FACTS devices, or Flexible Alternating Current Transmission Systems, are power electronics-based devices that are used in electric power transmission systems to enhance controllability and increase the efficiency of the system. These devices can modify and control the impedance, voltage, and phase angle of the power system in order to improve power flow, stability, and reliability. Some common FACTS devices include Static Var Compensators (SVC), Static Synchronous Compensators (STATCOM), Unified Power Flow Controllers (UPFC), and Thyristor Controlled Series Capacitors (TCSC). These devices are typically installed at key points in the power system, such as substations or transmission lines, to help regulate voltage levels, reduce line losses, and prevent voltage instability. Overall, FACTS devices play a crucial role in modern power systems by allowing for better control and operation of the grid, ultimately leading to a more stable and efficient electrical network.