**3. Grid Synchronization**

The phase angle of the grid voltage is a critical piece of information for grid-connected systems since it is used to obtain the control reference signal as previously pointed out. Numerous methods using different techniques for synchronization and grid-voltage monitoring have been presented in the technical literature about DPGS. Most of these studies are related to three-phase systems [22,28,58,59] than to single-phase applications [60,61]. Some of the methods are not always categorized properly, thus leading to confusion. In order to clarify, the most used techniques can be organized as presented in Figure 2.

**Figure 2.** PVSs synchronization techniques classification.

Commonly the synchronization and grid voltage monitoring methods are classified in two main categories: zero-crossing detection (ZCD) methods [62] avoiding the grid voltage phase control, phase-locked loop (PLL) methods based on a strict control of the grid voltage phase [63]. The PLL techniques are generally categorized as PLL based on ZCD, on the arctangent function [64] and on the Park transform [44]. Among these, the Park transform-based PLL provides the best performance. Actually, in the last years, a new family of PLLs known as enhanced PLL has been ranked as the most promising.

Looking at the single-stage PVS shown in Figure 1, two synchronization systems are required in order to manage correctly the disconnections and reconnections with the main power system: the first synchronization system is used to monitor the voltage grid, the second one is used to monitor the PVS voltage. Among the synchronization systems presented in literature, only some methods are compatible with the considered application.
