Oscillatory frequency characterization based on impedance analysis

The impedance-based analysis has been extensively used for frequency domain stability analysis in power systems with power electronics devices. Also, such analysis is used to determine the oscillatory mode frequencies. The impedance ratio analysis is a conventional approach employed to determine both stability conditions and oscillatory mode frequencies. In particular, oscillatory mode frequencies can be approximated from the gain and phase crossover of the impedance ratio. Such an assumption has been considered in the harmonic frequency range, but in the low frequency range, approximately under 100 Hz, this might not be valid. Low frequency interactions are mainly caused by voltage source converter outer loops and synchronous generator circuits and controls. This paper presents an alternative impedance-based analysis to identify mode frequencies and the participation of impedance components to the interactions. This method is compared to the approximation from the gain and phase crossover of the impedance ratio. Also, the simplification of synchronous generators as Thévenin equivalents is explored for low frequencies based on the impedance frequency response. An essential power system model with a voltage source converter, a synchronous generator, and a load is presented to show the advantages of this alternative impedance-based analysis compared to the gain and phase crossover of the impedance
Peer Reviewed
Postprint (published version)