In this paper a new method for power system stability analysis is introduced. The method is based on injection of a small voltage or current in an arbitrary point of the system. The apparent impedance is defined as the ratio between the voltage and current in the injection point. It is shown that the apparent impedance can be used to estimate the eigenvalues of the system that are observable from the injection point. The eigenvalues are obtained by applying the Vector Fitting algorithm to the measured set of apparent impedances. The proposed method holds some advantages over the well established impedance-based analysis method: It is no longer needed to estimate the source and load impedance equivalents separately, and it is not necessary to make any assumption regarding where the source and load are located. This reduces the required measurements and data processing. Furthermore, the stability analysis is global in the sense that the resulting stability margin does not depend on the injection point location. Finally, the method is well suited for real-time implementation due to low computational requirements. The method is outlined for DC-systems, while further work will extend the theory to cover single-phase and three-phase AC systems.
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