## 5 Stability Enhancement Using FACTS Devices
### Load Frequency Control (LFC)
It is concluded that the TCPS performs better than the ultra-capacitor in suppressing FD and providing LFC. Similarly, a study [75] compares the effects of TCPSs, SMES, and DIGG for LFC in multi-source multi-area power systems. Furthermore, SMES-TCPS is implemented in a two-area hydrothermal system in a deregulated environment in [76] to improve LFC. In response to such implementation, FD and settling time of the power system significantly reduce. Moreover, it is observed that the actual parameters of the tie-line (power exchange and power generation) are exact matches of theoretical parameters showing the effectiveness of TCPSs and SMES. In addition, under a steady state, the frequency error approached zero. In the literature [77], a study based on SMES and TCPSs also justifies their usefulness in restructured power systems. System dynamics in coordination with SSSCs and TCPSs for a two-area power system has been studied in [78]. Likewise, SSSCs with SMES in interconnected power systems are discussed in [79]. The authors conclude that SSSCs with energy storage devices notably enhanced LFC in the restructured power system. This paper also reveals that SSSCs with SMES not only perform better against realistic situations, generation rate constraint (GRC), and governor dead band but also show effectiveness against step load disturbance. Similarly, an SSSC with capacitive energy storage (CES) for LFC is presented under the open market scenario in [80]. Consequently, in [81] another approach named imperialistic competitive algorithm design is used for SSSCs and CES to provide LFC to multi-area power systems. This proposed approach effectively suppresses FD and justifies the superiority of power systems with SSSCs and CES. Furthermore, the authors of [82] prove that FD in multi-area power systems improve using SSSCs. Other FACTS controllers like UPFCs, generalized UPFCs (G-UPFCs), and IPFCs are also favorable in the electrical power system for LFC. UPFCs effectively enhanced LFC in an interconnected power system presented in [83]. Additionally, the LFC and robustness of a multi-area power system improve using UPFCs with an energy storage device, i.e., CES. In another study [84], a UPFC with an energy storage device (SMES) is used to assist the proposed controls (PI, fractional-order proportional-fractional filtered derivative action (FOPDF), and fuzzy PI-FOPDF) to provide LFC in an interconnected power system. Results clearly indicate that the proposed approach performed well despite the harsh conditions in the interconnected power system. However, the literature [85] proves that UPFCs and RFBs with the hybrid differential evolution and pattern search (DE-PS) algorithm for LFC are reliable in a deregulated environment. It is observed that FD is zero in multi-area networks when coordinated with UPFCs and RFBs. In the literature [84], UPFCs and SMES are added to the system for improving LFC. The reformed form of the UPFC is the G-UPFC [86]. The G-UPFC is also known as the multi-line UPFC because it can control the power flowing on more than one line or even on subnetworks [87]. In the literature [88], a fundamental frequency model of G-UPFCs was developed in an electromagnetic transient program (EMTP) and used to demonstrate its performance in a test power system. Also, in [89], the performance of G-UPFCs to suppress frequency oscillations was presented. The trend to adopt UPFCs and G-UPFCs increased in recent decades with the aim of regulating interconnected power systems [19]. However, SVCs and TCSCs are preferred due to the higher installation costs of UPFCs [90]. The authors of [91] concluded that the UPFC is not economically attractive in multi-line power systems due to its high cost. Therefore, the IPFC is considered preferable to the UPFC because it effectively controls the compensation and power flow of multi-area transmission lines. In addition, the authors of [92] used IPFCs to improve the LFC of a multi-unit power system coordinated with redox flow batteries (RFBs).