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http://www.zjyb-electric.com/double-wound-transformer-applications.html

Double winding transformer is a common transformer product with main functions include voltage change, impedance transformation, isolation and voltage stabilization. The manufacturing process is different according to different applied occasions (or different purposes). Three wound transformer has three wounds of each phase. When one wound connects to the AC power, the other two wounds can induce different electric potential. The transformer is used for load which requires two different levels of voltage. Usually in power plants and substations, there are three different levels of voltage, so three wound transformer is used widely in the power system.

High, middle and low voltage wounds of each phase are set in the same core column. In order to make the insulation reasonable, usually the high voltage wound is put in the outer layer, with middle and low voltage wounds placed in the inner layer. Based on the high magnetic conductivity of iron materials, most of the flux is confined in the core. Therefore, the two sets of coils can get a high degree of magnetic coupling. Because of the function of voltage boost and buck, the transformer has become one of the most important appendages of modern electric power system. The increasing electricity transmission voltage makes the long-distance transmission more economical. As for step-down transformer, it makes the usage of electricity more diversified. It can be said that if there is no transformers, modern industry is unable to reach such a great level at this time.