Comparison and evaluation of the PLL techniques for the design of the grid-connected inverter systems

The knowledge of the phase, amplitude and frequency of the utility voltage is a fundamental aspect for the design of the grid-connected inverter systems. In this paper are presented the basic features of the PLL technique. A particular attention is dedicated to the Synchronous Reference Frame-PLL scheme. About the generation of the orthogonal voltages system an evaluation of the most employed techniques is shown: Transport Delay, Inverse Park Transformation, Hilbert Transformation, Second Order Generalized Integrators (SOGI). Moreover some problems in filtering are treated. The internal filtering - due to the PD structure - and the external filtering - due to the harmonics presence in the grid voltage. In both the cases the most employed solutions are shown. For the internal filtering: Low Pass Filter, Resonant Filter, Moving Average Filter, Repetitive Controller. For the external filtering two alternative schemes are presented: the Dual SOGI-PLL and the Enanched-PLL (EPLL).