Interconnection of Direct-Drive Wind Turbines Using a Series-Connected DC Grid

This paper presents a "distributed high-voltage dc (HVDC) converter" for offshore wind farms. The proposed converter topology allows series interconnection of wind turbines with the need of neither ac transformer nor offshore platform at the sending end. Each wind turbine is equipped with a 5-MW permanent-magnet synchronous generator and an ac-dc-dc converter. The converter topology is a diode rectifier (ac-dc) cascaded with a single-switch step-down converter (dc-dc). The dc-dc stage allows the current to flow at all times in the dc link while simultaneously regulating generator torque. The inverter station, located onshore, is a thyristor-based converter that performs dc link current regulation. It also regulates the HVDC link voltage through supervisory inverter controls. A complete wind farm is simulated using the PSCAD/EMTDC software package. The 150-MW wind farm is modelled using six units of 25 MW with a rated dc link voltage of 125 kV at 1.2 kA. The simulation demonstrates the stable operation of the proposed configuration where each turbine is able to independently perform peak power tracking.