The integration of distributed generation and the diversity of loads generally associated with bidirectional power flow transmission result in uneven power flow distribution and line overload, which may deteriorate the normal operation of the transmission and distribution systems. As one of the practical solutions, the power flow controller enhances the network’s flexibility and controllability significantly within smart grids. This paper extensively investigates the topologies of power flow controllers used in AC grids. A comprehensive classification system is established, incorporating multiple hierarchical classifications to encompass various power flow control topologies. To provide a deeper understanding, thorough investigations of the flow control mechanism outline the structural characteristics and operational modes of corresponding representative topologies. Furthermore, analyses in terms of functionality and cost are performed to facilitate comparisons between different power flow controller topologies, thereby aiding in the identification of suitable application scenarios for each topology type. This study aims to provide a reference for selecting power flow control topologies based on different criteria. Moreover, the conclusion offers guidance for enhancing the relevant topologies, ultimately supporting the widespread implementation of flow control devices in power grids.