S9 series (35kV) three-phase transformer which is oil-immersed, self-cooled, double-winding and non-exited voltage-regulating, with a capacity of 800kVA to 16000kVA, is China's second generation of energy-saving product. Compared with S7 series product, its no-load loss has been reduced by 27.1%, onload loss reduced 11.2%, noload current reduced 41.4%, so the energy-saving effect is very noticeable and the working cost is to be greatly reduced. It is the main equipment of power system and we can get great economic efficiency in power distribution. It has been audited by the authority of province and it is one of the new products recommended by National of Economy & Trade to use in the field of reforming rural and city power distribution.