AC Fault Mechanism and Impact Analysis of Offshore Wind Power Connected to Flexible and Direct Systems

[Objective] In order to better understand the fault characteristics of offshore wind power access system, the impact analysis of AC fault characteristics of offshore wind power access system is carried out. [Method] The topology analysis of grid-connected transmission system of offshore wind power and the work control principle analysis of offshore wind power electrical structure were presented, including the work control principle analysis of PMSG, the topology and principle analysis of flexible and direct transmission system, the work control principle analysis of offshore converter station, the work control principle analysis of onshore converter station, and the work control principle analysis of offshore converter station. The fault characteristics and mechanism of offshore wind power access system were analyzed from two aspects: asymmetric fault and symmetric fault. [Result] The simulation model of PSCAD-based offshore wind power transmission through flexible direct transmission is established and verified. The correctness and universal applicability of the proposed mechanism analysis are verified through the simulation analysis of single-phase grounding fault and three-phase grounding fault. [Conclusion] The simulation results verify the correctness of the fault characteristic mechanism analysis. The symmetric component method can be satisfied with the fault analysis in the system with power electronic equipment access. In the case of asymmetric fault, the in-phase short-circuit current on both sides has a certain phase Angle difference, the negative sequence current on the soft straight side is the largest, and the short-circuit current on the wind side is generally small. There is only positive sequence current in the soft direct side and wind field side, and the phase of in-phase short-circuit current on both sides is almost the same, showing amplitude limitation.