Small-Signal Stability Analysis of the LCC-HVDC System With a STATCOM Based on Grid Strength

Abstract

Static synchronous compensators (STATCOMs) can typically provide voltage support for the line-commutated converter high-voltage direct current (LCC-HVDC) system in weak grids. However, the interaction between STATCOMs and LCC-HVDCs via ac grids may cause sub/super-synchronous oscillations that can degrade system stability. To this end, this paper proposes a grid-strength-based method for evaluating the small-signal stability of the LCC-HVDC system with a STATCOM. First, a closed-loop transfer function model of the LCC-HVDC system with a STATCOM is established for small-signal stability analysis. On this basis, an equivalent single-machine system is derived to represent the small-signal stability of the original system, which avoids establishing the detailed model of the original system and thus simplifies the complexity of stability evaluation. Moreover, we demonstrate that the small-signal stability of the LCC-HVDC system with a STATCOM can be evaluated by the difference between the short-circuit ratio (i.e., a grid-strength indicator) and its critical value for the equivalent single-machine system. From the perspective of grid strength, we explicitly reveal how the interaction between STATCOM and LCC-HVDC influences the system stability. Finally, several study cases via MATLAB/Simulink are provided to demonstrate the validity of the proposed grid-strength-based method.