Abstract
The expanding penetration of renewable energy sources and power electronic converters into power systems is raising concerns. These concerns are related to wideband resonance and harmonic stability issues, which result from dynamic interactions between the grid and the grid-connected inverters (GCIs). The dq impedance method, which is based on frequency-domain small-signal modeling in the synchronous reference frame, offers an effective analytical framework for studying wideband harmonic resonance in three-phase GCI systems. This paper develops a detailed dq impedance model to enable accurate stability analysis of GCIs, investigating the impact of key control parameters such as phase-locked loop (PLL) bandwidth and current-loop proportional-integral (PI) regulator settings on system stability.A step-by-step derivation of dq-axis small-signal models for GCI subsystems— including the filter circuit, PLL, and current regulator—is carried out before integrating them into a full converter impedance representation. The model is validated using a frequency sweep-based dq impedance measurement approach by injecting a perturbation voltage into the main power circuit. The bode plot of eigenvalues is applied to this impedance model to assess how grid impedance, current-loop coefficients, PLL parameters, and the inclusion of capacitive branches affect stability.
PLECS simulations are used to examine changes in output voltage and current waveforms under different parameter settings, showing good agreement with theoretical predictions. The proposed approach improves the physical understanding of dq-impedance behavior and offers practical guidance for impedance-based stability assessment in design.
| Date of Award | 15 Mar 2026 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jiajun Yang (Supervisor) & Giampaolo Buticchi (Supervisor) |