## 7 Conclusions  
Deregulation of electrical power systems becomes an essential need of the electrical world from time to time because of new challenges and the integration of new power resources or technologies. Therefore, problems related to power system stability, reliability, efficiency, or security arise that adversely affect power networks, both technically and economically. In this review paper, a detailed discussion about features of FACTS controllers, their role in deregulated power networks, and comparison of FACTS controllers against several techniques and power system parameters is provided. With a history of four decades or more, FACTS controllers have proved themselves as a reliable, secure, efficient, and cost-effective solution to some electrical problems. This study aims to elucidate the significance, advantages, disadvantages, and limitations of FACTS devices in LFC, voltage stability, and congestion management within deregulated power networks. In the context of LFC studies, third-generation FACTS devices are generally more prominent than other FACTS devices. However, the G-UPFC and IPFC are commonly integrated into subnetworks, irrespective of the UPFC, STATCOM, or SSSC, owing to their capability to control power flow across multiple transmission lines. An interesting finding is that FACTS devices, when combined with energy storage, are more beneficial than when used alone. Furthermore, when comparing second-generation FACTS devices, it is concluded that STATCOM is preferable in shunt configurations, whereas the SSSC is preferable in series configurations for LFC. Regarding voltage stability, it was concluded that shunt-connected FACTS devices are more beneficial than series-connected FACTS devices. However, among series-connected FACTS devices, the SSSC is superior in providing voltage stability compared to the TCSC. Similarly, series- and shunt-connected FACTS devices, such as the UPFC, offer better reactive compensation or voltage stability than series-only or shunt-only FACTS devices. Another notable finding of this study is that the combination of multiple FACTS devices is more advantageous than their individual performances. However, there is a trade-off between the selection of FACTS devices and their individual proposed benefits; for example, series-connected FACTS devices (TCSC or SSSC) and shunt-connected FACTS devices (SVC or STATCOM) are not economically viable owing to their cost for voltage stability. In terms of congestion  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**FACTS Devices**} & \begin{tabular}{c} **Methods/** \\ **Optimization** \\ **Algorithm** \\ \end{tabular}  & \multirow{2}{*}{**Findings**} & \multirow{2}{*}{**Limitations/Drawbacks/**Future Directions**} \\ \hline \multirow{4}{*}{[239]} & \multirow{4}{*}{M-SSSC} & GA, PSO, teaching-learning-based optimization (TLBO) & \begin{tabular}{c} Optimal M-SSSC configurations and placement in IEEE-14 bus system and a subarea of the Colombian power grid reduce congestion, enhance voltage stability, and system efficiency using L-index \\ \end{tabular}  & The M-SSSC is more beneficial than the traditional SSSC in terms of simultaneously addressing congestion issues, enhancing voltage stability, and optimizing power flow \\ \hline \multirow{4}{*}{[240]} & \multirow{4}{*}{SVC, TCSC} & \multirow{4}{*}{ \begin{tabular}{c} Symbiotic organism \\ search (SOS) algorithm, \\ PSO, GA, DE, GSA \\ \end{tabular}  } & Provide congestion management by minimizing the operating cost and system losses in IEEE-57 and 118 bus networks in the presence of SVCs and TCSCs in deregulated regime. & The major benefit of using a newer meta-heuristic optimization algorithm, SOS, is that it shows quicker convergence mobility and stronger local search ability and GSA \\ \hline \multirow{4}{*}{[123]} & \multirow{4}{*}{UPFC} & \multirow{4}{*}{\ \begin{tabular}{c} Modified moth flame \\ optimization (MMFO), MFO, PSO, GSA, GA \\ \end{tabular}  } &