Enhancing FRT Strategy for Onshore AC Grid Fault of Offshore Wind HVDC System Based on Iterative Calculation Method

The DC over-voltage of the offshore wind HVDC system caused by onshore AC system faults is an important safety issue facing the absorption of offshore new energy. Existing methods generally involve adding energy-dissipation devices or communication links, as well as improving the LVRT strategy of wind turbines. However, these methods reduce engineering economy or reliability and require changes to the proven control system of wind turbines. Based on this, this paper proposes an FRT strategy for the onshore AC grid fault of offshore wind HVDC systems based on the iterative calculation method. Firstly, a calculation model for the maximum transmittable active power of the onshore MMC under onshore AC fault conditions considering the LVRT strategy is provided. Then, an equivalent power flow calculation model for the offshore MMC during voltage sag is established. Meanwhile, a maximum active power model for the PMSG considering the LVRT strategy is also established. Based on it, an iterative calculation method for the AC voltage reference value of offshore MMC is proposed. Finally, simulations conducted on the RTDS platform have confirmed the efficacy of the presented method. This paper represents a significant contribution to the field of offshore wind HVDC system operational safety, while also facilitating advancements in offshore wind energy development.