Multi-connection assessment for impedance-based stability analysis through power system impedance determination in the frequency domain

The impedance-based stability analysis is emerging as a common method to evaluate the small-signal stability of networks containing large shares of inverter-based resources (IBR) in the sub-/super-synchronous and harmonic frequency ranges. The method requires the evaluation of frequency-dependent equivalent impedances of the power system, which can be done via “frequency scan” in the time-domain (EMT simulation) or via “frequency sweep” in the frequency domain. The major advantages of the frequency sweep are reduced computational cost and the use of look-up tables for the IBR equivalent impedances instead of highly detailed EMT models. In contrast to the single-connection assessment, the multi-connection assessment (MCA) evaluates the stability of a MIMO system, which can split the network into a passive or known stable part and the group of newly added IBRs. This paper presents the application of the frequency sweep for MCA with consideration of operating point dependencies and the coupling over frequency effect, where voltage perturbations at one frequency can cause currents at a shifted frequency via transfer impedances of IBRs. This is especially of interest for frequencies below the 2nd harmonic order. The according methodology is explained and derived theoretically. An example of a power system is provided as a showcase.