Self-Stability Analysis of Dual-Frequency-Band Grid Impedance Emulator

The impedance interactions between grids and converters may trigger resonance, and should be carefully investigated before field deployment of converters. To meet the testing requirement, a dual-frequency-band (DFB) grid impedance emulator has been proposed, featuring the coordinated operation of a high-switching-frequency converter and a high-power-level converter. In this article, instead of studying the commonly-seen impedance interaction between an emulated grid and a converter under test (CUT), the self-stability of a DFB grid impedance emulator has been analyzed. It is found that the unique error generation part of a DFB grid impedance emulator can degrade the internal controller, which may further cause control inconsistency and self-instability. To mitigate the adverse effect, an extra grid impedance term is inserted into the forward path of controller, and thus a straightforward design of control is guaranteed. Simulation results are provided to verify the analysis.