<![CDATA[The increasing demand for efficient, stable, and flexible power delivery in modern transmission networks has led to the widespread integration of Flexible AC Transmission Systems (FACTS) devices. This review presents an extensive assessment of five major FACTS devices—SVC, STATCOM, TCSC, SSSC, and UPFC—by analyzing their operating principles, technical architectures, system integration strategies, and real-world applications. Through the synthesis of findings from scholarly sources published between 2019 and 2025, the paper identifies critical challenges such as high implementation costs, dynamic response limitations, and system coordination complexity. In addressing these challenges, the paper proposes hybrid deployment strategies and cross-device compensation approaches, demonstrating how the strengths of one device can mitigate the weaknesses of another. A unique contribution of this study is the structured presentation of thematic comparative tables that highlight similarities in components, applications, and limitations of FACTS technologies, as well as their suitability for various power system problems. The paper is structured as follows: Section 1 introduces the motivation and scope of FACTS in modern grids. Section 2 provides a methodological framework and detailed classification. Section 3 presents a comparative review enriched with original figures and schematic diagrams. Section 4 offers thematic discussion on applications, cross-device solutions, and innovation trends. Finally, Section 5 concludes the study by summarizing findings and emphasizing the value of strategic deployment in solving transmission challenges. By synthesizing operational insight with implementation strategies, this work fills a vital gap in comparative FACTS literature and offers a roadmap for future academic and industry research.]]>
