Non-intrusive Methods for Mode Estimation in Power Systems using Synchrophasors

Abstract: The power system industry has been going through dynamic infrastructural and operational changes in recent years that have caused more prominent lightly damped electromechanical oscillations. 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. Mode estimation is widely accepted as one of the most important applications of wide area and measurement systems. This thesis investigates different approaches for improving mode estimation process by offering new methods and deepening the understanding of different stages in the mode estimation process.One of the problems tackled in this thesis is the selection of synchrophasor signals used as the input for mode estimation. The proposed selection is performed using a quantitative criterion that is based on the variance of the critical mode estimate. This approach differs from the existing techniques based on heuristics or on the analysis of the observability matrix of the power system. The proposed criterion and associated selection method, offer a systematic and quantitative approach for PMU signal selection. It is shown that not only the power system model affects the decision on signal selection, but also the characteristics of the ambient noise excitation that is neglected in observability-based methods. In addition, it is shown that the signal selection problem 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 a global PMU placement formulation.The thesis also analyzes methods for model order selection 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 power generation and transmission network.An improved accuracy of the mode estimation process can be obtained by intentionally injecting a probing disturbance. It is shown that 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 that can be imposed by various probing signal generating devices.Finally, the thesis provides a comprehensive description of a practical implementation of a real-time mode estimation tool. This includes description of the 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.