The Structure and Control of A Platform-less Voltage-Sourced Offshore Wind Energy Collection System With Series Connected Diode Rectifiers

This work proposes modifications to the conventional offshore wind farm (OWF) structure that can obviate large offshore platforms and significantly reduce installation costs. The proposed modifications involve replacing the two-winding transformer on each wind turbine (WT) tower with a three-winding transformer and connecting a diode bridge rectifier (DBR) at the secondary. The outputs of the DBRs are connected in series to form the HVDC link, thereby eliminating the requirement for the large offshore transformer used in conventional OWFs. This series-connected structure feeding power to a voltage-sourced DC bus poses a control challenge due to possible voltage mismatch at the DBR outputs arising from different wind speeds at individual WTs. To address this issue, the tertiary windings are connected in parallel through AC cables to enable redistribution of input power and balancing of DBR voltages. A frequency droop-based control strategy is presented and its robustness is assessed to ensure stable operation. A complete small-signal model of the proposed platform-less OWF is developed, and eigenvalue-based stability and parametric sensitivity analyses are performed to assess dynamic behavior. The proposed structure and control strategy are validated through PSCAD/EMTDC simulations and experimental studies on a scaled-down laboratory prototype under various normal and contingency conditions.