Introduction
Frequency and phase angle displacement are fundamental properties in safely controlling the ac grid voltage and power as well as ensuring the stability of the power system. Among the widely used techniques for estimating the angular frequency, ω, and phase, θ, of an ac grid are the Phase-Locked Loop (PLL) and the Frequency-Locked Loop (FLL) [1]. In addition to controlling the positive-phase sequence voltage at the Point of Common Coupling (PCC), the phase angle information is key in resolving the negative-phase sequence components under unbalanced operating conditions.