Low-Frequency Power Oscillations in DC Systems for Asynchronous Interconnection: Insights From the LUXI Back-to-Back HVDC Project

The Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) system is considered an efficient solution for providing functional support to asynchronously interconnected systems. However, this study demonstrates that frequency support from VSC-HVDC systems can lead to low-frequency power oscillations (LFPOs) due to the interaction between the frequency control and the frequency dynamics of AC grids. For convenience, a practical instance of a 0.25 Hz LFPO observed in the LUXI back-to-back VSC-HVDC project in the China is reported in detail. In this project, the VSC-HVDC system is designed with a frequency control (namely, Co-frequency control) to eliminate frequency divisions between the two asynchronous AC grids under normal operation conditions. Based on the damping torque method, the analysis concludes that the observed LFPO is induced by the interaction between the Co-frequency control and the frequency dynamics of the synchronous generators in AC systems, and is governed by the parameters of the Co-frequency control. To address the LFPO, two damping control schemes are proposed. Finally, all conclusions and findings are validated through EMT simulations. To the best of our knowledge, this is the first report of practical oscillations caused by the frequency control of VSC-HVDC systems, which is meaningful for engineers and researchers, by providing insights about the stable operation of asynchronously interconnected systems.