Slow-Coherency Based Composite Mode Oscillatory Stabilization by Means of a Hybrid PSS

This paper addresses the problem of the stabilization of composite mode electromechanical oscillations of generators. A Hybrid PSS of the two-level structure including of a local PSS and a global PSS is introduced. Time-scale decomposition is applied to the decoupling of the local and global stabilization problems, where determination of the global (inter area) control law is based on slow-coherency aggregation. Composite mode electromechanical rotor oscillations are observed, in practice, with weakly interconnected power systems. These oscillations are characterized by local and inter-area modes. In certain cases of operating conditions and/or network topology, the damping of these oscillatory modes may be negligible or even negative. The existing approaches to the stabilization of power system are basically in decentralized way, and the objective of the stability measures are largely concentrated on the damping improvement of local modes by means of a power system stabilizer (PSS), using only the local information. However, for those systems with poorly or negatively damped inter-area modes, applications of the locally tuned PSS cannot bring a total stabilization to the system, and the inter-area oscillations remain unaffected or possibly building-up to excessive margins. In this paper we have introduced the idea of the Hybrid PSS which is of a two-level control structure with local and global inputs, as in Fig. 1. The desired supplementary control signal for composite mode oscillations is obtained by a weighted combination of the local and global (inter-area) stabilization signals.