It is demonstrated in this paper that the change rate of the sum of three-phase active powers and reactive power are the cosine function and the sinusoidal function with respect to the phase difference between the two power systems during power swings. In this case, they cannot be lower than the threshold of 0.7 after they are normalized. However, they level off to 0 when a three-phase fault occurs during power swings. Based on this analysis, a cross-blocking scheme to rapidly identify the symmetrical fault occurring during the power swings is therefore proposed. By virtue of the improved two-instantaneous-value-product algorithm, the calculation of the active and reactive power is immune to the variation of the system power frequency. As the integration based criterion, this criterion has high stability. Simulation results show that this scheme is of high reliability and fast time response. The longest time delay is up to 30 ms
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