## 4 Grid Synchronization for the Secondary Control Unit  
The purpose of grid synchronization is to monitor and self-regulate the phase to prevent the deviations of voltage and frequency by minimizing the variance in voltage, frequency, and phase angle between the RES AC generators' output and the grid supply [126; 127]. The synchronization unit control loop is used to synchronize the microgrid's phase with the grid. Such synchronization  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Controller** & **Advantages** & **Drawbacks** \\ \hline \multirow{4}{*}{Centralized [66]} & All the decision making, collaboration, and prioritization are achieved more efficiently when information is collected at a single point through the central controller. & Performance degrades when failure occurs at a single point of communication. \\  & Provides fast and effective process, while retaining accurate power sharing. & Communication links among devices offer unreliable operation in terms of maintenance and cost considerations. \\ \multirow{4}{*}{Distributed consensus [116; 118]} & Increases flexibility and reliability of the microgrid system without taking line impedance consideration. & Sophisticated implementation. \\  & More reliable and has accurate restorations process. & High computational burden. \\ \multirow{4}{*}{Event-triggered [122; 123]} & Able to maintain superior closed-loop stability and performance. & High computational burden. \\  & Effectively updates voltage and frequency to nominal values. & Requires a new switch mechanism, which increases malfunction factor. \\ \hline \hline \end{tabular}
\end{table}
Table 3: Performance comparison of voltage and frequency of secondary control.  
control entails a cluster of all the controllable DG units employed in the secondary control level in the hierarchical control architecture. It provides relatively zero steady-state errors to be translated into the grid-connected operation mode. Grid synchronization is a subjective subtopic that can be classified into primary or secondary control families. However, in this paper, it is considered as a secondary control. As reported in [115], an ideal synchronization must conform to desirable characteristics such as (i) adeptly tracking the primary grid's phase angle, (ii) effectively tracking variations in the frequency and is immune to disturbance signals and high harmonics parameters, and (iii) fast response to the alterations on the primary grid. Numerous grid synchronization methods have been proposed over the years, either based on phase-locked loop (PLL) or non-PLL methods. On the other hand, synchronization without a dedicated synchronization unit has become an alternative for conventional PLL to provide fast and robust operation and simultaneously reduce the computational burden by nonlinear PLL characteristics. Therefore, numerous proposals for self-synchronization techniques have been presented over the years, as reported in [128; 129; 130; 131; 132; 133; 134].  
Table 4 address the advantage and disadvantages of grid synchronization with significant methods should be considered to present advanced control techniques that are capable of retaining power quality.