Revisiting the Effect of Grid-Following Converter on Frequency Dynamics -- Part I: Center of Inertia

Understanding the impact of grid-following (GFL) converters on system frequency dynamics is crucial, from both the center of inertia (COI) and frequency spatial variation perspectives. Part I of this series clarifies the mechanisms by which GFLs influence COI frequency dynamics. A multi-generator model of the power system with GFLs is developed, incorporating the local dynamics of GFLs and their interaction with synchronous generators via virtual tie lines. By aggregating the multi-generator model into the COI frame, the interaction between the COI frequency and the equivalent frequency of GFLs is revealed. The equivalent inertia and other components at the GFL side, determined by control parameters and operating conditions, support the COI through virtual tying power. Simulation validates the accuracy of the proposed modeling and demonstrates that the impact of GFLs on COI frequency is relatively weak. The equivalent inertia and other components of GFLs still significantly influence COI frequency dynamics, with their effects being both time-variable and adjustable.