A Modal Transformation Approach to Design Reduced Order Functional Observer-Based WADC for Low-Frequency Oscillations

Low-frequency oscillations (LFOs) are one of the major concerns in modern power system; hence, these should be appropriately treated in order to achieve reliable operation. In this regard, we propose a novel wide-area damping controller (WADC) to dampen LFO modes effectively. The proposed WADC utilized thyristor controlled series compensator (TCSC) based flexible ac transmission system (FACTS) device through which supplementary input is provided. The selection of modal feedback signals for WADC design is carried out using coordinated modal transformation and linear quadratic regulator (LQR) approach. However, these signals are not accessible directly through the phasor measurement devices (PMDs). Hence, this paper utilizes a reduced-order functional observer to estimate the feedback signals using PMD data. The estimated feedback signals are further modulated by simple gains to improve the damping of critical LFO modes. The proposed methodology is validated on benchmark 4-machine 11-bus system and a more practical 25-machine, 105-bus system representing the complex Eastern Regional Grid (ERG) of India. The results obtained on these nonlinear systems show a significant improvement in damping of the critical LFOs leaving the other modes almost unaffected.