Series-DC connection of Offshore wind generating units - modeling, control and galvanic isolation

•Control strategy for an offshore wind farm with the generating units connected in series with a VSC-HVDC transmission link.•DC-DC solid-state transformer to interconnected the wind generation units of an offshore farm in series with a DC transmission link.•Voltage source converter DC transmission systems for offshore wind farm inter- connection. This work proposes an air-core DC-DC solid state transformer to connect the wind turbines of an offshore wind farm in series with a DC transmission link. This emerging layout does not use a power transformer to increase the voltage from the offshore collector network to the transmission level, nor does it require a rectifier station to convert the electrical energy to DC. The diode-clamped rectifiers, connected to each permanent-magnet generator, are controlled to drain sinusoidal currents from the generators in order to extract the maximum power from the wind turbines. The air-core transformer, designed by means of a finite element software, ensures the galvanic isolation between the wind generator and the HVDC link. From the generator side, a single-phase full-bridge three-level inverter feeds the primary coil of the transformer, whereas the secondary coil is wound using by means of an XLPE-insulated cable. This paper also presents a strategy for designing the controllers that regulate the output voltage of each wind conversion unit. Results from digital simulations, obtained with PSCAD/EMTDC, are presented to validate the technical feasibility of the proposed DC-DC solid-state transformer scheme, as well as to demonstrate its performance under different operational conditions.