Comparative Impedance Characteristic Analysis of Grid-Following and Grid-Forming Inverters

This paper comprehensively analyses the impedance characteristics of grid-following (GFL) and grid-forming (GFM) inverters at around synchronous frequency areas considering various operating and grid connection conditions and control settings. Both analytical and from simulation extracted impedances are obtained for ensuring model plausibility. Equivalent single-input single-output (SISO) inverter impedances are derived for intuitive analysis. The work summarises already published results and reveals new aspects: 1) an inverter-dominated grid tends to exhibit sequence impedance coupling, which influences the inverter SISO impedances; 2) the impedances of GFL inverters are more symmetrical about the synchronous frequency compared to the GFM inverters; 3) inverter low-frequency SISO impedances are highly impacted by the inner-loop control; 4) the adopted GFM control exhibits much better passivity and much less interaction stability risk than the GFL control; 5) the equivalent SISO impedances of GFM inverter are much more influenced by the connected grid conditions (e.g. grid strength and X/R ratio) compared to the GFL inverter; 6) the SISO impedances of GFL inverter are much more sensitive to grid voltage and frequency deviations and operating modes/points compared to the GFM inverter; 7) apart from passivity, the inductive/capacitive feature of GFL/GFM inverters shows high impact on interaction risks.