Controller design for PSS and FACTS devices to enhance damping of low-frequency power oscillations in power systems
Low frequency electromechanical oscillations are inevitable characteristics of power systems and they greatly affect the transmission line transfer capability and power system stability. PSS and FACTS devices can help the damping of power system oscillations. The objective of this dissertation is to design an advanced PSS and propose a systematic approach for damping controller design for FACTS devices. Intelligent control strategy which combines the knowledge of system identification, fuzzy logic control, and the neural networks are applied to the PSS design. A fuzzy logic based PSS is developed and tuned by neural network strategy. The proposed PSS improved the damping of power system oscillations over a conventional PSS. But the same control strategy is not satisfactory for the FACTS damping controller design, mainly because of the different location and role of FACTS devices in power system oscillations compared to PSS. A systematic approach is proposed to design damping controllers for FACTS devices. The problem is considered from a control point of view and treated as a feedback control problem. A low order plant transfer function is obtained by PRONY method; proper control input is selected and a damping controller is designed combining the eigenvalue sensitivity analysis and the root locus method. A gain varying strategy is proposed to change the controller gain according to the transmission line loading condition for better damping effect. This approach is successfully applied in damping controller design for SVC, TCSC, and UPFC. Simulation results demonstrate good damping effects of these controllers Another work accomplished in this dissertation is the modeling of UPFC, a voltage-sourced converter-based FACTS device who simultaneously control bus voltage and power flows on transmission lines. The UPFC brings quite a few challenges to power system simulation and study including power flow calculations, modeling of converter control and UPFC dynamics, interfacing UPFC with the power system for transient simulation program development and physical and operating constraint modeling. The proposed model accurately represented the behavior of UPFC in quasi-steady state and well demonstrated the unique capability of the UPFC to control both the load flow and the bus voltage rapidly and independently.