Mechanism analysis of low-frequency oscillation in offshore wind power via VSC-HVDC transmission system for bulk power system simulation applications

Delivering offshore wind power via Voltage Source Converter-based High Voltage Direct Current systems (OWF-VSC-HVDCs) has emerged as an effective solution for the large-scale integration of offshore wind farms (OWFs). For the electromagnetic transient (EMT) simulation of a bulk power system with a high penetration of OWF-VSC-HVDCs, equivalent modelling of OWF-VSC-HVDCs is typically required to balance simulation efficiency and accuracy. However, the low-frequency oscillation mechanisms of OWF-VSC-HVDCs are not yet fully understood and require further analysis, which directly impacts the selection of key parameters and modelling approaches for the equivalent models of OWF-VSC-HVDCs. Therefore, to further provide a theoretical basis for the equivalent modelling of OWF-VSC-HVDCs in bulk power system simulation applications, this study develops a two-dimensional transfer function model that fully captures the converter control dynamics in an offshore wind power system connected via VSC-HVDC. Using the Schur complement of the transfer function matrix, an equivalent resonant circuit is derived to illustrate the physical origin of oscillations induced by wind-HVDC interactions. A stability margin index is introduced to quantify oscillatory risk. Analysis reveals that low bandwidths in both the wind turbine phase-locked loop (PLL) and the VSC’s outer AC voltage control loop can form a low-frequency resonant circuit. Improper controller tuning or high wind power output may result in negative equivalent resistance at the resonant frequency, increasing the risk of instability. Electromagnetic transient simulations confirm the accuracy of the proposed method and the validity of the findings. The finding provides a theoretical basis for EMT simulations, controller optimization, and low-frequency oscillation analysis in the integration of large-scale offshore wind power via VSC-HVDC transmission systems.