As the city became larger and larger, the whole world pay more attention on the heavy traffic and vehicle pollution issue. Therefore, the urban subway is wildly used and developed because of its large capacity, safety, high speed and good environmental protect effect. SUNTEN aimed at this market and grasped the opportunity to successfully develop and produce the first 12-pulse rectifying traction transformer. During the development of it, we studied the features of transformer-rectifier and cleared the relationship between the rectifying rate of continuous voltage and the parameter of transformers and rectifier. We solved lots of technical problem such as the balance of rectifying bridge and no-load voltage rise. The 2500kVA model pattern we researched and produces has passed the full on-load test. We gained a lot of precious technique and experience from it. In 1997, in order to improve the domesticalization and technical level of urban railway transportation equipment, reduce rectifying harmonic wave, improve the electric network quality and join in international competition, SUNTEN cooperated with several units such as Shanghai Metro-subway three line, Tianjin Electrified Railway Designing Institute, Shanghai Rectifier Works,Guangzhou Metro-Subway and so on. We based on the technique of 12-pulse rectifier transformer and march to the advanced 24-pulse rectifying mechanism research, harmonic wave content calculation, assembling unit system full on-load test, assessment and design of transformer phase shift method, balance, exchange and reliability of two units of transformer-rectifier, etc. We finished the sample tral-produce and analog machine on-load test and developed the 24-pulse dry type rectifier transformer successfully. It filled the domestic blank and reached the highest level of the world present railway transportation equipment. Product Type: ZQSC(B)9-800~4400 Main Technical Parameters: Voltage level:10kV, 20kV, 35kV, Rectified pulse:12 pulse and 24 pulse, The network harmonic wave pollution of 24-pulse rectifying circuit is 50% lower than that of 12-pulse rectifying circuit, the filter equipment here could be omitted. Main Service Range: Power-supply system of urban subway and railway transportation.