Xiang Jia Ba hydro power plant,the biggest air cooled hydro generator with an output of 890 MVA will be installed.The machine is designed with a 23 kV stator winding which finally allows an optimized electrical design of the entire machine considering the project specific speed and output. In general,rated voltages above 20 kV offer a new potential for the overall optimization of very large hydro units.The application of such a high voltage requires specific attention during design,the manufacturing,the testing and the installation phases. Alstom has continuously developed the stator winding technology towards increased field strength.Based on the feedback from installed generators and extensive tests in the laboratory the Alstom corona protection system has been confirmed as a reliable technology for voltages above 20 kV.For large,low speed hydro generators such as the units for Xiang Jia Ba project,the structural concept of the stator and rotor,the thrust bearing and the cooling system are of great importance.The 800 MW units incorporate a number of proven Alstom design concepts that have already been successfully implemented in air-cooled hydro generators above 600 MVA such as Longtan,GouPiTan and Xiaowan, and NuoZhaDu.For Xiang Jia Ba a completely air cooled generator has been investigated which reflects today the biggest air-cooled machines in China.Large machines are very sensitive to thermal expansion,centrifugal forces,and forces involved in fault disturbances e.g.short circuits and double earth fault on field windings.The Alstom design concepts,successfully applied on the generator for Three Gorges,are designed to meet requirements like roundness,concentricity and stability of the generator in all conditions and to minimize the impact of the abovementioned forces.The experiences made in the Three Gorges project show that all these requirements are fulfilled by oblique elements and that shrinkage of the rotor rim will not affect the coupling between rotor hub and the thrust block of the thrust bearing.