## 5 Stability Enhancement Using FACTS Devices
### Voltage Stability
Therefore, successful placement of FACTS devices helps to enhance the voltage magnitude profile and mitigate power losses (active and reactive losses) in a Nigerian 48 bus power system. Results show superior performance of TCSCs as compared to SSSCs for steady-state stability. In [144], STATCOM was proposed for improving the voltage profile and load-ability limits of the multi-bus power system using MATLAB. The PSO algorithm is used to find the optimal location of a STATCOM that enhances the upper limit voltage and load-ability. Results show that STATCOM is a better option to adopt regardless of the use of conventional techniques like power system stabilizers (PSSs). Similarly, in a study based on voltage stability conducted by Prof. [PERSON] [145], modal or eigenvalue and line outage contingency analysis methods were used to identify the weakest bus for STATCOM and SSSCs, respectively. This study reveals that STATCOM achieved a better voltage stability margin than SSSCs. As discussed previously, penetration of renewable energy into existing infrastructure causes voltage stability problems. Therefore, a comparative study for FACTS shunt devices (SVC and STATCOM) in wind farm generators connected to power systems is discussed in [146]. An IEEE-14 bus system is used in this case to evaluate the significance of FACTS devices for voltage stability. Results show that STATCOM performs better than SVCs. Similarly, the authors presented the impact of the integration of a large-scale PV system to displace the conventional generation and the effects on dynamic voltage stability of a power system [147]. Therefore, the authors used Dominion Virgin Power's system with various PV scenarios to show how dynamic voltage stability is affected. The same authors propose a novel concept of voltage control together with auxiliary damping control in which inverters of the PV farm are utilized as a STATCOM, called PV-STATCOM, for increasing the power transmission limits during the day and night [148]. Another study, which reveals the significance of PV-STATCOM for enhancing power transmission limits, is discussed in [149]. In [150], the authors used a combination of a TCSC and STATCOM to improve voltage stability in a wind power grid-connected system. The authors concluded that this TCSC-STATCOM combination not only effectively increased voltage stability but also enhanced the low-voltage ride-through capability. In addition, this combination performs better compared to the individual controllers. The authors of [151] summarized a comparison of four different FACTS devices (SVC, TCSC, STATCOM, and UPFC) for providing voltage control. They concluded that STATCOM is less useful than other FACTS devices under study. However, the UPFC is preferable for stability enhancement regardless of its cost. Another comparison of FACTS controllers (SVC, TCSC, SSSC, and UPFC) has been presented in [152]. The results concluded that the TCSC provides low voltage control as compared to other proposed FACTS controllers, but the UPFC secures 1 st place to provide stability enhancement. With this aim, another comparative study about various FACTS devices (SVC, SSSC, STATCOM, and UPFC) is discussed in [153] regarding voltage stability in the IEEE-14 bus system. As expected, results in this study also show the superiority of the UPFC over other FACTS controllers. However, the authors of this research study concluded that the worst response is shown by the SSSC. This result contradicts the findings presented in [151], because STATCOM shows the worst response. Therefore, we concluded that the selection of FACTS controllers for power systems not only depends on the parameters of the power system but also on the techniques used to find optimal locations of FACTS devices. Another study [154] is found in the literature with the objective of enhancing system stability and minimizing voltage deviation, line loading index, losses, and cost. These objectives were achieved by allocating FACTS devices (SVC, TCSC, and UPFC) to the IEEE-30 bus system.