Derivation of UPFC DC load flow model with examples of its use in restructured power systems

The unified power flow controller (UPFC) integrates properties of both shunt and series compensations, and can effectively alter power system parameters in a way that increases power transfer capability and stabilizes the system. The paper presents an approximate model for power flow studies that takes into consideration modeling of lossless UPFC-embedded transmission lines including the effect of line charging susceptance. The approximate model is then simplified to a DC load flow model that takes into account the modeling of the UPFC. The equations derived in this paper are applied to an optimal power flow of a small test system, where the results obtained by approximate load flow method are compared to those of DC load flow method. The results presented in the paper show valuable impacts of putting UPFC in service on out-of-merit costs and locational marginal prices (LMPs) in a restructured power system environment. The results show that the DC load flow relations can be modified to reflect the effects of the UPFC. In addition, results show that UPFC may reduce generation out-of-merit costs and let LMPs regain uniformity.