3. Selection and formulation design of epoxy resin pouring materials
3.1 Selection of epoxy resin mixture
The choice of epoxy resin mixture should be considered from the two aspects of performance requirement and casting process.
For dry-type transformers, due to the large volume of winding, high requirements for electrical performance and mechanical strength, so the choice of pouring material must meet the performance requirements of the transformer in all aspects, and the insulation level generally requires more than 3 classes.
In terms of technicality, it mainly requires that under certain technological conditions, the mixture has high fluidity, easy leaching and the applicability of the mixture, that is, the pouring time of the mixture can be controlled under certain conditions.
At present, the dry transformer winding pouring mixture (no packing type) is divided into: epoxy resin, curing agent, plasticizer, accelerator and color paste and other materials.
3.1.1 Epoxy resin
In the pouring process, there are many kinds of epoxy resin that can be used, but the dry transformer is limited by the technical conditions and process conditions, so the material performance of epoxy resin is very strict. If the content of organochlorine and inorganic chlorine is required to be low, because the higher content of organochlorine in epoxy resin will lead to the reduction of the electrical properties of epoxy resin cured at high temperature; If there is inorganic chlorine in epoxy resin, the electrical properties of the cured epoxy resin will become worse or the cured epoxy resin will crack at room temperature. At the same time, due to the presence of chlorine ions and other impurities in the epoxy resin, the gel time of the epoxy resin mixture in the pouring process is shortened, which makes the pouring process unable to proceed smoothly, leading to the premature gel of the mixture being scrapped.
At present, the epoxy resin used for pouring transformer windings is basically medium viscosity liquid bisphenol! Epoxy. The resin whether imported or domestic production, its performance is basically the same, are high purity epoxy resin.
3.1.2 curing agent
In the epoxy resin mixture, the excellent electrical and mechanical properties of the epoxy resin can be demonstrated only when the epoxy resin is cross-linked with curing agent to form an insoluble and insoluble network structure. Therefore, curing agent is an essential component in the casting process of epoxy resin.
The type and grade of curing agent are directly related to the main performance of pouring curing system. Because the content of impurities, volatiles and main components in curing agent varies, it will not only make the pouring process abnormal, but also reduce the electrical and mechanical properties of the solidified body. So, the choice of hardener must be careful, after a breed is decided, unfavorable change easily.
In order to meet the performance requirements of transformer, the curing agent selected for casting winding of transformer is basically cured at high temperature. This is because the curing agent used in the early stage is mostly solid anhydride, such as polynonylanhydride, phthalic anhydride and so on. They all have good electrical and mechanical properties, and the price is lower. But because of the poor casting process, such as sublimation problem, mixture temperature control problem, filter blockage problem is difficult to solve, so it has been replaced by all liquid high temperature curing agent.
At present, the curing agent used for pouring transformer winding is basically isomerized methyl tetrahydrogen (or hexahydrogen) phthalic anhydride. The curing agent has low viscosity, low volatilization, good casting process, good electrification and mechanical properties after curing.
3.1.3 plasticizer
A certain amount of plasticizer should be added more or less in the formula of epoxy resin mixture to improve the brittleness of epoxy resin curing body, prevent the curing body from cracking and meet the performance requirements of the product. However, the added amount should not be too much, because in the mixture, with the increase of plasticizer, the thermal deformation temperature of epoxy resin curing body gradually decreases, and the electrical properties also decrease accordingly. Therefore, in the epoxy resin mixture, the amount of plasticizer should be controlled.
Plasticizer suitable for pouring dry transformer windings must be a material with stable performance under high temperature conditions, and its viscosity should be small, conducive to infiltration, diffusion and adsorption. At present, plasticizers used for pouring transformer windings are mainly divided into two categories: one is side chain epoxy resin; The other group is polyols.
3.1.4 promoter
In the epoxy resin mixture, the main role of accelerators is to accelerate the crosslinking curing reaction and adjust the gel time to meet the requirements of casting process. At the same time, it also has the effect of reducing the curing temperature of the mixture, so as to reduce the shrinkage of the cured material, avoid the casting of the cured body crack.
In the high temperature curing mixture formula, the dosage of accelerator should not exceed 0.5% in general, especially for large casting parts such as casting transformer windings, which need to be strictly controlled. Because the castable USES a lot of mixture at one time, once the accelerator is too much, the mixture will soon gel solidification and scrap. In addition, even if the casting can proceed smoothly, it will cause the cured epoxy resin to break during the curing process due to the premature gelation, which will lead to the failure of the extended macromolecules during the reaction and lead to the cracking of the cured resin.
3.1.5 paste
A small amount of color paste is added to the epoxy resin mixture, the purpose of which is to color the casting to obtain the desired color.
The color paste of epoxy resin mixture must meet the following conditions: strong coloring power, that is, a small amount of color paste can be added to achieve the dyeing effect; It has good dispersion in epoxy resin. Excellent heat resistance, F class dry type transformer, should not decompose at 155 degrees Celsius, not fade; It has good electrical properties, that is, the addition of color paste does not affect the electrical properties and other comprehensive properties of epoxy resin cured body.
3.2 Formula design of epoxy resin casting materials
The formulation design of epoxy resin castable is the key to the whole casting process. The formulations are designed with emphasis on the performance requirements of the products to be poured and the process requirements in the pouring process.
3.2.1 Performance requirements of casting products
For dry type transformers, the casting winding is required to have excellent electrical performance, achieve the corresponding insulation level (domestic general requirements to achieve F class), have excellent mechanical performance. At the same time, the casting windings are required to be of good toughness and rigidity to ensure that the casting windings do not crack or delamination in the long run of the transformer. High temperature cured epoxy resin mixture is usually selected to satisfy the properties mentioned above.
3.2.2 Casting process requirements
The formulation design of epoxy resin mixture must also meet the technical requirements, mainly considering the viscosity and gel time of the mixture. For epoxy resin mixtures that also meet product performance requirements, a formulation with lower viscosity should be chosen. Because the low viscosity of the mixture to facilitate vacuum degassing, and good fluidity, permeability to the pouring parts.
Gel time is an important parameter in epoxy casting process. The gelation time is too short, which may cause the mixture not to pour, that is, the gel solidifies and cannot be poured, thus causing serious consequences. But the gelation time should not be too long, otherwise it will bring a lot of inconvenience to the mass production. Therefore, the gel time of the mixture must be controlled within an appropriate range. From the actual operating situation, the drying transformer winding pouring process generally requires gel time between 3~5h is more appropriate.
Several problems that should be paid attention to in the pouring process of winding
4.1 Preparation of castable
The pouring material should be prepared according to the process formula, and the amount of each component should be precisely controlled during the preparation, especially the accelerant. Although the amount of each component takes a small proportion, it has a great influence on the curing reaction. The amount of accelerator is related to the structure and volume of winding. Generally speaking, for small windings, the amount of accelerator can be more, and less on the contrary. The proportion of additions shall be determined by trial.
4.2 Control of pouring temperature
The vacuum casting temperature shall be determined according to the performance of the pouring material
The temperature should not only ensure that the pouring material has a low viscosity, make it flow well, facilitate vacuum degassing and in a short period of time to soak the pouring parts, but also ensure the service life of the pouring material. If an epoxy resin mixture can be cured at 80 ° C after the addition of accelerator, the casting temperature for the epoxy resin mixture of this component should not exceed 80 ° C. In the actual use, it is more appropriate to control the pouring temperature between 60 ℃ and 70 ℃.
4.3 Control of vacuum degree
The vacuum degree has a direct effect on the elimination of gas in the mixture and the quality of the castable during pouring. If the vacuum is low, the degassing time should be extended, but the higher the vacuum, the better, as it is limited by the saturated vapor pressure of the pouring material. If the vacuum degree exceeds the saturated vapor pressure of the material, it will lead to the volatilization of the material and lead to the change of mixture ratio, and the quality of the pouring material cannot be guaranteed or even scrapped. Therefore, only when the vacuum degree is lower than the saturation vapor of the highest component in the mixture, can the mixture ratio be stable.
4.4 Curing time and curing temperature control
After pouring, the winding must be cured. For curing process parameters and process, each plant has its own characteristics. However, according to the curing reaction characteristics of epoxy resin and the requirements on the electrical properties of windings, two-step curing process is generally adopted, that is, pre-curing at lower temperature and post-curing at higher temperature. The pre-curing time of METHA as curing agent is generally 3-5h, and the temperature is controlled between 90-110 ° C. At this time the curing process is the mixture of liquid state to solid state conversion process. The pre - cured epoxy resin is elastomer, which is easy to mold. The casting winding must be cured after demodulation. The purpose is to make the epoxy resin undergo further crosslinking curing, so that the epoxy resin will eventually form an insoluble and insoluble solid. In order to obtain the best electrical and mechanical properties, the curing process at this stage should adopt the step curing process, the step level and temperature gradient can be determined according to the size of the workpiece. The heating rate is generally controlled below 25 ° C per hour, so as to avoid the stress concentration caused by the rapid heating rate and the intense curing reaction.
5 conclusion
Over the years, the domestic epoxy resin casting process has done a lot of research work, and accumulated rich manufacturing experience, the technology has been widely used. With the improvement of dry-type transformer structure and the improvement of performance requirements, the technical specifications of epoxy resin and its auxiliary materials are also improved, and the higher requirements for pouring process are also put forward.
In order to meet the requirements of electrical and mechanical properties of dry-type transformer, in addition to the reasonable formulation of epoxy resin mixture, feasible pouring process and reasonable process parameters must be provided. Only in this way can we guarantee the excellent performance of pouring products, and many work in this field is still to be further explored and studied by us, in order to constantly improve the pouring process level and product quality.
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