High-Frequency Oscillation Suppression Strategy for Interconnected Grid-Forming Energy Storage and Grid-Following HVDC Systems Based on Impedance Analysis

Interconnecting grid-following (GFL) and grid-forming (GFM) converters help stabilize weak grids with a high penetration of power electronics, but dynamic interactions can induce high-frequency oscillations (HFOs). The HFO mechanism within interconnected grid-forming battery energy storage (GFM-BESS) and grid-following VSC-HVDC systems remains unresolved, with viable suppression strategies yet to be established. To address this, this study utilizes harmonic linearization to derive small-signal frequency-domain impedance models for both devices. The models are developed for high-frequency-oscillation analysis: the GFM-BESS model retains all control loops, while the GFL-HVDC model retains the current loop and the phase-locked loop, with the slow outer loop omitted as justified within the high-frequency band. Frequency-coupled 2 × 2 impedance matrices are established to capture the frequency coupling introduced by the power loops and the phase-locked loop. Stability is established by the argument principle, the exact Nyquist encirclement count and the roots of the closed-loop characteristic function. At a nominal control delay of 50 μs, the voltage-loop proportional gain has two finite stability boundaries: insufficient proportional action fails to damp the integral-loop mode, whereas excessive proportional action destabilizes the delay-affected filter mode. The resulting two-sided admissible interval is 0.0771 < k v p < 0.5261 p.u.; a robust engineering range of 0.15–0.30 p.u., with k v p = 0.22 p.u. as the nominal value, is recommended. Frequency-sweep results validate the impedance models, while the independently evaluated root locus and Nyquist count give identical stability classifications. Time-domain gain-step tests further verify both sides of the interval: changing k v p from 0.60 to 0.22 suppresses the HFO, whereas a further decrease of 0.15 p.u. to 0.07 re-excites it.