Revisiting the Effect of Grid-Following Converter on Frequency Dynamics -- Part II: Spatial Variation

TL;DR

This paper investigates how grid-following converters influence spatial frequency variations in power systems, extending existing models to include branch current effects and validating findings through simulations.

Contribution

It introduces an extended frequency divider formula incorporating branch currents and spatial effects, enhancing understanding of GFL impact on frequency dynamics.

Findings

GFLs have a relatively weak contribution to node frequency variations.

Superposition coefficient of GFLs is time-varying.

Branch frequency superposition exhibits complex patterns.

Abstract

Besides the center of inertia (COI) frequency dynamics addressed in Part I, the spatial frequency variation in power systems with grid-following (GFL) converters is also crucial. Part II revisits the effect of GFLs on frequency spatial variation. Leveraging the interfacing state variables and equivalent frequency defined in Part I, an extended frequency divider (FD) formula is proposed. The linearized mapping relationship between network node frequency and synchronous generator (SG) rotor frequency, as well as GFL equivalent frequency, is modeled. The superposition contribution from GFLs is determined by the electrical distance between the generator and the frequency observation node, as well as the system power flow conditions. Additionally, the frequency mapping for branch currents, which is overlooked in previous research, is addressed. Simulation results validate the accuracy of the proposed extended FD formula. They quantitatively demonstrate that the superposition contribution of GFLs to node frequency is relatively weak and that the superposition coefficient is time-varying. The branch frequency superposition reveals a complex and distinctly different pattern.