Fault-decoupled instantaneous frequency and phase angle estimation for three-phase grid-connected inverters

Frequency and phase angle estimation is a key aspect for grid-connected inverters that are required to guarantee low-voltage fault-ride-through capability. Over the past two decades, a number of estimation algorithms have been proposed, mostly based on the well-known phase-locked loop (PLL). It has been demonstrated that standard PLLs don´t perform correctly in abnormal grid conditions, due to the oscillations produced in the frequency and phase angle estimates by the voltage harmonics. This paper introduces a new, general approach to harmonic decoupling and presents a highly intuitive and simple scheme, applying it to an αβ-PLL; compensation of any desired number of harmonic components is possible. Two implementations of this decoupling scheme are presented. It is shown that the performances of the resulting fault-decoupled PLLs are comparable with those of other advanced frequency and phase angle estimation structures.