An Identification Method of Dominant Instability Factors in the New Power System Using the Apparent Power Phasor Trajectory

Recently, a new stability classification including the frequency, voltage, power angle, and impedance angle stability has been proposed. However, instability coupling among the voltage, power angle, and impedance angle may occur. Previous studies have investigated the coupling between the voltage and power angle. Nevertheless, system instability may also involve the voltage magnitude, power angle, and impedance angle. Dominant instability factor identification remains a research gap. Consequently, this paper proposes an Apparent Power Phasor Trajectory (APPT)-based identification method. Different from coupling analyses that mainly describe interaction relationships or stability boundaries, the APPT constructs comparable trajectory distance indicators in a unified apparent power phasor framework. Based on a two-node equivalent model of a grid-forming inverter, APPT sensitivity relationships and trajectory distance indicators are formulated under a unified metric. Numerical studies show that the proposed method identifies the dominant instability factor in representative scenarios. In the dynamic Kundur case, the overall consistency ratio reaches 95.01%. Simulation-based checks give accuracy values ranging from 97.07% in the reference setting to 90.94% under synchronization mismatch. Together with the IEEE 39 bus case, these results indicate that the APPT provides an interpretable basis for dominant instability factor identification in new power systems.