Non-intrusive methods for mode estimation in power systems using synchrophasors

Abstract

Power system industry has been going through dynamic changes in recent years that have caused more prominent lightly damped electromechanical oscillations. A real-time monitoring of electromechanical oscillations is of great significance for power system operators; to this aim, software solutions (algorithms) that use synchrophasor measurements have been developed for this purpose. It is widely accepted that the mode estimation is one of the most important applications of wide area and measurement systems. This thesis investigates different approaches for improving the process of the mode estimation by closely identifying causes of possible errors in the overall process. Also, it provides a comprehensive guide for implementation of the mode estimation tool by tackling implementation issues. One of the problems tackled in this thesis the selection of synchrophasor signals used as the input for mode estimation. The proposed selection is performed using a novel criterion that is based on the variance of the critical mode estimate. This approach introduces a paradigm shift since the previous methods for signal selection were mostly based on pure heuristics or on observability analysis that is proved to be inadequate. It is shown that not only power system model affects the decision on signal selection, but also characteristics of the ambient noise excitation that is neglected in the observability based methods. Also, it is shown that the signal selection is similar to the PMU placement problem for this particular application, which means that the proposed solution provides a way of including mode estimation requirements into the global PMU placement formulation. The thesis also analyzes methods for selection of model order used in mode estimation. Further, negative effects of forced oscillations and non-white noise load random changes on mode estimation results have been addressed by exploiting the intrinsic power system property that the characteristics of electromechanical modes are predominately determined by the transmission part of the system. An improved accuracy of the mode estimation can be obtained by intentionally injecting a probing disturbance. It is shown the further improvement can be accomplished by adequately shaping the frequency spectrum of the probing signal. The thesis presents an optimization method that finds the optimal spectrum of the probing signals. In addition, the probing signal with the optimal spectrum is generated considering arbitrary time domain signal constraints which can be imposed by various probing signal generating devices. Finally, the thesis provides a full description of the practical implementation of a real-time mode estimation tool. This includes description of the used hardware, software architecture, graphical user interface as well as details of the most important components such as the Statnett’s SDK that allows easy access to synchrophasor data streams.