The unified power flow controller (UPFC) is recognized as one of the most promising flexible ac transmission system (FACTS) devices for modern power systems, due to its versatile functionalities. Based on typical operation conditions, this paper investigates the global and local power flow operation rules of the UPFC embedded transmission system theoretically. First, the detailed global power flow model (GPFM) and the local power flow model (LPFM) are established concerning several selected critical points (SCPs) in the system. The theoretical power flow operation rules for the ranges, the movements and the trajectories of GPFM and LPFM are analyzed, respectively. Then, the case study is performed based on the proposed simplified model, and the results are analyzed from GPFM and LPFM perspectives respectively. The operation rules and the power flow ranges is studied. Further case studies based on the proposed original mathematical model are also carried out, and the results are listed from different viewpoints of two-dimensional planes and three-dimensional spaces. All types of results are also analyzed from GPFM and LPFM perspectives, which verify that the UPFC is adaptive to various operation conditions for power flow regulation.

This paper aims to discover the general steady-state operation characteristics, as well as improving the dynamic performance, of the modular multilevel converter (MMC)-based unified power flow controller (UPFC). To achieve this, first, we established a detailed power flow model for MMC-based UPFC containing each critical part and made qualitative and graphical analyses combining 2-dimensional operation planes and 3-dimensional spatial curve surfaces comprehensively to derive general power flow principles and offer necessary references for regulating UPFC. Furthermore, to achieve better performance, we designed a feedforward control strategy for the shunt and series converters of UPFC, both comprising two feedforward control blocks with the introduction of necessary compensating branches, and analyzed the performance in complex and time domain, respectively. The proposed power flow principles and control strategies were validated by a (power systems computer aided design) PSCAD model of 220 kV double-end system; the results reveal the MMC-based UPFC can realize the power flow principles and improve the control speed, stability, and precision of the power flow regulations under various conditions.

The power flows in the unified power flow controller (UPFC) embedded system is mainly regulated by the two variables containing the magnitude and the phase angle of the output series inserted voltage (OSIV) of UPFC. Different value combinations of the two variables can form multiple regulation modes of OSIV, and the regulation principles and efficiencies for power flows are distinct by different regulation modes. This paper dedicates to research the regulation principles of active and reactive power flow gradients (PFG) to multiple characteristic independent variables (CIVs) at several selected critical points (SCP) of the system in different operation conditions. The CIVs contains the magnitude of OSIV, the phase angle of OSIV, and the phase difference of the system. First, multiple power flow regulation modes of OSIV are designed, the mathematical models of the PFG to each CIV at each SCP are established, and the theoretical principles for the PFG to each CIV at each SCP are analyzed and compared. Next, four typical operation conditions of the system and four regulation scenarios are assumed and case studies for the PFG to each CIV at different SCP are carried out. The test results at each SCP are analyzed both in the two-dimensional planes and three-dimensional spaces. The regulation principles and efficiencies of PFG to each CIV at different SCP are compared with each other and summarized, which can offer useful references for practical engineering and applications of UPFC.