Distribution transformers that supply power directly to distribution networks are widely used at home and abroad. In China, the annual output of distribution transformers is about 50 million KVA, accounting for about 1/3 of the total annual output of transformers. Therefore, the operational reliability, product technical performance and economic indicators of distribution transformers will directly affect the economic construction of the country and the safety of power supply for urban and rural residents and enterprises and institutions.
In recent years, in order to meet the needs of the country in the transformation of urban and rural power grids, a number of new-type and high-quality distribution transformers have been developed, making the transformer equipment of distribution networks more advanced, providing more reliable power supply, and lower electricity prices in rural areas. The loss values ​​of distribution transformers developed in recent years have been declining, especially the value of no-load loss has decreased even more. This is mainly due to the improvement of the magnetic properties of magnetic materials, followed by the diversification of core types of magnetically conductive structures. Such as the use of thin high-magnetic-conducting silicon steel or amorphous alloys, stepped joints full inclined structure iron core, rolled iron core (planar, three-dimensional), annealing process applications. Reduce the noise while also paying attention to the noise level. In the case of dry-type distribution transformers, the partial discharge test is listed as an example. The user also has requirements for the partial discharge amount, which is an assessment index of the operational reliability of dry-type distribution transformers, which is more current than the requirements of the International Electrotechnical Commission. The requirements are strict. Therefore, predicting the development trend of various types of distribution transformers in China on the basis of existing ones and promoting the further development of distribution transformers should be an important task.
Sites that require fire protection and explosion protection, such as commercial centers, airports, subways, high-rise buildings, and hydropower stations, often use dry distribution transformers. At present, dozens of factories in China have been able to produce conventional epoxy cast dry type distribution transformers. Both non-excitation voltage regulation and load regulation. In the normal operation, it is a self-cooling cooling method. When a blower is installed, emergency conditions are provided (other transformers start the fan when there is a fault) to run as super-nameplate capacity. In China, the largest three-phase single-unit capacity can reach 20000kVA (35kV class) and the highest voltage class can reach 110KV (single-phase 10500kVA). The annual output of dry-type transformers has accounted for 20% of the annual output of the entire distribution transformer. In view of epoxy resin cast dry distribution transformers, there are also the following problems:
(1) The degree of freedom of design is small, and each winding must be scratched with a mold.
(2) A large amount of smoke will be generated once it is burned at high temperatures.
(3) Because the coefficient of thermal expansion of epoxy resin and wire is not the same, if the buffer layer is not properly set, it is easy to crack under the impact of hot and cold temperature, the casting layer cracks, the partial discharge increases, some of the company's individual products have such quality problems Exposure during operation.
(4) Environmental protection issues. Once the expected life of the epoxy castable dry-distribution transformer has expired or the windings of the transformer are damaged due to various faults, it is difficult to destroy the cast-formed windings. Currently, it is impossible to make the ring. Oxygen resin degradation. From an environmental point of view, this will be a growing problem.
(5) Epoxy-casting type dry-type distribution transformers are mostly class F temperature-resistant grades. Only individual enterprises can produce cast-type dry distribution transformers with class H temperature resistance. In view of the above reasons, some companies are currently developing open-air dry-type H-level distribution transformers [1, 2].
In recent years, in order to meet the needs of the country in the transformation of urban and rural power grids, a number of new-type and high-quality distribution transformers have been developed, making the transformer equipment of distribution networks more advanced, providing more reliable power supply, and lower electricity prices in rural areas. The loss values ​​of distribution transformers developed in recent years have been declining, especially the value of no-load loss has decreased even more. This is mainly due to the improvement of the magnetic properties of magnetic materials, followed by the diversification of core types of magnetically conductive structures. Such as the use of thin high-magnetic-conducting silicon steel or amorphous alloys, stepped joints full inclined structure iron core, rolled iron core (planar, three-dimensional), annealing process applications. Reduce the noise while also paying attention to the noise level. In the case of dry-type distribution transformers, the partial discharge test is listed as an example. The user also has requirements for the partial discharge amount, which is an assessment index of the operational reliability of dry-type distribution transformers, which is more current than the requirements of the International Electrotechnical Commission. The requirements are strict. Therefore, predicting the development trend of various types of distribution transformers in China on the basis of existing ones and promoting the further development of distribution transformers should be an important task.
Sites that require fire protection and explosion protection, such as commercial centers, airports, subways, high-rise buildings, and hydropower stations, often use dry distribution transformers. At present, dozens of factories in China have been able to produce conventional epoxy cast dry type distribution transformers. Both non-excitation voltage regulation and load regulation. In the normal operation, it is a self-cooling cooling method. When a blower is installed, emergency conditions are provided (other transformers start the fan when there is a fault) to run as super-nameplate capacity. In China, the largest three-phase single-unit capacity can reach 20000kVA (35kV class) and the highest voltage class can reach 110KV (single-phase 10500kVA). The annual output of dry-type transformers has accounted for 20% of the annual output of the entire distribution transformer. In view of epoxy resin cast dry distribution transformers, there are also the following problems:
(1) The degree of freedom of design is small, and each winding must be scratched with a mold.
(2) A large amount of smoke will be generated once it is burned at high temperatures.
(3) Because the coefficient of thermal expansion of epoxy resin and wire is not the same, if the buffer layer is not properly set, it is easy to crack under the impact of hot and cold temperature, the casting layer cracks, the partial discharge increases, some of the company's individual products have such quality problems Exposure during operation.
(4) Environmental protection issues. Once the expected life of the epoxy castable dry-distribution transformer has expired or the windings of the transformer are damaged due to various faults, it is difficult to destroy the cast-formed windings. Currently, it is impossible to make the ring. Oxygen resin degradation. From an environmental point of view, this will be a growing problem.
(5) Epoxy-casting type dry-type distribution transformers are mostly class F temperature-resistant grades. Only individual enterprises can produce cast-type dry distribution transformers with class H temperature resistance. In view of the above reasons, some companies are currently developing open-air dry-type H-level distribution transformers [1, 2].
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