The external characteristics of a synchronous generator generally refer to the curve of the voltage change at the generator terminal when the load current changes while the internal potential remains unchanged. It mainly tests the longitudinal synchronous reactance of the generator, which is also the internal impedance of the generator. It is an important indicator of the load carrying capacity of the synchronous generator. However, synchronous generators often use thyristor fast excitation and damping windings, and their longitudinal synchronous reactance is mostly transient, far less than the steady-state value. In addition, due to the regulating effect of the excitation system, the external characteristics can be manually manufactured, which can be positive or negative. The positive external characteristics are that the terminal voltage decreases with the increase of the load current, while the negative characteristics are that the terminal voltage increases with the increase of the load current. Generally, the excitation system can be adjusted within the range of plus or minus 15%.