Harmonic State Space Based Stability Analysis of LCC-HVDC System With Saturated Transformer

Transformer saturation caused harmonic instability has always been a pain point for the LCC-HVDC system. Although various analysis methods have been proposed, none of them examine the impact of complex harmonic coupling of nonlinear transformers under saturation conditions. To address this concern, the paper adopts the harmonic state space (HSS) technique to accurately describe the harmonic coupling in LCC-HVDC systems with saturated transformers. A saturable transformer model is first developed with the excitation curve described by polynomial fitting and then interconnected with the LCC-HVDC system to form the complete HSS model for the study. To facilitate the stability analysis, the HSS model is further transformed into a single input single output impedance. As a result, the influence of the harmonic coupling can be easily observed through the impedance curves. Finally, impedance based stability analysis is conducted to explain the reason why the LCC-HVDC is more easily to lose its stability under transformer saturation conditions and identify key factors influencing this kind of instability. Simulation and experiment results validate the theoretical analysis.