Hierarchical Coordination Controllers for UPFCs based on Non-linear Fuzzy Control

This paper investigates coordination control between two UPFCs (Unified Power Flow Controllers) in a multi-machine system. More than one UPFC installed in the power system may interact with each other and this interaction affects system performance or even lead to system instability. A two-level hierarchical control scheme is proposed which contains internal and external controllers. The internal controllers regulate the transmission line real/reactive power flow and the bus voltage. The external controllers generate supplementary reference signals to the internal controllers to mitigate the interaction between two UPFCs. The proposed external controllers using Takagi-Sugeno fuzzy strategy are more effective in reducing this interaction than conventional PI controllers. A three-machine nine-bus system example is used to demonstrate the interaction between UPFCs and the effectiveness of the proposed control scheme.