Analysis of Circulating Current Mechanism of Grid-Forming Offshore Wind Farm Based on DRU-HVDC

The alternating current (AC) generated from an offshore wind farm (OWF) is passed through a low frequency alternating current (LFAC) submarine cable and then transmitted to a diode rectifier unit (DRU) for rectification to direct current (DC), which is considered a viable solution for OWF power transmission. The voltage and frequency of the system are established by the grid-side converter of the wind turbine (WT), due to uncontrollable nature of the DRU. However, the inconsistency of output parameters between two wind turbines can lead to circulating currents causing an increase in power losses and a decline in system stability. This article provides a detailed analysis of the mechanism behind circulating currents that occur after OWF parallel connections. The analysis divides into four situations: voltage amplitude imbalance, phase angle imbalance, output impedance imbalance, and frequency imbalance. This research lays the foundation for future fundamental frequency circulating current suppression. Additionally, the theoretical analysis about the circulating current was verified using MATLAB/Simulink.