This paper compares the two criterions of generator loss-of-excitation protection based on impedance principle and inverse reactive power principle, by analyzing the variation of the steam turbine generator’s power output and generator-end impedance. The main criterions of the two protection principles are demonstrated the same principle and trip mode. The main action area of loss-of-excitation protection based on impedance principle is mapped from impedance plane to P-Q plane, in order to get the power action area. The function can save the setting calculation of the latter one from engineering experience. Finally, taking a 1 000 MW steam turbine generator as an example, the setting calculation of the two kinds of protection principles are calculated, which provides a specific reference for the dual principle loss-of-excitation protection configuration.