Out-of-step oscillation criterion based on frequency characteristics

The characteristics of voltage frequency during the out-of-step oscillation are researched for the conditions that the electromotive force amplitude of one side is equal and unequal to that of the other side,which shows that,under both conditions,the voltage frequencies at the same side of oscillation centre increase or decrease synchronously;the voltage frequencies of two sides have no intersection;the voltage frequencies of one side decrease while those of the other side increase.A criterion based on frequency characteristics is proposed for out-of-step oscillation,which,immune to system structure change and operating mode change,distinguishes the synchronous oscillation from the out-of-step oscillation and identifies the location of oscillation centre.Simulation on BPA verifies the correctness of the analysis.