The capacity,voltage,and length of modern electric power system are increasing ceaselessly.Synchronous generators will cause self excitation with the help of armature reaction,its stator current and voltage will increase.The self excitation conditions and the transient process are two aspects for studying self excitation.Now,self excitation conditions are mathematical formulas under certain postulate.The model of transient process of self excitation is linear.The conditions of self excitation are presented based on simulation,and the results of self excitation effected by the electromagnetic parameters of hydro generators operating extra high voltage long transmission line are simulated,based on a saturation model of hydro generators.The conclusions are very helpful for operation and design of hydro generators.