Core type transformer: the power transformer is divided into core structure and shell structure according to the structure of iron core, and the core of core structure is surrounded by winding. The structure features of three phase core transformer are that the three phase magnetic circuit is connected together. The magnetic circuit of each phase is used as its own circuit with the other two phase magnetic circuit. Compared with shell type transformer, it has the advantages of low consumption materials, low price, small occupation area and simple maintenance. Therefore, it is widely used. But in large-capacity giant transformers, three-phase group transformers are often used in order to facilitate transportation and reduce spare capacity. The core type transformer uses this advantage to be used for high voltage power transformer. For example, the 12 pulse wave rectifying device composed of three-phase five-pillar rectifying transformer is a good case. With core type's high reliability, sufficient power supply capacity, sufficient development space, less investment and low operation cost, the power plant upgrading and upgrading of core transformer are provided for chemical plants. The efficiency of electricity consumption is improved and the cost is effectively reduced.