Quasi Steady-State Frequency
Abstract
Accurate frequency estimation is critical for the control, monitoring and protection of electrical power systems, in particular, of systems with a high penetration of power electronics. This paper introduces the novel concept of Quasi Steady-State (QSS) frequency as a quantity that fills the gap between stationary and instantaneous frequency. QSS frequency coincides with the fundamental frequency of an AC voltage in any stationary conditions, including unbalanced and non-sinusoidal, and is able to capture the time-varying fundamental frequency in transient conditions. The paper also proposes a metric borrowed from fluid dynamics, namely, the time derivative of the circulation, to define the scope of validity of the QSS frequency. Analytical examples as well as a case study based on a fully-fledged EMT model of the IEEE 39-bus system serve to illustrate, respectively, the properties of the QSS frequency and its behavior in transient conditions.
Cite
@article{arxiv.2505.21461,
title = {Quasi Steady-State Frequency},
author = {Joan Gutierrez-Florensa and Alvaro Ortega and Lukas Sigrist and Federico Milano},
journal= {arXiv preprint arXiv:2505.21461},
year = {2025}
}