## 4 Grid Synchronization for the Secondary Control Unit
### Phase-Locked Loop  
Synchronization via PLL is the most widely adopted and is embedded as a dedicated synchronization unit [135; 136; 137]. It offers precise tracking of voltage, phase angle, and frequency, and provides simplicity, flexibility, accurate phase angle estimation, etc. However, PLL's performance depreciates when the power converter is integrated into weak grids [138; 139; 140; 141]. This scenario is caused by the presence of voltage imbalance and harmonic distortion, double frequency oscillation to the phase error signal, slow dynamic performance, complicated and slow coefficient performance, etc. Numerous modified methods have been presented over the years to mitigate these issues, as shown in [142; 143; 144; 38]. Methods that have been proposed include synchronous reference frame-PLL (SRF-PLL), fixed reference frame PLL (FRF-PLL), enhanced PLL (E-PLL), and second-order generalized integrator PLL (SOGI-PLL), among others. * SRF-PLL: Studies in [140; 145; 146; 147] have shown that this technique is the basic scheme in the three-phase system. It offers a rapid and accurate estimation of the phase angle/frequency in ideal grid conditions and offers a straightforward implementation. The operation of SRF-PLL is through the natural frame (_abc_) being transformed into the dq reference frame through Park's transformation. Despite fast and accurate phase estimation in ideal conditions, the performance is highly degraded against abrupt shifting in the phase angle as well as double frequency errors in \begin{table}
\begin{tabular}{c c c} \hline \hline
**Controller** & **Advantages** & **Drawbacks** \\ \hline \multirow{4}{*}{PLL} & Provides accurate synchronization. & \multirow{4}{*}{Uninearable in high grid faults. \\  & Better for grid harmonics and noise rejection. & \\  & Simple implementation. & \\  & & \\  & Immune to phase angle and frequency shifts, harmonics, and & \\  & uncertainty parameters. & \\  & Offers reliability under frequency variations, harmonics, and & \\  & unbalanced voltage. & \\  & & \\  & Provides robust phase tracking with fast transient under & \\  & highly polluted grid. & \\  & Accurate synchronization even under noise occurrence. & \\  & Faster and effective in updating frequency and phase angle & \\  & fault conditions. & \\  & Simple and easy to implement. & \\  & Reduces the computational burden. & \\  & Simplifies the overall controller by the withdrawal of the & \\  & synchronization unit. & \\  & Provides accurate synchronization process. & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The advantages and drawbacks of the grid synchronization techniques. negative sequence [148], which are caused by frequency deviation, distorted grid voltage, and the presence of harmonics. Several methods with different techniques and algorithms have been proposed to address these issues: decoupled double SRF-PLL (DDSRF-PLL) [149; 147], modified SRF-PLL (MSRF-PLL) [150], and adaptive lattice SRF-PLL (ALSRF-PLL) [151]. * SOGI-PLL: SOGI is widely used in the quadrature signal generator (QSG). Also called quadrature-PLL, it seamlessly detects the positive-sequence component to estimate the input signal in-phase and also the quadrature-phase amplitude. Dual SOGI-PLL (DSOGI-PLL) [137; 152] corresponds to the transformation of the voltage vectors in the stationary reference frame (\(\alpha\beta\)) into QSG, which presents the filtered voltage vector. The positive-sequence voltage vector is obtained via the positive-sequence calculator (PFC), and the q-component is equivalent to zero. As a result, it is able to estimate an accurate phase angle under severe fault conditions such as slow dynamics and large frequency overshoots. On the other hand, a frequency feedback loop is adopted for SOGI resonance frequency under a frequency shift. Some results in [153] demonstrate a rather fast dynamic response and harmonics filter capability.