## 2 General Characteristics of Electrical Power Systems  
Prior to examining the role of FACTS devices, their economic or technical benefits, stability enhancement, and congestion management, it is essential to present several key aspects of power networks [12]. 1. Generation and load should always be equal. This means that electric energy cannot be stored at the power system scale. 2. Ownership of electric energy identity is lost in the transmission network. 3. Power systems tend towards instability because of unsustainable electrical market competition. It means stability can only be achieved by sustainable compliance. 4. With proper investment both transmission capacity and reliable power supply will be enhanced. The reliability and quality of electric power delivery are extremely important to customers [24]. Generally, the term \"power system reliability\" refers to the capability of the power system to execute its intended function, specifically the generation and transmission of electrical energy to consumers without interruption [25]. According to the IEEE, the term \"reliability\" is defined as \"The probability that a system will perform its intended functions without failure, within design parameters, under specific operating conditions, and for a specific period of time\". A power system is a complex entity comprising numerous subsystems and components that collectively function to supply power to consumers continuously or over an extended duration. To achieve the desired functionality, power systems are categorized as short- or long-term performance-based systems. The short-term performance of a power system pertains to its capacity to withstand outages or contingencies, which is referred to as power system security. In essence, security denotes the power system's capability to manage sudden disturbances, which may include dynamic events (such as voltage stability, frequency stability, and angular stability), static events (such as voltage or thermal limits), or cybersecurity threats (affecting software or hardware infrastructure). Conversely, the long-term performance of a power system is evaluated based on its ability to consistently meet demand, necessitating adequate generation and transmission facilities with the requisite level of reserve. This capability is termed power system adequacy (PSA). In other words, adequacy refers to the availability of sufficient facilities to meet the power system's load demand, even when one or more components malfunction owing to sudden failure events. Therefore, it is pertinent to assert that power system security is achieved through stable \"operation\" of the system, while adequacy is ensured through the \"planning\" of sufficient facilities in the long term. Consequently, reliability is assured when the power system is both secure and adequate, thereby enabling the continuous delivery of energy. Conversely, the situation in which actual power deviates from the ideal waveform or deviates from an adequate power (voltage and current) level refers to \"power quality\" [26]. In the literature, various sources employ the term PQ interchangeably with different connotations, including voltage quality (VQ), current quality (CQ), supply reliability, service quality, quality of consumption, quality of supply, and supply reliability. Generally, PQ encompasses both VQ and CQ. VQ denotes deviations from the ideal voltage, characterized by constant amplitude and frequency. Similarly, CQ pertains to deviations from the ideal current waveform. Notably, CQ complements VQ, as the ideal current sine waveform is in phase with the voltage sine wave, maintaining constant amplitude and frequency. Therefore, any deviation in sinusoidal voltage or current waveforms constitutes a PQ disturbance. Such disturbances include three-phase imbalance, voltage sag and swell, voltage flicker, interruptions, harmonics, and switching transients. Moreover, low-frequency swings and power system faults can lead to blackouts, brownouts, and voltage sags that result in dissatisfaction of customers [27].