## 5 Stability Enhancement Using FACTS Devices
### Load Frequency Control (LFC)
While comparing strategies for LFC, robust control and artificial-intelligence-based methodologies showed better performance in terms of dealing with various deregulation power system modeling uncertainties, non-linearities, and load disturbances [111]. 9. Classical control methods are amenable and easy for practical implementation. However, investigation reveals that they exhibit poor performance against various system dynamics, non-linearities, and disturbances because they mostly consider root locus, Bode, and Nyquist approaches to obtain phase margins and gain of the controller, [112; 113]. That is why a gap is filled by FACTS devices for complex or deregulated power systems. In general, the above-mentioned conclusions are helpful for selecting FACTS controllers. However, no specific general conclusion has been drawn for selection of the control methodology, optimization algorithm, and energy storage devices for LFC. One of the reasons is system dynamics, tie-line parameters, location, etc. preventing generalization. Another major factor, cost, plays an important role in the selection of a specific FACTS device. Other factors such as fund availability can also thwart advancement in interconnected systems. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**FACTS Devices**} & \multicolumn{2}{c}{**Methods/**} & \multirow{2}{*}{**Findings**} & \multirow{2}{*}{**Limitations/Drawbacks/**} \\  & & & **Algorithm** & & **Future Directions** \\ \hline \multirow{2}{*}{[62]} & \multirow{2}{*}{SVC} & Model analysis & SVC used to damp the & \multirow{2}{*}{} \\  & & Observability and & low-frequency interarea & \\  & & controllability analysis & oscillations & \\ \hline \multirow{2}{*}{[64]} & \multirow{2}{*}{SVC} & Feedback signals are & \multirow{2}{*}{} \\  & & composed of frequency & & \\ \cline{1-1}  & & deviation and reactive & & frequency oscillations & In future, proposed methodology can be tested in multi-area power systems \\ \cline{1-1}  & & power variation & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: FACTS devices and their role in LFC. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **FACTS Devices** & \begin{tabular}{c} **Methods/** \\ **Optimization** \\ **Algorithm** \\ \end{tabular}  & **Findings** & \begin{tabular}{c} **Limitations/Drawbacks/** \\ **Future Directions** \\ \end{tabular}  \\ \hline \multirow{10}{*}{[114]} & \multirow{5}{*}{ \begin{tabular}{c} STATCOM \\ **Optimizer (ARO)** \\ \end{tabular}  } & FR (IEC 60034–1 standard) & It is concluded that a meta-heuristic & \begin{tabular}{c} \\ **provided to both New** \\ **England IEEE-39 bus system** \\ \end{tabular}  & \begin{tabular}{c} **algorithm, i.e., ARO-based PD-PIDA, is** \\ **superior to PIDA-based marine** \\ **predator algorithm.** \\ \end{tabular}  \\  & & \ \begin{tabular}{c} **optimized PD with** \\ **PID-acceleration-based** \\ **STATCOM** \\ \end{tabular}  & \begin{tabular}{c} **Along with FR, other parameters such as** \\ **voltage stability will be considered in the future.** \\ \end{tabular}  \\  & & & & \\ \cline{3-4}