During normal operation, the generator is not allowed to run over the rated capacity for a long time. When the generator voltage is lower than the rated value, the stator current is allowed to appropriately increase, but for long time running of the generator, the maximum current of the stator shall not exceed 5% of the rated value. In case the system has short-circuit fault, the generator is in out-of-step operation, and the groups of motor are started and forced to excite, the generator stator and rotor are likely to be short-term overload. If the overload makes the current of the generator stator and the rotor exceed the rated value more, there will be risk that the winding temperature exceeds the permissible limit, thus making the insulation age fast and even causing the mechanical damage. The greater the overload value is, the longer the duration is , and the more serious the risk is. But because the temperature of the generator under the rated conditions is lower than the highest allowable temperature of its insulating materials used, the certain allowance can be used for the short time overload.