Series-Parallel DC Collection Topology and Control for All-DC Offshore Wind Power System Considering MPPT and Surplus Power Distribution

All-dc offshore wind power system is a promising solution for long distance and high-power transmission. This paper proposes a series-parallel connected DC collection topology for all-dc offshore wind power system. On the one hand, a novel series-parallel modular multilevel converter is proposed for the medium-voltage high-power wind turbines. This converter adopts series-parallel connection to raise the voltage level of the wind turbine and the special structure with multi-winding transformers can eliminate the secondary power fluctuations, thus reducing the dc-link capacitor requirement. On the other hand, considering the wind power difference among the series wind turbines, a power balancing system is configured to balance the surplus power among the series WTs, thereby achieving maximum power point tracking of the series WTs. Compare to the traditional offshore wind power transmission system, the proposed topology cancels the expensive offshore platform, and solve the wind power curtailment problem. In addition, the corresponding control and modulation scheme of the proposed topology are implemented. Digital simulation and real time hard-in-loop simulation tests are conducted to verify the effectiveness and superiority of the proposed DC collection topology.