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AC transmission systems are often affected by stability issues. During the design phase of such systems, their ability to cope with the various kinds of disturbance is taken into consideration, however, sudden large disturbances may still result in power angle instability, which can greatly increase the risk of an overarching power system break down. Therefore, the development of a safety protection system is a priority in the field of power system construction. In response to this problem, we propose a new exploratory solution strategy -Phase Sequence Exchange Technology (PSET). The PSET can be summarized as follows: when the power angle of an equivalent dual power supply system swings to a suitable angle between 90 • and 180 • , disconnect the primary side phase of the communication line by power electronic equipment and causing the swift misalignment of the connection. The A, B, C three-phase sequence then, connects to the three-phase C, A, B sequence. The PSET instantaneously realizes this change and reduces the power angle by 120 • , pulling the formerly increasing power angle back to a smaller angle and preventing the system from becoming imbalanced. In this paper, the mechanism which improves the stability of the PSET is explained by using the equal area rule. The threshold of the PSET work angle is determined by the energy function and the unbalanced potential energy in the system that can be effectively reduced by the PSET. Based on these findings, the phase trajectory method is used to determine the evaluation criteria for the PSET. This allows us to establish whether the PSET is effective by examining the state of the system after the fault is removed. Finally, an example is given in order to verify the effectiveness of the PSET and to prove that PSET is able to prevent the system from becoming imbalanced while still maintaining its structural integrity.INDEX TERMS Transient stability emergency control, phase sequence exchange technology, power electronic equipment, effective criterion.

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