With the increasing penetration rate of renewable energy, the proportion of traditional thermal and hydroelectric power generation is continuously decreasing, leading to a reduc-tion in the frequency regulation capability of the power system. Due to the droop characteristic of the synchronous generator, fluctuations in PCC(Point of Common Coupling) voltage and current can cause fluctuations in the fundamental frequency of the system. Firstly, the port small-signal model of the synchronous unit and branch consisting of the inverter series line impedance is established, and the relationship between voltage, current, and fundamental frequency small-signal disturbances is constructed. Based on the GNC(Generalized Nyquist Criterion), a return ratio matrix containing fundamental frequency disturbances is constructed, and the maximum number of inverters for small-signal stability is found according to the eigenvalue trajectory of the return ratio matrix. Finally, the effectiveness of the small-signal model and stability discrimination method is verified through time-domain simulation.