Accurate asymmetric fault modelling and analysis for offshore HVDC systems

• Bridging the gap between dual-sequence control and the classic symmetric component modelling through rigorous theoretical derivation. • Revealing the loss of negative-sequence current in WPPs due to the voltage-dependent current injection specified by the FRT code. • Reporting the negative-sequence overcurrent of the MMC caused by indirect grounding of interface transformers. This paper provides an accurate asymmetric fault modelling method for offshore high-voltage direct current (HVDC) systems, which provides theoretical guidance for the control and protection design of offshore wind power plants (WPPs) and modular multilevel converters (MMCs). Different from the independent modelling under positive- and negative-sequence in classic power systems, the phase angle of the negative-sequence control in the WPPs is normally obtained from the phase-locked loop in the positive-sequence control, i.e., the control modelling under the negative-sequence is coupled with that of the positive-sequence even during fault steady state. On this basis, the controls of WPPs will interact with those of the MMC and will vary with the fault point parameter and operation mode switching, rendering cumbersome fault analysis and protection design. In this scenario, this paper reveals the dual-sequence coupling and multi-converter interaction with rigorous derivations, based on which it demonstrates the over-current of the MMC due to grounding resistance and the loss of negative-sequence current of the WPPs due to voltage-dependent current injection, those bring new thoughts to the control and protection design of offshore HVDC systems. Finally, the correctness of the theoretical modelling and fault analysis is proved by the simulation results.