Frequency Quality Assessment of GFM and GFL Converters and Synchronous Condensers
Taulant Kerci, Federico Milano
TL;DR
The paper addresses how to maintain frequency quality in grids with rising grid-forming and grid-following IBRs alongside conventional machinery. It introduces a stochastic differential-algebraic modeling framework, applying Itô calculus to capture wind and demand variability, and compares GFM, GFL, and synchronous-condenser configurations on the IEEE 9-bus benchmark under normal and abnormal conditions. Results show that GFM markedly improves both short-term metrics (RoCoF, Zenith) and long-term metrics ($\sigma_{f}$, minutes outside $\pm 100$ mHz) relative to conventional or GFL-only setups; however, a combination of GFL devices with synchronous condensers can achieve similar performance in many scenarios, with AGC relevance diminishing in GFM-dominated grids. These findings offer practical guidance for transmission system operators on technology choices and AGC design in future power systems, highlighting robustness benefits of GFM and potential economic trade-offs with GFL+condensers depending on system strength and market context.
Abstract
This paper compares the impact of different conventional and emerging technologies and control strategies on frequency quality. We study, in particular, the long-term dynamic performance of grid-forming (GFM) and grid-following (GFL) inverter-based resources (IBRs) as well as conventional synchronous machines. Extensive simulations and several realistic scenarios consider both short-term and long-term aspects of frequency quality. It is shown that, while overall GFM IBRs significantly improve frequency quality, a combination of GFL IBRs providing frequency support such as wind and batteries, and synchronous condensers, might be enough to meet similar frequency quality standards. Another result of the paper is that the need for automatic generation control (AGC) becomes less clear in GFM IBR-dominated grids from a frequency quality perspective.