Analysis of Transient Characteristics of Asymmetric Fault Current in Offshore Wind Power System Based on Positive and Negative Sequence Decoupling Control

To elucidate the impact mechanism of positive- and negative-sequence decomposition on fault currents in offshore wind AC systems during asymmetrical faults, this study proposes a time-domain method for calculating transient negative-sequence currents. Considering the influence of DSOGI-PLL-based sequence decomposition, the control equation for the negative-sequence transient circuit is derived in the dq-axis coordinate system. The relationship between the negative-sequence component of the grid-side converter (GSC) output voltage modulation signal and the negative-sequence component of the GSC terminal output voltage is analysed, revealing how this discrepancy affects the transient characteristics of the fault negative-sequence current. By integrating the circuit equations in the dq-axis coordinate system, the negative-sequence current control equation, and the relationship between GSC output voltage and modulation voltage, a second-order variable-coefficient differential equation is formulated, constituting the time-domain calculation model for transient negative-sequence current under asymmetrical faults. This model accurately reflects the influence of sequence decomposition and negative-sequence current control on fault currents. Simulation results demonstrate that the model effectively captures transient characteristics such as overshoot and attenuation in the negative-sequence fault current of wind farms under asymmetrical faults, with a steady-state error of less than 3%, providing a theoretical basis for protection scheme development.