Kleber Melo Silva

In this letter, a new algorithm is proposed to improve frequency estimation for relaying applications. The key idea is to apply successive filtering stages whose parameters are adapted following the estimated frequency deviation. Its performance is compared against other algorithms reported in the literature. Results show that the proposed algorithm provides an accurate and fast frequency estimation.

This paper presents a simplified analytical framework to explain the behavior of impedance measured by distance relays during power swings in systems with inverter-based resources (IBRs). By extending classical single-machine infinite-bus (SMIB) analysis, the impact of IBR current injection is isolated as an additional impedance component, Z_add = E_M/I_IBR. A qualitative analysis of this Z_add term—considering the magnitudes and relative phase angles of the synchronous machine voltage (E_M) and the IBR current (I_IBR)—is provided to elucidate how it distorts the impedance loci from the circular behavior observed in purely synchronous systems. Comparative simulations of a system with a synchronous machine versus one with an IBR of similar capacity demonstrate these significant alterations in the measured impedance. The findings highlight that the controlled, and often desynchronized, nature of IBR current injection during events such as low-voltage ride-through (LVRT) is a primary driver of these distortions. This work offers an intuitive, circuit-based understanding of these phenomena, providing a practical perspective for protection engineers assessing out-of-step protection in IBR-rich environments and underscoring the challenges for traditional impedance-based schemes.