Published in IET Generation, Transmission & Distribution Received on 17th November 2008 Revised on 5th February 2009 doi: 10.1049/iet-gtd.2008.0574 ISSN 1751-8687 Probabilistic performance indexes for small signal stability enhancement in weak wind-hydro-thermal power systems J.L. Rueda D.G. Colome ´ Instituto de Energı ´a Ele ´ctrica, Universidad Nacional de San Juan, Av. Lib. Gral. San Martı ´n 1109 Oeste (J5400ARL), San Juan, Argentina E-mail: joseluisrt@iee.unsj.edu.ar Abstract: Deterministic strategies are still largely used for small signal stability (SSS) assessment and enhancement in most power systems worldwide. However, the solutions obtained with such strategies are very limited since they are correct just around the particular conditions analysed. Therefore it is essential to develop comprehensive strategies to cope with more operating conditions and random factors in SSS studies. This paper presents the development and application of a probabilistic methodology for SSS assessment and enhancement. The approach accounts for uncertainties of generation and nodal load demands as well as the effects of system element outages. Probabilistic performance indexes based on a combination of Monte Carlo method and fuzzy clustering are calculated. It is shown how properly statistical processing of output variables of interest can be adapted to evaluate the proposed indexes, which are the instability risk index and two additional indexes concerning power system stabiliser location and transfer capability as affected by SSS. The results obtained using a 18-power plant power system are analysed and compared against the results obtained through a deterministic approach. Relevant discussion highlights the viewpoint and effectiveness of the proposed methodology in providing instability risk assessment and useful information that aims at minimising the occurrence and impacts of electromechanical oscillations in the context of power system operation around uncertain load conditions. 1 Introduction The electric utility industry around the world has evolved towards competitive markets with open access of transmission systems as a consequence of restructuring processes. Within this context, power systems in several countries are faced with great challenges concerning overall reliability and security, because the operating environment is undergoing more highly stressed and unprecedented operating conditions, as the systems are often forced to operate closer to the security limits for economical reasons. Despite increasing profitability, several factors make power system vulnerable to stability problems when disturbances occur as evidenced in recent publications [1]. This vulnerability is especially high in weak power systems such as Latin American systems, where the stability is predominantly a problem of insufficient damping for electromechanical oscillations [2]. The problem is commonly termed small signal stability (SSS) [3]. Current power system small signal stability assessment (SSSA) is carried out based on a reduced list of specified operating conditions and network configurations that are considered to reach the operating limits (e.g. worst-case scenarios). Next, widely used techniques such as modal analysis or Prony analysis are applied to understand the system oscillatory performance and to propose solutions [4]. Although conceptually easy, this deterministic approach does not reflect properly the stochastic nature of power system behaviour and needs since it does not consider uncertainties in the system. On the other hand, it is assumed that instability risk is low, but it is not possible to quantify how low it is [5]. Hence, the obtained results are too conservative and the provided solutions are made to IET Gener. Transm. Distrib., 2009, Vol. 3, Iss. 8, pp. 733–747 733 doi: 10.1049/iet-gtd.2008.0574 & The Institution of Engineering and Technology 2009 www.ietdl.org