Robust control designs for large-scale power systems

Robust decentralized excitation control and robust power system stabilizer (PSS) are developed based on the Lyapunov direct method. Exponential stability of power system under the proposed controls is shown. The controls require only the local measurements at each generator site and is globally robust against unknown system parameters, load variations and topology changes that may occur anywhere in the power system after the faults. Comparisons and simulations are carried out for robust mechanical control, robust excitation control and their combination in terms of critical clearing time and suppression of sustained oscillation. Their performances are compared to those under conventional exciter control and under PSS. The study illustrates that these nonlinearly-designed robust controls are superior to conventional controls in enhancing transient stability. The combination of robust controls and PSS can damp out very well sustained oscillations. Determination of the control gains is straightforward and optimal control gains can be found. A methodology to locate robust transient controllers is developed by introducing a performance index. The siting procedure admits all nonlinear control design methods and nonlinear dynamics of the power system. Its concept is simple and physically rooted. The power transfer capacity is investigated for systems with the FACTS controller UPFC (Unified Power Flow Controller). The effects of equipment constraints, the system topologies, and locations of UPFC on the real power transfer are discussed. The potential capability of enhancing system transient stability using UPFC is also studied.