As renewable energy sources like wind and solar power increasingly replace traditional energy sources and are integrated into the power grid, the issue of insufficient system inertia is becoming more apparent. This paper presents an online adaptive time window inertia constant identification method based on ambient measurements to identify the equivalent inertia constant of the time-varying inertia at Point of Interface (POI) level. The proposed method takes advantage of the online inertia estimation and the data-driven equivalent inertia constant identification techniques to simultaneously achieve online tracking and accuracy. With this regard, this paper first describes the inertia providers in modern system. Then, based on the frequency and power data measured by the Phasor Measurement Unit (PMU), this paper provides an improved data-driven equivalent inertia constant identification method. Subsequently, the paper proposes an ambient data smoothing method to cope with the numerical errors and provides, as a byproduct, an adaptive time window inertia constant identification. The adaptive time window is designed to enhance the accuracy of the method. Finally, the feasibility and accuracy of the proposed method of tracking synthetic inertia are validated by the simulation tests based on a grid in northwest China with high renewable energy penetration and a Virtual Power Plant (VPP). The experimental results show that the accuracy of this method is within $5\%$.

With the advancement of energy transformation and new power systems, large-scale integration of new energy to replace synchronous generators is leading to insufficient inertia in the system, threatening stable frequency operation. Evaluating the minimum inertia requirements of power systems is beneficial for improving the integration capacity of new energy and ensuring secure and stable operation. This paper establishes a mathematical model of frequency dynamics after a fault and advances an approach to assess the minimum rotational inertia requirements. The proposed approach is tested on the modified WSCC-9 bus system to validate its efficacy.