Quasi Steady-State Frequency

TL;DR

This paper introduces Quasi Steady-State (QSS) frequency, a new measure that accurately captures the fundamental frequency in stationary and transient conditions of power systems, especially with high power electronics penetration.

Contribution

The paper proposes the novel concept of QSS frequency and a fluid dynamics-inspired metric to define its validity scope, bridging stationary and instantaneous frequency analysis.

Findings

QSS frequency matches fundamental frequency in stationary conditions.

QSS frequency captures time-varying fundamental frequency during transients.

The metric defines the validity range of QSS 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.