The criteria for out-of-step separation equipment in current domestic engineering fields are acquired from the out-of-step two-machine equivalent power system. Its applicability under multi-frequency oscillation is yet to be found out. Expressions of electrical quantities are inferred based on a three-machine equivalent system. And it is found that the out-of-step center would migrate between different transmission lines and different positions even on one and the same transmission line. In addition

the more distinct the different modulations are

the more transmission lines will there be for the out-of-step center to migrate in between. Besides

the three out-of-step splitting criteria based on Ucos φ characteristic

the measured impedance locus and the apparent impedance angle can hardly adapt to the multi-frequency oscillation scenario. It is advised that the two splitting criteria

apparent impedance angle and phase angle difference cooperate with each other to improve the correctness of out-of-step separation equipment.