## 4 FACTS Controllers, Types, and Their Potential Role  
[PERSON] and [PERSON] are the pioneers of FACTS controllers. According to the Institute of Electrical and Electronics Engineers (IEEE), FACTS controllers are defined as: \"Alternating current transmission systems incorporating power-electronics based and other static controllers to enhance controllability and increase power transfer capability\" [42]. FACTS devices are a fast-growing result-oriented technology that is reliable, economical, secure, and efficient in the modern electric world [43]. These fast-acting power-electronics-based controllers dynamically control active (P) and/or reactive (Q) power flow in power systems, exclusively or simultaneously [44]. They can also control transmission lines current and voltage, transfer, or exchange of active power between lines and supply or absorb Q. The primary advantages of utilizing flexible AC transmission system (FACTS) devices are their ability to enhance operational efficiency and regulate power flow within transmission lines. The control of power flow in these lines is contingent upon various factors, including the impedance of the transmission line, the magnitude of bus voltage, and the phase angle between them. Consequently, FACTS devices can modify one of these parameters to achieve power control or augment the limits of transmission lines. Over recent decades, the electricity market has transitioned from a unidirectional to a bidirectional system, evolving from a regulated to a deregulated environment. This transition has intensified competition among market participants, not only concerning electricity costs but also in maintaining power quality. Furthermore, power generation in a deregulated context can vary based on fuel availability, environmental conditions, and economic factors, among others. Simultaneously, the load also fluctuates due to weather conditions, time of day, and expansion. The variability in both load and generation can introduce uncertainty, potentially resulting in congestion. Therefore, optimizing power generation by considering fuel consumption is crucial to meet the increasing electricity demand. Integrating FACTS devices into such deregulated power systems offers several benefits, including reducing the reactive power supply from the source to the load, minimizing voltage sag and swell at bus voltages, lowering harmonics in the source current, and decreasing real power loss. Essentially, there would be an increase in real power delivery to the load without expanding power generation systems. Without the addition of extra lines, FACTS devices can enhance system security by improving voltage regulation and reducing congestion through power flow regulation. Based upon applications, FACTS controllers are classified into three generations and four types [45] as shown in Figure 1 and Table 2, respectively. In brief, shunt-connected FACTS devices regulate reactive power through the control of susceptance. \begin{table}
\begin{tabular}{c c c} \hline
**Sr.** & **Type of Ancillary Services** & **Problem Associated with Power System** \\ \hline
1 & Voltage and reactive power control (Q) & Variations in voltage profiles \\ \hline
2 & Scheduling and dispatch (S \& D) & Lack of electrical energy storage systems \\ \hline
3 & Frequency control & Variation in power system frequency (f) \\ \hline
4 & Operating reserves and spinning reserves & Difference in energy demand and generation \\ \hline \end{tabular}
\end{table} 
Table 1: Power system problems and their associated ancillary services. This indicates that shunt- or parallel-connected controllers manage the voltage at a node by injecting reactive current, regardless of the transmission lines connected to it. Consequently, the operational principle of a parallel controller involves supplying reactive power to the transmission line, with the objective of enhancing active power transfer by improving the power factor while maintaining voltage levels within safe limits under extreme loading conditions [46].