With a substantial increase in the proportion of power electronic grid-connected devices in a power system, the output characteristics on the power supply side are obviously different from the output characteristics of traditional power supply represented by synchronous generators. The fault analysis of power electronics dominated power systems, especially the network asymmetrical fault analysis, faces new challenges. However, the existing fault analysis methods basically do not realize the time-varying amplitude/frequency characteristics of the devices' internal voltage, and the available time-varying amplitude/frequency symmetrical components method based on time-varying amplitude/frequency signals is just a set of mathematical decomposition formulas without explicit physical connotation as a support. Under this background, the characteristics of the relationship between the three-phase instantaneous values formed by the positive-and negative-sequence time-varying amplitude/frequency rotating vector are analyzed, and the