Understanding the Impact of Hydro-Reservoirs and Inverters on Frequency-Constrained Operation
Valeria Aravena, Samuel Cordova, Maximiliano Kairath, Matias Negrete-Pincetic
TL;DR
This paper tackles the challenge of frequency regulation in power systems with high shares of inverter-based resources by developing a data-driven Frequency-Constrained Unit Commitment (FCUC) that explicitly models hydro-reservoir dynamics and Grid-Forming (GFM) inverters. It introduces a dynamic frequency model with hydro water-hammer and a GFM-based Virtual Synchronous Machine, and embeds this into an FCUC where RoCoF, Nadir, and QSS constraints are enforced; notably, Nadir is handled via a data-driven linear boundary learned from dynamic simulations. Computational experiments on Chilean system data for 2024 and 2035 demonstrate substantial operational cost savings (up to 28%) compared with industry-standard FCUC, and reveal hydro-reservoirs’ critical role as thermal units retire. A planning analysis shows GFM inverters outperform Synchronous Condensers in investment returns, especially when retrofitting existing assets, guiding policy toward deploying GFM-based frequency support. Overall, the work provides practical insights for secure, cost-effective operation and planning of decarbonized grids with significant hydro and inverter-based resources.
Abstract
The increasing participation of renewable energy sources in power systems has entailed a series of challenges resulting from the replacement of conventional synchronous machines with carbon-free Inverter-Based-Resources (IBRs). In this context, the present work contributes to the existing literature on Frequency-Constrained Unit Commitment (FCUC) models by studying the role of hydro-reservoirs in the ongoing decarbonization of power systems. For this purpose, a novel FCUC model is developed, which captures hydro-reservoir dynamics and their impact on frequency Nadir requirements through a data-driven approach. Moreover, the proposed FCUC model is used to simulate a series of future scenarios in the Chilean power system, and to understand the role that hydro-power units, thermal units, Grid-Forming (GFM) inverters, and Synchronous Condensers (SCs) will play in the future in terms of frequency regulation. Exhaustive simulations on the Chilean power system for years 2024 (current scenario) and 2035 (carbon-free scenario) illustrate the benefits of the proposed approach, which include: (i) significant cost-savings resulting from the using the proposed FCUC model relative to the industry standard, achieving operational cost reductions of up to 28% without compromising system security; and (ii) providing insights into the key role that hydro-reservoirs will play in the future. As a final analysis, the developed FCUC model is used to evaluate the financial benefits of incorporating SCs and GFM inverters to provide frequency regulation support to the grid in 2035. The results show that, while both technologies contribute to reducing the system's annual operational costs, GFM inverters significantly outperform SCs in terms of investment return.