Abstract: Aiming at the calculation problem of Process Immunity Time (PIT) for frequency-converter sensitive equipment after photovoltaic grid connection, this paper proposes a PIT calculation method based on mixed potential function theory. By constructing the large-signal average value model of the system, the stability criterion is derived in combination with the hybrid potential function theory, and the critical instability time of the system is iteratively solved using the numerical simulation method. The simulation results show that the proposed method can accurately calculate the PIT under different voltage sag depths, and the error between the theoretical value and the simulation value is less than 5%, verifying the effectiveness of the method. This method provides theoretical support for the dynamic stability analysis and operation planning of photovoltaic grid-connected systems.