Comparison of different robust control methods in design of decentralized UPFC controllers
The unified power flow controller (UPFC) integrates properties of both shunt and series compensations, and can effectively alter power system parameters in such a way that increases power transfer capability and enhances system stability. In practice, simple proportionalintegral (PI) controllers are used to control the UPFC. However, the PI control parameters are usually tuned based on classical or trial-and-error approaches and as such, they are incapable of obtaining good dynamic performance for a wide range of operating conditions and various loads in power systems. Hence, in this article robust control approaches are proposed based on the quantitative feedback theory (QFT), H? loop-shaping and ?-synthesis, to design UPFC controllers (power-flow and DC-voltage regulator). The three mentioned methods are compared with each other and a supplementary damping controller is developed to improve damping power system oscillations. Here, a single-machine infinite-bus (SMIB) power system, installed with a UPFC (with system parametric uncertainties) is considered as a case study. The system parametric uncertainties are obtained following 40% simultaneous alterations in parameters and load from their typical values. The simulation results indicate satisfactory verifications of the robust control methods in dealing with the uncertainties considered. When the above three methods and the PI controller are compared in performance in several time-domain simulation tests, the results show clear superiority of the three methods over the PI controller, with the QFT presenting the best performance amongst the three robust control.
robust control unified power flow controller quantitative feedback theory h? loop-shaping flexible ac transmission systems ? synthesis