8 minutes after about 30 minutes of pouring, and then paused for 8 minutes after pouring for another 30 minutes, and the total pouring time is controlled within 1.5 hours; Semi-gel: The temperature of semi-gel is Tn℃, and the curing time is long. When the epoxy resin solidifies to a viscosity of about 28pa.s to 33pa.s, the "semi-gel" process is completed; Secondary vacuum pouring; Curing: The curing process is step-by-step and curing under normal pressure; the curing furnace is heated to the curing gel temperature Tn℃ for 40 minutes, and the curing time is 7 hours; the temperature is raised to (Tn+15)℃ for 30 minutes, and the curing time is 3 hours; the temperature is raised to (Tn+35)℃ for 30 minutes, and the curing time is 3 hours; the temperature is raised to 135℃ for 30 minutes, and the curing time is ≥12 hours; the temperature is lowered to (Tg+20)℃ as the curing furnace cools, and the cooling time is ≥3 hours; Demolding: The demolding sequence is: terminal mold → panel → outer mold → core mold → airway rod. After demolding, the coil is moved back to the curing furnace and slowly cooled to room temperature in the furnace.

2. The step-casting process of the epoxy resin coil with filler as claimed in claim 1, characterized in that: The Tg is the glass transition temperature.

3. The step-casting process of the epoxy resin coil with filler as claimed in claim 2, characterized in that: After the vacuum pouring is completed, the vacuum degree is adjusted to 1-2 mbar and maintained for ≥40 minutes, then the vacuum is broken, and then pressurized to 2 standard atmospheric pressures and maintained for 30 minutes; and then the atmospheric pressure is broken.

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

5. The step-casting process of the epoxy resin coil with filler as claimed in claim 4 is characterized in that: The epoxy resin preparation includes pre-drying of silicon micropowder, preparation of A / B tanks, mixing and degassing. The pre-drying temperature of silicon micropowder is 105°C, the drying time is ≥10 hours, and it is cooled to (Tn-15)°C with the furnace for standby use. The A tank (resin tank) and the B tank (curing agent tank) of the pouring system are respectively batched, stirred and degassed according to the required proportions; the vacuum degree of tank A is 2-3mbar, the temperature is (Tn-10)°C, and the degassing time is ≥4 hours; the vacuum degree of tank B is 3-4mbar, 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 2mbar and a temperature of (Tn-12)°C. The mixing tank is a dynamic mixing tank and needs to be stirred for 45-60 minutes. The mixing is completed after no bubbles overflow.

6. The step-casting process of the epoxy resin coil with filler as claimed in claim 5, characterized in that: This process is suitable for coil casting of dry-type transformers with larger capacity and higher voltage level, such as coils with capacity of 6300kVA and above and voltage level of 35kV and above.

7. The step-casting process of the epoxy resin coil with filler according to claim 6 is characterized in that: This graded casting process is applicable to all types and formulas of filled epoxy resins used for casting dry-type transformer coils. The specific temperature value of the curing gel temperature "Tn" varies according to the type and formula of the epoxy resin, and is the recommended data of the epoxy resin provider.

8. The step-casting process of the epoxy resin coil with filler as claimed in claim 7, characterized in that: During the semi-gelling process, the actual time deviation of semi-gelling at Tn℃ is only allowed to be within 10 minutes. The semi-gelling time should include the time for vacuuming of the subsequent secondary vacuum pouring and other transfer connection time. Vacuuming can be started 30 minutes before the end of the required time.

9. The step-casting process of the epoxy resin coil with filler according to claim 8, characterized in that: The graded casting process can be divided into two-stage or multi-stage casting and gelling processes. The specific casting and gelling stages are preferably such that the amount of filler resin in the first stage is 250-350 kg.

10. The step-casting process of the epoxy resin coil with filler according to claim 9, characterized in that: When the room temperature is below 15℃, immediately wrap the outer ring of the coil with a thin blanket after removing the outer mold; cover the pulled-out part with a thin blanket every time a part of the airway rod is pulled out to slow down the cooling rate. The demoulded coil is quickly moved back to the curing furnace and slowly cooled to room temperature in the furnace.

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