ANALYSIS OF INTER-AREA OSCILLATIONS VIA NON-LINEAR TIME SERIES ANALYSIS TECHNIQUES Daniel Ruiz-Vega* Arturo R. Messina** Gilberto Enríquez-Harper*,*** *Programas de Posgrado en Ingeniería Eléctrica SEPI-ESIME-Zacatenco. IPN. Mexico City, Mexico. e-mail: drv_liege@yahoo.com. **Graduate Program in Electrical Engineering Cinvestav, IPN. PO Box 31-438, Guadalajara Jal. 45090, Mexico. e-mail: aroman@gdl.cinvestav.mx ***Unidad de Ingeniería Especializada Comisión Federal de Electricidad Río Ródano 14, col. Cuauhtémoc Mexico City, Mexico. e-mail: gilberto.enriquez@cfe.gob.mx Abstract – This paper compares the characteristics and information provided by different modal identification tools, in the analysis of a very complex forced inter-area oscillation problem recorded in the Mexican intercon- nected system. These oscillations involved severe frequency and power changes throughout the system and resulted in load shed- ding and the disconnection of major equipment. This pa- per reports on the early analytical studies conducted to examine the onset of the dynamic phenomena. Instances of variations in the amplitude and frequency of the excited inter-area modes are investigated and per- spectives are provided regarding the nature of studies required to identify and characterize the underlying nonlinear process. It is shown that nonlinear analysis tools are able to identify aspects of the dynamic behavior of the system that are needed in the validation and characterization of the observed phenomena, even in cases where power system dynamic characteristics change several times due to load shedding and generation tripping operations. Keywords: Power system dynamic behavior, Inter- area oscillations, Modal identifications tools, Non- linear modal identification tools. 1 INTRODUCTION This document details the analytical studies con- ducted to examine the onset of major inter-area oscilla- tions in the Mexican system during the winter of 2004. The study focuses on the use of time-frequency repre- sentations to extract the key features of interest directly from the actual system response. Of primary interest here is the analysis of the time evolution of recorded signals, since this allows replicat- ing the events leading to the onset of the observed oscil- lations, and analyzing the influence of particular operat- ing conditions on system behavior. The non-stationarity of the data following the trig- gering event makes reliable estimate of the frequency and damping characteristics of the observed oscillations difficult. Traditional methods of time series analysis do not address the problem of non-stationarity in power system signals, and often assume linearity of the process [1]. To circumvent these problems, time-frequency representations are used to give a quantitative measure of changes in modal behavior on different time scales. Two main analytical approaches have been investi- gated to extract the underlying mechanism from the observed system oscillations. The first approach is based on the use of time-frequency representations of time series. These models are capable of explaining the nonlinear nature of the observed oscillations and permit the tracking of evolutionary characteristics in the sig- nals and the development of measures like instantane- ous characteristics to capture mode interaction. The second approach uses conventional analysis techniques currently used by the electric industry. Particular atten- tion is paid to the suitability of these techniques as a detector of nonlinear modal interaction. Analyses of observed measurement data via nonlin- ear spectral analysis techniques reveal the presence of complex dynamic characteristics in which the dynamic characteristics of the dominant modes of oscillation excited by the contingency change with time. The mechanism of interaction characterizing the transition of these modes involves strong nonlinear behavior aris- ing from self –and mutual interaction of the system modes. This is a problem that has received limited at- tention in the power system community. A challenging problem in studying this transition concerns the identification of the primary modes in- volved in the oscillation and the study of the nature of the coupling among interacting components giving rise to nonlinear, and non-stationary dynamics. The implica- tions of such complex spatio-temporal behavior can throw much light on the dynamic patterns of the system and information about the local behavior in both the time and frequency domains can be extracted. Numerical simulations with nonlinear spectral analy- sis techniques show good correlation with observed system behavior and also point to the importance of nonlinear effects arising from changing operating con- ditions. These predictions are the basis for additional studies currently being undertaken involving small- signal and large signal performance and are expected to improve modeling and analysis techniques used in power system dynamic analysis studies. 2 DESCRIPTION OF THE EVENT 2.1 General description of the system The Mexican National Electric Power System is com- posed of 9 control areas. Six of these areas (namely 15th PSCC, Liege, 22-26 August 2005 Session 32, Paper 2, Page 1