## 5 Stability Enhancement Using FACTS Devices
### Voltage Stability  
Voltage stability is also a major problem associated with multi-network power systems. As we know, increased power demand has led to power network upgrading in recent years. However, upgrading requires a huge amount of reactive compensation to solve problems like regulation and voltage control. Therefore, series and shunt compensation both have the capability to provide reactive compensation and increased voltage stability [124; 125]. In other words, FACTS controllers can reduce voltage stability problems in power networks [126; 127; 128; 129; 130; 131; 132]. A series of publications were found in the literature for improvement in voltage stability using FACTS devices [133; 134; 23; 135]. The authors of [136] used SVCs to increase voltage stability in IEEE-14 and 30 bus systems. In addition, researchers also focused on improving the other two system parameters, i.e., increasing loading the margin and reducing active power losses. However, it is concluded that the SVC does not improve overloaded line conditions in case of contingencies (i.e., transmission line outage, generator outage, and overloads). Furthermore, a comparison between phase shifting transformers (PSTs) and SVCs has been made, and the results show that the SVC performs better than the PST in terms of increasing the line loading margin and bus voltage deviation. Furthermore, the genetic algorithm is used to find the optimal location of SVCs in the IEEE-30 bus system to improve the voltage profile and minimize losses [137]. Another study using genetic algorithms was conducted to provide voltage stability, improving voltage profile, by optimally placing an SVC in the IEEE-14 bus system. Similarly, in [138] line-stability-index-based voltage stability analysis for load variation is carried out to find the weaker bus in the IEEE-14 bus system so that the TCSC is optimally placed. Results show that the TCSC significantly contributes to voltage stability enhancement in the power system under study. Consequently, another study [139] based on the line stability index determined the most severe line in the power system (IEEE-14 bus system) and placed a TCSC to enhance voltage stability. The authors of [140] presented a comparison analysis of three FACTS controllers (TCSC, SSSC, and STATCOM) to enhance voltage stability in power systems. A modified IEEE-14 bus system is used to validate the results in this study. It is concluded that STATCOM provides a higher voltage stability margin as compared to the TCSC and SSSC. While comparing the TCSC and SSSC, it is concluded that the SSSC provides a better loading margin and voltage profile. Another comparative study of SVCs, TCSCs, and STATCOM for the voltage stability boundary of IEEE-14 and 30 bus systems is presented in [141]. The results depicted that the effects of the SVC and STATCOM are similar, but TCSC compensation depends on the compensation provided. In [142] a mutual effect of FACTS devices, i.e., SVC and SSSC, is discussed for enhancement of power system stability, which shows that a combination of FACTS devices is more useful than individual  
\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**} \\  & & & & Area frequency oscillation \\  & & & stabilization, reduced load \\  & & & disturbance, and enhancement \\  & & & in dynamic power flow using \\  & & & 3 degrees of freedom of \\  & & & proportional, integral, and \\
[123] & UPFC & \ \begin{tabular}{c} Grasshopper optimization \\ algorithm (GOA), moth \\ swarm algorithm (MSA), PSO \\ \end{tabular}  &
\begin{tabular}{c} 3 ones. Moreover, the authors of [143] used the voltage stability sensitivity factor to find the optimal location of FACTS devices (TCSC and SSSC).