Small-Signal Stability of Sequence-Decomposed Grid-Forming IBRs with DC-Link Voltage Dynamics

The proliferation of technical specifications for grid-forming (GFM) inverter-based resources (IBRs) is accelerating globally to facilitate their effective integration in power grids. Notably, the precise regulation of negative sequence currents is currently mandated for GFM IBRs, necessitating the incor-poration of sequence-decomposed modules (SDMs) into the GFM control architecture. However, SDMs have an adverse impact on the dynamics of the GFM control loops, resulting in performance loss of system stability. Specifically, GFM IBRs with DC-link voltage control (DVC) are susceptible to losing their steady-state operating points due to the presence of SDMs. This paper investigates the effects of SDMs on the small-signal stability of GFM IBRs with DVC. A single-input single-output small-signal model considering interactions of the entire control loops is developed to unveil the unstable sub-synchronous oscillation (SSO) issues induced by SDMs. Subsequently, potential remedies for the SSO issues are explored. To mitigate instability risks, an active power reference feedforward control method is proposed to actively dampen the oscillations inherent in the dynamics of active power and DC-link voltage. Controller hardware-in-the-loop (CHIL) experiments are conducted to corroborate the theoretical findings and the efficacy of the proposed control method under different grid conditions.