Study On Coordination Of Multi-objective Control And Transient Stability Strategy Of Unified Power Flow Controller (UPFC)

As a typical representative of the third generation of FACTS devices, Unified Power Flow Controller (UPFC) has strong ability to regulate both load flow and bus voltage, and to improve the power system stability dramatically. This paper focuses on the system level control of UPFC for load flow, bus voltage and transient stability.Firstly, this paper briefly reviews the principle, characteristics and operation mode of UPFC. Then the dynamic mathematical model is established. A controller of load flow and voltage based on the PI loop is proposed and implemented in PSASP. The simulation results show that the load flow and voltage can be adjusted quickly and efficiently by UPFC, but there are interactions among different control objectives.To solve the problem of interactions, the corresponding coordinated control strategy is proposed, whose purpose is to maintain the other control objectives when one of the control objectives is adjusted. The coordinated control schemes include the coordination of active power control and reactive power control, the coordination of the load flow control and the voltage control, and the coordination of active power exchange of VSCs. Each coordination scheme is clear and is easy to implement, which shows good results through simulation. It also shows that the coordination of the load flow control and the voltage control can improve the response speed of the active power regulator, and the coordination of active power exchange of VSCs can improve DC capacitor voltage stability under transient conditions. The UPFC damping controller based on the principle of phase compensation is designed, and the damping effect is analyzed through simulation for various operation modes. Then the nonlinear optimal control is introduced into the UPFC. The control law of UPFC nonlinear control is resulted from exact linearization via state variable feedback, using all local signals. The simulation results show that the UPFC nonlinear control can improve the small signal stability and transient stability of power system. At last, the influence of parameters upon the control effects is analyzed.