Investigation of SSTI Between Practical MMC-Based VSC-HVDC and Adjacent Turbogenerators Through Modal Signal Injection Test

With increasing applications of voltage source converter-based HVDC (VSC-HVDC), the potential risk of subsynchronous torsional interaction (SSTI) between turbogenerator and VSC-HVDC has been extensively studied in previous work. However, contradictory results were reported by different researchers. Unlike previous studies, wherein the oversimplified models of VSC and generator were used, this paper uses the full-scale model of a practical modular multilevel converter (MMC)-based VSC-HVDC and the multi-mass shaft model of a turbogenerator to analyze the characteristics of SSTI. Moreover, a different approach, namely, the modal signal injection test, is proposed to identify the damping of torsional modes and the impact from decisive factors such as the loading levels, control strategies of converters, and the coupling strength between VSC-HVDC and ac networks. Comparative analyses between the full-scale and the equivalent models have revealed how system modeling affects SSTI characteristics and helped to explain the inconsistencies among the results of previous studies.