Structural Impact of Grid-Forming Inverters on Power System Coherency

TL;DR

This paper investigates how integrating grid-forming inverters affects power system coherency and oscillatory behavior, revealing that their placement can significantly alter system dynamics and stability.

Contribution

It provides a mathematical analysis showing the impact of GFMs on system eigen-structure and validates findings with IEEE test system simulations.

Findings

GFM integration preserves the Laplacian structure but alters its entries.

Placement and penetration of GFMs significantly influence system coherency.

Numerical validation confirms theoretical predictions on IEEE test system.

Abstract

This paper addresses the following fundamental research question: how does the integration of grid-forming inverters (GFMs) replacing conventional synchronous generators (SGs) impact the slow coherent eigen-structure and the low-frequency oscillatory behavior of future power systems? Due to time-scale separated dynamics, generator states inside a coherent area synchronize over a fast time-scale due to stronger coupling, while the areas themselves synchronize over a slower time scale. Our mathematical analysis shows that due to the large-scale integration of GFMs, the weighted Laplacian structure of the frequency dynamics is preserved, however, the entries of the Laplacian may be significantly modified based on the location and penetration levels of the GFMs. This can impact and potentially significantly alter the coherency structure of the system. We have validated our findings with numerical results using the IEEE 68-bus test system.