Micro-sector Analysis of Harmonic Instability for HVDC Systems Interconnected to Weak Grids

This paper proposes an enhanced harmonic stability assessment method that combines the advantages of loci-based with individual configuration analyses. Building on established impedance loci methods, the approach introduces the concept of micro-sectors to improve accuracy and efficiency when analysing VSC-HVDC connections to weak, IBR-dominated AC grids.

The method exploits the natural sparsity of impedance data within conservative harmonic impedance sectors. Instead of broad sectors that may introduce non-existent resonances, impedance loci are subdivided into micro-sectors along impedance magnitude and angle dimensions. Phase margin (PM) is evaluated in magnitude mapping micro-sectors where PM is defined, while gain margin (GM) is assessed in phase angle mapping micro-sectors where GM exists. Micro-sectors without relevant impedance data are automatically excluded.

This structured segmentation enables automatic identification of all critical PM and GM values for individual configurations, while grouping similar critical cases without manual postprocessing.

Time-domain simulations using synthetic models are presented to validate the frequencydomain results. The proposed methodology achieves accuracy comparable to individual configuration analysis while retaining the efficiency and clarity of loci-based methods, making it well suited for harmonic stability assessment of VSC-HVDC systems connected to weak AC grids.