## 5 Stability Enhancement Using FACTS Devices
### Load Frequency Control (LFC)  
In interconnected power systems, LFC is one of the major issues [57; 58]. LFC is defined as regulating the generating unit power in reaction to changes in system frequency or interchange patch line power within their stated limits [53]. LFC has two objectives in deregulated power networks, i.e., power exchange control in interarea tie-lines and maintaining the frequency within their limits [57; 59]. With this aim, the role of LFC has become significant in recent years. One of the solutions to tackle LFC is using FACTS devices [60] because classical control methods (P, PI, and PID) are not enough to control sudden variations in deregulated power systems. FACTSs have the capability to maintain the dynamic stability of the system and improve power transfer capability [22]. The main purpose of using FACTS devices in deregulated power networks is to enhance power system stability and security, diminishing transmission losses and increasing their load ability [53]. Therefore, FACTS devices installed in those power systems face overloading power problems at transmission lines [61]. With this aim, the static VAR compensator (SVC) is installed in interconnected power systems to control various parameters and enhance systems capabilities [62]. Similarly, in [63], interarea oscillations (frequency and time domain) are effectively damped out using an SVC, controllable capacitor (CSC), and phase shifter (PS). In [64], an SVC is used to improve LFC by reducing frequency oscillations. To achieve this goal, a feedback signal for an AGC-based SVC was used to stabilize the power system. Furthermore, an SVC is used in the IEEE New England 39 bus, 10 machine power system to analyze and improve its performance for frequency regulation [65]. Moreover, the authors also compare conventional SVC performance with a modified SVC in this restricted system and conclude that the SVC enhanced frequency regulation. However, the modified SVC performs better in case of large disturbances. With this aim, [PERSON] [66] proposes a modified SVC with thyristor firing angle law and implements it on a 150 bus, 27 machine power system. In this study, the author further compares the performance of the conventional SVC with the newly adapted methodology. Results show that the proposed SVC not only outperforms the conventional SVC but also enhances FR in deregulated power systems. STATCOM is an advanced version of an SVC [67] and superior to an SVC [68]. STATCOM is a shunt-connected FACTS controller. It has the capability to control the frequency or damp out frequency oscillations within their permissible limits [69]. [PERSON] et al. provided a detailed discussion and claimed that the STATCOM controller is a quantitatively superior controller compared to the SVC due to its various attributes [67]. To justify their statement, the authors reviewed 300 research papers to describe the state of the art of STATCOM technology and its future potential and summarized significant conclusions. Furthermore, a detailed review presented in [70] discussed various aspects of STATCOM such as installation, control, stability, and frequency control. Similarly, in [71] STATCOM with superconducting magnetic energy storage (SMES) is used to improve the LFC in deregulated power networks. Results show that settling time in FD and overshoots improved by 40% in the presence of STATCOM-SMES. With this aim there are other FACTS controllers like the thyristor-controlled phase shifter (TCPS), static synchronous series compensator (SSSC), unified power flow controller (UPFC), and interline power flow controller (IPFC) that also play a vital role in LFC. In [72], TCPS-based LFC in a multi-area network is studied. Results reveal significant support against various problems, such as frequency deviation and tie-line power flow, using TCPS. [PERSON] et al. [73] discussed TCPSs and SMES in interconnected power to provide LFC. Correspondingly, in [74] a detailed study on ultra-capacitors and TCPSs is presented for deregulated power systems.