On passivity of bulk power grids: Challenges and new findings

Stability analysis of the bulk power grid with progressively increasing interconnections of new devices is challenging as it repeatedly needs the complete system modeling and assessment. In this context, passivity theory is appealing since it gives a sufficiency condition for stability and individually passive components ensure passivity of their interconnected system. In literature, a significant attention is focused on improving component-level passivity while the grid is represented by Thevenin's equivalent algebraic models that are assumed passive. We have shown that bulk power grid is not necessarily passive, especially in the electromechanial frequency range when a dynamic model is considered. Moreover, the passivity property can alter with changing operating conditions. It is shown that power system stabilizers (PSSs) can ensure passivity in one condition and loose it in another. This highlights the need for considering passivity in control design and ensuring robustness in that process.