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This paper presents an analysis and comparison of Phase Locked Loop techniques used in grid utility applications to find the voltage vector angle generated from the supply voltages. The Phase Locked Loop (PLL) has a wide range of applications as Distributed Generation (DG), Flexible AC Transmission Systems (FACTS), static VAR compensators, cycloconverters, Active Power Filters (APF's) and others systems connected to the utility. The performance of these systems in grid connected applications is strongly influenced by the adopted PLL strategy. For this reason, the goal of the proposed paper is to present a comparison of different PLL-based techniques for utility applications to indicate the appropriate solution dedicated to a specific application. The criteria to compare the Phase Locked Loops techniques is the performance under distorted and unbalanced supply voltages.

A new stationary frame control scheme for threephase pulsewidth-modulation (PWM) rectifiers operating under unbalanced voltage dips conditions is proposed in this paper. The proposed control scheme regulates the instantaneous active power at the converter poles to minimize the harmonics of the input currents and the output voltage ripple. This paper's novelty is the development of a new current-reference generator implemented directly in stationary reference frame. This allows using proportional sinusoidal signal integrator (P-SSI) controllers for simultaneous compensation of both positive and negative current sequence components. No phase-locked loop (PLL) strategies and coordinate transformations are needed for the proposed currentreference generator. Experimental results are presented for a 20-kVA alternative current (ac)/direct current (dc) converter prototype to demonstrate the effectiveness of the proposed control scheme. A comparison with two other existing control techniques is also performed. Fast dynamic performance with small dc-link voltage ripple and input sinusoidal currents are obtained with this control scheme, even under severe voltage dips operating conditions.Index Terms-AC/DC converter, current-reference generator, voltage dips.