Communication-Less Proportional Voltage Balancing to Set up a Voltage-Sourced Platform-Less DC Collection System for Offshore Wind Farms

This paper focuses on an alternative control strategy of the DC wind energy collection system (DWECS) for offshore wind farms with the aim of (a) eliminating the offshore platform, (b) ensuring stable and reliable operation of the DWECS in voltage-sourced mode without depending on any fast communication. The target is to reduce the installation cost and increase operational flexibility without compromising on reliability. Input-parallel output-series connection of isolated DC-DC converters placed on the wind turbine (WT) nacelles is considered here. Voltage-sourced operation of the series-connected DWECS may lead to voltage mismatch and overvoltage in some WT converters, as the spatial distribution of WTs and the wake effect result in unequal input power. To address this, a voltage balancing scheme is proposed here, which involves modifying the input voltages of the WT converters in proportion to their output voltages. Unlike existing strategies, the proposed method does not depend on any fast communication among the WTs and/or between the onshore grid and the turbines. Small-signal stability analysis of the DWECS is carried out to justify the necessity of the proposed control. Finally, effectiveness of the proposed scheme is validated through simulations with realistic system data and experiments on a scaled-down laboratory prototype.